Monday, September 06, 2004

Rim myth?

Is the heating up of rims/ tube failure really an issue on descents or is that myth?

9 comments:

Liz Schleeper said...

If wheels are current technology and tires/ tubes in good condition, I am wondering if feathering (heavy feathering let’s say) of brakes on mountain descents can cause equipment failure. The four people I know who have flatted decending Wintergreen, flatted because of routine gravel in tube, not breaking. But descents in Blue Ridge/ Appalachia are often steep but not long, so is this more an issue in long mt passes? I held on to breaks pretty good down long descents in Alps and Rockies. Am I just lucky?

Ruth Stornetta said...

Descending Wintergreen on a tandem- using the brakes too much- rim heated up- tire blew big time- un reparable- (we were OK- Roger didn't even dump the bike;-)moral of the story- don't do a big tandem descent without disc brakes!

CTClink said...

If the tire is not over inflated to start with, excess pressure due to heating _by_itself_ should not be a problem. However, it is possible that the heat could liguify the rubber of the tire (as well as the brake pads). Liquified rubber would be a good lubricant between the tire bead, the tire carcass, and the rim. At the same time, the force from the road is trying to push the tire around the rim. The tire and tube are anchored to the rim by the valve. Excess tire could accumulate at the valve. Pow!
Wouldn't we love to see some slow-motion video of this?

ChrisWil said...

Being the only person in the area who has ever ridden DAILY in large hills on a road bike I can say that breaking will cause a tire to blow do entirely to heat. The math on how much heat is released due to potential energy is easy and most of that must be transmitted by the aluminum rims in contact with the break pads to the road. If you have ever liquified your break pads during a descent you can understand the energy evolved, the idea that rubber tubes will hold under that indefinitely is absurd. Trick to coming down mountains is to learn to use drag to consume as much energy as possible this means going fast as drag is a function speed to the second power ( in a first order approximation anyway).
.
and
.
If any of you mention Bernoulli in your response I will hunt you down.

Hammer Tillubleed said...

"Break" or "Brake"? Hey, it's your byke...

ChrisWil said...

u r rt i 8 2 grm ck

alltoowell said...
This comment has been removed by a blog administrator.
alltoowell said...

Aside from heat related overinflation while riding steep downhills I have had brake pads overheat and fail to stop my bike when approaching a busy intersection at 40+ mph. My rims have been scored due to excessive steep downhill braking. I've seen the glue fail with "glue on" tires due to excessive heat.

alltoowell said...

(I had no idea that button would delete my entry.)Not a myth in my opinion. RE: first post -- Maybe not so much luckier as lighter... Given the same downhill, speed, wheels, brakes, etc.: a 125 pound rider's tubes will not reach the same pressure caused by braking as a 225 pound rider. Considerably more energy is dissipated in decelerating the heavier rider, producing more heat, resulting in higher tube pressure. Regardless of weight, downhill braking at higher speeds typically results in higher tube pressures. Perhaps the apparently "routine gravel in tube" would not have produced a flat if the tube pressure had been lower.