Go Back  Bike Forums > Bike Forums > Touring
Reload this Page >

TPU tubes with rim brakes?

Notices
Touring Have a dream to ride a bike across your state, across the country, or around the world? Self-contained or fully supported? Trade ideas, adventures, and more in our bicycle touring forum.

TPU tubes with rim brakes?

Old 08-23-22, 12:00 PM
  #51  
IPassGas
Full Member
 
Join Date: Feb 2015
Location: Pittsburgh, PA
Posts: 367

Bikes: Schwinn, Nishiki, Santana, Trek, Rodriguez

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 179 Post(s)
Likes: 0
Liked 95 Times in 60 Posts
Not to inflame an already heated discussion. It would be good for people to reference more authoritative work. For example Wilson's Bicycling Science is a great source. Here are figures for (1) rim temperature increase during constant braking versus speed and (2) terminal velocity versus %slope. From these graph 1 you see for constant braking to maintain 40 MPH, a single bike does not heat rims more than 50C even on a 20% slope. My concern would be temperature increases closer to 100C (melting temperature of butyl rubber, 120C). I have been concerned of late with this on a tandem since we often bike in mountains. For a tandem, >10% slope can cause temperature increases that are much higher. Wilson does not mention pulsing brakes. I have heard anecdotal talk that pulsing does allow air flow between pad and rim/disc to prevent brake pad melting.



Last edited by IPassGas; 08-23-22 at 12:18 PM.
IPassGas is offline  
Old 08-23-22, 04:29 PM
  #52  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
"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.
Pick any speed you like. Yes, a bike that is pulse braked is going to go faster than one that where the brakes are dragged for any given road or slope. That’s the reason people drag their brakes…to keep the average speed lower than if they let gravity and air resistance determine the speed. It’s not an “unfair advantage” but a fact of the two approaches to 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."

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???
Stop with the “magic blanket” stuff. I never said anything about some “magic blanket”. The rim that does not have friction applied to it has a higher heat outflow than the rim that is undergoing constant friction. No friction…i.e. no braking…results in more heat being pulled from the rim by air flow than if the rim is under constant friction. To use your (bad) analogy in the proper way, if you have a hose constantly running water into the top of your pool and draining water out of the bottom but not enough to drain away the water being added, the pool overflows. If you pulse water into the pool when it has emptied a bit, then don’t put water in until the level drops again, you don’t overflow the pool.

Just to be clear, the hose constantly running water into the top of the pool is analogous to dragging the brakes.

"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.
First, I misread your point. But, to be fair, you are misreading mine as well. If you are constantly braking to keep the speed at 35mph and not slowing for corners, the pulse brake scenario only needs to slow to that same 35 mph for a corner, so the heat input on the pulse braking isn’t that much greater than the constant braking scenario. The difference is that once the pulse braker slows, friction isn’t being applied. No friction means an overall rim temperature decrease.

On a long descent like the one you (partially described…no distance yet), I’d probably not touch brakes for a very long time. If anything, I’d probably be pushing the pedals to get to a maximum speed. Any speed control on my part would be minimal and/or relying on air resistance. My rims wouldn’t get hot because I wouldn’t be using the brakes at all. The pulsed braking strategy doesn’t lose meaning just because the road doesn’t have corners. If I had to stop or slow for some reason on that kind of downhill, I would still brake hard and then get off the brakes.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.
cyccommute is online now  
Old 08-23-22, 05:06 PM
  #53  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
the funny thing throughout all of this is that all of us chiming in here actually have substantial real life experience riding loaded in mountains, mr gas on a tandem to boot.

we all have figured out how to manage our braking, within what is comfortable for each of us vis a vis speed, cornering and judging each specific situation.

In my case, from my past motorcycle paved circuit racing experience, where maximum braking is just part of the game, I've boiled brake fluid, had to back off to let my brakes come back to me, I've also locked the front and crashed, locked the front and not crashed--and a bunch of you have mountain bike experience, where flirting with the edge of traction also is just part of the game and having to judge your braking ability/feedback with the speed and conditions and how sharp you are feeling (reaction time)------so really, we all have this figured out and know how to deal with this stuff.

I guess the main thing is that between mountain biking and motorcycling racing, we really learn how to get the MAX braking from using front and rear, which frankly, a looooooot of bike riders don't do. I think its fair to say that some of us have a much higher comfort zone with speed, while being very aware of what speed we can do for the conditions and environment--but yes, things can change in just a few seconds, so yes there are dangers, but at least managing our brake systems properly and reducing the amount of brake fading is a good skill we've all learned (or at least be sensitive to noticing the signs of it happening and making adjustments to stay safe)

ps, and yes, when I've balled up the rubber of my rim pads, it was because I was descending Tour de France Pyrenees hairpin descents and riding like racing a motorcycle--last second max braking into the hairpins over and over, going faster downhill than cars around the corners etc--but thats the racer in me that just come out, still happens a bit as an old fart.
I have learned though to leave more margin of error, and to pay attention more to brake fade (and to my reaction times) and just simply back off when its better to do so.

Last edited by djb; 08-23-22 at 05:10 PM.
djb is offline  
Old 08-23-22, 05:17 PM
  #54  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by cyccommute View Post
Pick any speed you like. Yes, a bike that is pulse braked is going to go faster than one that where the brakes are dragged for any given road or slope. That’s the reason people drag their brakes…to keep the average speed lower than if they let gravity and air resistance determine the speed. It’s not an “unfair advantage” but a fact of the two approaches to braking.

Stop with the “magic blanket” stuff. I never said anything about some “magic blanket”. The rim that does not have friction applied to it has a higher heat outflow than the rim that is undergoing constant friction. No friction…i.e. no braking…results in more heat being pulled from the rim by air flow than if the rim is under constant friction. To use your (bad) analogy in the proper way, if you have a hose constantly running water into the top of your pool and draining water out of the bottom but not enough to drain away the water being added, the pool overflows. If you pulse water into the pool when it has emptied a bit, then don’t put water in until the level drops again, you don’t overflow the pool.

