Hello all,
We are trying to do a four wheel drive robot, with four CIM/CIMpleBoxes one on each wheel.
We have a rectangular wheel base 27x37 with about even weight distribution.
The problem is when we turn, well, complete failure, the wheels skid and it locks up…
So an suggestions/help would be appreciated
With your current configuration, assuming you are using 4 high traction wheels (either kitbot wheels or AndyMark wheels), the easiest solution would be to switch out one set of wheels for omniwheels of the same diameter. You can keep your gearboxes and code by doing that.
Good thing you’re finding out now and not at competition! :eek: Good luck with the robot!
Direct driving any wheel with the CIMpleboxes is a bad idea. The wheels will not have nearly enough of a gear reduction and will not have the torque needed to turn.
We are going though the KOP shain + sprockets to btw.
So we need more torque to be able to turn or to go to omniwheels?
it would be a good idea to use the sprockets to add another gear reduction between the CIMple box and the wheels.
You might want to think about changing all 4 wheels to omniwheels at this point, since changing just 2 will give you a somewhat funky drive (turning point very close to the non-omni wheels).
The obvious disadvantage is that you can not resist being pushed sideways.
you should think about your game strategy and decide which change is best for you.
summarizing:
- change the sprockets so you get more gear reduction between the cimple box and the wheel.
- change 2 or 4 of your wheels to omni wheels.
Good luck and keep us posted if you need help.
-Leav
p.s.
At the regional, be sure to bring a camera and take pictures of drivetrains and mechanisms you find interesting, veteran teams a fountain of inspiration and knowledge for rookies! 
Have you considered converting to a 6WD? If you’re using the kitbot in long configuration this can be done quite easily.
Just add in a center pair of wheels and have them chained to either a cimple or corner wheel. 1/8" drop 6WD with little effort!
Congratulations, you have rediscovered the extreme difficulty of using skid steering with four traction wheels and a long wheelbase. 
The problem is that you have too much side friction and not enough force to overcome it. One solution is to change your gearing to give you more force, though that will reduce your maximum speed.
The quick fix is to replace your front or rear wheels with omniwheels (mecanum wheels could also work, but they’re a lot more expensive and might not fit in the same space). This will let you turn easily, though you’ll need some driver practice to get used to the fact that the robot will turn around the end still having the traction wheels. Depending on the design of the rest of your robot, you will probably find one end or the other to be more appropriate for the omniwheels. Some teams have found that putting omniwheels on diagonally opposite corners works okay too, which would put the center of rotation at the center of the robot, but still has the risk of the other wheels skidding/jumping while turning.
It would also be possible to modify the surface of one pair of wheels to make them easier to slide, such as cutting horizontal grooves into the tread or wrapping a bunch of zip ties around it. Omniwheels are easier.
6wd would be the way to go, if you’re using the kit chassis (or another chassis that could be modified for 6wd). But you didn’t tell us what your chassis is–we all have to guess.
How about some pictures? we love pictures
solution (A. convert to 6wd tank, if you have the weight this drive-train is excellent and has taken many teams to world finals. however… if you don’t have the weight this late in the season.
solution (B. Slick rear wheels, if you are using the KOP wheels than you need to buy a pair of these, or contact a team from a few years ago that might have some laying around
i think that this is the plastic tread wheel, if someone could verify for me
http://www.andymark.com/ProductDetails.asp?ProductCode=am-0494
if not than any team around last year or 2008 has a set of 8 inch sick wheels.
although omni is a good option as well, i have found that putting a pair of slick wheels on is cheaper, more effective, and lighter.
Those are three dirty words for many teams :eek:
Slick wheels are not particularly good at providing force for a robot. Honestly, I don’t see why teams that use them bother even driving them with chain.
Omni wheels are better, because they do have traction in one direction. They have no traction in the direction you want them to have no traction in!
Thanks for the info everyone,
we are using the kit frame, however going 6 wheel drive might be an issue with the way built our forklift.
I will get pictures up later for you all the see
the center wheel does not need to be centered front to back, it could be off center by 6" or so and still do the job of making the robot easy to turn.
Okay after talking it over with the team,
we are going to be trying out some omniwheels then if that doesn’t work going 6 wheel drive. 6 wheel drive is a problem since our tower supports are in the way.
Mk.32 we used a robot last year with front traction wheels and rear omni wheels and really didnt run into any problems. its true pivot point was in between the front wheels and it gave us exceptional control and finesse when driving in the competition. its a good, simple fix to that problem. we competed fairly well and for the game it worked well, but im personally biased to a 6 wheel tank for its simplicity.
Sweet, good to know.
Love the ChiefDelphi community, for all their help.
Also what would be preferable, 4 CIMs all powering the traction wheels with 2 gearboxes, OR powering every wheel with its own CIMple box + CIM.
i would recommend doing 2 motors per gearbox, driving all 4 wheels. im not sure how fond of chain drives you are, but getting power to those back wheels, omni or not, will give you a little more pushing power that might come in handy. its never a bad thing to have the extra umph, whether you use it or not
This is a great whitepaper for you to check out: http://www.chiefdelphi.com/media/papers/1443
Also, if you search CD-media there are tons of technical resources similar to this one to help teams out.
It offers some level of redundancy, sort of. plus it allows you to reclaim some of the normal force lost to undriven wheels.
Hey all,
we just got our omni-wheels and installed them.
Everything pretty much works perfectly the way we wanted to well just the drivetrain at least…