4 Wheel Drive Too Much Traction

[JesseK]

There are several things wrong with this drive train setup, and there are multiple ways to remedy it. The root cause is pointed to by David, and the result is that your entire robot is current limited while turning (pulls more than 120A while turning @ 12V...which causes battery voltage to drop*). I can confirm Mr. Gregory's gear ratios because last year that's exactly what we ran (12:32) using higher traction 6" wheels and no issue with turning (though our wheelbase was MUCH wider than it was long...).

Start by examining the attached photo that was created with this tool (not to toot my own horn -- but there are a couple of tools on CD that should come on a KOP CD or something ... namely, JVN's to start with).

In no particular order, you could do one or more of the following:

1. Use omni wheels
2. Use smaller wheels
3. Offset your center of gravity
4. Swap Gearboxes to Toughbox Mini's/Nano's (hard to tell exactly what gearbox you have -- looks like CIMple boxes since the CIM shafts stick out)
5. Make a 'lessons learned' so your team never designs a wheel base that is longer than it is wide
6. Use 12:32 or 14:38 sprocket reduction, in picture 2
7. Change the cogs on the gearbox to 16-tooth cogs with the same tooth profile

*One day I'll get around to updating my sheet to model this, now that I understand it better

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

[thmeans06]

Find your center of mass and (looks like it is toward the right side in the picture) and shrink your wheelbase by moving either your front or back wheels closer to the center mass.

[Kevin Sevcik]

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's pretty obviously bent and welded sheet steel. I know we're all about the lightness in FRC, but has it actually reached the point where people can't recognize steel when they see it?

Originally Posted by Kevin Sevcik

That's pretty obviously bent and welded sheet steel. I know we're all about the lightness in FRC, but has it actually reached the point where people can't recognize steel when they see it?

I recognized it as sheet steel right away, I was just curious because I was talking to someone on 253 earlier and they said they were using aluminum.

[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?

[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

[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

[Taylor]

Originally Posted by JesseK

In no particular order, you could do one or more of the following:[list=1][*]Make a 'lessons learned' so your team never designs a wheel base that is longer than it is wide

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?

[JesseK]

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?

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 (w/o dropped centers) until they do. Team 25 won Einstein last year, and they typically have the same setup (8WD, all touching the ground, all long configurations from year to year). The difference is, they understand what works, what doesn't, and why.

[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