Just to be clear, the hose constantly running water into the top of the pool is analogous to dragging the brakes.

First, I misread your point. But, to be fair, you are misreading mine as well. If you are constantly braking to keep the speed at 35mph and not slowing for corners, the pulse brake scenario only needs to slow to that same 35 mph for a corner, so the heat input on the pulse braking isn’t that much greater than the constant braking scenario. The difference is that once the pulse braker slows, friction isn’t being applied. No friction means an overall rim temperature decrease.

On a long descent like the one you (partially described…no distance yet), I’d probably not touch brakes for a very long time. If anything, I’d probably be pushing the pedals to get to a maximum speed. Any speed control on my part would be minimal and/or relying on air resistance. My rims wouldn’t get hot because I wouldn’t be using the brakes at all. The pulsed braking strategy doesn’t lose meaning just because the road doesn’t have corners. If I had to stop or slow for some reason on that kind of downhill, I would still brake hard and then get off the brakes.
Your argument is simply absurd. No, a bike that is drag brake is NOT necessarily going slower than a bike that is pulse braking. A rider can drag as hard or lightly as they like, just as a rider and pulse as hard or lightly as they like. The rider is in control of the brake lever. The rider can do what their brain desires.

The entire premise of this discussion is that the cyclist with the HIGHER AVERAGE SPEED will reach the bottom of the descent with cooler rims, REGARDLESS of the differing approaches to timing the brake usage. From my very first post in this thread, many days ago, I've repeated this OVER AND OVER AGAIN. Have I somehow failed to communicate clearly? I say this in every post I write. How is this not getting through???

Let me lay it out again for the umpteenth time for you:
  1. The descent is a 10 mile downhill at a steep 20% slope. The run has many tight turns but the road surface is good.
  2. Rider 1 practices pulse braking. In the straights he maintains a natural terminal velocity of 60 mph. In the corners he slows to 40 mph to avoid crashing. At the bottom of the descent his computer shows an average speed of 50 mph.
  3. Rider 2 is a pro rider. He has pro technique and balls the size of watermelons. He is able to anal pucker bomb down the entire descent at the natural terminal velocity of 60 mph, never touching his brakes once. He is simply that good, he is a downhill God. However on this day he wishes to experiment with drag braking. He uses one pinky finger to drag his brakes the entire way down. At the bottom of the descent his computer shows an average speed of 59.5 mph. His one pinky drag braking slowed him down by 0.5 mph compared to no brakes at all.
  4. The wheel temperatures of both cyclists are now measured.
  5. Which cyclist has cooler rims? The pulse braker or the drag braker?

Please answer so we can put this debate to bed and end this discussion thread.

Last edited by Yan; 08-23-22 at 05:22 PM.
Yan is offline  
Old 08-24-22, 08:33 AM
  #55  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
Your argument is simply absurd. No, a bike that is drag brake is NOT necessarily going slower than a bike that is pulse braking. A rider can drag as hard or lightly as they like, just as a rider and pulse as hard or lightly as they like. The rider is in control of the brake lever. The rider can do what their brain desires.
If there is no speed difference, then why drag your brakes? If there is no need to slow down you should just let the bike go. It doesn’t matter if the pressure on the brake is hard or light, any friction on the wheel is going to be slower than no friction on the wheel.

The entire premise of this discussion is that the cyclist with the HIGHER AVERAGE SPEED will reach the bottom of the descent with cooler rims, REGARDLESS of the differing approaches to timing the brake usage. From my very first post in this thread, many days ago, I've repeated this OVER AND OVER AGAIN. Have I somehow failed to communicate clearly? I say this in every post I write. How is this not getting through???
That is your premise. I don’t agree with it. And yes, you have “repeated this OVER AND OVER AGAIN.” You have been wrong every time you repeat it. You have failed to communicate because you are incorrect. No friction (or segments of no friction) on a descent results in cooler rims than constant friction.

Let me lay it out again for the umpteenth time for you:
And, again, you are incorrect.

  1. The descent is a 10 mile downhill at a steep 20% slope. The run has many tight turns but the road surface is good.
  2. Rider 1 practices pulse braking. In the straights he maintains a natural terminal velocity of 60 mph. In the corners he slows to 40 mph to avoid crashing. At the bottom of the descent his computer shows an average speed of 50 mph.
  3. Rider 2 is a pro rider. He has pro technique and balls the size of watermelons. He is able to anal pucker bomb down the entire descent at the natural terminal velocity of 60 mph, never touching his brakes once. He is simply that good, he is a downhill God. However on this day he wishes to experiment with drag braking. He uses one pinky finger to drag his brakes the entire way down. At the bottom of the descent his computer shows an average speed of 59.5 mph. His one pinky drag braking slowed him down by 0.5 mph compared to no brakes at all.
  4. The wheel temperatures of both cyclists are now measured.
  5. Which cyclist has cooler rims? The pulse braker or the drag braker?

The pulse braker. But your whole premise is rediculous. No pro rider would drag brakes down a hill. Why would they? And why would the pulse braker have to slow so much for corners?

Let’s add two more scenario to your list.

1a. A rider keeps his speed at 40 mph all the way down the hill.
3a. The downhill God doesn’t touch his brakes all the way down the hill.

Now: Which one has the coolest rims? For extra credit, rank the bikes from coolest rims to hottest. (Hint: position 3 and 4 are not what you think they are)

But people who drag brakes down hills aren’t “experimenting” with dragging brakes. They are dragging brakes because they are too afraid the speed that they would reach on a downhill for whatever reason. They think that they are being “safe” by slowing down without realizing that they are overheating their rims and are far more likely to reach a temperature where they could experience a blowout…even if that is extremely difficult to do.

Please answer so we can put this debate to bed and end this discussion thread.
I have answered. Several times. It’s just “not getting through” to you.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.

Last edited by cyccommute; 08-24-22 at 08:41 AM.
cyccommute is online now  
Old 08-24-22, 08:35 AM
  #56  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by cyccommute View Post
The pulse braker.
Hahahaha.... That's what I was waiting for.

