TOGOAT drive

How a rubber band in the spine tube turns the rear wheels, through a 2:1 string pulley. Simplified parts at real millimeter sizes. Drag to turn it, pinch to zoom, tap a part.

Ready: band slack, nothing wound yet
Tap a partor a label to see what it does.
Axle turns0.0
String off axle0 mm
Shuttle travel0 mm
String : shuttle2 : 1
Band length44 mm

One run, in order

  1. Wind. Roll the car backwards about 24 turns. The string wraps onto the rear axle and drags the shuttle back down the tube, stretching the band. That stretch is the fuel.
  2. Release. The band pulls the shuttle forward. One string leg is tied to the chassis at the dead end and can't move, so all the string the shuttle frees comes off the axle, and the axle spins the wheels.
  3. Coast. When the string runs out, its loop slips off the drive peg. The axle spins free and the car rolls on. Most of the distance is this coast, which is why low friction matters so much.

Why 2:1

½ the pullat the axle, so the tires grip instead of spinning at launch.
2× the stringoff the axle, so the band's energy comes out over more turns.

The string loops round a bearing on the shuttle, so the shuttle hangs on two legs. Move the shuttle 1 cm and 2 cm of string comes off the axle. It's the same trade as a low gear on a bike.

How far

  • One #33 band, doubled: about 45 mmodel estimate
  • Two #33s, doubled: about 74 mmodel estimate
  • The assignment minimum is 5 m. The hallway runs give the real number.

Simplified for clarity: the tube is see-through, the wheels are ghosted, and the wound string is drawn thicker than real braid. Positions and sizes come from cad/v3spec.py.