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Originally Posted by sanddrag
It is all in the weight distribution. If you get it right, your clearance can be next to nothing.
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No matter how you configure it or how you distribute the weight, you will, at some point, high side your robot without the proper ground clearance. The only thing that will keep you from STAYING high sided, as Ken pointed out, is inertia. The only thing that will accomplish what you have described is a shift-on-demand center of gravity (CG).
A six wheel design (two front, two middle, two rear) still has points in between the wheels where the frame can, and if not designed properly, will contact the transition edge from ramp to platform. 2mm of clearance will not allow for the proper clearance and your CG will not shift at the right point.
With regards to your CG you've got three realistic options: in between the front and middle wheels, directly over the middle wheels, and in between the middle and rear wheels.
If your CG is in between the front and middle wheels your robot will high side itself in between the front and middle wheels as it makes the transition from ramp to platform.
If your CG is directly above the middle wheels, as your robot moves onto the ramp it will shift back to between the middle and rear wheels until the robot is on a level surface again. As the robot transitions from the floor level playing surface to the ramp, the CG will shift to the rear of the robot. Once this happens, the CG will stay between the middle and rear wheels until it is horizontal again. Since the rear wheels will have to make the transition from ramp to platform to accomplish this, the high siding will occur between the middle and rear wheels.
If your CG is in between the middle and rear wheels, your robot will high side itself in between the middle and rear wheels as it makes the transitions from ramp to platform.
Obviously there are other variables in this equation. Ken brought up inertia as being one way to overcome the need for adequate clearance but then you need enough inertia to overcome the friction between your robot frame and the ramp/platform ridge. Another variable is going up the ramp at an angle versus perpendicular to the incline. This may allow you to lower your clearance if designed properly.
For a simple topic about tracks versus wheels this has become a very detailed discussion. I love it!
Sean