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TPU tubes with rim brakes?

Old 08-17-22, 06:47 AM
  #26  
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To interrupt the discussion and get back to the original question for a moment, using the new polyurethane tubes is a choice, like so many others we make.

Riding in the big-big combination is a user error. But I do it a few times a month, so I'll take the extra weight hit to make sure my chain's long enough to accommodate it.

I'll take a spare set of brake pads on a long tour because I've needed it. On a shorter, flatter tour, if I changed the pads last month, I wouldn't bother and would save that much weight.

I've looked ridiculous riding with the derailer cable's extra length coiled up because I didn't have the tool to cut it for a few days. But since I had the spare cable, I was still riding!

TPU on a coastal tour of the Gulf or southeast Atlantic coast? Sure. If you're worried about patching, take a butyl spare tube (or two).

Despite the entertaining techno-babble, I've personally seen and felt darn hot rims. (Don't feel them at the stop sign at the bottom of a 3/4 mile 8% grade!) And I've seen the aftermath of a couple of blowoffs caused by braking while touring. While some might immediately think, "User error!" my first thought was "There but for the grace of God go I." While I don't know the exact mechanics of how a rim blows when overheated, I can't see the tube weakening or melting at a lower temperature would be beneficial in any way. You might save a half a pound riding TPU tubes and carrying TPU spares. I'd think that was not a smart place to save weight on a mountainous tour. That personal choice thing again!
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Old 08-17-22, 06:55 AM
  #27  
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Originally Posted by pdlamb View Post
To interrupt the discussion and get back to the original question for a moment, using the new polyurethane tubes is a choice, like so many others we make.

Riding in the big-big combination is a user error. But I do it a few times a month, so I'll take the extra weight hit to make sure my chain's long enough to accommodate it.

I'll take a spare set of brake pads on a long tour because I've needed it. On a shorter, flatter tour, if I changed the pads last month, I wouldn't bother and would save that much weight.

I've looked ridiculous riding with the derailer cable's extra length coiled up because I didn't have the tool to cut it for a few days. But since I had the spare cable, I was still riding!

TPU on a coastal tour of the Gulf or southeast Atlantic coast? Sure. If you're worried about patching, take a butyl spare tube (or two).

Despite the entertaining techno-babble, I've personally seen and felt darn hot rims. (Don't feel them at the stop sign at the bottom of a 3/4 mile 8% grade!) And I've seen the aftermath of a couple of blowoffs caused by braking while touring. While some might immediately think, "User error!" my first thought was "There but for the grace of God go I." While I don't know the exact mechanics of how a rim blows when overheated, I can't see the tube weakening or melting at a lower temperature would be beneficial in any way. You might save a half a pound riding TPU tubes and carrying TPU spares. I'd think that was not a smart place to save weight on a mountainous tour. That personal choice thing again!
all good points really
and yes, I have burned my finger touching my rim after a sketchy loaded descent with melting pad material balled up. Just couldnt resist seeing how hot the rim was.
(but wasn't braking properly, dragging the brakes too long on a big long downhill, so was my fault, learning experience)
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Old 08-17-22, 08:13 AM
  #28  
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Originally Posted by pdlamb View Post
You might save a half a pound riding TPU tubes and carrying TPU spares. I'd think that was not a smart place to save weight on a mountainous tour. That personal choice thing again!
The weight saved will vary pretty widely depending on the tire size and the number of spare tubes. Also it will vary if you compare regular butyl tubes or light weight ones. Someone running 25mm tires, light butyls tubes, and one spare might save 3 or 4 ounces. Someone with 2" tires, and 3 regular butyl spare tubes, may save close to 2 pounds. But, yeah, it is in the realm of personal choice and certainly not a slam dunk given the cost.
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Old 08-17-22, 09:09 AM
  #29  
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Originally Posted by Yan View Post
It makes absolutely no difference whether your pulse your brakes or apply light continuous pressure. As far as heating up your rims, the only thing that matters is your speed of descent. At the top of the hill you have a fixed amount of gravitational potential energy. By the time you reach the bottom of the hill you have a fixed lower amount of gravitational potential energy. During your descent that stored energy was converted to heat via friction. The conversion can happen in one of two ways: via air friction or brake friction.
It does make a large difference. As pointed out above, when the brakes are off, no heat is being put into the system. Think of it this way: Pulsing the brakes hard, heats the surface of the rim. The inner part of the rim is still mostly cool. When the brakes are released, the heat easily radiates away from the rim to the air. The core of the rim will heat some but not much.

Applying continuous pressure soaks heat into the core of the rim. There is some air radiation but the constant friction keeps the rim hotter over a longer distance. Constant braking of any vehicle results in overheated brakes.

At the bottom of the hill, you’ve done the same conversion of kinetic energy to heat but with pulsed braking, you are shedding that heat all along the way. With constant braking, you are keeping the heat in the rim.

If you bomb the hill at high speed and barely use your brakes, all that energy is converted by air friction and your brakes stay cool. Air resistance is the square of the speed, so the faster your bomb the hill the exponentially greater the air friction. If you use your brakes, the energy is converted to heat in your brakes, so they heat up. If you do this for long enough your brakes will overheat. However don't forget your rims are being continuously cooled by the air. The faster you're moving, the more cooling is taking place. The hotter your rims, the faster they transfer heat to the air. On the other hand if you descend like a grandma and take an hour to get to the bottom of the hill, you've spent so long converting your energy that your rims cooled themselves the whole time.
You almost got it but kind of stumbled at the end there. Part of the equation is that air resistance. At higher speeds, heat is swept away front the rim faster and the rims cool faster. At lower speed, less air flow and, thus, less cooling. The continuous braking combined with lower cooling due to less air flow results in more heat being held by the rim. Taking an hour to do a 3 mile hill, for example, would result in cool brakes but taking an hour to do that kind of downhill is a bit excessive.

You can take the above and plot a graph. High average descent speed results in cool brakes. Low average descent speed results in cool brakes. Medium average descent speed results in hot brakes.
You kind of got it. But it’s not the speed that results in hot brakes. It’s their application. A medium average descent speed is usually associated with constant braking to keep your speed at a certain level. High speed descents really aren’t possible with constant brake application.

[What you do in the mean time doesn't make any difference whatsoever. Apply the brakes continuously lightly, pulse it every one second, pulse it every five seconds. Alternate between front and rear. Use both at the same time all the time. As long as your speed stays the same it doesn't matter. When people say they pulse the brakes, what they're really doing is increasing their average speed. They're bombing down the hill, slowing occasionally so they don't die. Higher average speed = cooler rims.
And then you don’t get it. Pulsing brakes isn’t about doing it on a set time interval. It’s about using the brakes when they are needed and then getting off them so that the rim can cool. Yes, you bomb down a hill and hit the brakes so that you don’t run off the road but you do it because it doesn’t over heat the brakes. This also works for disc brakes and is probably more important there. The rotor is much smaller. It has a lot less surface area to radiate the heat and it has a lot less thermal mass for the heat to flow into. Hub mounted rotors are much, much easier to cook.
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Old 08-17-22, 10:11 AM
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This is utterly wrong. The brake pads cover less than one square inch of the rim. They're not some blanket that prevents heat from escaping. Your rims are losing heat at the same rate regardless of whether the brake is on or off. You don't need to "get off the brakes" so they can start cooling. They are already cooling, all the time. All this talk about the surface of the rim vs the core of the rim is just a bunch of mumbo jumbo. Metal is an excellent conductor of heat. Aluminum is used as a heatsink in many applications. The heat is distributed throughout.

