Just a quick question: is differential drive achievable with a 4WD platform? It's an outdoor robot (half a meter in both width and length) and the tires are probably going to be rubber ones. The wheels will be fairly close but I'm still afraid the grip from the tires will be too much. On the other hand I don't want to invest in a complicated drive system.
It will work, but it wont be smooth…
It will work, but it wont be smooth…
Since at least two of the wheels will have to skid along the surface everytime you make a turn, the robot wil most likely jump/shake while turning. This will wear out your rubber tires faster than necessary.
You might also get som unpredictable behaviour. Let’s say you have tested and found out that a 2 seconds burst of both left and right motors in oposite directions will spin the robot excactly 180 degrees. But when you try it on a slightly different surface the robot will rotate sligtly less then 180 due to the extra friction on the wheels that has to slip.
The weight distribution will also play a role, the wheel carrying the lightest weight will always be the wheel that spins during turns.
If you wanna do some research on this subject, I suggest you try out the Mr. Basic kit from DAGU Electronics. It’s cheap and has 4WD skid steering. Then you can try it out on a small scale before spending big bucks on something you might not end up liking.
But then again, this hobby is all about trial and errors so I suggest GO AHEAD, (it’s more fun that way)
4w differential drive
You could the two back wheels differentially and have the two front wheels free-wheel individually. Then you could you use a belt/sprcket/chain to connect each back-wheel to a front wheel. This would only require an extra sprocket on each wheel and two chains/belts.
My idea/suggestion is similar to kiwiclive’s
But rather than chains, use the four motors as one usually would and when making a turn, for example to the left, drive the front right forward and the back left in reverse allowing the other two to freewheel. By freewheel I mean send them 0 0. I would think that would keep them from locking, but, maneuver has a valid point. You will likely run into problems that all skid steer platforms have. That of imprecise turns based on ‘road’ surface.
Thanks a lot for all the
Thanks a lot for all the replies. I think I’ll settle with the differential drive because I have no problem with the fact that it won’t be as accurate as some other system. I have also watched plenty of vids on youtube and thre are some fairly decent 4WD platforms with a differential drive. Having two motors go loose when turning is also a great idea 
Keep in mind that if you’re using geared motors they might not free wheel at all, depending on the ratio.