Wide Rims and Skinny Brakes

The current trend in "aero" wheels is to use a wider rim. We're not referring to rim depth but the actual width of the rim. Historically, most rims on wheels in the aero category were around 20mm wide give or take. The modern rims we're seeing are measuring 23mm and even wider. The claimed advantage is that this allows the rim manufacturer to build a more aero shape with the forethought of a mounted tire. If a tire is 23mm, the rim should be too, that's the idea. Apparently it's all true.

There are two issues that should be considered. One is tire size when using tubular tires. Most manufacturers are recommending 23mm or larger tires. A lot of tubular tire riders are used to running 21mm tires. The reason behind the larger tire is to keep from hitting the rim on the ground and to maintain good aerodynamics. HED, Zipp and many others are all recommending 23mm.

The second issue is brakes. Shimano and Campagnolo brakes are working fine with these wider rims but Sram and many of the aftermarket maker's brakes don't open quite wide enough to give adequate clearance between the brake pad and the rim. The most problematic are Zero Gravity brakes. The Zero Gravity brakes don't have much spring tension to begin with and when the brakes have to be set up that wide, they don't return well. Couple that with the added cable friction from internal cable routing like most time trial frames have and you've got a pretty annoying problem. We've had to shave down brake pads about half way down in order for the brakes not to rub.

Advice? Check with the rim maker and see what tire size they recommend and stick with that. Run brakes that work well. Odds are you'll be hearing more about this in the coming months due to the number of wider rims that are on the way.