Constant vs non-constant braking is also meaningless. It depends on how hard you're applying the brakes. Friction = coefficient x normal force. If you only lightly feather the brakes, there will be hardly any friction. You can apply the brake continuously for 8 hours straight and your rims will still be cool as a cucumber.

Keeping a high speed in the straights and braking at corners has nothing to do with getting off the brakes so they can "start cooling". Your rims are always cooling regardless. All you're doing is increasing your average speed and thereby increasing the percentage of energy converted via air friction vs via brake friction. Less energy going into the brakes = less rim heat. The heat that would have gone into your rims went into your face and body skin instead.

Originally Posted by cyccommute View Post
It does make a large difference. As pointed out above, when the brakes are off, no heat is being put into the system. Think of it this way: Pulsing the brakes hard, heats the surface of the rim. The inner part of the rim is still mostly cool. When the brakes are released, the heat easily radiates away from the rim to the air. The core of the rim will heat some but not much.

Applying continuous pressure soaks heat into the core of the rim. There is some air radiation but the constant friction keeps the rim hotter over a longer distance. Constant braking of any vehicle results in overheated brakes.

At the bottom of the hill, you’ve done the same conversion of kinetic energy to heat but with pulsed braking, you are shedding that heat all along the way. With constant braking, you are keeping the heat in the rim.

You almost got it but kind of stumbled at the end there. Part of the equation is that air resistance. At higher speeds, heat is swept away front the rim faster and the rims cool faster. At lower speed, less air flow and, thus, less cooling. The continuous braking combined with lower cooling due to less air flow results in more heat being held by the rim. Taking an hour to do a 3 mile hill, for example, would result in cool brakes but taking an hour to do that kind of downhill is a bit excessive.

You kind of got it. But it’s not the speed that results in hot brakes. It’s their application. A medium average descent speed is usually associated with constant braking to keep your speed at a certain level. High speed descents really aren’t possible with constant brake application.

And then you don’t get it. Pulsing brakes isn’t about doing it on a set time interval. It’s about using the brakes when they are needed and then getting off them so that the rim can cool. Yes, you bomb down a hill and hit the brakes so that you don’t run off the road but you do it because it doesn’t over heat the brakes. This also works for disc brakes and is probably more important there. The rotor is much smaller. It has a lot less surface area to radiate the heat and it has a lot less thermal mass for the heat to flow into. Hub mounted rotors are much, much easier to cook.

Last edited by Yan; 08-17-22 at 10:25 AM.
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Old 08-18-22, 08:44 AM
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Originally Posted by Yan View Post
This is utterly wrong. The brake pads cover less than one square inch of the rim. They're not some blanket that prevents heat from escaping. Your rims are losing heat at the same rate regardless of whether the brake is on or off. You don't need to "get off the brakes" so they can start cooling. They are already cooling, all the time. All this talk about the surface of the rim vs the core of the rim is just a bunch of mumbo jumbo. Metal is an excellent conductor of heat. Aluminum is used as a heatsink in many applications. The heat is distributed throughout.
A “heatsink” is just what it sounds like…a place to sink heat into. It can be a heat exchanger but not necessarily. Metal can “sink” (or absorb) a whole lot of heat because of the metal’s high melt point. Some heat will radiate away from the metal if the metal isn’t insulated but if you keep putting heat into the metal, it will just keep absorbing the heat to the point where it eventually melts. Granted that takes an enormous amount if heat but, for the purposes of this discussion, the more fiction your apply to the metal, the hotter it will get and the hotter it will stay…i.e. you “sink” more heat into the system.

“Getting off the brakes” or not dragging the brakes, if you like, doesn’t constantly “sink” heat into the rims like constant fiction does. The whole point of stopping in the middle of a downhill to let your brakes cool is to stop the friction on the metal so that it has a chance to radiate away. The same thing is accomplished by pulse braking without the bother of stopping. In fact, the heat exchange with the air (different from the heat sink) is more efficient when air is flowing over the rims which is what makes pulse braking more effective than constant braking with a stop to cool the rim.

Constant vs non-constant braking is also meaningless. It depends on how hard you're applying the brakes. Friction = coefficient x normal force. If you only lightly feather the brakes, there will be hardly any friction. You can apply the brake continuously for 8 hours straight and your rims will still be cool as a cucumber.
Yes, that is the equation for friction. But there is more to the story than just the amount of friction. There is the rate at which the friction is applied and the rate at which it can be radiated away. You’ve already addressed that above with your comment about medium speed resulting in hotter the wheels will become. Part of that is because you can’t reach high speeds to take advantage of the greater air flow if you constantly use the brakes to keep your speed low. Attaining high speed is only possible if you use the brakes sparingly…i.e. with pulsed braking.

Keeping a high speed in the straights and braking at corners has nothing to do with getting off the brakes so they can "start cooling". Your rims are always cooling regardless. All you're doing is increasing your average speed and thereby increasing the percentage of energy converted via air friction vs via brake friction. Less energy going into the brakes = less rim heat. The heat that would have gone into your rims went into your face and body skin instead.
If your brakes will stay cool with constant braking, then why do people who practice that type of braking need to stop in the middle of a descent to let their brakes cool. A 3 to 5 mile downhill isn’t all that long. What you describe…braking hard for corners and letting the bike run without brakes in the straights…is the definition of “pulse braking”.
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Old 08-18-22, 10:09 AM
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Originally Posted by cyccommute View Post
A “heatsink” is just what it sounds like…a place to sink heat into. It can be a heat exchanger but not necessarily. Metal can “sink” (or absorb) a whole lot of heat because of the metal’s high melt point. Some heat will radiate away from the metal if the metal isn’t insulated but if you keep putting heat into the metal, it will just keep absorbing the heat to the point where it eventually melts. Granted that takes an enormous amount if heat but, for the purposes of this discussion, the more fiction your apply to the metal, the hotter it will get and the hotter it will stay…i.e. you “sink” more heat into the system.

“Getting off the brakes” or not dragging the brakes, if you like, doesn’t constantly “sink” heat into the rims like constant fiction does. The whole point of stopping in the middle of a downhill to let your brakes cool is to stop the friction on the metal so that it has a chance to radiate away. The same thing is accomplished by pulse braking without the bother of stopping. In fact, the heat exchange with the air (different from the heat sink) is more efficient when air is flowing over the rims which is what makes pulse braking more effective than constant braking with a stop to cool the rim.

Yes, that is the equation for friction. But there is more to the story than just the amount of friction. There is the rate at which the friction is applied and the rate at which it can be radiated away. You’ve already addressed that above with your comment about medium speed resulting in hotter the wheels will become. Part of that is because you can’t reach high speeds to take advantage of the greater air flow if you constantly use the brakes to keep your speed low. Attaining high speed is only possible if you use the brakes sparingly…i.e. with pulsed braking.

If your brakes will stay cool with constant braking, then why do people who practice that type of braking need to stop in the middle of a descent to let their brakes cool. A 3 to 5 mile downhill isn’t all that long. What you describe…braking hard for corners and letting the bike run without brakes in the straights…is the definition of “pulse braking”.
Your technique is correct but your understanding of the physics is wrong. You're getting confused by the braking at corners. You're not braking at the corners because you're concerned about heat. You're braking at the corners because if you don't you'll crash. Later when you're sitting in your armchair you mull it over and pretend you did it for heat control.

