4 Wheel Drive Too Much Traction

[Lil' Lavery]

I'm curious as to how those pneumatic wheels/hubs are fixed to the output shafts on the CIMs. Care to share the details?

[Ether]

Originally Posted by Taylor

I agree that wider bases have less scrub, but never longer than wide?

Yes, under certain conditions wheelbase can be longer than trackwidth

For the robot pictured, if you make the following simplifying assumptions:

1) all four wheels identical and driven independently with the same torque

2) coefficient of friction of the wheels the same in X and Y directions

3) Center of Mass lies on the the longitudinal axis, somewhere aft of the geometric center of the 4 wheels

... then the analysis becomes straightforward:

http://www.chiefdelphi.com/forums/sh...ad.php?t=99089


JesseK's advice is good though:

If the team doesn't understand the fundamentals of the forces of scrub in a skid steer drive train, then it's probably best they steer clear of longer wheel bases

[theawesome1730]

Originally Posted by Andrew Lawrence

I know this is a little off topic, but is your robot made out of aluminum? It doesn't look like aluminum to me.

That without a doubt is steel

[Racer26]

Originally Posted by Lil' Lavery

I'm curious as to how those pneumatic wheels/hubs are fixed to the output shafts on the CIMs. Care to share the details?

If I had to guess, I'd say those are AM hubs with the 8mm bore /2mm keyway in them, and a snapring just inside them, bolted to the wheel (ie. the snap ring is on the end of the motor shaft, inside the body of the wheel.

[jblay]

The easiest fix is to put tape on your wheels and leave it there so that the tape is what contacts the ground, the won't be able to push anyone but you will be able to turn, we did this a few years ago wrapping our wheels in duct tape at the competition when we realized it was becoming a really big issue

[billbo911]

Running through the JVN calculator, your current drive train has an overall reduction of 4.67:1. It yields a top speed of 24.13 ft/sec. You need to at least twice that amount of reduction.

My I suggest swapping out the belt drive to chain with the following sprockets.

12 tooth on the transmissions and a 30 tooth on the wheels.

That will increase your reduction to 11.67:1, drop your top speed to 9.65 ft./sec, and give you enough torque to turn.

The sprockets you want are available from both AndyMarh and VexPro.

[Madison]

Originally Posted by billbo911

Running through the JVN calculator, your current drive train has an overall reduction of 4.67:1. It yields a top speed of 24.13 ft/sec. You need to at least twice that amount of reduction.

My I suggest swapping out the belt drive to chain with the following sprockets.

12 tooth on the transmissions and a 30 tooth on the wheels.

That will increase your reduction to 11.67:1, drop your top speed to 9.65 ft./sec, and give you enough torque to turn.

The sprockets you want are available from both AndyMarh and VexPro.

This is the best, most straightforward advice you've received so far. Do this.

[JesseK]

Originally Posted by billbo911

Running through the JVN calculator, your current drive train has an overall reduction of 4.67:1. It yields a top speed of 24.13 ft/sec. You need to at least twice that amount of reduction

...

Originally Posted by Madison

This is the best, most straightforward advice you've received so far. Do this.

This doesn't make any sense. One mentor makes a conclusion about turning based upon a tool that doesn't tell him anything about how/why a robot turns, then another veteran mentor backs him up? What is this, the Apple forums?

[Madison]

Originally Posted by JesseK

This doesn't make any sense. One mentor makes a conclusion about turning based upon a tool that doesn't tell him anything about how/why a robot turns, then another veteran mentor backs him up? What is this, the Apple forums?

Bill's advice regarding a ratio change mirrors the advice you provided. I respect that you gave the team options, but swapping the drive ratio is the most straightforward fix for this team at this time. They can triage *why* their drive failed to turn later; for now, it's probably more important that they get a machine working so they can continue testing other systems.

They're also likely to have chain and sprockets around from years passed. They may not have other wheel options and moving gearboxes or CoM around is a ton of work.

[Ether]

Originally Posted by JesseK

This doesn't make any sense. One mentor makes a conclusion about turning based upon a tool that doesn't tell him anything about how/why a robot turns, then another veteran mentor backs him up? What is this, the Apple forums?

I did the calculation, using what I thought to be conservative numbers:

I assumed the Center of Mass was at the Center of Geometry of the wheels

I assumed the trackwidth/wheelbase ratio was 1/2 (it appears to be a bit better than that from the picture.

I assumed the coefficient of friction of the wheels he's using is 1.2

I assumed the wheels are 6".

I assumed a total weight of 150lbs.

I assumed 10% loss of torque in the gearbox and sprockets.

Using Billbo911's 11.7:1 gear ratio, I calculated ~180 oz-in torque required at each motor.

[billbo911]

Originally Posted by JesseK

This doesn't make any sense. One mentor makes a conclusion about turning based upon a tool that doesn't tell him anything about how/why a robot turns, then another veteran mentor backs him up? What is this, the Apple forums?

Jesse,
Instead of being offended by your tone, I think I'll build on your point.

Jesse's post will lead you to determining the root cause of the problem. It will teach you how to analyze a drivetrain and design it better in the future. That said, the solution it would lead you to at this point in the season would be impracticable to implement. It is correct, but not necessarily an optimal solution for your current situation.
You really should learn to use the analyses tools he is giving for future designs.

In the mean time, what I gave you will get you going in the shortest amount of time and least amount effort, something that is needed with the amount of time you have left. And, if you apply the information I gave you to Jesse's approach, you will see that it works, too. Please give it a try once you get the drive working to your satisfaction.

[JesseK]

Originally Posted by Ether

I did the calculation, using what I thought to be conservative numbers:

I get it, the math is doable. The point is that advice was given in the form of exact gear ratios based upon esoteric experience rather than math. On top of that, it's not the best advice for that team because we don't know that team's experience with chain.

Replacing the belt pulley on the gearbox with a 12-16T pulley has the same net effect (the robot turns) at potentially less expense -- time & money wise.

[Madison]

Originally Posted by JesseK

Replacing the belt pulley on the gearbox with a 12-16T pulley has the same net effect (the robot turns) at potentially less expense -- time & money wise.

It also requires a different tooth count on the belt, all other things being equal. The flexibility of chain is a great asset here.

[JesseK]

Originally Posted by Madison

It also requires a different tooth count on the belt, all other things being equal. The flexibility of chain is a great asset here.

They could also slide the gearboxes more towards the center, so long as they're not welded to the frame (can't really tell from the picture if the glare is a weld joint or not). It doesn't look like the two gearboxes on the side will interfere, but it's best to judge it hands-on.

[r0306]

We solved the problem by using free spinning low traction wheels in the front and doubling up the motors in the back for more drive power. Although the robot still struggles to turn sometimes, it is a noticeable improvement to before. We can even turn in one spot. Thanks for the help everyone!