Gary, The Challengers use a system called 'Differential Steering". No clutches or brakes involved at all. It is a mechanical system that's nearly as old as the hills. I believe a guy involved with design of the very early Oliver crawlers was the first to use it on an ag tractor. In its simplest form, diff-steer involves essentially inserting another differential. Then by holding the 'ring gear' from turning, one output turns CW and the other CCW. They add a set of gears on one of the axles to reverse rotation that is encountered when you hold the pinion on a car and turn the other wheel. In a Cat, when you hold the "ring gear" from turning, both drive axles turn the same way and at the same speed. Where the early Oliver used a large crank, that the skinner turned when he wanted to change directions, the Challengers use a hydraulic motor to spin the "ring-gear'. Turning it one way causes one track to speed up and the other to slow down. Turning this motor faster creates more of a differential in speed between the two belts. At its most extreme, the tractor can be standing still when you turn the steering wheel and one track will go forward while the other goes backwards. That situation gives you "Spot-Steer" where the tractor turns in its own length.

Now, as to how the steering motor gets its instructions on when and how much to rotate, that depends on which Cat vintage you're speaking about. The first and second generation Challengers; (65 and up to the 95E) and the row crop Cats (35 to 55) all had a hydrostatic steering controller valve very similar to the gerotor type hydrostatic controller that is so common on today's tractors and combines. Turning the steering wheel flows oil to the valve that controls quantity and direction of high pressure oil supplied to the steering motor that's responsible for turning the "ring-gear" when you want to turn and holding it when you want to go straight. The North American market MT series of Challengers use a proportional voltage controlled valve mounted up to the steering motor. Encoders in the steering wheel supply a signal to the Tractor Control Module that works its way through a couple on-board computers, before being sent to the valve, via the CAT-BUS. When the signal gets to the valve, the voltage that the valve sees controls how much and which way, the hi-pressure oil is fed to the motor. The European market MT tractors have a redundent steering system that cobbles a mechanically coupled steering valve to the steering wheel and is used as a back-up if the fly-by-wire system fails. I've never been around one of those, but they look expensive and complex. Something I'm glad we don't have to deal with.

Now to your original question: How does the CAT ag tractor steer?

Pretty darned nice, actually. Its about time to get over the SF again, and with the rain we had last night, you should have just enough time to get up here and see for yourself. I only charge folks $1 per acre for the privelige of driving the rig to see for themselves. After a thousand acres, you should have a pretty good idea of how it works