But have you ever ridden a descent with no corners at all? I have. Here it is on the map. And here is a photo of the top with an elevation sign. It's a desert pass over the Altyn-Tagh. The word "pass" is being used loosely here. It's actually one way slope up onto the Tibetan Plateau. 8000ft elevation change top to bottom. Most of it is dead straight on alluvial fans. Part of it is through some rocky hills but the road is engineered for high speed traffic so the curves are large radius and don't require cornering. You can get a tuck on this descent, never touch your brakes, and bomb 50+ mph the entire way down. But you won't do that because you don't have a death wish right? What if you hit something on the road and blow a tire at that speed? You're dead. But what braking technique do you use when there are no corners to force you to brake? Do you pulse, alternate, or light drag? The answer is it doesn't matter. Heat management is like dieting. If you eat 15 cheese burgers a day you're going to get obese. It doesn't matter what interval you stuff your face at. All that matters is total energy in energy out. If you dump more energy into your rims per minute than they can lose, you will overheat eventually. It doesn't matter what braking pattern you use. If you maintain a high average speed you'll lose more energy to air resistance and have to dump less energy into your rims. That's the only thing that matters.

Here's another thought experiment: is it better to alternate drag front and rear, or drag both continuously at half the hand pressure? The answer is, again, it doesn't matter. You're dumping the exact same amount of energy per minute into your two rims. When you get to the bottom of the descent and measure your rims, you'll find that the temperature is identical between the two techniques.

For the record I alternate drag between front and rear. It's not for heat management. It's for my hand muscles. I never do the fast-slow-fast-slow boink boink Slinky style. Riding like that is stupid and annoying, for both myself and anyone else riding with me. I sit up straight to catch the air and try to not use my brakes at all. If I have to use my brakes I heat manage with my ears. When your brakes start to overheat you'll hear the sound change. At that point I pull over and take a photo break. When you are on a big mountain descent the scenery is usually pretty good.

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Last edited by Yan; 08-18-22 at 10:28 AM.
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Old 08-18-22, 12:44 PM
  #33  
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Originally Posted by Yan View Post
If I have to use my brakes I heat manage with my ears. When your brakes start to overheat you'll hear the sound change. At that point I pull over and take a photo break.
It's also possible, if you've got nice thick brake pads, to ride the brakes hard at that point. If you're lucky the surface of the pad will melt and slough off, leaving nice fresh brake pad material. But even though it's possible, I don't recommend it because your tire might blow off first.
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Old 08-19-22, 06:30 AM
  #34  
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Originally Posted by Yan View Post
Your technique is correct but your understanding of the physics is wrong. You're getting confused by the braking at corners. You're not braking at the corners because you're concerned about heat. You're braking at the corners because if you don't you'll crash. Later when you're sitting in your armchair you mull it over and pretend you did it for heat control.
No, my understanding of physics isn’t “wrong”. I know that I’m braking at corners to maintain control. I’m not braking in the straights to avoid heat build up…at least not braking constantly in the straight parts to avoid heat build up. I pulse brake in the straights if the speed is getting too high for comfort. My tolerance for speed is probably a bit higher than most people, however.

But have you ever ridden a descent with no corners at all? I have. Here it is on the map. And here is a photo of the top with an elevation sign. It's a desert pass over the Altyn-Tagh. The word "pass" is being used loosely here. It's actually one way slope up onto the Tibetan Plateau. 8000ft elevation change top to bottom. Most of it is dead straight on alluvial fans. Part of it is through some rocky hills but the road is engineered for high speed traffic so the curves are large radius and don't require cornering. You can get a tuck on this descent, never touch your brakes, and bomb 50+ mph the entire way down. But you won't do that because you don't have a death wish right? What if you hit something on the road and blow a tire at that speed? You're dead. But what braking technique do you use when there are no corners to force you to brake? Do you pulse, alternate, or light drag? The answer is it doesn't matter.
Yes, I’ve done lots of relatively straight long descents. Probably not 8000 ft ones but lots of others. You don’t say how long this 8000 foot descent is but isn’t going to happen in 3 miles. Very long descents don’t really require that much in terms of braking nor would I expect the wheels to heat much in those conditions. Yes, I agree that your brakes are going to have lots of opportunity to cool on that descent. But you also aren’t likely to drag your brakes over a very long descent like that. If you did, the descent would take forever and your brakes and wheels would get very hot. Dragging brakes on a very long descent is not a good idea.

Heat management is like dieting. If you eat 15 cheese burgers a day you're going to get obese. It doesn't matter what interval you stuff your face at. All that matters is total energy in energy out. If you dump more energy into your rims per minute than they can lose, you will overheat eventually. It doesn't matter what braking pattern you use. If you maintain a high average speed you'll lose more energy to air resistance and have to dump less energy into your rims. That's the only thing that matters.
The hamburgers is the wrong analogy.

However you are so close to understanding. I agree that if you dump more energy into your rims per minute than they can lose, they will overheat. But if you aren’t dumping friction into the rims…i.e. pulse or “on/off” braking…you aren’t dumping any energy into the rims when the brakes are off. If you are constantly braking whether lightly or strongly, you are constantly dumping energy into the wheels. The wheels can’t shed all of that energy and they overheat, thus the need to stop and “let them cool down”. That cool down period is a time of no energy being put into the system which allows the heat to radiate away…just without the air flow you’d get if you were riding without the brakes on.


Here's another thought experiment: is it better to alternate drag front and rear, or drag both continuously at half the hand pressure? The answer is, again, it doesn't matter. You're dumping the exact same amount of energy per minute into your two rims. When you get to the bottom of the descent and measure your rims, you'll find that the temperature is identical between the two techniques.
You are missing the third scenario…not dragging the brakes at all. In the scenario where you are pulse braking…i.e. brake hard, then no braking, then brake hard, ad infinitum…you are putting the same energy into the system but there are periods of high energy input and periods of no energy input or even negative energy input as the air pulls the heat away. At the end of the descent, pulse braking rims will be cooler than if you drag the brakes all the time.

For the record I alternate drag between front and rear. It's not for heat management. It's for my hand muscles. I never do the fast-slow-fast-slow boink boink Slinky style. Riding like that is stupid and annoying, for both myself and anyone else riding with me. I sit up straight to catch the air and try to not use my brakes at all. If I have to use my brakes I heat manage with my ears. When your brakes start to overheat you'll hear the sound change. At that point I pull over and take a photo break. When you are on a big mountain descent the scenery is usually pretty good.
Who is taking about your “Slinky style” of braking? I don’t do that kind of braking, nor would I suggest it. When I brake hard, it isn’t to a stop or even to a super slow speed just to go around a corner. I know how to corner fast. In my experience, brake draggers are usually the ones who do the “slow-slower-slow-slower Slinky style” braking on steep, curvy downhills.

Frankly, I don’t pay too much attention to my brakes while riding. I don’t hear a sound change because I never get my brakes and rims hot enough to make that sound. I pull over to take pictures because I want to, not because I need to.
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Old 08-19-22, 06:53 AM
  #35  
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Originally Posted by Yan View Post
You can get a tuck on this descent, never touch your brakes, and bomb 50+ mph the entire way down. But you won't do that because you don't have a death wish right? What if you hit something on the road and blow a tire at that speed? You're dead.
very telling sentence here.
I get it Yan, you've toured in mountains and have gone down big descents successfully a lot, but this comment shows that you are not really comfortable with speed and your assessment of danger is different than mine.

