Skid Steering
Tanks, many outdoor robots, and some four-wheel bases have no steering. They turn by running the left and right sides at different speeds, like a differential drive. But their long tracks or fixed wheels touch the ground front to back, so to turn they must drag sideways across it.
Why skid steering slips
The robot turns at rate \(\omega = (v_R - v_L)/b\) for track spacing \(b\). A point on a track at distance \(x\) ahead of the turning center moves sideways at \(\omega x\), but the track can only roll forward, so it must slide. Summing friction along both tracks of length \(\ell\) gives the power lost to sliding:
What to notice
The arrows along each track show how fast that part slides sideways. They are zero at the middle and largest at the ends, so long tracks waste the most energy turning. Driving straight wastes nothing. Spinning in place wastes the most. High-friction ground like rubber on concrete makes turning hard, which is why skid-steer robots can scrub up floors and struggle to turn on grippy surfaces.
In exchange, skid steering is simple and very rugged, with no steering parts to break, which is why it is popular for outdoor and rough-terrain robots.