Mister I think there is no point in us continuing this. You're clearly living in another plane of existence where reality is irrelevant to you.

Good day to you.
______

Edit: I see you edited your post so in the interest of fairness I'll respond to your question. Order of coolest to hottest rims would be:

1. Downhill god no brakes 60 mph terminal velocity
2. Downhill god one pinky drag braking 59.5 mph
3. Pulse braker slinky riding between 60 and 40 mph averaging 50 mph
4. Drag braker maintaining 40 mph continuously.

Lower average speed = hotter rims. As you can see drag vs pulse is irrelevant. The drag braker can be cooler or hotter than the pulse braker. It depends on how hard they drag. Average speed is the only relevant factor. For a graphical illustration refer to the chart IPassGas generously provided a few posts above.

Last edited by Yan; 08-24-22 at 12:34 PM.
Yan is offline  
Old 08-24-22, 03:42 PM
  #57  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by IPassGas View Post
Not to inflame an already heated discussion. It would be good for people to reference more authoritative work. For example Wilson's Bicycling Science is a great source. Here are figures for (1) rim temperature increase during constant braking versus speed and (2) terminal velocity versus %slope. From these graph 1 you see for constant braking to maintain 40 MPH, a single bike does not heat rims more than 50C even on a 20% slope. My concern would be temperature increases closer to 100C (melting temperature of butyl rubber, 120C). I have been concerned of late with this on a tandem since we often bike in mountains. For a tandem, >10% slope can cause temperature increases that are much higher. Wilson does not mention pulsing brakes. I have heard anecdotal talk that pulsing does allow air flow between pad and rim/disc to prevent brake pad melting.
While I admire Dr. Wilson’s work, I think he has drawn a wrong conclusion in this case. He seems to assume that the temperature decreases with speed is due to the increase airflow. However, he neglects the effect of the brakes on the rim itself. On a 20% slope (or any slope for that matter) maintaining a slower speed requires more of the forward momentum to be converted to heat than does a faster speed. You can’t maintain close to terminal velocity with the brakes on as hard as they would be at 20 mph. The temperature increase at slower speed would be friction related rather than related to the air flow. At higher speed, there is a lower temperature increase above ambient because there isn’t any friction being put into the system.

He makes the statement “…going fast avoids heating the rim unless emergency braking has to be applied, in which case the danger of overheating the rim is sudden and serious.” The problem with that statement is that if you are going 60 mph, you aren’t using the brakes so the heat being put into the system is zero. Hard application of the brakes to slow for some reason would be transitory and would not cause the temperature of the system to rise to the same level as if the brakes were being used to maintain a given speed.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.
cyccommute is online now  
Old 08-24-22, 04:16 PM
  #58  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
Hahahaha.... That's what I was waiting for.

Mister I think there is no point in us continuing this. You're clearly living in another plane of existence where reality is irrelevant to you.

Good day to you.
That’s your opinion. You are wrong.
______

Edit: I see you edited your post so in the interest of fairness I'll respond to your question. Order of coolest to hottest rims would be:

1. Downhill god no brakes 60 mph terminal velocity
2. Downhill god one pinky drag braking 59.5 mph
3. Pulse braker slinky riding between 60 and 40 mph averaging 50 mph
4. Drag braker maintaining 40 mph continuously.
Nope. But again your scenarios are wrong and ridiculous. Why would someone want to “pinky drag” to keep their speed 0.5mph lower? Why would the pulse braker have to slow 20 mph? The only thing you have right is the drag brake scenario would have the hottest brakes…by a whole lot.

Lower average speed = hotter rims.
And why is the lower average speed equivalent to hotter rims? Because the brakes are being used more. More brakes being used equals more heat being poured into the system. The less the brakes are being used, the cooler the rims stay for two reasons. One is because the air flow is higher but the other is that heat isn’t being put into the system.

As you can see drag vs pulse is irrelevant. The drag braker can be cooler or hotter than the pulse braker. It depends on how hard they drag. Average speed is the only relevant factor.
No, the drag braker isn’t going be cooler or hotter than the pulse braker. Someone dragging the brakes isn’t going to go as fast as someone who isn’t using the brakes. The average speed could be the same but the average heat input of the two systems isn’t going to be equivalent. One is losing heat between applications of heat and the other is loosing less heat.

For a graphical illustration refer to the chart IPassGas generously provided a few posts above.
That graphic is only relevant for drag braking. It’s states quite clearly that it is about the constant application of brakes. Perhaps Wilson didn’t know about pulse braking or he didn’t include it in the graph because it is harder to model. We can still use the graph to look at the effect of pulse braking vs drag braking, however. Let’s say that someone is riding down the slope in the graph at 30 mph through the constant application of the brakes (yellow arrow). The temperature rises to about 70°C over ambient and stays there. It’s constant from the top of the hill to the bottom.

Now let’s say the brakes aren’t used at all and the bike is allowed to run to terminal velocity (red arrow). The temperature rise is zero over ambient.

Now let’s say that someone is pulse braking…your so called “slinky” braking…(green arrow). They apply the brakes and momentarily slow from 60 mph with zero temperature rise over ambient to 40 mph with a temperature rise of up to about 60°C over ambient. But they get off the brakes, gain speed and are back up to 60 mph. The rims have zero input of heat and quickly fall back to ambient temperature. I’ll agree that there could be some latent heat which would rise the temperature slightly over ambient but it would be small compared to the heat poured in by constant braking. And the longer the bike stays at 60 mph, the cooler the rims will be come. The main point I’ve been making (for a week) is that the temperature of the rims doesn’t raise as much as if the brakes are left on continuously.