Internet discussions are great, but every descent is different and how fast one can go down a descent and how much one needs to control speed will depend on lots and lots of factors, and one factor is the individuals riding skill and competence at speed, not to mention judgement.

of course, all the real world factors prevail--
-line of sight
-your eyesight and reaction time
-your experience going fast and competency with high speed bike handling
-how you brake, ie using front brake HARD for effective bleeding off speed, (back brake too) and letting off fully to let heat dissapate.
-road surface
-grade
-upcoming turn radius
-possibility of side arriving traffic
-potholes, no potholes
-wind, especially side gusts
-any and all traffic
-intimately knowing the condition of your tires and bike in general
-knowing how to manage your braking system ie proper braking, and constantly assessing the possible changing effectiveness of your braking system and adjusting for that

so one riders "completely comfortable" can easily be another riders "terror filled descent", with death looming, as your blanket comment does seem strongly to suggest your view.

BUT, this aint a pissing contest or who has bigger balls, descend as you are comfortable and competent at, who cares? As long as one is safe and is able to assess from experience what is safe, then its all good.

Improper use of braking though can have serious implications, and that's what this usual discussion always leads back to.....but interblabs writing will never really know how one person brakes.......
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Old 08-19-22, 07:05 AM
  #36  
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Originally Posted by cyccommute View Post
Who is taking about your “Slinky style” of braking? I don’t do that kind of braking, nor would I suggest it. When I brake hard, it isn’t to a stop or even to a super slow speed just to go around a corner. I know how to corner fast. In my experience, brake draggers are usually the ones who do the “slow-slower-slow-slower Slinky style” braking on steep, curvy downhills.
very much so, this is my observation also.
and from all the people I ride with who do this, it comes down to fear of using the front brake.
the fear of "flipping over the handlebars" is so prevalent in most people I know, or the fear of locking the front wheel and losing the front.

developing the skill of developing feel for traction is why I've always felt that skiing, mountain biking on loose surfaces, riding motorcycles on loose surfaces, is such a huge advantage to have done in life, simply because it can give you the experience of knowing by feel how hard you can brake or turn on various , constantly changing surfaces.

mind you, I have friends who have skied their whole lives, but just dont have the inherent reaction times, reflexes, eyesight, and are still slow as molasses and do the slow, slow, slower thing--so there's that too, some people just like that (and again, thats ok, just not when they make blanket statements on what is safe that they think apply to others.
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Old 08-19-22, 07:23 AM
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Originally Posted by Yan View Post
But have you ever ridden a descent with no corners at all? I have. Here it is on the map. And here is a photo of the top with an elevation sign. It's a desert pass over the Altyn-Tagh. The word "pass" is being used loosely here. It's actually one way slope up onto the Tibetan Plateau. 8000ft elevation change top to bottom. Most of it is dead straight on alluvial fans. Part of it is through some rocky hills but the road is engineered for high speed traffic so the curves are large radius and don't require cornering. You can get a tuck on this descent, never touch your brakes, and bomb 50+ mph the entire way down. But you won't do that because you don't have a death wish right? What if you hit something on the road and blow a tire at that speed? You're dead.
I don't know about that pass since I haven't been there, but suspect I'd be tempted to let it roll all the way down if the surface was decent at all. I have done so on some a long descents with at least one of over 5000' of descending in the Sierras with only some minimal braking for the high speed well banked and wide switchbacks. It was a blast. I do have a long history of risk sports including but not limited to a variety of motorcycle and bicycle racing disciplines. I was never a very good bicycle racer, but did descend like a demon especially on a MTB or in a canoe or kayak. I have been accused of having a death wish pretty frequently by my buddies and compeditors. So suffice it to say, I tend to like to let the speed max out.

As far as certain death from a 50mph crash. That is nonsense. The biggest danger would be from going over the side of a dangerous drop off at any speed even a lower one, or blowing a turn and hitting a stationary object. From your description it sounds like the liklihood of that is low there.

By the way if you have panniers on the bike i'd question the 50+ miles per hour. It is pretty hard to get to that speed due to the aero drag IME even if you tuck.
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Old 08-19-22, 07:28 AM
  #38  
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Making yourself as big and non-aero as possible has a role in descending without overheating your brakes if you're worried about speed.
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Old 08-19-22, 08:49 AM
  #39  
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Originally Posted by cyccommute View Post
You are missing the third scenario…not dragging the brakes at all. In the scenario where you are pulse braking…i.e. brake hard, then no braking, then brake hard, ad infinitum…you are putting the same energy into the system but there are periods of high energy input and periods of no energy input or even negative energy input as the air pulls the heat away. At the end of the descent, pulse braking rims will be cooler than if you drag the brakes all the time.
This is exactly slinky braking you're describing right here. I'm sorry but if I had to ride like that I would go insane.

You just don't get it. Pulse braking, you're braking hard some of the time and completely off the brakes at the other times. Drag braking, you're never off the brakes, but you're applying less force than in pulse braking so you're putting heat into the rims at a slower rate. End of the day you put the same total amount of energy into the rims. Therefore it makes no difference what technique you use. Same temperature.

Applying the brakes doesn't put a magical blanket on the rims that stops them from losing heat. What you're saying is the same as saying, during the time you're sitting down to eat, you body is losing no calories. No mister, your body doesn't care if you happen to be eating at any given moment. It's burning the same 85 calories per hour all day long. Your rims don't care if you're adding heat to them at any given moment. They're losing heat continuously the entire descent. When or how you add the fixed amount of energy into the rims doesn't matter. The only thing that matter is the total amount of energy you put in. Drag lightly the entire time vs brake hard pulsing, doesn't matter. Same total heat energy.

Originally Posted by djb View Post
very telling sentence here.
I get it Yan, you've toured in mountains and have gone down big descents successfully a lot, but this comment shows that you are not really comfortable with speed and your assessment of danger is different than mine.

Internet discussions are great, but every descent is different and how fast one can go down a descent and how much one needs to control speed will depend on lots and lots of factors, and one factor is the individuals riding skill and competence at speed, not to mention judgement.

of course, all the real world factors prevail--
-line of sight
-your eyesight and reaction time
-your experience going fast and competency with high speed bike handling
-how you brake, ie using front brake HARD for effective bleeding off speed, (back brake too) and letting off fully to let heat dissapate.
-road surface
-grade
-upcoming turn radius
-possibility of side arriving traffic
-potholes, no potholes
-wind, especially side gusts
-any and all traffic
-intimately knowing the condition of your tires and bike in general
-knowing how to manage your braking system ie proper braking, and constantly assessing the possible changing effectiveness of your braking system and adjusting for that

so one riders "completely comfortable" can easily be another riders "terror filled descent", with death looming, as your blanket comment does seem strongly to suggest your view.

BUT, this aint a pissing contest or who has bigger balls, descend as you are comfortable and competent at, who cares? As long as one is safe and is able to assess from experience what is safe, then its all good.

Improper use of braking though can have serious implications, and that's what this usual discussion always leads back to.....but interblabs writing will never really know how one person brakes.......
Yes that's the thing about touring. You're always riding on unknown roads, by definition. Unless you consider riding the same loop every week around your house to be "touring"? The world has many countries, life is short. I don't waste my life touring somewhere I've already toured before. Therefore the road conditions are always unknown to me.

