Hi guys, new poster here.
I have read through this whole thread and am absolutely fascinated by what you guys are doing.
Although the concept is very different, what some of rock crawler guys are using in the way of engines and transmissions may be relevant to building a small tracked vehicle on a very tight budget.
One successful way to go about this would be to use the complete engine and transmission from a front wheel drive vehicle, turned around, so that the original driveshafts pointed towards the front and back. Then couple those to identical front and rear differentials (with one or the other inverted). These extra differentials provide four wheel drive, as well as creating the very low overall gearing needed for a rock crawler with reasonably large wheels and tires.
Earlier in the thread, it was suggested that using cut down tractor tires as rubber tracks works reasonably well, but in mud or snow the driving wheels slip within the tracks. If the rubber tracks were driven from both ends (four wheel drive) that problem might well be solved without too much extra complication.
Anyhow, getting back to a REAL tracked tractor, with rear drive sprockets. A front wheel drive engine and transmission, either manual or automatic, with the engine size and type of your choice, should be able to be bought as a complete package at a bearable cost. The differential of that could be welded up, so only one driveshaft could go to a rear differential to give the suitably low overall gear ratios required for a tractor.
Now the rock crawler guys usually run the engine offset to one side, and have the driver on the other side. These have a reasonably wide chassis, so that is a practical arrangement for them, and can have a low center of gravity.
A tracked vehicle is going to be much narrower in the chassis, so the engine could be located centrally at the front, in the usual position, and the driver could either sit on top of the engine in a really small tractor, or behind in something a bit larger and longer. The driveshaft will be offset to one side at the front, and the rear differential could also be offset, or central. It should work either way.
The rock crawler people prefer the transmission (of whatever type) to have a cable operated gear shift, which really simplifies the gear linkage problem, or use an automatic. One last thought. A factory power steering pump might be a bonus, and allow for some very basic hydraulics.
Sorry to dirty up the thread, but the whole concept has proven to work very well for the rock people. Their only complaint seems to be that the overall gearing usually ends up being a bit too low. But with tracks instead of wheels the overall gearing of this system may turn out to be pretty ideal. It seems to me to be a low cost approach using mostly standard readily available parts to build something powerful, simple and very robust.
