4WD Turning Difficulties

[Mr_I]

Purely anecdotal, but tonight we put our bot on regolith, and managed to steer without too much difficulty. It is a 4WD in standard ("portrait") orientation, with the difference being we incorporate a shorter wheelbase (to increase the space for the gullet). To use the now-standard way of drawing:

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| [[. . . . . . . . .[[ |
| . . . . . . . . . . . .|
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________________

While I can't say I've read every word of this thread (and especially not all the links off of it), I have to wonder what we did "wrong" to get it to steer. Could it be the shortened wheelbase?
(Yes, pictures coming)

[MrForbes]

The shortened wheelbase will make a big difference.

We got our new wood chassis rolling tonight

http://www.youtube.com/watch?v=Yb8IPQYp_M0

The concrete floor is pretty smooth, and it probably has a bit more friction than the proper material (which we need to buy soon!). Handling is pretty good...all things considered...but having it light, no trailer, wrong floor, everything will be different in the game.

[Paul Copioli]

Mr. I,

Did you have the trailer attached during your driving? We noticed a considerable difference with and without the trailer. Driving without the trailer was quite fun, but driving with the trailer was just painful.

Paul

[Mr_I]

Originally Posted by Paul Copioli

Mr. I,

Did you have the trailer attached during your driving? We noticed a considerable difference with and without the trailer. Driving without the trailer was quite fun, but driving with the trailer was just painful.

Paul

Good point. Tonight, maybe?

[jreuter]

"I have to wonder what we did "wrong" to get it to steer. Could it be the shortened wheelbase?"

The spreadsheet model (http://www.chiefdelphi.com/media/papers/1917) predicts that you should NOT be able to turn a "portrait" tank drive robot, if you use the published coefficients of friction. However, I've seen several posts which claim that the .14 lateral coefficient of friction is overstated. If you put lateral coefficients more like the in-line coefficients, the model computes that you will indeed be able to turn.

[Brandon Holley]

Originally Posted by jreuter

"I have to wonder what we did "wrong" to get it to steer. Could it be the shortened wheelbase?"

The spreadsheet model (http://www.chiefdelphi.com/media/papers/1917) predicts that you should NOT be able to turn a "portrait" tank drive robot, if you use the published coefficients of friction. However, I've seen several posts which claim that the .14 lateral coefficient of friction is overstated. If you put lateral coefficients more like the in-line coefficients, the model computes that you will indeed be able to turn.

The fact that other people have not produced the lateral coefficient of friction in their tests, and indeed have produced a smaller number...combined with the fact that your robot DOES turn would lead me to believe .14 is wrong.

[kramarczyk]

In addition to the CoF values, where is your CG at? Moving it rearward helps your turning significantly.

[RobotDevil1985]

Originally Posted by Teamcap.2395

Okay, so here is a crazy thought I had...
If you did a 4WD in the same configuration as a omni-wheel drive...
...(like this) (In the same fashion as "The Pre" )
|= Chassis Perimeter
// = Wheels
. = space filler
___________
|//...........\\|
|................|
|................|
|................|
|\\...........//|

Would the low amount of friction on the wheels allow
the robot to move like an omni-drive?

I don't know how slick the flooring is because I was not able to attend a Kick-off.
But from what I have heard, it sounds pretty slippery.
Any guesses on how well this would work?

Edit: Hachiban beat me to it!

We tested that theory, granted we were on a smooth tile instead of the flooring, and found that without the ability to guarantee that all the wheels are on the floor with the same pressure. Otherwise the robot will veer uncontrolably depending on which wheels are in contact.

[amos229]

Originally Posted by Alex.Norton

Simple skid steer is almost never the correct route to take. If i was going skid steer I would still be tempted to do something like a 6wd with a lowered center wheel to make the control of the robot more consistent (turning around the center) and to make the turning easier.

I would need to try some tests but even if I had the resources to build anything I'm pretty sure I would still go for the 6wd drive over a swerve simple cause the traction is so low a swerve could get excessively hard to control. Then again this isn't really a problem for me considering I'm mentoring a team with very little machining capabilities.

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

[MrForbes]

Originally Posted by amos229

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

Why is that a problem? You can locate the center wheels towards the front or rear of the robot to take advantage of this.

[Larry Lewis]

Originally Posted by squirrel

Why is that a problem? You can locate the center wheels towards the front or rear of the robot to take advantage of this.

But you will still most likely only be driving on four wheels due to the weight in the back of the robot. If that is the case why not just put 4 wheels on your robot but place them closer together. You would get the same effect I think. But I am not sure if that will help out the driving situation.

Has anybody out there built up a 4WD chassis and tried it out on the game surface? I would like to hear about the why things handle if so.

[Racer26]

The front wheels would just be there to stop the front edge of the chassis from slamming the ground when CG shifts (due to collisions/abrupt deceleration/whatever else), you are correct that the vast majority of the time, you'll be on your wheels closer to the trailer.

[sdcantrell56]

Basically in a 6wd this year with a dropped center wheel, the front 2 wheels will be similar in functions to casters, just to keep the robot from slamming forward like has already been mentioned. The advantage of powering is that now when the robot rocks forward, the robot will still be distributing power to 4 wheels, versus if the front wheels were unpowered, only 2 wheels would have power but the friction force on those wheels would be half of the friction force that 4 powered wheels have to work with.

[Dominicano0519]

Originally Posted by amos229

Center drop wheels arent the answer this year since because you have the wieght of the trailer attached to you then your always going to be on the back four wheels.

your right wow

i almost forgot the trailer

[MrForbes]

The trailer wheels are near the center of the trailer, so it won't have a lot of tongue weight. You can position the wheels on your robot wherever you want to, they dont need to be exactly in the center. You can position heavy parts of your robot wherever you want to, so the center of gravity need not be in the center of the robot. What all this means is that you can balance the robot how you want...you can have a 6 wd robot that rides on the front 4 wheels almost all the time, if you so desire.