You have no idea what's coming up on the road. You can be bombing down a descent in a wealthy country with the best roads in the world, but there was a rock fall the previous week and it smashed a hole in the pavement. You have no idea what's in front of you.

I don't have the balls to bomb down a 8000 ft descent without touching the brakes. If you do, good for you. I don't think you're lying. I believe you. Enjoy your exhilaration. Try not to do a Fabio Casartelli.

Originally Posted by staehpj1 View Post
As far as certain death from a 50mph crash. That is nonsense. The biggest danger would be from going over the side of a dangerous drop off at any speed even a lower one, or blowing a turn and hitting a stationary object. From your description it sounds like the liklihood of that is low there.
The danger would be from crashing on a curved section of road, hitting something stationary, and going splat like a bug. If you're going at motorcycle speeds you will have a motorcycle style aftermath. If you feel curious there are plenty of motorcyclist decapitation / delimbing / disembowelment videos on the cesspool of R*ddit. If you haven't watched any of these videos I highly suggest you do. Modern society is so insulated from death that most people have no idea how weak the human body is. We are meat jello. Land in the wrong way and it doesn't take much energy to break your back. But yes it's situational. Ideally you would just have a bunch of road rash. Yikes.

And by the way, motorcycles have suspension and big tires, touring bikes don't.

Last edited by Yan; 08-19-22 at 09:29 AM.
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Old 08-19-22, 09:25 AM
  #40  
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Originally Posted by Yan View Post
This is exactly slinky braking you're describing right here. I'm sorry but if I had to ride like that I would go insane.

You just don't get it. Pulse braking, you're braking hard some of the time and completely off the brakes at the other times. Drag braking, you're never off the brakes, but you're applying less force than in pulse braking so you're putting heat into the rims at a slower rate. End of the day you put the same total amount of energy into the rims. Therefore it makes no difference what technique you use. Same temperature.

Applying the brakes doesn't put a magical blanket on the rims that stops them from losing heat. What you're saying is the same as saying, during the time you're sitting down to eat, you body is losing no calories. No mister, your body doesn't care if you happen to be eating at any given moment. It's burning the same 85 calories per hour all day long. Your rims don't care if you're adding heat to them at any given moment. They're losing heat continuously the entire descent. When or how you add the fixed amount of energy into the rims doesn't matter. The only thing that matter is the total amount of energy you put in. Drag lightly the entire time vs brake hard pulsing, doesn't matter. Same total heat energy.



Yes that's the thing about touring. You're always riding on unknown roads, by definition. Unless you consider riding the same loop every week around your house to be "touring"? The world has many countries, life is short. I don't waste my life touring somewhere I've already toured before. Therefore the road conditions are always unknown to me.

You have no idea what's coming up on the road. You can be bombing down a descent in a wealthy country with the best roads in the world, but there was a rock fall the previous week and it smashed a hole in the pavement. You have no idea what's in front of you.

I don't have the balls to bomb down a 8000 ft descent without touching the brakes. If you do, good for you. I don't think you're lying. I believe you. Enjoy your exhilaration. Try not to do a Fabio Casartelli.
Line of sight of course is a big deal, seeing far enough ahead to react, and to moderate speed to allow a reaction.

So we do have an idea of what's ahead, by looking, and judging things, always ready to change our speed or line or whatever.

But it's all about the time and place, and assessing the given situation.
And heck, we all have a million times more chances of getting taken out from behind by a texting driver---whether with regular tubes or TPU tubes.

But yup, life has dangers, most people think we are nuts bicycling touring on roads with cars.
Touch wood
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Old 08-19-22, 09:41 PM
  #41  
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Originally Posted by staehpj1 View Post
By the way if you have panniers on the bike i'd question the 50+ miles per hour. It is pretty hard to get to that speed due to the aero drag IME even if you tuck.
Agreed. The fastest I’ve done on a single bike is 52 mph in a full tuck. I’ve hit 55 on a tandem on an 11% grade. Add panniers and it’s tough to get past about 45 mph. But, like you, I’d just let it fly.
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Old 08-19-22, 10:43 PM
  #42  
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QUOTE=Yan;22615701]This is exactly slinky braking you're describing right here. I'm sorry but if I had to ride like that I would go insane.[/QUOTE]

You are making too much out of what I’m calling “pulse braking”. I brake when needed. I’m not constantly braking braking on and off which is what you seem to think I’m describing.

You just don't get it. Pulse braking, you're braking hard some of the time and completely off the brakes at the other times.
Well you do seem to get it. But…

Drag braking, you're never off the brakes, but you're applying less force than in pulse braking so you're putting heat into the rims at a slower rate. End of the day you put the same total amount of energy into the rims. Therefore it makes no difference what technique you use. Same temperature.
Your analysis if flawed. It’s the same energy but not the same temperature. That’s because there is a significant amount of time where no heat is being put into the wheel during braking with pulse braking. I agree that the amount of energy put into the system is the same but there are long periods of no energy input. Constant braking doesn’t give the same effect. In general, people who overheat rims generally practice your constant force braking, aka dragging their brakes. You’ve stated that you have melted pads. I’ve never melted a pad nor even gotten close to overheating a rim.

Applying the brakes doesn't put a magical blanket on the rims that stops them from losing heat.
I have never said that. But I have said that constant input of heat from friction causes heat to build more than if there are large intervals of no input. Both rims will radiate heat but the constant braking scenario can’t shed heat faster than it is input. If you can’t shed heat faster than it is input, the heat builds up faster. The heat is additive. As you pointed out above, friction is equal to the coefficient of friction X the normal force. If there is no normal force (i.e. the brakes are off), there is no friction.

What you're saying is the same as saying, during the time you're sitting down to eat, you body is losing no calories. No mister, your body doesn't care if you happen to be eating at any given moment. It's burning the same 85 calories per hour all day long.
I don’t like your food analogies. A biological unit is different from a non-biological unit.

Your rims don't care if you're adding heat to them at any given moment. They're losing heat continuously the entire descent. When or how you add the fixed amount of energy into the rims doesn't matter. The only thing that matter is the total amount of energy you put in. Drag lightly the entire time vs brake hard pulsing, doesn't matter. Same total heat energy.
We don’t need an analogy because we have an excellent example right here. Your problem is that you are confusing “heat” with “temperature”. They aren’t necessarily the same thing. You can have the same amount of heat put into two systems but end up with two different temperatures based on the rate.
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Old 08-20-22, 10:50 AM
  #43  
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Of

[just back, catching up]

1. Tubolito warns that their tubes aren't suitable for rim brakes. Aerothan are ok.
2. I can remember of only one flat during the past five years, riding 10+kms on butyl. Got three flats this summer after switching to TPU. Might be because of the unusual prevalence of broken glass on the road, random bad luck or because TPU is thinner and therefore more vulnerable than butyl
3. I am probably giving up on TPU because a patch didn't hold. Interesting in theory.

Might
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Old 08-22-22, 10:55 AM
  #44  
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If you fill a glass with water, it's the same amount of water whether you drip it in slowly, or fill the glass in a few big pours spaced out in time. Dripping continuously vs several big pours does not have any effect on how fast water evaporates out of the glass. Whether water happens to be dripping into the glass at any given moment has no effect on the way water evaporates out. As long as there is some water in the glass, it's evaporating out at the same rate no matter what you do. The drops of water dripping in are not a magic blanket preventing evaporation.

