staehpj1

It seems the recommendations vary on this. Some brands say disc brakes only because of potential heat problems. Anyone have thoughts or personal experience with this? It seems the advantages are significant if money is no object and you don't want to go tubeless. I'd hate to plop down that kind of cash ($20-40 per tube) and find they didn't work out due to hot rims from rim brakes or some other reason. I'd try them on my disc brake bike first, but it is set up for tubeless. Also I'd never give a bike a very good test of heating up the brakes with local riding here in Florida and have no immediate travel plans to the mountains.

The weight savings of two tubes installed and one or two spares is pretty significant and they are supposed to be more puncture resistant. Are they just the latest fad or will they here to stay?

If I were to buy a new bike or build up a new wheel set the point may be moot as I might go tubeless, but for some of my existing bikes TPU tubess seem like a sensible option.

cyccommute 

Thermoplastic polyurethane (TPU) tubes shouldn’t have issues with increases in pressure due to heat as the pressure increase is much smaller than people think…on the order of 1 psi increase for each 10°F increase in temperature. TPU does start to soften at 80-85°C (185°F). That temperature would be difficult to achieve on a rim brake wheel due to the thermal mass of the rim, tire, and tube as well as due to the radiant cooling provided by the surface area of the same. If you drug the brakes down a very long hill, you might be able to get to that temperature but it would be tough.

TPU tubes aren’t all that new. I recall them being a thing in the early 90s. I had one for a tube in a belly boat in that era. They never really caught on for a couple of reasons. Since they are not completely puncture proof, patching them was a problem which seems to have been at least partially solved. Sizing was also a problem as the polyurethane tubes of old did not stretch. The size of the tube had to be matched to the size of the tire very carefully. And then, as now, they were expensive.

Germany_chris

I run them on a rim brake bike

staehpj1

Thanks, That is helpful. When you say "start to soften" do you think that is actually close to failure? Or is there yet more room for error?

It is possible to get a rim above 185 F but I am pretty sure it isn't the norm. I have seen a rim hot enough to make spit sizzle when a wetted finger touched it, but that was an extreme case that could and should have been avoided. That wasn't on my bike. I am thinking that if I do decide to run TPU tubes I'd have the heat issue in the back of my mind and never go that far without stopping to let the rims cool if there was ever a need to. Not that I ever expect to heat the rims anywhere close to that hot by braking. I have gone down some pretty long and steep grades with a load and checked the rims halfway down and they were sometimes pretty warm, but never all that hot that I'd be worried they were close to 185 F.

I see that some brands say they are for rim and disc brakes and some say disc only. I doubt the difference is the tubes and figure it has more to do with how worried they are with avoiding possible lawsuits.

cyccommute 

I can’t say for certain but if the tube were heated outside of a tire, it likely would deform permanently. Inside a tire, the tube is constrained and likely wouldn’t blow out. The tire would have to come off the rim which is unlikely.

[QUOTEIt is possible to get a rim above 185 F but I am pretty sure it isn't the norm. I have seen a rim hot enough to make spit sizzle when a wetted finger touched it, but that was an extreme case that could and should have been avoided. That wasn't on my bike. I am thinking that if I do decide to run TPU tubes I'd have the heat issue in the back of my mind and never go that far without stopping to let the rims cool if there was ever a need to. Not that I ever expect to heat the rims anywhere close to that hot by braking. I have gone down some pretty long and steep grades with a load and checked the rims halfway down and they were sometimes pretty warm, but never all that hot that I'd be worried they were close to 185 F.

I see that some brands say they are for rim and disc brakes and some say disc only. I doubt the difference is the tubes and figure it has more to do with how worried they are with avoiding possible lawsuits.[/QUOTE]

It would be possible to get the rim hot but you’d also have to get the tire that hot as well. Rubber doesn’t conduct heat all that well

staehpj1

I guess that one place where I am real likely to use one first is as a spare tube for my tubeless equipped MTB. As it is I don't carry a spare since a suitable butyl tube is so big and I am pretty unlikely enough to need it that I don't take a big heavy butyl tube. A TPU tube is enough smaller and lighter that I might be willing to pack one in the little seat wedge for normal around home trail riding. Then again the tubeless system has been super reliable so unless going pretty far off the beaten path...

staehpj1

Yeah, not gonna happen. I will forget about that worry.

himespau 

I always thought the "start to soften" concern was more about them becoming more permeable to air.

