Drivetrain help
 

We were so hopeful, six 8-inch pneumatic wheels from Andy Mark configured in a drop center, about 1/8" being directly driven by a CIMPle Box with 2 cims on each side, front and rear tires chained up. Our hopes were dashed when it pulled the batteries way down and refused to do much moving. We loosened chains, thinking maybe there was some binding, but not much better results. We are troubleshooting this, and the most likely remedies we've come up with are to lower the center wheel some more or switch to omnis on the corners. I read somewhere that direct driving any wheel from a CIMPle Box is a bad idea because you lack torque. I'm wondering if the gering may be our primary problem that we could correct by not direct driving and using a smaller give on the CIMPle Box. We currently have 26 tooth gears on each of the wheels. Thanks in advance for your insight. Hope all of your builds are going well.

Brian Holmgren

Re: Drivetrain help
 

Even when you were trying to go straight forward, with no turning??

If so, check to make sure your motors are not fighting each other. It's a common mistake.


Even if that turns out to be the problem and you get it fixed, you will probably have trouble turning. 1/8" is likely not enough with pneumatic tires.

Re: Drivetrain help
 

It did give us some headaches even going straight forward off and on. Motor direction was something we checked as we hooked it up, so we're OK there.

Re: Drivetrain help
 

The CIMple Box ratio is 4.67:1. This is far too high for 8" wheels.

For 8" wheels, you should be somewhere in the 12-16:1 neighborhood.

Re: Drivetrain help
 

May I ask, how did you check it?

Re: Drivetrain help
 

Agreed, but the symptoms he described seem to go way beyond that, no?

Re: Drivetrain help
 

You want some more reduction, as Madison said. The kit setup uses the CIMple box and a 12:22 (?) chain reduction for 6" wheels.

also see about your tire pressure, it can make a big difference...

Re: Drivetrain help
 

I misunderstood your original post when you said "Our hopes were dashed when it pulled the batteries way down and refused to do much moving." I read that to mean the robot wouldn't go forward.

If the primary problem is turning, then your problem is most likely a combination of (a) need more gear reduction (as others have posted) and (b) 1/8" drop is not sufficient for pneumatic tires

Re: Drivetrain help
 

So if we were to substitute a tough box for the CIMPle Box we'd be closer to OK?

Re: Drivetrain help
 

Yes. Standard toughbox is 12.75:1.


I just re-read the above. Are you saying that the center wheels are direct-driven and the front&back wheels have a gear reduction?? If so, that's not a good idea.

Re: Drivetrain help
 

I'd go with removing some traction at one end of the robot, too--I've seen one of the Toughbox's closer relatives, the 2006 KOP gearbox, have trouble turning a 6WD drop-center when all 6 wheels (6", in that case) were pneumatic. Replacing one end with lower-traction kit wheels worked in that case.

Re: Drivetrain help
 

We checked the motor direction by unplugging the PWMs and checking them one at a time on each side to ensure they were going in the same direction.

Re: Drivetrain help
 

How much was the center dropped?

Re: Drivetrain help
 

Can't argue with that :-)

Re: Drivetrain help
 

Can't remember--I want to say it was in the 1/8 range, but it's been 6 years. Might have been more. It turned with the 6 pneumatics... but it bounced all over the place when it did.

As shipped (6 pneumatics): http://www.chiefdelphi.com/media/photos/23376
After Arizona Day 1, where the change was made (4 pneumatics, 2 KOP on the front): http://www.chiefdelphi.com/media/photos/24202 (far right robot, the wheel in the air that's visible)