I don't know why you're so caught up in your magic blanket theory. It's simply plain wrong.

Originally Posted by cyccommute View Post

Your analysis if flawed. It’s the same energy but not the same temperature. That’s because there is a significant amount of time where no heat is being put into the wheel during braking with pulse braking. I agree that the amount of energy put into the system is the same but there are long periods of no energy input. Constant braking doesn’t give the same effect. In general, people who overheat rims generally practice your constant force braking, aka dragging their brakes. You’ve stated that you have melted pads. I’ve never melted a pad nor even gotten close to overheating a rim.

I have never said that. But I have said that constant input of heat from friction causes heat to build more than if there are large intervals of no input. Both rims will radiate heat but the constant braking scenario can’t shed heat faster than it is input. If you can’t shed heat faster than it is input, the heat builds up faster. The heat is additive. As you pointed out above, friction is equal to the coefficient of friction X the normal force. If there is no normal force (i.e. the brakes are off), there is no friction.

I don’t like your food analogies. A biological unit is different from a non-biological unit.

We don’t need an analogy because we have an excellent example right here. Your problem is that you are confusing “heat” with “temperature”. They aren’t necessarily the same thing. You can have the same amount of heat put into two systems but end up with two different temperatures based on the rate.
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Old 08-22-22, 06:20 PM
  #45  
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Originally Posted by Yan View Post
If you fill a glass with water, it's the same amount of water whether you drip it in slowly, or fill the glass in a few big pours spaced out in time. Dripping continuously vs several big pours does not have any effect on how fast water evaporates out of the glass. Whether water happens to be dripping into the glass at any given moment has no effect on the way water evaporates out. As long as there is some water in the glass, it's evaporating out at the same rate no matter what you do. The drops of water dripping in are not a magic blanket preventing evaporation.
Again, your analogy is flawed. Let’s try to fix it. Instead of water, let’s use a liquid that evaporates at a rate that is just slightly slower than the rate it drips in. Let’s start with the glass empty. With constant drips, the glass will slowly fill over time until it pours over the edge. It will take a while but eventually, the drips in will exceed the evaporation out. That’s the constant dragging of brakes scenario.

Now let’s pour in a large pour of this magic liquid and then let it sit for a while. The liquid is evaporating at the same rate as before but you aren’t adding any liquid in. The liquid level goes down. Now you pour in a bit more of the liquid and let it sit again. Same thing happens as before. If you carefully balance the input, the overall evaporation rate is higher than the overall liquid input and you end up with a glass that has less liquid in it than the constant input.

But we don’t need an analogy. We have the comparison of pulse braking vs constant braking. Even if the friction input is equivalent, constant input of heat results in a higher temperature of the rims than does large inputs of heat with significant periods of negative heat…i.e. radiant cooling. The result is a lower rim (or any other friction surface) temperature.

The heat input may not be equivalent, however. In the pulse braking method, the speed is faster and the time spent riding down a hill is lower. Less time results in less heat input. Additionally, the movement of the air over the rims is higher. This results in a higher exchange of heat and greater cooling than if the brakes are dragged constantly.

I don't know why you're so caught up in your magic blanket theory. It's simply plain wrong.
You call it a magic blanket. It isn’t. As for it being wrong, why don’t professional racers, who descent far faster and far longer than us mere mortals, stop in the middle of a downhill to let their brakes cool off? They pulse brake all the time. They brake hard before corners and stay off the brakes the rest of the time.
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Old 08-23-22, 09:55 AM
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Originally Posted by cyccommute View Post
Again, your analogy is flawed. Let’s try to fix it. Instead of water, let’s use a liquid that evaporates at a rate that is just slightly slower than the rate it drips in. Let’s start with the glass empty. With constant drips, the glass will slowly fill over time until it pours over the edge. It will take a while but eventually, the drips in will exceed the evaporation out. That’s the constant dragging of brakes scenario.

Now let’s pour in a large pour of this magic liquid and then let it sit for a while. The liquid is evaporating at the same rate as before but you aren’t adding any liquid in. The liquid level goes down. Now you pour in a bit more of the liquid and let it sit again. Same thing happens as before. If you carefully balance the input, the overall evaporation rate is higher than the overall liquid input and you end up with a glass that has less liquid in it than the constant input.

But we don’t need an analogy. We have the comparison of pulse braking vs constant braking. Even if the friction input is equivalent, constant input of heat results in a higher temperature of the rims than does large inputs of heat with significant periods of negative heat…i.e. radiant cooling. The result is a lower rim (or any other friction surface) temperature.

The heat input may not be equivalent, however. In the pulse braking method, the speed is faster and the time spent riding down a hill is lower. Less time results in less heat input. Additionally, the movement of the air over the rims is higher. This results in a higher exchange of heat and greater cooling than if the brakes are dragged constantly.

You call it a magic blanket. It isn’t. As for it being wrong, why don’t professional racers, who descent far faster and far longer than us mere mortals, stop in the middle of a downhill to let their brakes cool off? They pulse brake all the time. They brake hard before corners and stay off the brakes the rest of the time.
Your modification to the analogy is wrong. You're taking a fair scenario and changing it to be unfair. In your version you end up pouring less overall volume of water into your glass compared to the drip side. Your scenario forces the drip side to slowly overflow, but then you say, "well, I control my side and limit my pours so the glass never overflows".

Well yeah of course you can control how much you pour each time so you never over flow. But I can ALSO slow down my drip rate so it never overflows. So what's your point??? You are allowed to control your pour rate but I'm not allowed to control my drip rate? What the hell?

The pro cyclists are descending at nearly 100km/h. Air resistance increases at the square of the speed. It's an exponential increase. At those speeds almost all of the gravitational potential energy of the altitude is being burnt off via air resistance. The pro cyclists barely tax their brakes at all. That's why their brakes don't over heat. It has nothing to do with pulse braking.

Last edited by Yan; 08-23-22 at 10:01 AM.
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Old 08-23-22, 10:28 AM
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Originally Posted by Yan View Post
Your modification to the analogy is wrong. You're taking a fair scenario and changing it to be unfair. In your version you end up pouring less overall volume of water into your glass compared to the drip side. Your scenario forces the drip side to slowly overflow, but then you say, "well, I control my side and limit my pours so the glass never overflows".

Well yeah of course you can control how much you pour each time so you never over flow. But I can ALSO slow down my drip rate so it never overflows. So what's your point??? You are allowed to control your pour rate but I'm not allowed to control my drip rate? What the hell?
It’s not my fault it’s a poor analogy. Although I’ve explained the real world situation several times and it is easier to understand as well as more valid, here it is again:

Let’s assume the heat input is the same for both dragging brakes and pulse braking. With constant braking the heat is put into the system continuously and is radiated away continuously. The heat build up is linear and additive. The longer the brakes are rubbing against the rim, the more heat goes into the rim. The radiation of the heat is also additive but, since it has a negative sign, is subtracted from the heat input. But the constant input of heat never lets the radiative losses out pace the heat input. The temperature of the rims increases as a result. (Heat and temperature isn’t the same. This link explains why.)

With pulse braking, the same amount of heat put into the rim but there are intervals where there is no heat being put into the system and heat is being removed from the system. No heat being added and heat being subtracted results in less overall heat in the system. The temperature of the rims doesn’t increase and may even decrease over the same distance traveled.