__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.
cyccommute is online now  
Old 08-24-22, 04:52 PM
  #59  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by cyccommute View Post
While I admire Dr. Wilson’s work, I think he has drawn a wrong conclusion in this case. He seems to assume that the temperature decreases with speed is due to the increase airflow. However, he neglects the effect of the brakes on the rim itself. On a 20% slope (or any slope for that matter) maintaining a slower speed requires more of the forward momentum to be converted to heat than does a faster speed. You can’t maintain close to terminal velocity with the brakes on as hard as they would be at 20 mph. The temperature increase at slower speed would be friction related rather than related to the air flow. At higher speed, there is a lower temperature increase above ambient because there isn’t any friction being put into the system. He makes the statement “…going fast avoids heating the rim unless emergency braking has to be applied, in which case the danger of overheating the rim is sudden and serious.” The problem with that statement is that if you are going 60 mph, you aren’t using the brakes so the heat being put into the system is zero. Hard application of the brakes to slow for some reason would be transitory and would not cause the temperature of the system to rise to the same level as if the brakes were being used to maintain a given speed.
Originally Posted by cyccommute View Post
That’s your opinion. You are wrong.
Sure, sure. Whatever you say.

Someone with a post-doctorate and who was a professor of engineering at MIT wrote an entire 480 page book, but of course you know better. You are right and the MIT engineering professor is wrong. How dare they disagree with you... you're the smartest man you know... you've never been wrong in your life before so how could you be wrong now... What a gap of knowledge the scientist had! Oh boy... oh boy... Massive pat on the back for you old boy! You put him right you did. What a shame the MIT professor never had a chance to be educated by you... Good on you... good on you...

Last edited by Yan; 08-25-22 at 08:17 AM.
Yan is offline  
Old 08-25-22, 08:55 AM
  #60  
pdlamb
Senior Member
 
Join Date: Dec 2010
Location: northern Deep South
Posts: 8,039

Bikes: Fuji Touring, Novara Randonee

Mentioned: 34 Post(s)
Tagged: 0 Thread(s)
Quoted: 2165 Post(s)
Liked 1,389 Times in 885 Posts
Originally Posted by IPassGas View Post
Not to inflame an already heated discussion. It would be good for people to reference more authoritative work. For example Wilson's Bicycling Science is a great source. Here are figures for (1) rim temperature increase during constant braking versus speed and (2) terminal velocity versus %slope. From these graph 1 you see for constant braking to maintain 40 MPH, a single bike does not heat rims more than 50C even on a 20% slope. My concern would be temperature increases closer to 100C (melting temperature of butyl rubber, 120C). I have been concerned of late with this on a tandem since we often bike in mountains. For a tandem, >10% slope can cause temperature increases that are much higher. Wilson does not mention pulsing brakes. I have heard anecdotal talk that pulsing does allow air flow between pad and rim/disc to prevent brake pad melting.

I wonder, was this model ever validated against experiment, or is this one of those "assume a spherical cow" type of homework problems that made it into a book?
pdlamb is offline  
Old 08-25-22, 09:26 AM
  #61  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
Someone with a post-doctorate and who was a professor of engineering at MIT wrote an entire 480 page book, but of course you know better. You are right and the MIT engineering professor is wrong. How dare they disagree with you... you're the smartest man you know... you've never been wrong in your life before so how could you be wrong now... What a gap of knowledge the scientist had! Oh boy... oh boy... Massive pat on the back for you old boy! You put him right you did. What a shame the MIT professor never had a chance to be educated by you... Good on you... good on you...
As a scientist, I know how the process works. Even the most learned scientist is not entirely right all the time nor is what they write (or study) all inclusive and the last word on the subject. Nor would any scientist worth his salt assume that he has covered everything when writing an article or even a book.

His graph and his studies don’t explicitly say that he studied the use of pulse braking. There are gaps in what he writes and he seems to have missed the point of the fact that lower rim temperatures are less about the air flow than about the decreased input of heat from constant braking. Something you keep missing as well.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.

Last edited by cyccommute; 08-25-22 at 09:54 AM.
cyccommute is online now  
Old 08-25-22, 09:40 AM
  #62  
IPassGas
Full Member
 
Join Date: Feb 2015
Location: Pittsburgh, PA
Posts: 367

Bikes: Schwinn, Nishiki, Santana, Trek, Rodriguez

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 179 Post(s)
Likes: 0
Liked 95 Times in 60 Posts
Originally Posted by pdlamb View Post
I wonder, was this model ever validated against experiment, or is this one of those "assume a spherical cow" type of homework problems that made it into a book?
The model is combination of basic physics and empirical models for brake/rim cooling, so fairly a sophisticated step up from a spherical cow. The model depends on your components, but Wilson is fairly conservative. The terminal velocity chart does not need brake/rim data and is a more straightforward application of physics. Wilson is a well-known expert on bicycle physics and especially HPV. If you are fascinated with bicycles, the whole book is a good read. The calculation for the amount of energy to dissipate when braking to a stop for a specific: speed, slope, mass, front cross sectional area, etc, is a homework problem....56 kJ for us on a tandem, @ 45 mph on a 10% slope. Using the heat capacity of our rims gives a 50C increase over ambient...so about 75C... which you can touch for a few second without much pain, which we have done, so kind of tested
IPassGas is offline  
Old 08-25-22, 09:46 AM
  #63  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by pdlamb View Post
I wonder, was this model ever validated against experiment, or is this one of those "assume a spherical cow" type of homework problems that made it into a book?
On page 256 of the 3rd edition he states

Wilson (1993) studied rim temperature attained during steady downhill braking of the type required in cycling on mountain roads. His results showed that temperatures reached are likely to be dangerously high for standard road bicycles that rely on rim brakes alone…
The citation is Wilson, (1993), “Rim temperatures during downhill braking.” Human Power 10, no. 3 (Spring/Summer), 15-18.

The paper is about mathematical modeling rather than measurement. He makes no mention of the idea of pulse braking and even says “Letting the brakes off to stop heating the rims would lead to a totally impracticable speed,” in reference to a tandem ride in New Zealand. He presents various other anecdotes which seems to indicate that he is not a bold descender.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.

