# Minimum Clearance?

So with the angle of the ramp, and assuming you have a bot that is the maximum dimensions (28 * 38), how much ground clearance do you have to have between the underside of the robot and the ground to clear the ramp without getting stuck? I know it’s probably an easy math problem but I haven’t figured it out and other teams would probably benefit from this answer being public.

So… anyone wanna take a crack at it?

okay. Whats the distance between the wheels? And do you want to know if it will get started up or when it hits the top? You might run into issues with a bumper…

I take it you mean driving perpindicular to the wall.

(ie: Straight onto the ramp)

Interesting question.

I’ll see what I can come up with in CAD, but to effectively do that you can use some algebra and angle it all out. Good Luck.

I attached an Excel sheet I created back in 2003 (along with a bunch of other such handy reference calculations) to figure these out. I never had time to document these and put in a white paper. I hope it is pretty self explanatory.

Ground Clearance.xls (18.5 KB)

Ground Clearance.xls (18.5 KB)

BTW - keep in mind that the clearance could be greater while driving on the transition of the ramp parallel to it rather than perpendicular to it. For the example in the sheet I attached, where the robot front is 38 wide, would require greater clearance in the front clearance orientation than in the side orientation.

I believe the minimum clearance you need is:

0.134*(the distance between your wheels) = minimum clearance

I wouldn’t rely on that math without checking it.
To explain how I got that:
0.134 = tan(15)/2

If the ramp is 30 degrees, the angle at the top is 150 degrees
When the corner is evenly between your wheels, the angle between the bottom of your robot and the ground is 15 degrees on either side of the corner: (15*2 + 150 = 180)

tan(15) = (clearance distance)/(1/2)*(distance between your wheels)
Solving for (clearance distance) yields the above equation

That look right?

well the bumper cant be any lower than 2.5 inches, correct??

If you (or your teammates) don’t trust your math skills completely, you can use physical models. It doesn’t take long to put a couple of wheels on the ends of a stick and play with them on a ramp. I was surprised to find how little clearance it takes for a six-wheel robot to make it over the top.

Thats one of the advantages of a six wheeled robot, given that the wheels are aligned with three on each side, for even if one pair loses traction, there are still two pairs in contact, and thus the robot will not end up getting hung up on the top of the ramp. On the bottom, it acts identical to a four wheeled robot.

One issue that Eric and I didn’t consider at first but is certainly important is that you need to look at the clearance between not just in the side view, but also in the front view of the robot. The clearance should be calculated for both the case when the robot is facing directly up the ramp, and when it is perpendicular to it as the relevant distance between the wheels changes between the cases.

Chances are your forward direction will have a shorter wheel base and require lower clearance than the sideways direction. This means that you may be able to safely drive up the ramp without bottoming out but then turn 90degrees while still half on the ramp and bottom out.