Originally Posted by ericm979

A clincher rim is U shaped in cross section. The air pressure in the tube is trying to pry the sides of the U apart. That puts both tension and compression on the U. Carbon works well in tension but not in compression.

The second problem is that carbon does not conduct heat well. On a long twisty descent with a lot of braking, the heat generated on the brake track will be conducted through the rest an aluminum rim. With carbon the heat will just build up on the rim walls. Sometimes enough to cause the rim to deform. Or it can cause the tire to come off the rim.