The pro cyclists are descending at nearly 100km/h. Air resistance increases at the square of the speed. It's an exponential increase. At those speeds almost all of the gravitational potential energy of the altitude is being burnt off via air resistance.
And the pulse braking system results in a higher overall speed as well. If you are dragging your brakes, you are trying to keep your speed down over a given distance. And, yes, air resistance comes into play at higher speeds as well. Someone practicing pulse braking can take far greater advantage of the wind resistance and wind braking so that heat doesn’t go into the wheels as it does with constant braking.

The pro cyclists barely tax their brakes at all. That's why their brakes don't over heat. It has nothing to do with pulse braking.
Sorry but that is incorrect. They tax their brakes (and rims during the age of rim brakes) far more than us regular cyclists do. They brake harder for corners than we do and they are traveling at a higher speed. Their brakes don’t over heat because they don’t use them all the time. They don’t drag their brakes down hills to control their speed. They brake hard for corners and then let the bike fly. If they need to control speed in straights…not something that happens very often…they tap their brakes, slow a little, and then get off their brakes again. That is the very definition of pulse braking.

If they tried to drag brakes down a hill, they would lose.
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Old 08-23-22, 11:08 AM
  #48  
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Originally Posted by cyccommute View Post
It’s not my fault it’s a poor analogy. Although I’ve explained the real world situation several times and it is easier to understand as well as more valid, here it is again:

Let’s assume the heat input is the same for both dragging brakes and pulse braking. With constant braking the heat is put into the system continuously and is radiated away continuously. The heat build up is linear and additive. The longer the brakes are rubbing against the rim, the more heat goes into the rim. The radiation of the heat is also additive but, since it has a negative sign, is subtracted from the heat input. But the constant input of heat never lets the radiative losses out pace the heat input. The temperature of the rims increases as a result. (Heat and temperature isn’t the same. This link explains why.)

With pulse braking, the same amount of heat put into the rim but there are intervals where there is no heat being put into the system and heat is being removed from the system. No heat being added and heat being subtracted results in less overall heat in the system. The temperature of the rims doesn’t increase and may even decrease over the same distance traveled.

And the pulse braking system results in a higher overall speed as well. If you are dragging your brakes, you are trying to keep your speed down over a given distance. And, yes, air resistance comes into play at higher speeds as well. Someone practicing pulse braking can take far greater advantage of the wind resistance and wind braking so that heat doesn’t go into the wheels as it does with constant braking.

Sorry but that is incorrect. They tax their brakes (and rims during the age of rim brakes) far more than us regular cyclists do. They brake harder for corners than we do and they are traveling at a higher speed. Their brakes don’t over heat because they don’t use them all the time. They don’t drag their brakes down hills to control their speed. They brake hard for corners and then let the bike fly. If they need to control speed in straights…not something that happens very often…they tap their brakes, slow a little, and then get off their brakes again. That is the very definition of pulse braking.

If they tried to drag brakes down a hill, they would lose.
It's a great analogy. You just didn't get it, I'm sorry. The same way you're not getting how it works with the rims. I tried to make it easier to understand but it didn't help.

"With pulse braking, the same amount of heat put into the rim..."

Wrong. With pulse braking, it's NOT the same amount of heat being put into the rim. You are squeezing the brake lever WAY harder during pulse braking, compared to someone drag braking. When you pulse brake you squeeze the brake HARD but leave gaps in time. Someone else who is drag braking leaves no gaps in time, but they are LIGHT on the brakes.

Hard braking force = more friction = more heat per second put into the rims (but with gaps in time)
Light braking force = less friction = less heat per second put into the rims (but with no gaps in time)

It balances out, you see? Why is this simple concept so difficult to understand?

You're pulse braking alternating between 25 and 40 mph. Every time you need to slow down you give your brakes a firm application. A large burst of heat enters your rims. Then you're off your brakes for a while. You repeat the cycle while always keeping your max heat under a limit. Ok. When you get to the bottom of the descent, you check your computer and see that with your pulse braking, your overall average speed was 35 mph.

Someone else is drag braking maintaining a constant 35 mph. They're not squeezing their lever the same way you're squeezing your lever. They're only slowing themselves by 5 mph compared to natural speed. They barely have a casual one finger lean on that brake lever. They are NOT generating heat like you are during one of your pulses. Whatever small feathering amount of heat they generate continuously, that heat is able to continuously dissipate in the 35 mph wind. They ALSO keep their max heat under a limit. The end heat result is the same as your 25-40 mph slinky riding.

Get it now?

Yes, pro cyclists don't overheat because they only use their brakes momentarily. If you add up the amount of seconds they brake on an entire descent, you'll realize that summed up, they didn't really use their brakes much. That's EXACTLY my point! Pro cyclists don't use their brakes. So OF COURSE they won't over heat! Yes they squeeze their brake HARD when they corner, but if they only used their brakes for a grand total of 30 seconds during an entire descent, then it doesn't matter how hard they squeezed in those few seconds. 95% of their gravitational potential energy dissipated into the air, not their brakes. But WHY did 95% of their energy go into the wind? Because HIGH AVERAGE SPEED. It's the only thing that matters.

What you're doing in your argument, is comparing yourself pulse braking at 35 mph average speed, to someone else drag braking at 25 mph average speed. You're artificially manufacturing this unfair advantage in the scenario to make it favorable to your own side, and then pretending it supports your theory. I'm sorry, but no.

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Old 08-23-22, 11:47 AM
  #49  
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Originally Posted by Yan View Post
It's a great analogy. You just didn't get it, I'm sorry. The same way you're not getting how it works with the rims. I tried to make it easier to understand but it didn't help.
There is no need for an analogy when you have a real world example. Your analogy isn’t easier to understand because it is wrong.

"With pulse braking, the same amount of heat put into the rim..."

Wrong. With pulse braking, it's NOT the same amount of heat being put into the rim. You are squeezing the brake lever WAY harder during pulse braking, compared to someone drag braking.
Your examples have been predicated on the idea that the conversion of kinetic energy of the bicycle/rider system is the same in pulse braking and in continual braking (drag braking). So have mine. Yes, someone practicing pulse braking squeezes the brakes WAY harder during the braking phase but they are not squeezing the brakes at all when the brakes aren’t applied. During the time when no braking is being done, the heat input is negative. If both bikes travel the same distance to the same final speed (stopped or some other speed), the conversion of the kinetic energy is the same. The one caveat is that with pulse braking, the time of travel is likely shorter due to the higher average speed and the air flow is higher due to that higher average speed so more heat is lost in pulse braking.

When you pulse brake you squeeze the brake HARD but leave gaps in time. Someone else who is drag braking leaves no gaps in time, but they are LIGHT on the brakes.

Hard braking force = more friction = more heat per second put into the rims (but with gaps in time)
Light braking force = less friction = less heat per second put into the rims (but with no gaps in time)

It balances out, you see? Why is this simple concept so difficult to understand?
You are so close to getting it. It is the gaps in time during pulse braking that you aren’t understanding. During those gaps, the heat input is negative. More friction with periods of no friction results in a higher heat transfer away from the rims than if you have constant friction at a slightly lower friction input. It’s the higher negative bits that changes the balance. You have the same heat input but you have a higher heat outflow with pulsed braking.

