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Originally Posted by Cory
actually, 4 wheel drive could be worse for getting on the ramp than 6 wheel drive. Depending on how you mount your wheels, the point at which you'd normally bottom out would be occupied by the middle wheels on the robot, whereas if you had a 4 wheel drive robot you'd get stuck.
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Very true. For the past two days straight (and probably for one more), we have carefully been considering this. For a traditional 6wd skid steer (tank steer) robot to make it up the ramp effectively, the end of it that goes up the ramp first must be much heavier so that when the middle wheels reach the edge between incline and horizontal, gravity pulls the "front" right down on top of the horizontal platform. If this is how it's done, and the wheels are the front-most and back-most thing on your robot (no frame rails in front of them) then it is possible to have an incredibly low frame (1/4" off the ground) If the "back" of the robot is much heavier, you will either tip over or get hung up on the frame between the middle wheels and the back wheels.
In a six wheeled robot, when the middle wheels get on that edge where the ramp ends and the platform begins, there is a bit of a shaky transition because the robot is sort of like a teeter totter. For some brief moment in time, the robot will only be on it's two center wheels.
For a traditional 4 wheeled robot, the transition from incline to horizontal surface is quite pleasant, and all 4 wheels remain on the ground at all times. There is no teeter tottering going on at all. However, you will need clearance under the center of your frame.
And then there is the question of long vs wide. For turning's sake, you'll probably want 6wd for a long robot (or 4wd with a couple low friction or omni wheels) where 4wd would be just fine for a wide robot. But a wide robot may tip going up that ramp whereas a long robot may not. The it presents the question if you want a wide robot, how do you keep from tipping. Wheelie bar? Extending frame?
So many questions and so little time to answer them all.