GMTI

Gear Motor Tread Idlers

£2.78 £2.32
You only need one motor per side on a tracked vehicle, but you'll need at least one idler wheel to stretch the tread between two points on your robot!
Or

You only need one motor per side on a tracked vehicle, but you'll need at least one idler wheel to stretch the tread between two points on your robot!

This idler matches the same dimensions as the driver cog (38mm dia x 15mm wide), but is drilled out to 1/4" (6.4mm), which lets you use a standard #8 bolt as the idler axle.

38mm diameter x 15mm wide