[RRLedford]

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