Drivetrain issues

I’m part of 1802 Stealth Robotics and we have come to the conclusion that we have TOO much traction because we are using 4 cim motors to drive our bot and it keeps poppin chains and sheering keys and bolts, any ideas to how we can fix that???

We need a bit more details about your drive train. How many wheels, configuration of wheels, tread etc.

The number of motors doesn’t determine your traction. Adding more motors would help with current issues, generally.

What material is on your wheels?

What gear ratio is your transmission running at?

Are your chains properly tensioned? (Doesn’t sound like it)

There are a few things that pop out at me from your post. If you keep popping chains and shearing keys it sounds like something is out of alignment. The issues I can think of that could cause this is sprocket misalignment, gearbox misalignment, Not securely mounting the gearbox (ie. no top support plate) or even perhaps something in binding in your gearbox.

A picture of your drive train would go a long way in helping you diagnosis the problem.

I have run 4 CIM drivetrains for a number of years with none of the problems that you are experiencing and it is obviously something other than the number of motors in your drive.

Several years ago we had the same issue, and just like Greg indicated, it was due to something being out of alignment. One sprocket had been shifted over by about 1/8 of an inch on one shaft. It caused the chain to repeatedly break - usually during a match! Once we realigned it, we were fine.

Best regards,


If you don’t have the quality issues already mentioned such as misalignment or improper assembly, then your problems could be the result of too high of a gear ratio. A CIM motor just doesn’t have enough torque to cause that kind of damage if applied with a typical gear ratio (6 to 15 depending on wheel size). If you are experiencing sheared keys and broken chains, then your drive could be seeing too much torque. Are you doing something unusual in the way of transmission? What is your gear ratio, wheel size, and robot weight? All three must be appropriate to work together. A picture or more info would probably answer this. If you suspect your drive ratio is too high, then lowering it will have the benefit of saving your chains and keys, and making you faster as well.

well we have 8 high traction wheels which i think i have already determined to be part of the problem, the alignment was fine and was adjusted as needed, im working on getting pictures of the drivetrain

8 high traction would do it. Maybe the 4 end wheels can be shaved down somehow? That would make it easier for you to turn. Pictures would be great.

If you have 8 traction wheels on the ground at a time you will be putting a ton of strain on your motors. Try raising the end wheels up 1/8 to 1/4 inch. This will shorten your effective wheelbase making skid style turning easier.

Since their frame is already machined I would rather suggest adding strips of roughtop tread to your center 4 wheels or shave off the end 4 as Bharat suggested. Wheel scrub can be a pain if you’re flat on all 8 all the time. Either way you’d be raising your end 4 up.

Don’t know what size you are using, but you can do what team 134 did-

Replace four traction wheels with KoP or slick wheels.

Good luck.

Not necessarily…

We have 8 of our traditional high traction wheels on our drive base this year and can turn with all 8 wheels down using only one motor on each side. We proved this in testing our drive base while determining if the second motor for our drive would be a CIM or a Fischer. Our prefered method however, is to lift the the outer 4 to reduce current draw but it is not necessary.

If you are breaking keys I would suggest that maybe your keys are too small for the application, or as Greg already mentioned it’s your gearing. I highly reccomend the use of hex shafts for power transmission when ever possible. with the fact that AndyMark sells hex bearings this is much easier to execute now as well…

Duct Tape…

We are running a 4-wheel/4-chain tank drive with dual CIMed super shifters on both sides. They are AM 8" Plaction wheels with 22 tooth sprockets and 18" axle-to-axle wheels. We did spacer our two rear wheel sprockets to align with the outer set of 15 teeth of the dual sprockets of the gearboxes using 1/4-20 nuts and FTC kit shaft collars initially. Our bot is 116 lbs too. Our chains have stayed tight, with one exception (more later). We did do some programming changes to have more gradual acceleration, and I will post those details later, along with bot PICs

At week 3 we started driving this setup long enough to confirm that it had so much torque, and the chain tension was so great, it was distorting our framing and wheel well mount scheme. So, we had to totally rework this to beef it up using 30mm extruded aluminum framing in an overlapping, drilled- thru, double-bar H pattern under gearboxes thru-bolted with grade 8 1/4-20 bolts. A piece of 30mm extruded slotted framing was also added between the wheel wells on outside near axle height. Because our chains were angled down toward wheels causing a downward cantilevered force on the output shaft we also had to ad structure to resist this stress tilting the gearboxes over toward the outside. This set our build schedule back a week or more, but it proved to be invaluable.
We often do (inadvertent) wheelies in high gear, and our bot was the fastest tank with good maneuverability at the Midwest regional. Our traction is excellent. We mostly played defense (kicker winder issues) and were able to harass the more nimble swerve and mecanum bots by getting where they were heading nearly as fast, and then pushing them around before they could accomplish their goals.
As we progressed into the quarterfinals on Sat., just one of our chains began getting looser than the rest. By the time we realized how fast the loosening was progressing, we were queued in the semifinal matches had no time to add shim washers under the gearbox. I later realized we had focussed on the wrong end of the chain. What really had been happening was that the 6-screw sprocket mount had been slowly coming apart.
Students later informed me that some of the (cheap grade) 10-32 screws were not quite long enough to fully engage the nylon of the Nylock nuts, so they had just made them “extra tight” instead of getting proper length screws. Near the end of the 2nd semifinal match, the sprocket finally came loose enough to wedge the chain, shear the screws, shatter the sprocket, and pop the master link. With our partner 1625 also encountering a chain failure in the same match, our alliance (111, 1625, 3135) was eliminated. Still, even whith this we did NOT shear the key of that gearbox!

I don’t know how closely your gear ratios match our setup, but we are NOT shearing any keys so far. With our now well anchored gearboxes and wheel wells, and with the horizontal chain tension forces canceling each other out
we seem to have come close to a max optimized tank scheme.
Before North Star begins, we will be replacing all 24 of our sprocket mount screws with high strength ones and proper 1/2" OD single nylon spacers between the wheel & sprocket, where needed. We are also replacing chains with fully-riveted ones (no master link) to give 400+ pounds of working tension versus 250 lbs with the master link.
More to follow
-Dick Ledford