I had a good rummage and came up with an ex radio-controlled jeep. Perfect.
I put the transmission to one side, its a two-speed gearbox and planetary differential. Highly useful for a tank-style bot, just drive the transmission and brake the tracks to turn, simple and efficient.
The hubs pressfitted right onto some 12v NEMA steppers which was handy. They will be the least trouble with all the wet grass flying around driving the wheels directly, and precisely...
Speaking of wet grass it needs a powerful motor. An old cordless drill provided a beefy 18v motor.
I tried to get the brass gear off but it wouldnt play, so in the end I hammered a plastic pipe over it and used that to make a better hub for the strimmer wire than I'd hoped. I was going to replace the gear with a drilled tube, tapped to take a bolt to tighten it onto the motor shaft.
I tried a bit of strimming using a battery with the wires just poked into the connector... Worked a treat. ;-)
Gave up trying to engineer four steerable hubs using servos to swivel them after a fair bit of cussing and decided to go low-tek instead.
A couple of Pololu's make driving the steppers easy. Three signal wires; Enable, Direction and Step, plus the motor and MCU power leads and grounds. They are 3v tolerant and can handle I think up to 30v through the stepper coils.
That battery was a bit hot without any regulation, so I decided to recycle the battery from the drill. I had other plans for it but never mind.
It drives the motor really nicely...
I forgot though, its 18v and the steppers are 12v. Probably be fine but I dont want to risk burning one to find out. I chopped in the motor controller from the drill, its monolithic and has the trigger built into it so I set it to 12v with a cable tie round the trigger rather than use a 12v regulator as well as the 5v one for the Pi.
Thats bolted handily to the chassis.
I still have a few more interconnects to do between Pi and Polou's, and hook up the power to the regulator before it rolls, but I tested the mowing bits.
With the chassis level, the strimmer wires tend to wrap around the hub a bit as they dont have a chance to straighten out after contacting something.
I'm going to replace them with a disc with two screws in it, and use the plastic mower blades available everywhere cheaply. They work, and are simple to install in this...