Niles H.

On the long, steep downhill rides that I have to do (I live in the mountains), my rims get very hot. What I am wondering is: how hot can they get before causing problems (assuming my tire pressures are low enough to begin with, so they can handle the extra pressure after heating)?

And what, exactly, are the problems that can occur?

And, is there equipment that will handle the extra heat?

It's really a drag to keep stopping and letting them cool, and to have to monitor them....

vpiuva

Learn to bomb faster.
But seriously, they can overheat. A riding partner of mine took his 650c bike out West and had his valve stem area melt to the rim because of the braking. The tire pressure change is not enough to cause problems, however. Sit up and use wind resistance, alternate your braking f/r. A deeper rim profile (heat sink) would probably help, too.
Post this in Road Cycling. There are several mountain riders on that forum.

HillRider

Apparently, intermittent hard braking, like just before a turn, followed by getting off the brakes completely when you can is easier on the brakes and keeps the rims cooler than continual moderate braking. I believe Lennard Zinn did an instrumented study on this and that was the conclusion.

ken cummings

There are a few genuinely geeky solutions. I have seen small drag parachutes for people who live on flat country and need hill training. An old wind resistance trainer for use on rollers clamped to the seat post and had a roller that rubbed on the rear wheel. Roller shaft had two squirrel-cage type fans on it. It would make a great air brake. Very baggy clothing helps to slow sky divers. An extremely baggy set of overalls could help on descents.

Michel Gagnon

What happens ? Basically, the more heat you create, the more pressure there is in the tires. Eventually, you'll have a blowout.

Solutions.
Speed. I think it's somewhere on Damon Rinard's pages. Basically, you need to go either very slowly or very fast. On one hand, you brake hard to keep the bike to a walking pace, but wheels turn so slowly that there is little heat buildup due to friction. At the other extreme, wind resistance is function of your speed exponent 4 (or 3?). There is a speed that you can't exceed due to extreme wind resistance, usually in the neighbourhood of 50-80 km/h depending on grade and weight vs frontal area. So the closer you are to that speed, the less braking you need to do.
In general, 30-40 km/h (20-30 mph) is the worst speed, i.e. the speed at which you are the most likely to overuse your brakes and overheat your rims.

Pulsing brakes, braking before curves... It's almost mathematically impossible to prove and indeed many physicians will tell you that it should not make any difference, but all truckers will tell you that you can survive most hills with on and off braking but you can't do that by keeping a light constant pressure on the brakes. You might want to use more the rear brake: it's less effective for hard braking, but fairly good to keep your speed in check. And since a rear-wheel blowout is much less dangerous than a front-wheel blowout, that's a good reason to use more the rear brake than the front one in that situation.

Stopping to cool down the brakes. Especially useful and interesting while touring fully loaded. Why not have a break every 10-15 minutes to give your brakes a break?

My strategies. In a steep hill, I tend to apply the rear brake on an off, but at a relatively light pressure to avoid any skids in potholes, frost heaves and the like. Before a curve or any other point where I have to slow down more I use both brakes (more pressure on the front lever). So far, it worked fine, except in one 8-km long winding 16 to 20 % grade when I had a fully loaded triplet. In that case, I stopped twice to cool the brakes just in case.

Technical solutions are expensive because they may require a modification of your bicycle. They do exist and are routinely installed on many tandem bicycles because rim overheat is a more serious problem.
Disc brakes. In theory, disc brakes have the same hazards: they are also likely to overheat. However, the consequence is disc warp, which is expensive (need a new disc) but not dangerous. I think that disc brakes allow you to go down slightly steeper hills (without disc warp) than rim brakes.
An ideal solution would be to install two rim brakes and a disc brake, or two disc brakes and a rim brake, but I have never seen it. You would need to find a way to activate three brakes with two hands; either with a splitter, which I don't recommend, or with 3 levers (like this).

Drag brake. The best solution available right now is the Arai drum brake. It's a very poor brake that never overheats. While you can't really use it to stop at the next intersection (it's sloooow), you could apply it at the top of a hill and keep it there for 10-20 km without any problem. A typical tandem installation uses a shifter lever to activate it; set it and forget it! Basically you decide the ideal drag brake pressure, then you use the rim brakes only near tight curves.

pm124

Just to back up Michel, you can have a blowout. These are typically caused by rim wear and high pressure tires. Once you've gone through your 4th set of brake pads, take the bike in to have the rims inspected to make sure they aren't wearing thin.

Folding bikes and mini-bikes tend to have this problem, especially those with 349 (Brompton) or 355 (Birdy) wheels.

froze

Stab braking does work and has been proven by the CHP in California. Stab braking works not only for large semi trucks, but for older cars with drum brakes, but even bicycles. It's long term friction that generates the most amount of heat as found in trucks and cars that try to apply constant or riding the brakes down a steep grade. Instead, with stab braking, you hit the brakes hard without locking up and slow the vehicle or your bike to about 5 mph lower then the speed you want to go, then release and let the bike regain speed to about 5 over the speed you want to go then repeat the process. Also always try to brake before a curve just as race car drivers do.

I lived in So Calif for about 25 years and rode all over the mountains in that area and practiced the stab braking method and never had a problem on a bike or even on older cars with drum brakes.