Last edited by cyccommute; 08-25-22 at 09:53 AM.
cyccommute is online now  
Old 08-25-22, 09:58 AM
  #64  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
well, what I take from all this is that tpu tubes arent really worth the mullah.

hey stae, did you see the thread by gauvins about "what worked, what didnt" ? on his I think 8 week trip?
He wasnt impressed with the zuper dooper light tpu tubes
djb is offline  
Old 08-25-22, 11:28 AM
  #65  
staehpj1 
Senior Member
Thread Starter
 
staehpj1's Avatar
 
Join Date: Aug 2006
Location: Tallahassee, FL
Posts: 11,132
Mentioned: 7 Post(s)
Tagged: 0 Thread(s)
Quoted: 956 Post(s)
Liked 423 Times in 326 Posts
Originally Posted by djb View Post
well, what I take from all this is that tpu tubes arent really worth the mullah.

hey stae, did you see the thread by gauvins about "what worked, what didnt" ? on his I think 8 week trip?
He wasnt impressed with the zuper dooper light tpu tubes
Yes. I did see that. I think there are applications where they may make sense and ones where they don't. Knowing which are which may be tricky

All that said I have not opened my wallet for any at this point. I do think there are some use cases where I might in the future.
__________________
Pete in Tallahassee
Check out my profile, articles, and trip journals at:
https:/www.crazyguyonabike.com/staehpj1


staehpj1 is offline  
Old 08-25-22, 05:57 PM
  #66  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by cyccommute View Post
There are gaps in what he writes and he seems to have missed the point of the fact that lower rim temperatures are less about the air flow than about the decreased input of heat from constant braking. Something you keep missing as well.
You seem to think that simply keeping the brakes constantly applied will always result in a large amount of heat to be generated. Causing the drag braking technique to always out-heat the pulse technique. "Because it never has a chance to cool".

The gap in your science knowledge is in not understanding that the amount of heat is not solely based on brake on vs brake off, but also related to how hard the brake is being applied when it IS on. In middle school or high school physics you should have been taught, assuming your state's curriculum is similar to that of other regions, that frictional force = coefficient of friction x normal force. The important factor here is the normal force. If you keep the normal force very small, the friction force will also be very small. That means the amount of heat generated will be very small.

Here the question is not "but why would a rider only drag brake by 0.5 mph?" The question is, "what would happen if he simply chooses to do it?" The answer is that the heat put into the rims would be very small, with the graph eventually converging on zero as the normal force is infinitely reduced.

It should be obvious to most people by now that drag vs pulse braking is not the determining factor on how much heat is retained in the rim. You can drag brake in a way that ends with cooler rims, just as you can pulse in a way that ends in cooler rims. The rider's action determines the temperature result, it is not fixed to "drag always hotter no matter what".

I apologize for being sarcastic in my previous post, but until you catch up on this part of basic physics, you're simply not going to understand how this works. Maybe give that book another read. Not sure if this is covered or if he just assumes you already know. Good luck!

Last edited by Yan; 08-25-22 at 06:00 PM.
Yan is offline  
Old 08-25-22, 08:30 PM
  #67  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
Originally Posted by Yan View Post
It should be obvious to most people by now that drag vs pulse braking is not the determining factor on how much heat is retained in the rim. You can drag brake in a way that ends with cooler rims, just as you can pulse in a way that ends in cooler rims. The rider's action determines the temperature result, it is not fixed to "drag always hotter no matter what".
I'm going to have to disagree with you on this, certainly not from my experience, and not with my experience of how pad wear occurs.

This whole term of pulse braking is a vague one, it sounds like the description of "pumping the brakes" in car talk.

But in the end, unless one of you dudes gets a grant to do an extensive study on this-- which would actually be really cool (sic) to do btw! I can't see Internets description of braking techniques ever really being clear, as I'm pretty certain that how I brake to maintain a safe speed for me, is not what some of you guys are doing, with probably quite different pad and rim temps and wear.
All I know is that over decades, I've consistently gotten really good pad life out of my rim and disc systems, so for me this is the bottom line proof that I minimize wear and overall friction/ heat , despite really throwing out the anchors, probably stopping harder than most of you, just because of my racing experience.

And while this has become a bit of an arm wrestle discussion, I still find it fun to think through braking techniques and putting down thoughts and descriptions of it all.

Last edited by djb; 08-26-22 at 12:31 AM.
djb is offline  
Old 08-25-22, 10:49 PM
  #68  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
You seem to think that simply keeping the brakes constantly applied will always result in a large amount of heat to be generated. Causing the drag braking technique to always out-heat the pulse technique. "Because it never has a chance to cool".
Not necessarily a large amount of heat but more heat than if you didn’t apply the brakes at all…or even intermittently. Yes, continuous braking is continuously pumping heat into the rims. Pulse braking is pumping heat into the rims intermittently with periods of cooling. The temperature the rim reach with continuous braking is also dependent on the speed you are trying to keep the bike at. The slower the speed, the more braking is needed, the more friction generated, and the higher the rim temperature.

The gap in your science knowledge is in not understanding that the amount of heat is not solely based on brake on vs brake off, but also related to how hard the brake is being applied when it IS on. In middle school or high school physics you should have been taught, assuming your state's curriculum is similar to that of other regions, that frictional force = coefficient of friction x normal force. The important factor here is the normal force. If you keep the normal force very small, the friction force will also be very small. That means the amount of heat generated will be very small.
You don’t have to be insulting. I have a degree in chemistry and it came with a whole bunch of instruction on heat and heat management. I spent 40 years doing bench chemistry including designing reactors. I had to learn how to deal with heat a whole lot so I know how heat works.

Additionally, again, I also have decades of mountain riding including mountain biking, loaded touring, and loaded mountain bike touring. And, because I don’t drag brakes, I have no experience with over heated-rims and melted brakes. I don’t consider that to be a bad thing.

There isn’t a “gap in [my] science knowledge”. Yes, you’ve stated frictional force before. I’m well aware of it. The part you are missing is the effect of time. Yet again, constant braking results in constant heat being poured into the system. Granted the system isn’t adiabatic so there is heat loss at the same time as heat is being added but while heat is being added not all of it can go away. If no heat is being added…i.e. the brakes aren’t being applied…, the overall effect is cooling of the rim more than if the brakes are being constantly applied. Since the system isn’t adiabatic, the rim also loses heat and, since no heat is being added, the overall loose of heat is greater.

