The Laws of Physics


Physics considerations of a Soap Box


A gravity racer obtains its energy solely from the force of gravity (assuming no starting push is permitted), which is effectively equal for any combination of racer design and course. However, performance can be greatly affected by the combined effects of various forms of drag and also the moment of inertia of the wheels, and also of course by the skill of the driver in choosing optimal lines. Drag breaks down into aerodynamic drag and rolling resistance. The latter can be minimised by careful attention to the wheel bearings, tyre sizes and pressures, and brakes (to avoid parasitic brake drag). High tire pressures and narrower tyres will typically lower the tyre contact rolling resistance, usually at the expense of grip. Larger wheels have lower rolling resistance than small ones, and rolling resistance is also proportional to overall weight. If permitted, a 3-wheeler design will have 25% less rotational inertia than a 4-wheeler, all else being equal. Aerodynamic drag can be minimised by designing a fully enclosed body that reduces the drag coefficient, but given that this can be a highly technical thing to achieve especially for the amateur builder, and given that some aerodynamic drag is inevitable, a heavier cart will always do better than a lighter one with the same drag coefficient - its terminal velocity will be higher. For this reason there are usually class weight limits in most controlled forms of the sport. Since some of the potential energy due to gravity must be converted to the kinetic energy of the rotating wheels, a vehicle with lighter wheels will accelerate faster than one with heavier wheels. If a race is started on some form of ramp, having a tail-heavy car can also improve final acceleration, as the effective height of the centre of gravity of the vehicle is raised.

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