Bicycle wheel rims meant for use with disc brakes or hub brakes sometimes lack this surface. The Westwood pattern rim was one of the first rim designs, and rod-actuated brakes, which press against the inside surface of the rim were designed for this rim. These rims cannot be used with caliper rim brakes. The cross-section of a rim can have a wide range of geometry, each optimized for particular performance goals. Aerodynamics, mass and inertia, stiffness, durability, tubeless tire compatibility, brake compatibility, and cost are all considerations. If the part of the cross-section of the rim is hollow where the spokes attached,
Wheel hub motor designs have minimized the electrical steel content of the motor by using a coreless design with Litz wire coil windings to reduce eddy current losses. This significantly reduces wheel motor weight and therefore unsprung weight. Another method used is to replace the cast iron friction brake assembly with a wheel motor assembly of similar weight. This results in no net gain in unsprung weight and provides a car capable of braking up to 1G. A good example of this is the Michelin Active Wheel motor as fitted to the Heuliez Will, the first electric car with an Active Wheel drive,