I've seen some people talk about using them as a low weight/small size spare to pack for tubeless disk bikes, but that seems pretty expensive for a spare. I guess I'm not far enough down the weight weenie rabbit-hole yet.

Germany_chris

They do ride a bit better than butyl tubes but not $20 more per tube better

staehpj1

That hadn't occurred to me. I wonder if that is an issue and if so how much.

They are also a lot smaller. This is a bigger deal for a bike with bigger tires so for a MTB it amounts to a good bit of space when you might want to fit some basic tools and spares in a tiny seat wedge. As far as expense goes I figure it is likely to be a one time or at least once in a very great while expense since my tubeless system has been very reliable and the tube possibly will never be used. In the 8 years I have had the tubeless setup I have never needed to use a tube. If I used one every 8 years it would be a pretty low expense.

staehpj1

Cheaper than converting to tubeless unless you are already tubeless compatible. Worth it? That depends on the rider. I think it just might be for me in some situations.

Germany_chris

I think that depends on the tires...
Love 'em or hate 'em I mostly ride Rene Herse extra lights and TPU tubes have made the ride better and when replacement time comes around I will replace al my with them but it's not a priority.

staehpj1

Sounds like a sensible approach. I probably wouldn't replace tubes with them when not changing tires or at least remounting them for some other reason. Also with a light supple tire they make sense. With a stiff heavy tire the benefit wouldn't be noticed. I doubt anyone woyld use a TPU tube in a Marathon Plus tire.

pdlamb

I've seen steam coming out of spoke holes after running through a ford before climbing and then descending a steep, twisty road. Considering the braking sidewall would have to generate the heat that was conducted down to the rim tape? Let's just say I wasn't going to measure sidewall temperature with my finger! But I suspect the sidewall in contact with the tube was well over 212F.

To Pete's question, I doubt you'd need to worry about 1-3 mile grades of 12-15% with hairpin turns in Florida. But I'd be happy to have butyl tubes in my wheels when I encountered them elsewhere.

cyccommute 

Just because you see steam doesn’t mean that the rim is at the boiling point of water. Water vapor can condense at various temperatures depending on the dew point at the time the water vapor evaporates from the wheel. Here in dry Colorado, you would likely never see “steam” in a situation like you describe. The amount of water in the air is too low for the dew point to be low. In the more humid areas of the world, seeing “steam” in that situation would be more likely at a lower temperature. Remember that you can see very dense “steam” at cold temperatures and the water vapor is only at 98°F.

I have years of steep downhills of much longer distance than 3 miles under my belt. Rabbit Ears Pass is just one…7 miles of 7% grade (but no turns). Colorado National Monumen is another with 6 miles on the front entrance and 7 miles on the back entrance, both of which have tons of switchbacks with 15% grades. Done both on downhill (two separate occasions). I’ve been down mountain bike trails and jeep roads with cantilevers. Saxon Mountain outside of Georgetown is 6 miles long with an average of 8% grade but with some switchbacks as high as 15%. It’s a damned steep road. The one thing that all those rides…and the thousands of other descents I’ve done…is that I have never once overheated a rim brake. I’ve never even gotten them hot to the touch. If you use your brake properly, you can descend anything without heating the wheels.

pdlamb

Not the "steam" you're describing, but what ChemEs call "live steam" -- water vapor that is pushing droplets out by the force of the steam pressure trapped inside the wheel.

On a couple other occasions, I've wasted some water out of my bottle to wet down a finger, then flicked it against the rim like grandmother used to do with her cast iron pan to see if it was hot. Like the cast iron pan, the water hissed and fizzled before quickly evaporating. I've also seen the remains of a wheel after a tire blew on the Vesuvius grade. Not me! I stopped to let my rims cool for 10 minutes halfway down.

cyccommute 

Not sure what or who ChemES is but I find your description hard to believe. Even if the rim were at the boiling point, you would have to get water into the rim which would be difficult considering that the water is cooler than the rim which means the water would vaporized before it got into the rim…think water drops on a hot skillet. If the water did happen to get into the rim, it would quickly cool the rim below the boiling point so the whole system couldn’t be “live” any more.

If you are talking about Mt. Vesuvius, I’ve got similar (or worse) within 100 miles of my house. Saxon Mountain that I referred to above has a similar drop (2450 feet) over a slightly shorter distance (5.7 miles) and a higher % grade (8.5%) with far more switchbacks that are far steeper (some up to 15% grade). I’ve done it with cantilever brakes and never over heated the rims.

Oh, and by the way, Saxon Mountain is dirt with a mostly ungraded surface.

