6 wheel drive question

[ChuckDickerson]

Originally Posted by Stephen of REX

By offset, do you mean that the center wheel is dropped down lower than the other wheels? If so, then the center off mass should be above the center wheels, so that the chassis can turn with only two wheels on the floor. If you moved the center of mass back, then it would favor the middle and back four wheels, meaning the front wheels wouldn't touch the ground as much.

I would respectfully have to disagree and say that it would probably have to depend on the game and your objectives. Our 2006 robot was 6WD (6 IFI 2x6 traction wheels) with the center pair dropped and built on an IFI KitBot chassis so whatever the standard KitBot drop is (IIRC 3/16”). We put as much mass down low over the back half of the robot including the battery, 2 AM shifters, 4 CIMs and every bit of electronics we could squeeze in there. The ball collector/ejector was in the front between the front two wheels so it was very light in the front. The result was a dropped center 6WD with very little rock. We were driving on the back 4 wheels 99% of the time which made driving more “predictable”. Basically the front wheels were only in play when we were on the ramps. The whole thing worked great and the drivers didn’t complain about any annoying rocking that I remember. Point is design for what you want depending on the game.

[Stephen of REX]

Originally Posted by DeepWater

I would respectfully have to disagree and say that it would probably have to depend on the game and your objectives. Our 2006 robot was 6WD (6 IFI 2x6 traction wheels) with the center pair dropped and built on an IFI KitBot chassis so whatever the standard KitBot drop is (IIRC 3/16”). We put as much mass down low over the back half of the robot including the battery, 2 AM shifters, 4 CIMs and every bit of electronics we could squeeze in there. The ball collector/ejector was in the front between the front two wheels so it was very light in the front. The result was a dropped center 6WD with very little rock. We were driving on the back 4 wheels 99% of the time which made driving more “predictable”. Basically the front wheels were only in play when we were on the ramps. The whole thing worked great and the drivers didn’t complain about any annoying rocking that I remember. Point is design for what you want depending on the game.

Oh, I didn't mean that you always want center off mass over the center wheels. Yours is a perfect example of when you'd want weight towards the back.

[EricH]

DeepWater hit on exactly why you'd want the CG in the back (or front, depending on your objectives). Now, here is why you probably don't want it in the middle:

The rock is a lot harder to control. You have, in effect, an inverted pendulum with a barrier on each side. Balance it right, you're fine. Tap it lightly, or blow air on it, and it goes crazy. FRC robots get that kind of tapping from normal handling. Now, let's say that you want to pick up an object and put it somewhere with a reasonable degree of precision. You're rocking probably about 1/8" in either direction from dead horizontal, due to the "height" of the drop with relation to the end wheels. So you have to allow for a 1/4" variation in location, just in normal rocking--and that's not including the multiple other places that you could get an error.

So you really don't have a huge amount of precision, while by moving the CG back about a foot, you still have that amount of variation, but you know that it will be momentary at worst, and you'll end up back in your "normal" position.