Braking Heat on Steep Descents
Although tandems with their higher weight to wind drag ratio have this problem more often, steep mountain roads, especially ones with poor or no pavement require so much braking that single bicycles blow off tires from overheating. For tubulars the problem is not so much over pressure than rim glue melting as all pressure sensitive glues do with heating. As glue softens, tires slip on the hot rim and pile up on the valve stem. This is the usual indicator that tubular tire wheels are too hot. The next is that the tire arches off the rim in the area just before the stem.
This is a serious problem both for tubulars and clinchers because most clincher tires, given enough time on a hot rim will blow off if inflated to recommended pressure. Pressure that gives good rolling performance (hard) while tubulars roll off from lack of adhesion to the rim. The faster the travel, the more descending power goes into wind drag and the better the rims are cooled. Going slowly does not help, unless speed is reduced below walking pace.
On steep descents, where rims stay too hot to touch for more than a minute, reducing tire inflation pressure is a sure remedy. However, tires should be re-inflated once the rims cool to normal. The blow-off pressure is the same for small and large tires on the same rim, it being dependent only on the opening of the rim width. Also, tires with a smaller air volume become hot faster than larger ones.
There is no way of descending continuously and steeply without reducing inflation pressure, unless there is an insulator between the tube and rim of a clincher. Insulating rim strips are no longer offered because they were an artifact of dirt roads that often required riders to descend so slowly that all potential energy went into the brakes and almost none into wind drag. These rim strips were cloth tubes filled with kapok, their insulating purpose being unknown to most people when they were last offered.
from here:
Descending by Jobst Brandt