Gravity racer

A gravity racer or soapbox car is a motorless vehicle which is raced on a downhill road either against the clock or against another competitor.

Originally, gravity racers were built from wooden soap (or apple) crates and rollerskate wheels, but have grown more sophisticated over time, with materials like aluminium, fiberglass and even CFRP being used.

In addition to being built by children, there are organised competitions and races ("soapbox or billycart derby") that often engage the enthusiasm of adults.

Many, but not all, events impose the following rules: Soapbox cars weigh an average of 150 pounds (68 kg) and reach top speeds of 20 to 30 mph (30 to 50 km/h).

Steering is typically actuated using a rope connected to the ends of the steerable beam (which can then double as a useful manual pulling device).

Races will usually take place downhill and the most efficient and skillfully driven cart will win – gravity applying equally to all.

[2] In Australia, Billy carts tend to conform, even in the 21st century, to a more traditional or rudimentary specification which rose to prominence throughout the country in the late 1800s and early 1900s, often constructed by juveniles from scavenged or relatively inexpensive materials.

Traditional designs are very minimalistic, with an open board-like body and wheels made up of naked ball bearings (no tyres allowed), usually discarded parts obtained from auto mechanics.

[4] In 1933 Dayton Daily News newspaper photographer Myron Scott of Dayton, Ohio, United States had covered a race of boy-built cars in his home community and was so taken with the idea that he acquired rights to the event; the national-scale Soap Box Derby grew out of this idea.

In the UK gravity racer derbies have recently become more popular, brought to the masses by large events such as the Red Bull Soapbox Race and that held between 2000 and 2004 at the Goodwood Festival of Speed.

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.

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.