Q:

Is friction required for a gravity-propelled vehicle?

I have a physics project that requires me to construct a vehicle out of household materials (i.e. CDs, duct tape, wooden dowels, popsicle sticks). This vehicle must not contain any sort of propulsion generator such as an engine or a motor. Instead, once the vehicle is built, it will be release from the top of a wooden wedge that will allow the vehicle to be propelled by gravity. Once the vehicle reaches the bottom of the wedge, it will continue onto a level tile surface and will only rely on the velocity that the vehicle gained from the wedge for it to move. The goal for this project is to get the vehicle to the farthest distance possible from it’s starting position. 
When I am constructing this vehicle, should I aim for a very low coefficient of friction between the wheels of the vehicle and the surface that it is rolling on, or should I allow some friction? I know that friction is required for the wheels of the vehicle to roll, but will friction keep the vehicle from reaching its maximum distance?

A:

For the most part, the amount of friction between the wheel and ground doesn't matter. If the wheel rolls without slipping, then varying the coefficient of friction changes nothing since static friction does not do work (Exercise for the reader: why does static friction not do any work?).
However, the frictional force between the wheels and ground needs to be enough to overcome the frictional torque between the axle and the rest of the vehicle. If there is too much friction against the axle of the wheel, the wheels won't turn and you'll slide down the incline, kinetic friction robbing you of kinetic energy. The way to deal with this is to minimize friction against the axle and make the vehicle heavy so the wheels get a lot of torque against the ground.

