This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
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.