Rim heating from braking
 

Hi All,

I just moved to the French alps (sucks I know! :) ) and did my first long descents. The roads around here have plenty of sharp bends and are up to ca. 15 % gradient which means near constant braking. After only a few 100 meters of descent my rims were almost too hot to touch.

Anybody ever experience a blowout or other problems from heating caused by braking? I am curious if I am worrying too much about it.

cheers,

Jim

carbon rims?

alternate front and rear braking. usually the spinning rim will cool down sufficiently when you're braking with the other wheel. i dont live in the alps tho...

You can definitely blow out tires from overheating. It's a bit rare on a single bike with aluminum rims, and not carrying a load.

If the rims get really hot, you'll smell and hear the pads off gassing, at which point you need to stop.

The key is to not brake constantly. Brake firmly before a turn, then let your speed build, until the next turn, and then brake firmly again. This allows the pads and rims to cool a bit.

What you don't want to do is "ride the brakes" which it sounds like you're doing.

how much u weigh?

I weigh about 80 kg, and the bike has alu rims. Having thought about this a bit I reckon I just need to grow a pair and let the thing roll between turns! It is pretty intimidating to go through the hairpins at speed though, especially as the centre line in France seems to be more for guidance than a hard and fast rule for motor traffic...

Yep, merlin was right. Whenever your bike is nearly upright, you can brake hard. You can choose lines that allow you to go straighter for a longer period if you want to slow even further.

For the turns, let it go. Braking in a turn lowers your cornering traction anyway (actually can make you more likely to slide out). You can also take advantage of a larger turn radius (curve might loosen up a bit) to go nearly straight again momentarily, and grab some brake there.

Lastly, if you get in a bad spot and must brake in a hard turn, don't use any rear brake. Much more likely to slide out that way. Just smoothly grab some front and hope for the best. Keep everything smooth (not gradual necessarily, just smooth).

Get this stuff figured out before your first rain descent.

As you get more comfortable going faster your rims will not get as hot. The faster you go the more energy will be dissipated through aero drag. If you go slower your rims will naturally get hotter regardless of how you brake (alternating front and rear or using both). So let the bike coast on straight stretches and use both brakes hard before the corner to scrub off enough speed to take the corner safely.

If it's a major concern and you don't already have them, you might also consider getting black rims.
According to Kirchhof's Laws of Thermodynamics, dark surfaces are more apt at absorbing and radiating heat away. The black rims will of course feel hotter to the touch, but that's because they're doing their job.
It's the same reason the bottom of the space shuttle and the backs of most refrigerators are black.

Chris can you explain how black things are better at radiating than white or silver ones? I'm halfway to a PhD in physics and I haven't heard that one yet.

Back on the physics kick though... deeper Al rims will have more surface area than shallow ones, and will thus radiate heat better.

Also, stay away from flimsy plastic rim strips like these. I've melted some on the hills outside of san francisco and it makes for a long walk home. They get hot and shrivel up and your tube punctures on the exposed holes in the rim. Haven't had any problems with cloth ones or the thick sturdy plastic ones.

EDIT: Upon a quick internet search, Chris is right.: Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorptance at the same temperature. Yay college!

That's why they paint almost all radiators black, refrigeration units included - because they dissipate more heat.

Automotive 101 - musta missed that one.........

"which means near constant braking"


This is the main problem. I have never had much trouble as long as I don't just ride the brakes (same can be said for brakes on a car on steep downhills). This isn't easy at 15% grades but even then, intermittent braking usually solves the problem. This often means slowing down more than you think you should when you do hit the brakes, using the front brake more, being more comfortable at higher speeds, etc..

This is a great problem to have though. You are lucky.

I'm sure that y'all are fully correct about the heat absorption of black radiators, fridge backs, etc, but there seems to be one thing missing from your consideration: Those surfaces are black to absorb heat energy from a fluid passing through it thereby cooling the fluid, and cooling the engine, or refrigerator. Since the rims themselves are what the OP is trying to cool, he should have silver or white rims to lessen the heat as there is no secondary fluid which he is attempting to draw heat away from.

Happened to me ONCE. Learned my lesson... major blowout even left a welt on my calf. Singed my fingers when I touched it too.
All the advice about alternating braking etc here is good.

Thing is though, no one is really sure WHY the tire blows off when the rim gets that hot, only that it does (and some people even dispute that, contending that it only happens on a bad tire installation)

1. Yes, pressure goes up with heat, but not enough for a blowout (blowout pressure on cold rims is *really* high. Also, rim temperature does not equal air temperature in the tube.
2. Yes, rims expand when they get hot... expansion of the rim width could be a player.
3. Maybe the rim softens up with heat enough to relax the shape just enough to let go of the bead?
4. Does the bead itself expand - creating a larger effective diameter? (doubtful but who knows)

Lots of heated argument threads on this subject in the archives.

How, pray tell, does one stop when the rims are so hot that the pads are audibly sizzling away?

No you're wrong about the absorption. The rims (black, white or any other color) absorb heat due to friction not radiation. They cool themselves through radiation (black helps this) and also conduction to the brake assembly (color makes no difference here).

I must respectfully, but strenuously disagree. The space shuttle was also given as an example, and there is no secondary fluid behind its black tiles.
Click here for more info. (You'll notice there is no mention of secondary fluids anywhere in the description.) Furthermore, silver & white surfaces are poor heat emitters and therefore exactly what you don't want when in a situation the OP describes.

The primary cooling method is convection into the surrounding air, not radiation. Again, color makes effectively no difference.

Also, any comparisons to the heat transfer problems associated with the space shuttle don't pass the straight face test.

I think it's probably conduction to the air but the black would still help with radiation.

Conduction is the heat transfer mechanism that burns your fingers when you grab the rim.
Convection is heat transfer into the air flowing over the wheel.
And radiation accounts for a PUNY fraction of the total heat transfer - the difference between the radiation of black and silver rims is PUNY squared.

??

Conduction is going on with convection (convection = conduction + flow of material).

(I'd guess that the radiative heat loss is small and that most of the heat loss is due to convection.)