4WD Turning Difficulties

[GarrettF2395]

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!

[Racer26]

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!

I'm pretty sure that this would "work" but, without really being a physics person, and not having crunched the numbers, I would think it would be even worse than a 4wd with the wheels on the corners.

[Steve_Alaniz]

You are correct. This will work, however, the math proves you are not getting the full force of straight in line wheels and mechanums have a tendency to be difficult to steer straight plus a tendency for the operator to over correct. That's why so many teams experiment with a gyro feedback to help drive straight and side to side. The low friction of the surface means that the gyro would try to stop a skid but only manage to spin the wheels really fast... the exact opposite of what you need. Slower speeds seem to give more control but they're... slower. speed will be a factor in this game almost as much as friction.

Just my 2 cents

Steve

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!

[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:

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

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.