You're pulse braking alternating between 25 and 40 mph. Every time you need to slow down you give your brakes a firm application. A large burst of heat enters your rims. Then you're off your brakes for a while. You repeat the cycle while always keeping your max heat under a limit. Ok.
You can’t definitively say what speeds pulse braking is alternating between. That depends on how much speed you are willing to carry through corners or how much you are slowing down. Some corners are may need to slow that much but some may not.

And, again, you are missing how much impact the “off [the] brakes for a while” has. Yes, you repeat the cycle while keeping the heat from building in the rim too much.


Someone else is drag braking maintains a constant 35 mph. They're not squeezing their lever the same way you're squeezing your lever. They're only slowing themselves by 5 mph compared to natural speed. They have a casual one finger lean on that brake lever. They are NOT gushing heat into their rims like you are during one of your pulses. Whatever small feathering amount of heat they generate continuously, that heat dissipates in the 35 mph wind. They ALSO keep their max heat under a limit.
If someone is traveling at 35 mph and they only have to slow to 30mph to go around a corner, why does the rider at 40 mph have to slow to 25mm? Why can’t they take the same corner at the same speed as the 35mph scenario? If you can corner at 30mph, I could corner at the same speed, thus the heat input on at higher speed isn’t much different.

But you are wrong on the 35mph scenario keeping their heat “under a limit”. That’s the point. The heat dissipates some but not nearly as much as during pulse braking for two reasons. The first is that the airflow is higher so the heat lost at higher speeds is greater. Second, there are periods when there is no heat going into the rim while at the same time heat is being lost. In the end, the pulse braking ends up losing more heat without putting in a whole lot more heat than constant braking.

Get it now?
I got it before. You are just wrong on your assumptions.

Yes, pro cyclists don't overheat because they only use their brakes momentarily.
In other words, they practice “pulse braking”.

If you add up the amount of seconds they brake on an entire descent, you'll realize that summed up, they didn't really use their brakes much. That's EXACTLY my point! Pro cyclists don't use their brakes. So OF COURSE they won't over heat! Their gravitational potential energy dissipates into the air, not their brakes.
Um…same thing occurs during pulse braking. Higher speed, some air resistance braking, shorter periods of brake application, more time for the rims to cool. If you don’t use your brakes, you aren’t putting heat into the system. If you use your brakes constantly, you aren’t allowing heat to dissipate away at the same rate and thus end up with hotter wheels.

Again, I do mountain riding. I do mountain bike riding with rim brakes. I’ve done mountain riding on tandems with rim brakes. I have never…in 40+ years of mountain riding…cooked a rim brake. I’ve never even come close. I’m unfamiliar with the sound change you describe because I’ve never experienced brakes hot enough to make that sound. It’s not because I don’t go down hills…I do at that highest possible speed I can…but because I know how to manage the heat input to avoid cooking brakes.
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Old 08-23-22, 11:59 AM
  #50  
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"The one caveat is that with pulse braking, the time of travel is likely shorter due to the higher average speed and the air flow is higher due to that higher average speed so more heat is lost in pulse braking."

What you're doing in your argument, is comparing yourself pulse braking at 35 mph average speed, to someone else drag braking at 25 mph average speed. You're artificially manufacturing this unfair advantage in the scenario to make it favorable to your own side, and then pretending it supports your theory. I'm sorry, but no.

"You are so close to getting it. It is the gaps in time during pulse braking that you aren’t understanding. During those gaps, the heat input is negative. More friction with periods of no friction results in a higher heat transfer away from the rims than if you have constant friction at a slightly lower friction input. It’s the higher negative bits that changes the balance. You have the same heat input but you have a higher heat outflow with pulsed braking."

There it is with that magic blanket theory of yours again. Higher heat outflow? Where the heck are you getting this wild idea from? The heat outflow does not care whether you happen to also be adding heat at the same time. If you hose water into the top of your pool while draining from the bottom, does the water care whether hose is on or off? NO! That drain hole is working the same way all the time! Hello???

"If someone is traveling at 35 mph and they only have to slow to 30mph to go around a corner, why does the rider at 40 mph have to slow to 25mm? Why can’t they take the same corner at the same speed as the 35mph scenario? If you can corner at 30mph, I could corner at the same speed, thus the heat input on at higher speed isn’t much different."

This is exactly what I've been saying all along. You're slowing down for CORNERING purposes, and then later when you get home you sit in that armchair and pretend you were managing your heat. This is exactly why a few days ago I brought up the no-cornering descent in Tibet. Take corning out of the equation and your whole strategy loses meaning.

Originally Posted by cyccommute View Post
There is no need for an analogy when you have a real world example. Your analogy isn’t easier to understand because it is wrong.

Your examples have been predicated on the idea that the conversion of kinetic energy of the bicycle/rider system is the same in pulse braking and in continual braking (drag braking). So have mine. Yes, someone practicing pulse braking squeezes the brakes WAY harder during the braking phase but they are not squeezing the brakes at all when the brakes aren’t applied. During the time when no braking is being done, the heat input is negative. If both bikes travel the same distance to the same final speed (stopped or some other speed), the conversion of the kinetic energy is the same. The one caveat is that with pulse braking, the time of travel is likely shorter due to the higher average speed and the air flow is higher due to that higher average speed so more heat is lost in pulse braking.



You are so close to getting it. It is the gaps in time during pulse braking that you aren’t understanding. During those gaps, the heat input is negative. More friction with periods of no friction results in a higher heat transfer away from the rims than if you have constant friction at a slightly lower friction input. It’s the higher negative bits that changes the balance. You have the same heat input but you have a higher heat outflow with pulsed braking.

You can’t definitively say what speeds pulse braking is alternating between. That depends on how much speed you are willing to carry through corners or how much you are slowing down. Some corners are may need to slow that much but some may not.

And, again, you are missing how much impact the “off [the] brakes for a while” has. Yes, you repeat the cycle while keeping the heat from building in the rim too much.

If someone is traveling at 35 mph and they only have to slow to 30mph to go around a corner, why does the rider at 40 mph have to slow to 25mm? Why can’t they take the same corner at the same speed as the 35mph scenario? If you can corner at 30mph, I could corner at the same speed, thus the heat input on at higher speed isn’t much different.

But you are wrong on the 35mph scenario keeping their heat “under a limit”. That’s the point. The heat dissipates some but not nearly as much as during pulse braking for two reasons. The first is that the airflow is higher so the heat lost at higher speeds is greater. Second, there are periods when there is no heat going into the rim while at the same time heat is being lost. In the end, the pulse braking ends up losing more heat without putting in a whole lot more heat than constant braking.



I got it before. You are just wrong on your assumptions.



In other words, they practice “pulse braking”.



Um…same thing occurs during pulse braking. Higher speed, some air resistance braking, shorter periods of brake application, more time for the rims to cool. If you don’t use your brakes, you aren’t putting heat into the system. If you use your brakes constantly, you aren’t allowing heat to dissipate away at the same rate and thus end up with hotter wheels.

Again, I do mountain riding. I do mountain bike riding with rim brakes. I’ve done mountain riding on tandems with rim brakes. I have never…in 40+ years of mountain riding…cooked a rim brake. I’ve never even come close. I’m unfamiliar with the sound change you describe because I’ve never experienced brakes hot enough to make that sound. It’s not because I don’t go down hills…I do at that highest possible speed I can…but because I know how to manage the heat input to avoid cooking brakes.

Last edited by Yan; 08-23-22 at 12:04 PM.
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