[kitare102]

Thanks for the new term. Without knowing how you guys have assembled your drive train, we can't be sure what's causing the problem, but I'll put in my experiences. From what I've seen, the one reason for drift is slight misalignment of the bearing holes causing uneven pressure perpendicular to the shaft. This makes one side of the robot need more power to get to the same speed. Thus, if you don't have an encoder monitoring both sides, they get the same power from the CIMs and the more misaligned side travels slower. Very tight chains could contribute to a binding axle. This can also happen from improperly assembled gearboxes.

An easy way to figure out the location of the problem is to turn both sides of the drive train by hand to see whether one has more friction than the other. Then you can take the gearboxes off and see of they feel different. If so, carefully reassemble the gearboxes (including adequate lube). If not, then the problem most likely lies in the axles of the chained or belted wheels.

One time, we couldn't discern a notable difference in the friction of either side, but eventually found out that on one side of the drive train were a set of practically new CIMs, while the other gearbox had old CIMs. One of them in particular produced a noticeably smaller output at the same power input, so we switched it out with a newer one and that mostly fixed the problem.

At the core of the problem, however, it can be very hard to effectively control for all of the variables that can cause drift in a drive train. The only surefire way you'll get great results on a regular basis is using encoders and PID to close the information loop and allow the system to correct for drift itself. PID can't make up for serious problems with the mechanical side of the drive train, but it works wonders for small symptoms with no discernable cause.