pwa wrote:I remember reading something some years ago about braking tactics on descents to minimise heat in the rims, and the conclusion was that there are basically two things that work.
One, which I find more practical in most circumstances, is the traditional fast descending, letting the brakes go and the speed build up, then braking hard and late as a bend approaches. That leaves the rims cooler than if you have been dragging the brake all the way down.
But there was a second suggested solution, and that was to descend very, very slowly, at 2 or 3mph, which it was claimed did not produce the heat build up that constant braking at 15mph would produce. I can see that being useful on some twisty lanes in Wales. Anyone got any ideas why that might work? Maybe it is to do with the fact the descent will take much longer and the rim will be able to lose heat at the same rate it picks it up.
The heat a rim gets too depends on the balance of heat in vs heat out.
The total amount of power is simply the rate of descent - how fast you are gaining energy through gravity.
This heat either goes into heating the rim, or heating the air.
Heating the air can come from air resistance, or heat transfer from the hot rim.
Now, consider a very slow descent.
There is very little heat entering the rim. That heat is transferrred to the air through heat transfer, which easily keeps up without the rim getting very hot.
Next, consider a very fast descent, with no braking at all.
No heat enters the rim. All the heat is dissipated in air resistance.
Finally, consider a medium speed descent. Perhaps half of the energy from gravity enters the rim. Heat transfer can't keep up without the rim gaining a lot of temperature.
So there is some rate of descent for any given hill, cyclist and bike which will produce a maximum temperature in the rim. Any descent either slower or faster will result in cooler rims. At the limit, either a very, very slow descent or a very fast descent without braking, will result in rims at ambient temperature.