The other factor to consider is that anything you do with constant braking, I can do with pulse braking. I don’t have to brake hard all the time or even that often. Your examples keep suggesting that I would have to slow more for a corner than you would under constant braking. But if you can go around a corner at 35 mph, for example, I can only have to slow to your speed. If I’m going down the hill at 40 mph and have to slow 5 mph, that’s not much of a braking demand. On the other side of the equation, I remove the friction after the corner and take full advantage of the cooling available without heat input. The overall effect is that the pulsed rims stay cooler.

Additionally, how much normal force is need is dependent on the speed you want to maintain. If the normal force is kept small like you maintain, you can’t keep the speed down. The magnitude of the normal force as well as the friction generated, is going to be proportional to the speed. The lower you keep the speed, the higher the normal force and the more friction that is generated. That is why Wilson’s graph shows higher heat generated at lower speed. As the speed approaches terminal velocity, the normal force, and the friction, decrease to the point where the brakes aren’t being applied and the rims have no heat going into them.

Here the question is not "but why would a rider only drag brake by 0.5 mph?" The question is, "what would happen if he simply chooses to do it?" The answer is that the heat put into the rims would be very small, with the graph eventually converging on zero as the normal force is infinitely reduced.
I object less to the argument than to the absurdity of a speed reduction. No one who practices constant braking is doing so to keep the speed down by 0.5mph. In your Altyn-Tagh example, you use your brakes because you fear letting the bike hit terminal velocity. That means slowing more than 0.5mph…reducing speed that much would make little difference in the event of having to avoid a collision or to reduce the risk of injury in a crash.

It’s a silly argument.


It should be obvious to most people by now that drag vs pulse braking is not the determining factor on how much heat is retained in the rim. You can drag brake in a way that ends with cooler rims, just as you can pulse in a way that ends in cooler rims. The rider's action determines the temperature result, it is not fixed to "drag always hotter no matter what".
Dragging the brakes could only end up with a rim as cool as pulse braking if you use the 0.5mph argument above or if you measured the temperature immediately after the brakes have been pulsed hard for control.

Additionally, if you can drag brake in a way that ends up in cooler rims, why do you need to stop in the middle of a hill to let the rims cool?

I apologize for being sarcastic in my previous post, but until you catch up on this part of basic physics, you're simply not going to understand how this works. Maybe give that book another read. Not sure if this is covered or if he just assumes you already know. Good luck!
Even a cursory reading of my posts would indicate that I know at least a little about physics and heat (which isn’t just in the purview of physics, by the way).

I have read the book…have you? Wilson only addresses constant braking. And from the paper I linked to above, his only experience is with constant (and hard) braking on downhills. He does not consider what the effect of intermittent braking would have on the rim temperature.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.

Last edited by cyccommute; 08-25-22 at 10:52 PM.
cyccommute is online now  
Old 08-26-22, 04:26 AM
  #69  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
From your post #55:
Originally Posted by cyccommute View Post
The pulse braker.
From you post just now, your answer has changed to:
Originally Posted by cyccommute View Post
Dragging the brakes could only end up with a rim as cool as pulse braking if you use the 0.5mph argument above.
You're starting to understand. Well done. FYI it's not "as cool as", it's "cooler". If you're not comfortable with emotionally accepting dragging being able to win the heat competition with 0.5 mph slower, feel free to change that to 0.1 mph, or 0.0001 mph slower. The point is, as the braking force reduces to approaching zero, the heat being put into the rims also approaches zero. At that point the rim temperature approaches zero degrees above ambient, even though the brakes are still being "constantly dragged". This is clearly visualized in the graph in the book. If you need a reminder go back and have a look at it again.

Your next rebuttal will likely be "you're using an infinitesimally small force now? Can't the pulse braker also use an infinitesimally small force to win the temperature competition?" Why of course. That's exactly my point. Whichever rider uses less braking force will be cooler. Less braking force, less heat. Either rider could win the temperature competition depending on how hard they use the brakes. That's what I've been trying to explain to you this entire time.

You keep mistakenly coming back to sentences such as "but why would a rider brake in x way, it's ridiculous." "Why would the pulse braker have to slow from x speed to y speed?" "This isn't realistic." Etc... All these questions are missing the entire point. Nobody is saying that these examples being used in a physics discussion are trying to imitate real life braking. The point of these examples is to demonstrate to you that your dogma of "dragging always hotter than pulsing NO MATTER WHAT" is wrong. Yes, drag braking CAN be cooler than pulse braking. It depends on HOW you use the brakes. We're are not locked in to a result of pulsing ALWAYS being cooler, as you believe.

If you go back to the prior discussion, we WERE using more realistic hypotheticals, but you didn't get it, so now we are forced to illustrate using extreme values to make things clear and help you understand. After you understand, then we can go back and start exploring real life discussions. If you don't understand the foundation there is no point.

Last edited by Yan; 08-26-22 at 04:55 AM.
Yan is offline  
Old 08-26-22, 05:39 AM
  #70  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
hey there Yan, one thing that I've been meaning to write but never got around to --- for those of us who have toured in far off places, we all know how we must always keep in mind a safety factor to avoid a dangerous situation somewhere where help would be sketchy or worse.
A few times I've made mistakes and gone too fast, and then realized that I had to be more careful and leave more safety margin--so lesson learned.

The one factor that helps with me being comfortable with X speeds for a given specific situation is that compared to my old touring bikes, the present one is so much more stable, handles really well even with a reasonably load on it (to avoid a pothole or whatever) and the mechanical disc brakes slow it down so, so much better and easier than my older bikes.
The last part about it braking so much better is the main factor, making descents so much safer for me, as I know I can safely and quickly bleed off speed with little effort or fanfare.

