Turning with 4WD / Pneumatic Tires / Wide Config

Our team is running a wide configuration 4WD with 8" AndyMark pneumatic wheels. It has a 15" wheelbase and a 32" trackwidth. It is geared for about 5 and 12 ft/s. Tires are fully inflated.

It skips and jumps when it turns. We can still get around just fine, and maybe we will smooth out the turning one way or another. But my main question is should we have seen this coming? I had assumed that it would turn well with a wheel base that was twice as wide as it is long.

Did you mean to say its trackwidth is 32" wide?

Wheelbase is the distance from the front to the back wheels. Trackwidth is the distance from the left to the right wheels.

should we have seen this coming? I had assumed that it would turn well with a wheel base that was twice as wide as it is long.

Pneumatic wheels, because of their higher strain/stress ratio compared to solid wheels, exhibit much more stick/slip friction behavior than solid wheels do. That is a major contributor to the hopping behavior of skid steer.

It depends. How far apart are your wheels? Our wheelbase on our wide robot this year is longer than the one for our long robot last year (we used 8" wheels then, and 4" wheels now). To answer your question: You couldn’t have seen it coming, although with some thinking you maybe could have. Pneumatic tires are proven to have a higher coefficient of friction against the carpet than the traction wheels do, so they will be harder to turn. I can’t say for sure, but maybe letting a bit of air out of the tires will help a bit. If anything, it will make it easier to go over the bump, which I assume you’re doing, since the deflation will create its own shock absorb system for the robot.

Yes, I meant trackwidth. Thanks for the clarification.

Do not let air out of the tires. Increasing the friction patch with non-ideal tires that interact with the carpet beyond the surface will only increase traction.

We had the exact same configuration - 4 WD wide with pneumatics. We intentionally put it on the kitbot first because we were worried about turning (having never used pneumatics before), and we found the same thing you did. We changed out to different wheels and went 6 WD wide just to make sure - 4 WD wide with center of gravity centered can still struggle to turn with the wheels at the corners.

Inflate the tires to max pressure to minimize the contact patch. That will help some. Beyond that though, you may need to change your center of gravity (see if moving your batter to the opposite end of the robot helps), or change your wheels.

Same initial situation here. We ended up swapping out two of the pneumatics for a set of old grey kit wheels, and we can now turn nicely. It also behaves better crossing the bump as long as we lead with the kit wheels and the pneumatics are fully inflated.