When you build your car, make sure there’s enough clearance between the wheels and the body that the wheels can freely spin. If you tighten the axles too much, your wheels won’t be able to spin without making contact with both the car body and the axle hub.
Don’t make those wheels too loose, though. If there’s too much of a gap between the car body and your wheels, the wheel will slide back and forth along the axle as the car rolls down the track. This will make it hard for the car to roll straight.
The easiest way to get the right wheel clearance is to make a wheel gauge. Take the lid from a margarin tub and cut a business card sized piece from it. In the narrow end of the car, cut a slit that’s about a quarter inch long and just a little wider than your axles.
When you install your wheels, put the axle in the car body and then set the gauge between the wheel and the car body with the slit around the axle. Tighten your axle until your wheel is tight against the gauge and then pull the gauge out. Your wheel will now have an optimal gap.