But if I have to carry more weight than usual, then I add in more safety margin, same with if the road is sketchy or whatever--but we all know and do this, so despite this back and forth about scientific stuff, we all have enough experience to adjust our riding to the specific environment (and that does include taking into account being in a remote area or even a non remote area but where any medical help is far, far off coming, and sketchy even if present)
djb is offline  
Old 08-26-22, 06:56 AM
  #71  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by djb View Post
hey there Yan, one thing that I've been meaning to write but never got around to --- for those of us who have toured in far off places, we all know how we must always keep in mind a safety factor to avoid a dangerous situation somewhere where help would be sketchy or worse.
A few times I've made mistakes and gone too fast, and then realized that I had to be more careful and leave more safety margin--so lesson learned.

The one factor that helps with me being comfortable with X speeds for a given specific situation is that compared to my old touring bikes, the present one is so much more stable, handles really well even with a reasonably load on it (to avoid a pothole or whatever) and the mechanical disc brakes slow it down so, so much better and easier than my older bikes.
The last part about it braking so much better is the main factor, making descents so much safer for me, as I know I can safely and quickly bleed off speed with little effort or fanfare.

But if I have to carry more weight than usual, then I add in more safety margin, same with if the road is sketchy or whatever--but we all know and do this, so despite this back and forth about scientific stuff, we all have enough experience to adjust our riding to the specific environment (and that does include taking into account being in a remote area or even a non remote area but where any medical help is far, far off coming, and sketchy even if present)
Yes I agree completely. The bike makes a big difference. I can't use disc brakes unfortunately. Parts availability are still a problem in a lot of places. It's not just the disc brake anymore. The bike industry has made a lot of standard changes in the last few years that have made things very problematic. The new "gravel" trend is the bike companies realizing they're going to run out of middle aged white men to sell carbon road bikes to. So now they're applying the same bling bling n+1 philosophy to the touring and adventure riding segment, which has led to big changes. Nowadays essentially all of the high end bikes available are using thru-axle disc brake hubs.

For example the Co-motion Americano. High end US based builder, supposedly their heaviest duty expedition touring frame. $5000 bike. Thru-axle disc hubs, which automatically eliminates it for an actual world tour.
https://co-motion.com/collections/si...ucts/americano

These are simply not available in developing countries. There was a guy posting on the R*ddit bike touring sub a few months ago who got stuck in Georgia (the country) and had to get a replacement wheel mailed to him from western Europe. That's ten+ days of time lost, messing up your entire vacation schedule.

So no disc brakes, no hydraulics obviously, no electronic shifting, no weird wheel sizes like 650b or 29+, no proprietary frame standards in the bottom bracket or headset shells, no boost drop out spacing. Everything has to be absolutely generic standard from 15 years ago. I ride a custom frame. I am using a modern drivetrain, but I'm using bar end shifters so I can slap an 8 speed cassette on there and keep going.

For brakes I'm using some Shimano XT BR-M760 v-brakes. These were the pinnacle of Shimano v-brakes from 20 years ago, from the last year before Shimano high end mountain groups went all in on disc brakes. They are the last generation of the old parallelogram v-brake design. They don't squeal like the early versions, and they work extremely well. But end of the day they still can't match disc brakes.

Last edited by Yan; 08-26-22 at 07:07 AM.
Yan is offline  
Old 08-26-22, 07:56 AM
  #72  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
I tend to agree with you on stuff like thru bolts or whatever, my bike in question is from just before this stuff, so QR, 26in wheels, and I use simple shifters (Gevenalles) so other than the mechanical discs, my bike is a bog standard bike from 15 years ago also, bb shell for example)
This is exactly why I bought this bike, and heck, I drove 3 hours to go buy it used......

oh, the other thing with my bike is that it generally is very stable. It can get a tiny wobble at times but it goes away with proper loading and its really a confident riding bike, whether at 5kph struggling up a killer uphill or going close to 80k--this confidence in its handling is a big factor with why I'm comfortable going fast at times when the conditions allow it.
djb is offline  
Old 08-26-22, 09:30 AM
  #73  
cyccommute 
Mad bike riding scientist
 
cyccommute's Avatar
 
Join Date: Nov 2004
Location: Denver, CO
Posts: 25,863

Bikes: Some silver ones, a red one, a black and orange one, and a few titanium ones

Mentioned: 138 Post(s)
Tagged: 1 Thread(s)
Quoted: 5254 Post(s)
Liked 2,824 Times in 1,666 Posts
Originally Posted by Yan View Post
From your post #55:
I still stand by that statement.

From you post just now, your answer has changed to:
Perhaps I should have said “almost” as cool. It would be mostly a wash (more on that later). But continuously squeezing the brakes to reduce the speed only 0.5mph is an absurd argument.


You're starting to understand. Well done. FYI it's not "as cool as", it's "cooler".
Nope and I’ll show you why in a bit.

If you're not comfortable with emotionally accepting dragging being able to win the heat competition with 0.5 mph slower, feel free to change that to 0.1 mph, or 0.0001 mph slower. The point is, as the braking force reduces to approaching zero, the heat being put into the rims also approaches zero. At that point the rim temperature approaches zero degrees above ambient, even though the brakes are still being "constantly dragged". This is clearly visualized in the graph in the book. If you need a reminder go back and have a look at it again.
Slicing the speed ever thinner doesn’t make your point any less absurd. There are seeds of what I’ve been saying in your comments however. You are correct in saying when braking force is reduced to zero, the heat being put into the rims is zero. That’s the point of pulse braking.


Now we get to the “later”. Drag braking can’t result in cooler rims under the same conditions. Let’s assume that all conditions are the same…system weight, straight run, air resistance, etc. We’ll use Wilson’s chart even though the slope is extreme. He has a graph and I can use those numbers. When in doubt, make a graph. In the graphs below I’m assuming they start at steady state and end at steady state. No acceleration to get to speed nor deceleration after the time interval.

First up, let’s say the pulse braked bike is traveling at 45 mph and the ride need to slow to the same speed as the riding using constant braking. The temperature would spike quickly and then decay.


You can see in the graph that the continuous braking results in a constant 70°C rim temperature. With pulse braking, the temperature increases momentarily and then decreases. The amount of heat generated would be the area under both curves. My calculus is too rusty to do the actual integration but the area under the pulse braking curve is significantly lower than that under the constant braking.