Overheating wheels on a downhill is a user error. I don’t check my rims after (or during) a downhill because I know that I haven’t overheated them. Braking on a bicycle should be down in short, hard pulses rather than a long sustained pressure.

pdlamb

So, you've never heard of a chemical engineer being called a Chem E; you missed the part about going through a ford, which let water get into the rim; you skipped the part about water from my fingertip sizzling on the hot rim; you're conflating dynamic heat transfer (heat going through the rim from the brake surface) with equilibrium thermodynamics; and you've never overheated a bike wheel on a downhill; and you don't mention ever seeing or hearing about an overheated rim leading to a tire blowout.

Remind me why I should consider your opinion on how hot a bike tube can get "informed?"

cyccommute 

I’ve never heard of a chemical engineer being called a ChemE nor of a group of them being called ChemEs. I’ve heard of them being called a chem. e. (with or without the punctuation). You were unclear in your useage.

That said, I am familiar with what “live steam” is and its uses. I don’t know of anyone who would call steam rising off a hot surface as “live steam” nor just boiling water as live steam. That designation is more for a boiler under pressure.

Nope. Didn’t miss it. Just don’t believe it. Water that hits a surface that would be at (or above) the boiling point of water isn’t going to allow water to get into the rim. The water hitting a metal surface that hot would be subjected to the Leidenfrost effect which is the same as water dropped onto a hot pan. It creates a layer of vapor under the water and the water is held away from the surface. If the rim is cool enough for the water to get into the rim, the rim is too cold to make “live steam”.

Nope. Not at all. I’m taking dynamic heat transfer into account. Yes, heat would go into the rim from the friction pads rubbing on the side of the rim. But the heated portion of the wheel would have to travel a long way through air that is actively transferring heat away from the rim. The rim isn’t a small piece of metal and has significant surface area for radiating heat away. By the time the part of the rim that is heated at the friction point to reach the bottom of the wheel where it would be in contact with the water, the rim would have cooled significantly. Contact with the water would cool it even further as boiling…or even just heating…water is endothermic. It sucks heat away from the metal very quickly and cools the metal rapidly below the boiling point.


Correct. Because I know how to use my brakes from years of using them on many, many, many steep downhills.

Nope. Never heard of someone doing it nor have I experienced it. I’ve heard myths about how it “could” happen but I’ve never met someone to whom it has really happened. As I stated above, the increase in pressure due to temperature is actually quite a lot lower than most people think it is. Using the 1 PSI increase for each 10°F increase in temperature, going from 70°F to 212°F would only result in about a 14 psi increase in pressure. As long as you aren’t using something silly like a hookless tire, a 14 psi increase isn’t likely to cause a tire to blow off the rim at the rated pressure.

Because I know a lot of stuff.

Going back to staehpj1‘s initial question, while the TPU material softens at around 80°C (185°F), it doesn’t melt until about 140°C. Soften doesn’t mean that it would be close to failure nor would it be close to tire blowoff. Melting would probably be bad but, for the metrically challenged, 140°C is about 285°F. That’s a whole different temperature and would be even harder to attain through braking.

And, to be clear, I probably wouldn’t use a TPU tube. They are too expensive and still a bit of a pain to patch.

Yan 

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.

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 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.

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.

This is how to descend properly: use both brakes to take advantage of maximum thermal mass, descend at a pace you are comfortable with. Listen to your brakes. When the braking sound changes, it's a sign the pads are overheating and their physical properties are changing. At that point consider stopping and taking a photo break. I have no scientific proof to support my method. However I did do a bunch of expedition touring in the Himalayas and on the Tibetan plateau so I've gone down a hill or two in my life.

Carbonfiberboy 

Having blown a few tires off a tandem, I think you oversimplify the issues and therefore the method. What you're trying to do primarily is not blow the tires off. That's not good. IME if you work toward that goal, the brake pads will be fine. As you say, a hot rim will transfer heat more quickly into the atmosphere.

Tire pressures can certainly increase to the point where they considerably exceed the max allowable pressure, otherwise they wouldn't blow off. If one accepts that the goal is to minimize the amount of heat transferred to the air in the tire, then pulsing the brakes is obviously the way to go. When the brakes are off, the rim can cool. When the brakes are on, it can't. So the thing to do is to let the bike run as much as possible and then hit the brakes hard before corners. If it's a twisty descent, one alternates front and rear. The idea is to briefly hit the surface of the rim and then allow that heat to dissipate into the atmosphere as much as possible before heating the rim surface again. This helps to prevent the transfer of that heat into the bead and then into the air.

