4 Wheel Drive Too Much Traction

[Racer26]

I think we're going to see alot of robots with this approximate design (chute with indexer, dropping into a linear shooter at a ~30deg angle).

This particular robot seems much heavier than it needs to be though... Why two gearboxes to a side unless you were putting mecanums on it? The whole chassis appears to be steel tube and bent sheet steel, which, might be more viable this year than ever before, with the reduced perimeter, but still.

[Lil' Lavery]

Originally Posted by Taylor

Can you please explain your thought on this? It's common practice for teams to have robots longer than they are wide - 48 had a (in)famously long robot last year, and they were multiple regional winners. I agree that wider bases have less scrub, but never longer than wide?

Wheel base, not robot/frame. I believe 48 had a dropped center, so their wheel base was still wider than it was long.

[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

[Mk.32]

6 wheel dropped center drive

Or you can switch the back or front to omins which is what we did rookie season and it worked wonderfully.

And a photo would help

[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.

[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!

[arun4444]

As a quick fix, i'd recommend putting all four motors on the rear wheels and swapping the fronts for omnis - just a shaft, no gearbox.

its hard to tweak four independent motors and gearboxes, especially without encoders etc.

and definetly you need to gear down to about 11-12 fps max, maybe sprockets?

[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.