The above graph is pulsed braking slowing from 45 mph to 35 mph over 5 minutes while braking at for 5 seconds at 30 sec intervals (sorry about the units on the time scale. Having problems with the spreadsheet). Again, the amount if heat is determined by the area under the curve. The area under the pulse braking curve is far lower than the area under the constant braking curve.




The above graph (90 sec interval) is if one rider were to constantly pulse the brakes vs constant brake application. The difference in area is smaller but the area under the pulse braking is still lower. Less heat put into the rim would mean lower rim temperature. This, by the way, would be the “slinky” pulse braking you have been referencing previously.


Now let’s get to the 0.5mph speed reduction (90sec interval). The pulse braking is being applied every 5 seconds. Temperature generated on the rims at 59.5mph is estimated to be 10°C increase. Pulsing the brakes every 5 seconds to maintain a 50 mph average would heat the rims to 25°C



If we convert that an area graph, it would be hard to say which rim would end up cooler




The graph could change a lot over a longer time frame with harder braking on the part of the pulse braker and longer cooling intervals.

Putting the absurd example aside, it is plain to see that even if you pulsed constantly, the heat put into the rim…and thus the rim temperature…is always going to be lower than if you drag the brakes continuously.

I look forward to you telling me how wrong I am. Please provide graphs if you do so.
__________________
Stuart Black
Gold Fever Three days of dirt in Colorado
Pokin' around the Poconos A cold ride around Lake Erie
Dinosaurs in Colorado A mountain bike guide to the Purgatory Canyon dinosaur trackway
Solo Without Pie. The search for pie in the Midwest.
Picking the Scablands. Washington and Oregon, 2005. Pie and spiders on the Columbia River!
Days of Wineless Roads. Bed and Breakfasting along the KATY
Twisting Down the Alley. Misadventures in tornado alley.

Last edited by cyccommute; 08-26-22 at 03:05 PM. Reason: graph axis corrections
cyccommute is online now  
Old 08-26-22, 10:04 AM
  #74  
Yan 
Senior Member
 
Yan's Avatar
 
Join Date: Jan 2006
Posts: 2,397
Mentioned: 11 Post(s)
Tagged: 0 Thread(s)
Quoted: 1008 Post(s)
Liked 248 Times in 164 Posts
Originally Posted by cyccommute View Post
I look forward to you telling me how wrong I am. Please provide graphs if you do so.
First of all let me say, although we disagree, I appreciate the effort of the graphs.

In your first few sets of graphs you're comparing someone pulsing between 45 and 35 mph, versus someone dragging continuously at 35 mph. Here of course the dragging would be hotter, due to the dragging having a lower average speed. You and I have never disagreed on this since day one.

Let's focus on the last pair of graphs, the final scenario. The problems I see are as follows:
  1. You're saying that dragging the speed 0.5mph slower than terminal velocity results in a 10C increase in rim temperature? If you refer to Wilson's graph you'll see that this is not the case. If you reference the axis, you'll see that if you're barely slowing the bike down from terminal velocity, then you're also barely raising the temperature. So please, let's not engage in this kind of exaggeration in an attempt to support our own point of view. It doesn't help anyone.
  2. You're letting the pulse braker's rim cool from 25C all the way back to 2.5C above ambient in the five seconds the brakes are off? If they are cooling from 25C to 2.5C in five seconds, does that mean in a few more seconds they will be at 0? I don't have rim cooling data, however let us be honest with each other and quit with all the BS. If you go down a big descent, even if you use your brakes optimally, your rims are going to be hotter by the time you finish compared to before you started. They're not going to be at 2.5C above ambient five seconds after you end your run. 2.5C you can't even perceive with your skin. If you invent a rim that can shed 22.5C of temperature in 5 seconds, you should stop talking about bikes and call Formula 1. They want to buy your technology.
If you fix these two errors you'll see that pulsing has a greater area under the graph compared to dragging. I'm at work right now but later I'll draw a graph for you by hand.

Edit: here it is, comments below:
  1. Standard heavy drag braking. Temperature increases until delta T is sufficiently high that cooling matches heating. At that point the temperature reaches a steady state and remains flat. Slope decreases as delta T increases.
  2. Standard pulse braking. Each interval follows the same curve as described above. That goes for both the heating and cooling phases. If you zoom out and squint you'll note that the entire shape also follows the same curve, despite it being jagged. As long as you're continuing to work that same on/off pattern with the brakes, your temperature is never going to go back down to ambient. It's going to zig zag up and down in some stabilized higher range.
  3. Drag braking at different strengths. Same graph as #1 above, just with multiple scenarios graphed together. If you're applying minimal brake lever force, then temperature stabilizes at barely above ambient. As brake lever force increases, the stabilization temperature increases. As you can see depending how hard you squeeze your brake lever, the temperature can stabilize higher or lower than in the pulse scenario. It just depends on how hard you squeeze that lever. If you squeeze with infinitesimally small force, the stabilized temperature will also be infinitesimally little above ambient. All the way until we trend down to zero brake force, at which point the temperature rise also reaches zero. All this time you're still technically continuously dragging. As you can see, dragging does not "ALWAYS" end up hotter than pulse braking, as you believe. It simply depends on how hard or lightly you drag. Either of the two braking techniques can end up hotter or cooler, depending how how hard you operate the brake levers.


Last edited by Yan; 08-26-22 at 02:40 PM.
Yan is offline  
Old 08-26-22, 03:21 PM
  #75  
djb
Senior Member
 
Join Date: Jul 2010
Location: Montreal Canada
Posts: 12,599
Mentioned: 32 Post(s)
Tagged: 0 Thread(s)
Quoted: 2496 Post(s)
Liked 784 Times in 653 Posts
you guys must remember the Monty Python character with the white handkerchief tied onto his head---"MY BRAIN HURTS!"

that's what my Bachelor of Fine Arts brain is saying right now ;-)
djb is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell or Share My Personal Information -

Copyright © 2023 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.