Page 330 - Robot Builder's Bonanza

BASIC DESIGN PRINCIPLES OF ROLLING ROBOTS 299

Figure 26- 2 The construction of the typical
wheel is a rubber (or other material) tire
mounted on a hub.

PICKING THE RIGHT WHEELS

Wheels are made up of tires (or tyres, in the United Kingdom), mounted on hubs (Figure 26- 2).
A tire is rubber, plastic, metal, or some other material, and the hub is the portion that attaches
to the shaft of the axle or motor. The hub usually has a hole for an axle, which is most often
the driveshaft of a motor.
Some wheels for robots are molded into one piece. Others, such as the Dave Brown
Lite- Flight wheels, are composed of separate pieces assembled at the plant. The Lite- Flight
wheels use a plastic hub that attaches to the motor shaft or axle, and onto the hub is mounted
a foam tire.

Wheel Materials
The first order of consideration is the materials used for the wheel. The least expensive wheels,
like those used on low- cost toys, are molded in one piece, usually a hard plastic. The wheel
doesn’t have a separate tire and hub. While these wheels are acceptable for some robots, you
probably want a softer tire surface. This requires a softer rubber or foam, over a rigid hub.
Rubber over plastic: The hardness of the rubber greatly influences traction. One common
measure of hardness is the Durometer, tested by a device called (get this!) the durome-
ter. There are several durometer scales, each labeled with a letter, such as A or D. A
Durometer of 55A is relatively soft and pliable; 75A is medium, and 95A is quite hard.
Rubber over metal: Typical of wheels made for R/C racing, these are heavier and stur-
dier, and are well suited to bigger robots. You can also get small rubber tires mounted
on aluminum hubs. These are typically sold at hobby stores as tail wheels for model
airplanes.
Foam over plastic: Foam wheels are also a mainstay in the R/C racing field. Like their
rubber counterpart, hardness varies.
Rubber/foam over spoked wheels: As the size of the wheel increases, so does its weight.
Spokes are used to reduce the weight of very large wheels. Smaller bicycle or wheel-
chair wheels are suitable for larger robots.

26-chapter-26.indd 299 4/21/11 11:52 AM