Braking technique is about more than just how hard to squeeze without locking either wheel or when to apply them; perhaps more imortant than either, especially on long descents, is the question of how to avoid getting so much heat into the rim that the tyre blows off.
This phenomenon is well known and is a particular risk for clincher users, since a blowout invariably leaves the bare rim riding on Tarmac; a fall is almost inevitable should this happen to the front wheel on a curve. Tubular tyres, being able to take very high pressure without blowing, are prone, when the rim gets hot, to the alternative problem of the glue softening and allowing the tyre either to roll around on the rim or to roll off entirely.
A fine example of the clincher blowout occurred on my recent autumn tour, where Andy Kay’s front tyre went bang at the foot of the Port Way, a particularly steep (25%) and long descent off the western edge of the Long Mynd. This descent is especially tricky because the gradient combines with an undulating surface, steady deviation to the left and single track to make releasing the brakes, even for a second, a dangerous thing to do.
Even using the trick of alternating front and rear brakes is fraught with risk since neither on its own is quite up to the task of reducing speed. Merely holding it in check all that can be hoped for.
Of course, dragging the brakes is what many cyclists do on long descents, and it is precisely what will cause a blowout. Braking generates heat through friction and there’s a surprising amount of heat to be dissipated; slowing from 60mph to 10mph generates around 50kW, all of which has to go somewhere. It mostly goes into the rim, heating it up. How hot? Hard to say but, going by Boyle’s Law and the likely blowoff pressure of a Specialized Armadillo, the air in the tube must get to more than 160degrees C….
The rim, being aluminium, not only heats up fast, it also cools fast in airflow. When Andy reached the foot of the hill, his tyre was still intact. However, instead of riding a few hundred metres to let the rim cool down, he stopped and felt it, at which point the tyre went bang.
It blew, not because the rim got hot, but because it stayed hot for long enough to heat the tyre, inner tube and air inside. This is why drag braking is a bad idea and the correct technique, where it can be applied, is to brake late, hard and for as short a period as possible.
Although the amount of heat going into the rim may be greater in a shorter period, the tyre and tube, being rubber, are slow to conduct heat from the rim to the air inside. Release the brakes while the cycle is moving and the rim will immediately begin to cool off. Provided this happens before the air in the tube gets hot enough to blow the tyre, then it, the tube and the air inside will begin to cool and the pressure will go down.
And if the hill is too steep to allow you to release the brakes and cool the rim, it may be better to stop entirely at frequent intervals than to risk a blowout while still moving. Or do what Andy did and keep going so you can find out how hot the rims are at the foot of the hill.