You should also learn to use your body as an air drag brake to increase wind resistence to slow you down in combination with stab braking. Also learn to slide your butt off the rear seat to put more weight on the rear wheel will help prevent the rear from locking up, especially if your using just the rear brake.

You can also alternate stab braking; this is where you would on a slower grade apply the rear brake fast and hard-ie stab braking, then use the front brake for moderate grades, and then both for really steep grades.

urbanknight

My uncle, who also loves the mountains, alternates brakes on steep, long downhills. He'll use the front for a few seconds then switch to the rear. This lets them cool off pretty well as long as you're not needing both brakes to keep you from plunging down the hill.

C_LOGAN

Get a single speed fixed wheel bike and pedal slowly down hills.

Carusoswi

Is it safe to assume that disc brakes would handle this situation better than rim brakes because the rotors are designed for a single purpose? I have discs on my bike. The rotors are slotted - I assume to dissipate heat, and, for sure, those rotors serve no other purpose except to provide a surface against which the caliper pads rub to slow the bike.

I am also guessing that well-designed rotors should be able to handle heating without warping - I could be wrong about that, too.

Curious to hear other comments.

Caruso

C_LOGAN

Fit a foot brake - or you could even hook up a special device that sprays water on your rims when you apply the brakes.

The possibilities are endless.

Niles H.

Thanks to all for the interesting and useful replies.

DannoXYZ 

Have you actually measured the temperature-rise inside the tyre? And what was the resultant pressure-rise?

DiabloScott

I blew out my rear once on a long downhill. I got off and started to pull off the wheel to change the tire and singed my hand on the rim it was so hot. Since then I've always feathered my brakes and I've found that the rims cool off very fast as soon as you let go of the brake. The recipe for a blow out is to just drag the brake constantly at some intermediate level; far better to squeeze and release, and alternate front and back.

DevilsGT2

A set of Avid BB7 Road's would serve perfectly. If that isn't enough, it's only a matter of about 40 dollars to upgrade to a larger rotor.

neilster1

replacement discs can be as cheap as $20.. shimano centerlocks will cost more.

froze

I don't get it; for a hundred years folks have been going down steep mountain passes not only in America but in Europe where there ever worse then here, PLUS in Europe they do loaded touring more then we do and way longer then we have and WE and THEY ALL DID IT ON JUST RIM BRAKES! Now suddenly no one can stop without blowing up a tire thus it's time to go to disk? Why not learn how to use the brakes you have and save your money? Have you noticed any disk brakes yet at the TDF? Gee, they race down steep mountain grades in that race and no disk...maybe someone needs to wake them up-those fools!

Michel Gagnon

No I haven't, but remember reading a couple of "scientific" measurements. I have also heard from quite a few tandemists who were cycling down big hills and who said that water dropped on the tire would evaporate on contact, and I also heard a few reports of blowouts due to rim overheating.

If water evaporates when it touches the tire, it means that the tire is at a temperature above 100°C. Roughly speaking, if you have 100 psi at 20°C, you'll have 140 psi at 120°C.

urbanknight

+1 I have never blown a tire from heating up rims (wasn't that mytch disproven by a bunch of phyics geeks on a thread a few months ago?). I have glazed brake pads, though, so it's nice to know how to avoid doing that.

DannoXYZ 

The thing is, a tyre's pressure-rating is typically 1/2 of the blow-off pressure. A tyre rated @ 120psi won't blow off until 240psi. the 140psi resulting from the 100-degree rise is well short of the blow-off pressure.

CdCf

Am I the only one wondering how the OP gets UP the hill? Sounds like it's a monster!

I'd go for disc brakes in that situation anyway.

What about the rim? Would the tyre pop off before the rim sidewalls get bent outwards?

DannoXYZ 

What would bend out the rim? Cerainly not the increase in pressure. The 120-C is well below any temperatures that would weaken the rim the slightest bit.

Now what we've seen alot when people mention spontaneous tyre explosions in their garages or shortly after changing a tyre is due to pinched tubes between the tyre and rim. After several rides, the tube may blow. Or it may blow the next time they pump it up. Certainly it can blow as well due to an extra 40psi? In these cases, it's exacerbated by the extra heat, but is certainly not caused by it.

A properly-mounted tube & tyre on the other hand would definitely be able to handle 140psi.

CdCf

Rims come with max pressure ratings for different tyre sizes. The same pressure with a larger tyre diameter creates a larger separating force on the rim, so larger tyres can't have as much pressure for a certain rim.

If this is a problem in practice, I don't know.

DiabloScott

The details of what actually happens is unknown, but it surely does happen. The pressure in the tire goes up, the hot rim expands, the hot rim gets softer, the bead may also be affected by the heat. It's kind of a beginner mistake but an experienced rider could do it on demand if he wanted to.

Anybody who claims to prove it doesn't happen could also probably prove that bees can't fly.

urbanknight

^ Sorry, I was only stating that the tires don't add that much pressure due to a heat increase. I'm sure a tire could be forced off the rim, but it needs to involve not only heat from brakes, but hard cornering to pull at the beads and possibly even a weak spot in the bead or porly mounted tire.