I run TPU tubes on my single, but probably wouldn't on our tandem. I've experimented with latex on it, but have come back to butyl.

It's not just rim brake bikes. Disc brakes can also overheat from the constant application of the pads, though that has nothing to do with this thread.

From the above one can see that one's best defense against overheating tires is to use deep section alu rims. The greater surface area allows faster heat dissipation. Again, the main reason to pulse the brakes is to allow aero drag to generate more heat, plus to some extent the quick temporary heating of the rim transfers the heat of braking more into the atmosphere than into the tire.

Yan 

You're saying the rim cannot cool when the brake is on?

No sure where you got this idea from. The rim is dissipating heat all the time, whether you are applying the brakes or not. When you use the pulsing technique all you're doing is increasing the average speed so the wind does more of the work instead of the brakes.

You can ride down the entire descent with your brakes lightly dragging. As long as you maintain the same high average speed as the pulsing technique, your rims will be fine.

Anyway we are arguing over semantics. Realistically, you're going to brake more at the corners.

djb

the issue isn't "pulsing", (how often you do this and how long is completely open to interpretation) but like cycco, as someone who has done a lot of loaded big downhill descents, I have always found it better to let the bike run, then do good hard braking applications (using front stronger) to bring speed down very quickly (quickly as in a relatively short braking application) --then letting bike run again with no braking to let heat dissapate.

this always keeps rim or rotor temps down

sure, hairpins and whatnot sometimes/often mean more frequent braking, and the odd really technical, windy, hairpinny descent can mean heated up brakes, but by doing really hard and short braking action, generally means less heat buildup overall---and I've learned this from making the opposite mistake, both in cars and on two wheeled things.

and everyyyyytme this topic comes up, we get to the fine detail that looooooooooots of people don't use their front brake properly, and are "front brake scared", so they don't use it hard enough and therefore have less braking effect with the same amount of actual braking time--which leads to more braking needed, more heat buildup etc etc etc

but interwebs blah blah is never going to teach front brake scared folks how to brake properly, and those who think they are braking properly just dont understand using the front brake really hard--or they think they do, but they really arent., well, not to the most effective use of them anyway.

good old interwebby arguments, gotta love em

staehpj1

Okay so in a different vein... I have said here and elsewhere how my tubeless setup has been completely trouble free. Well today I had a tire failure that is probably on the verge of fatal. This one might or might not mean using a tube if it occurred on a tour. The fact is it wouldn't have happened on tour, but still... It does raise the possibility a bit higher. Most likely for a tire like this the tube would be used only until a new tire could be obtained.

Here are the circumstances:

• The tire is well worn. Probably beyond what I should be using it for the terrain I was using it for. The knobs were mostly gone in the critical areas. I may have continued to use it for gravel roads or pavement, but was still riding on some singletrack with lots of roots, tight steep off camber turns, small drop offs, and other technical challenges.
• As a result of the techincal nature of the trail and the worn state of the tire I was running rather low pressure in order to maintain some grip on the surface.
• None of it was very high speed, but the roots and so on were too much for the low pressure and bit the sidewall at the rim.
• Sealant isn't good at sealing that kind of leaks, but it was slow enough to continue riding with frequent pumping (or CO2 which I use around home). So I may not have even installed a tube even on tour unless another tire was a fairly long way off. At a quick glance the damage doesn't look severe, but air is leaking through the sidewall at the rim. It may be able to limp along for a long time or may be ready for a catastrophic failure. I am not inclined to bother investigating much or experimenting since the tire is worn out any way.

In any case I am ordering some new tires. Sadly the ones I have been using (Stans Ravens) are no longer available or I may have bought them again. They were pretty good for the local trail conditions and surface I have benn riding on and they seated easily even with a hand pump on the Stans wheels. Wear life was so so though. The other MTB tires I have been using on another bike and happy with here (Kenda Slant Six) are apparently also no longer made. So I am ordering some Kenda Small Block Eights. They sound like they will suit the mix of surfaces I use the bike for.

djb

and in related news, I went mtbiking with my fatbike yesterday and ran too low pressures in front, for same terrain and reason, and got a slow pinch flat---so there you go.....gotta get that giant tube out and find the holes.
but at same time, take off these nicer tires for winter use, and put on the cheapo tough Kendas or whatever that came with the bike, as well as the much heavier stock tubes back in also--more appropriate for summer rooty and rocky stuff anyway.