4-wheel drive trouble

We´re having trouble turning our robot. It doens´t turn at all, but it´s very good with full foward.

It uses the gearbox provided in the kit, and it connects the wheels with chains. The wheels sticks to the ground very well (more than the pneumatics one), and they are made of Poliuretane.

You can see a picture of it here http://www.chiefdelphi.com/forums/showthread.php?t=25245

Further images can be seen at www.repio.com.br/under/en ( Gallery )

Are all four wheels run from one motor or what? How is it set up?

You may want to read this thread on 4-wheel drive and side friction.
Good luck!

We´re running each pair of wheels (each side) with one motor, so, 2 drill motors for 4 wheels.

We have too much side traction, and our traction point is not located in heaviest part of the robot (as you can see in our picture).

Any suggestions?

it does sound like you have too much traction…a solution to this might be to notch your wheels so that they have less surface area touching the ground at any given time

If you are only using the 2:1 reduction given in the kit, then you probably don’t have enough torque at the wheels to turn. If you were to increase your gear reduction or reduce your traction, you would be able to turn.

Hi guys, nice robot!
[OFF topic]We tried contacting you by the school phone today but nobody answered :slight_smile: [/OFF topic]

There are three things I’d suggest you to try:

a) Leave the high traction wheels in the heaviest part of the robot and switch the other two with a pair of wheels with a smaller coefficient of friction;

b) Chamfer or fillet the corner of the wheels (that’s what we do to our treads), that really helps a lot. This is a picture:


c) As Joe Ross pointed out, try changing the gear ratios for your system. It may be a little late and it’s definetely the hardest mod, but it’ll work for sure. Plus, your circuit breakers will thank you :smiley:

We’re considering solutions, the faster solution to tell the truth, 'cos we have to deliver our robot thuesday (because of the carnival holidays)

Thanks all.

A technique we used last year was to have a foot descend pnuematically when we wanted to turn. The foot had a plastic or delrin surface that had little friction with the carpet. This would lift one set of wheels off the ground and let the other two do the driving. The side friction is not only killing your turns but your current skyrockets during the manuever as well. That shortens your running time and drags down everything else on the robot that runs off the battery. In the past (pre-backup battery) this would have reset the robot controller.

Reduce the traction of either the front or rear tires. Or get a stronger transmission. As rookies we just went with two wheel/castors and didn’t mess with 4 wheel. Put more weight on one end maybe? ALOT MORE. Shrink your distance between your front and rear wheels. Gear down Front/rear slightly less than the Rear/Front. Flip the Gears in the Tranny so you are geared a lot lower. (kit tranny)

If you’re using the kit drive system from FIRST… it sounds like your solution might be easier than you think: Is your robot in high gear?

I just have to ask.

If it’s not in low… you’ll be hurting to turn with those ratios.

Good luck.


With 4-wheel, tank-drive, as you are using, your robot must be wider than it is long, or you will have trouble turning. Make sure your robot is 36" wide and 30" long, not 30" wide and 36" long. This will make a huge difference in the ability of your robot to turn.

If you cannot do this, then ensure that the lateral friction of your wheels is less than the longitudinal friction. That is, make sure that it is easier to push your wheels sideways on the carpet than it is to push them forward or backward. You may be able to do this by modifying the tread pattern on the tire, as mentioned in an earlier post.

i agree… we were having the same problem last year and probably this year too… that really helped us out…

another thing, think of different places that you can place pneumatics in of the robot which might help you to get up to the platform… :wink:

Actually, its the distances between wheels that matters (technically, it’s the distance between the contact points of the wheels that matters). Measure the distance between two wheels one side of the robot and call that y and measure the distance between two wheels on opposite sides of the robot, but both in the front (or back), and call that distance x. If y > x, your turning efficiency is drastically reduced. If x > y, you should be good.

To understand why, let’s look at some physics. Let’s say your pivot point is the center location between all 4 wheels. Then, the distance to each wheel is called r. To find the torque used to turn the wheel you take the cross product between Force and Distance (both vectors). Look at the atttached drawing. The force that the ground exerts on the wheel (friction) is always parallel to the wheel. Draw a line from the wheel’s contact point to the center. When the angle between force and this line is brought closer to 90 degrees, you can turn better. (A cross product between vectors **A and B is equal to ABsin(d) where d is the angle between the two vectors. So when d is closest to 90 degrees, sin (d) is closest to 1, so the cross product has its largest value.)

Moral: Move the left set of wheels farther from the right set of wheels. And make the wheels on the left closer to each other, and the wheels on the right closer to each other. Or don’t use 4-wheel drive. Our team made this “mistake” the first two years - we could hardly turn or maneuver. This year we went with track drives and they turn on a dime…

4wheeltorque.bmp (22.7 KB)

4wheeltorque.bmp (22.7 KB)

Put in a freshly charged battery and then find out the following:

What is your OI reading for voltage when you are tring to turn?? You may be spiking out the system with using only two drill motors for 4 wheel drive?

If it is reading anything less than 6 or 7 Volts then I suggest you either redesign your tranny:ahh: or for goodness sakes, make some wheel adjustments like stated above!!

Try wrapping a pair (one from each drive train) of tires with something radially. I understand this is kind of a dumb solution, it makes more sense to just get wheels or tires with the right treads, but its easy, cheap, and quick.

At first we tried 1/16" steel cable, but it wasnt big enough to lift the wheels off the ground. We probably shoulda have tried again with thicker cable, but that stuff is REALLY tedious to work with (think 100+ ft per pair and something like 130 individual wraps). So we had some zip tries lying around… They are excellent at reducing cross-wise friction (it turns on high gear now) but they did take away significantly from traction in the direction we need. Regardless, we can climb 6 inches with our two drills and 12.5" skyways, so im happy.

How bout this:
Replace 2 wheels with omni’s…
Probably the rear wheels. This will retain your 4 wheel strength while making your turning much slicker. I also think that with a small chamfer on the front wheels should do the trick.

are there any off the shelf 12 in omni or transwheels? i’d love to have them. never seen any big sized ones tho. probably too heavy. we can’t machine our own, but id really like them

Why not try zip ties? They’re cheap, slick, durable, and quick to install!

Is that tuesday or thursday?