The tire mounted on the hub measures in at 31.2mm (1.23in) diameter and overall
width of 13mm (0.5625in) wide. The hub is drilled for a 3mm axle ideal for
the 150:1 Mini Metal Gear Motor or
the 298:1 Mini Metal Gear Motor. It's a nice, fat, beefy wheel ideal for small
robots! Secure it to your motor shaft with the included 4-40 hex grub/setscrew
(requires 0.050 hex wrench). The entire assembly weighs 12g.
Although you can simply install the motor from the outside, it was designed
so you could internally mount to the hub. The internal
cutout is a little thick, but a bit of filing on the
edges of your gear motor lets it fit right in. By hand-filing the edges down,
you can fine-tune the installation so the brass edges of the gearbox act as
alignment bushing points that assist in keeping the wheel spinning true.
$4.99 
150:1 Micro Metal Gearmotor
298:1 Mini Metal Gear Motor