Wheel rim temperature
 

Hi Jan, How much heat build-up occurs in descending, say a 7% LONG grade, like Hurricane Ridge, on a single bike? My concern is having a tire blow-up. To minimize heat building up in the rims: Is it better to break intermittently hard and release versus applying a lighter constant pressure? Thank you!

What type of brake?

who is Jan? you may have wanted Bike Quarterly not here..

Do your own research if it matters.. need a thermal imaging tool or maybe even a thermometer .

real science is a cost, you got resources?





NB, you are tapping into 'FAQ', i vote that this has worked for decades : "It is better to break intermittently and release "

you can keep the speed down on the down hill and then the braking effort wont have be that hard..


Tandems have drag brakes, they used to have a big drum brake with a massive aluminum heat sink.

now the tandem bike companies offer an oversized disc for the rear so the disc has time to cool
before it gets back around to the caliper, again..





.....

Impossible to answer without details, except that countless riders do descents like that and worse without issues, even on a tandem.

Here are some hints.

The first thing is to use the "air brakes".
Every bike and rider has a terminal velocity where wind drag is is equal to the effects of gravity. At terminal velocity you stop accelerating and coast at constant speed until something changes. So the best way to descend without hitting brakes is to get comfortable nearer to your terminal velocity, where the wind is doing most of the work.

Also keep in mind that wind drag is proportional to the square of wind speed, so once you reach 15mph or so it ramps up vertical quickly with small increase in speed.
....

Use the brakes intermittently to trim off some speed then let the bike accelerate back towards terminal velocity and repeat the process as needed. You can also use your body and posture to slow your bike by being more upright and raising your wind drag as much as possible.

So, once the wind becomes the main brake, and your rim brakes the auxiliary, you won't be heating the rims nearly as much.

Sadly I see so many riders dragging their brakes and heating their rims at speeds just a few miles an hour below terminal velocity where they wouldn't need their breaks at all.

wheel rim temperature
 

Hi Jan, How much heat build-up occurs in descending, say a 7% LONG grade, like Hurricane Ridge, on a single bike? My concern is having a tire blow-up. To minimize heat building up in the rims: Is it better to break intermittently hard and release versus applying a lighter constant pressure? Thank you!

I blew up a rear tire once... ONCE. Holy hell it was loud and left a welt on my leg. Then I grabbed the wheel to change the tire and burned my hands. I'll never forget again.

I've heard people in the alps say they've seen steam coming from hot rims... so that's 100C... maybe a little less because altitude.

Brake on, brake off. The rims generally cool pretty fast if you give them a chance, but if you drag the whole time, you're asking for trouble.

wheel rim temperature
 

calipers

Oh, it depends. But from experience, when the wheel gets that hot, it causes the valve stem to start to separate from the inner tube, and causes a leak. I stop the bike quickly once I hear that sound, but its not like there is an :eek:

Here's a video of a test of what you're asking ... 2sec with Google.


edit: interesting as that's how i'd do the test exactly as well. I'd run them until they snapped.

Duplicate threads merged.

Had that happen first time in Austria. After dropping the best part of a mile in altitude through a series of switchbacks I rolled through a puddle and was greeted with hisses and fizzing. Tire and tube did OK though.

I've seen that as well. Kinda cool to watch, though it cooled off (or the water trapped in the rim tape all evaporated) pretty quickly. I'd just come off a twisty ridge averaging 12-15% grade for just under a mile. You can well believe I made a point of not touching either rim for another half hour!


Oh, and no leaks from the tube.


Better to BRAKE and coast, lather, rinse, and repeat. Dragging your brakes down a long slope can heat the rim up pretty quickly. OTOH, no braking, no heat in the rim. I've come down an eight mile road coming off a ridge with up to 6% grade, and had no temperature buildup. Curves were such I barely had to brake.

I was riding behind a guy who went down a short, steep hill in downtown Memphis on the bluffs (maybe 20 foot vertical drop in a couple hundred feet, 10%-ish grade?), and I'm guessing he was using the rear brake only. His inner tube exploded like a gunshot and startled everyone else riding near him. Luckily he kept it upright, and we were coming right upon the ride destination.

Even with a 4 pannier touring load, I've never had a problem, like that, in over 50 years of riding.

As FBinNY says, there are too many variables to say precisely but we can make some estimates. Based on acidfast7's video, the temperature increase is about 80°C. Based on Gay-Lussac's law and assuming a 100psi starting pressure, the increase in pressure for that kind of temperature increase is about 25psi. In other words, the pressure goes from 100 to 125psi. Is that enough to blow off the tire? Maybe but maybe not. It would depend on too many factors.

Something to consider in acidfast7's video link is that the rim is a carbon rim. Carbon doesn't dissipate heat as quickly...or, perhaps, at all...as aluminum. The heat build up from braking an aluminum rim would be lower and it would cool more quickly. That will result in a smaller pressure increase.

Look at the video again. About 3 seconds after the squeal starts...who could stand that for more than a couple of seconds:rolleyes:...the temperature jumps up to about 80°C. That would be an increase in pressure to 113psi. After 15 to 20 seconds, the temperature is 100°C which gives a pressure of 120psi. It takes 30 or 40 seconds of braking to get to 120°C.

Constant braking is not your friend. Punch the brakes hard, scrub speed, release the brakes, repeat as needed.

Had you been braking steadily and constantly or did it just get hot from spinning rapidly unbraked for a long time? I don't really understand the OPs question I guess.

Yeah, I was using constant light braking on the rear to control speed for about 2 miles - textbook wrong thing to do; tire blew off the rim and tube exploded. Having learned my lesson, I now visualize every time I let off the brake, I'm adding cool to the rims.

I don't think the pressure increase from temperature is the only thing that makes the tire blow-out, all that heat makes everything react differently.