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Unread 29-04-2009, 19:40
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Re: Winning Drive Train

Quote:
Originally Posted by sdcantrell56 View Post
I will counter this and say that this year, more wheels did actually return more traction.
I've been following this thread and have notice that, as Adam and JVN pointed out, a lot of this is speculative and isn't being backed up by actual numbers. Allow me to break tradition and share some of our testing methods and summarized results...

When we first started brainstorming we we very focused around being as simple and easy to repair as possible. We wanted no headaches at competition. Therefore we couldn't find a good way of justifying investing the time in developing and producing a crab/swerve drive. INHO those kind of systems need to be developed over the off-season not in the time precious 6 week build season. So the first question was: 4 or 6 axle drive...

4 axle vs. 6 axle Testing
We took our 2008 robot, made rover wheel hubs to fit its 7/16" hex shafts. This became our test platform since we could test 6 axles or remove the center wheels and test 4 axles.

We set up a 8' x 24' FRP track. We did a few different tests:
A) Straight Tests
B) Turning Tests
C) Push/Pinning Testing

We ran the robot down the 24' end of the FRP 10 times, each time on a different area so the FRP wouldn't wear in and give us more traction. We timed each run at full speed so theoretically the wheel slip time would be the same. We found that the straight-away times between 4 and 6 wheels were pretty much the same.

For our turning tests we would go from a straight away, into a turn and time how long it took to make a 180 degree turn. Then we would time how long it took for the driver to regain control in a straight-away after making the turn. We found that the 6 axle version made the turns quicker, but took longer to regain control after making the turn. The 4 axle version made much wider turns, but took no time to regain control after making the turn.

For our Push/Pinning Test we moved the FRP over a large piece of carpet. We set up a wall made of tables. We put our 2008 robot (with rover wheels) in different configurations on the wall (front two wheels on carpet, one side of wheels on the carpet, etc). This wasn't so much of an objective test as much as it was a scenario test... We found that the rocking motion of the 6 axle robot hurt it in pinning situations, especially when the robot was pinned front or back against the wall. The rocking motion meant that there were times when you would potentially have no wheels on the carpet and therefore lose any chance you had at getting out of a pin.

This led us to go with a 4 axle robot. Then came the question of how many wheels to use....

Breaking Force Testing for Multi Wheel Axles
We actually did push/pull/breaking force tests for the rover wheels on FRP. We created a jig that allows us to test 1, 2, 3, 4, 5 and even 6 wheels. We locked the wheels in place so we could test the breaking force the jig. We weighed the jig down with ~40lbs (the amount of weight per axle on a 4WD machine). We dragged the fixture with a fish scale across a section of FRP. The weight measured at the point where the wheels started dragging would be the breaking force.

We found that each wheel added to the fixture greatly reduced our breaking force. 2 wheels cut our breaking force in about half. After seeing it continue to drop after testing 3 wheels and 4 wheels we decided to stop adding more wheels since it was obvious that more wheels per axle meant reducing our breaking force.

After further research on running multiple wheels per axle it turns out that Trucks use them to reduce the load per wheel, not for traction purposes. Think about it... Semi-trucks are the most common vehicle for running 'dualies'. This is because each wheel on the axle reduces the load on the wheel. And if you notice, they're typically only running dualies on the trailer end (where a vast majority of the weight it located) of the truck because they want the traction on the cab.

This is what led us to going with a 4 axle, 4 wheel robot.
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Last edited by Jon Jack : 29-04-2009 at 19:45.