Originally Posted by daveryanwyoming .
Carbon fiber rims are waterproof in the sense that the cured material in a fully built rim does not absorb water and water does not penetrate or seep through the rim material. Water can get inside carbon fiber or alloy rims via the spoke holes and should be drained out after a wet ride but the rim material itself is waterproof.
The biggest downside of full carbon rims in the rain is braking performance. Alloy rims may lose a little braking power in the rain but their behavior is predictable. With wet carbon braking surfaces there isn't much friction or braking power when you first apply the brakes, so we tend to squeeze the levers harder without thinking about it in an attempt to get the brakes to respond. Eventually enough water is squeegeed out between the brake pads and the carbon rim material and all of the sudden those brakes we're squeezing pretty tightly grab all at once. Definitely not a good feeling in a hard braking situation on a wet day. That said, equipped with a decent set of carbon brake pads like the SwissStop yellow pads carbon fiber rims will still work pretty well in wet conditions as long as you allow for slightly longer braking distances than normal and recognize they won't feel quite the same as alloy braking surfaces.
I have no idea what you mean by: "What happened to the foam of the rim if it rains?" what 'foam' are you talking about? If that's a typo and you mean the 'form' of the rim then it's the same wet or dry as carbon rims or carbon bike frames don't absorb water and don't deform when wet.
-Dave
