4WD Turning Difficulties

I wanted to bring this helpful “white paper” to the attention of people who are busy designing their robots in the mistaken belief that turning on a slippery surface will not be an issue.

If you build a 4WD skid steer robot in the “traditional” or “narrow” (wheels aligned with the long axis of the robot) orientation there is a very good chance that your robot WILL NOT TURN.

I know this might seem counterintuitive, but run the spreadsheet in the whitepaper with the coefficients of friction for the wheels (KoP list / section of the manual as I recall) and look at the results.

Now keep in mind that in order to spin the trailer about your robot you will need to apply about a 3 lb force (30lb trailer weight estimate x .1 transverse dynamic coefficient of friction) at a distance a couple feet away from your robot’s centre of rotation. So, say about 6 lb feet of torque.

In this case even a 6wd robot (with the middle wheels dropped a bit) will have difficulties turning in place… it will do it, but slowly.

And thank you to Mark from 1189 for posting the white paper and spreadsheet.


I also ran this scenario through a drive train simulator and came up with similar results. Straight was perfectly as expected, but turning was near impossible and took forever. Add the sliding effect to it and your robot drive accuracy will take a nosedive.

what if you double or tripple up wheels?

stack wheels side by side so making a rough drawing using symbols

= wheels
| and _ = edges of robot
. =taking up space

| . . . . . . . |
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . . . . . .|
| . . . . . . . |

shouldn’t something like that work? since there would be more surface area?

It’s not likely to make any difference at all, since surface area has very little effect on friction with hard materials such as the wheel treads and arena floor.

sigh 'ight thanks

so much for that idea, lol

Have you crunched the numbers on a wide 4wd chassis? I think it would work a lot better because the wheels have a lareger moment on the robot’s cg, providing more turning force, and the trailer has a smaller moment, reducing the friction. I haven’t done any of the math, though, and it may turn out that skid steer just isn’t the way to go.

A couple members of our team did calculations for the majority of last night and more wheels don’t help you at all. I would say 4 is the max you’d need for a drive train this year. We toyed with the concept of a 2 or 3 wheeled drive train but stayed with the 4 wheeled system mainly because of the contact with the side carpet.

I’d love to hear other opinions on this and how many wheels people are planning on using.

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.

Yes, the numbers for a wide chassis are better, and for six wheel drive (with the middle wheels dropped a bit) are better still. Six wheel drive on a wide chassis, with the centre of gravity over the middle wheels naturally, is the best of the skid steer options. That goes for high friction OR low friction environments.

I suggest everyone who is interested in this download the spreadsheet and try their own numbers in it. You can simulate a six wheel drive with centre drop by assuming it is a four wheel drive with two wheels up in the air. Thus the wheelbase is the distance from the middle wheels to whichever wheels (front or back) happen to be on the ground. The CoG will also move towards the centre axle.

Of course these numbers are calculations and simulations, and need to be confirmed by experimentation. I suspect we’ll be seeing some video of the first protobots up on youtube by the end of the week.


What about wheels like this?


When you want to turn, spin the front and back wheels (wheels on short sides). When you want to drive forward spin side wheels.

Another thing we’ve considered is “toe.”

If the wheels are angled slightly at each corner, spinning all of them in the same direction will allow you to spin in place.

It will also take advantage of the superior transverse friction when driving forward. And will provide some braking to prevent the robot from coasting when making a hard stop.

The wheel’s tranverse friction is more than twice the inline (apparently).

Even though their friction sideways is probably less than the omniwheels we’re used to on carpet, that doesn’t mean they will slide sideways easily in this case.

It doesn’t matter that the transverse friction is really low, it matters that relative to that, the inline friction is substantially lower.

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” :smiley: )
|= 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. :frowning:
But from what I have heard, it sounds pretty slippery.
Any guesses on how well this would work?

Edit: Hachiban beat me to it!

No problem, although I must give credit where it is due. Chris Hibner put together the white paper upon which my spreadsheet is based. http://www.chiefdelphi.com/media/papers/1443
Chris did all of the heavy lifting.

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.

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.

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.

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.

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.

your right wow

i almost forgot the trailer