Why does one motor go slower than the other?
 

I've got an IBU2U Pro. When I advance the throttle, one track begins moving before the other one, and when the other starts moving it is slower than the first one.

What would cause this?

Thanks,

Steve

These motors have a discrepancy in the characteristics.
Moreover, the divergence of parameters can be very significant.
If this is not a problem of the motors themselves, there may be losses in the gearbox, check the smooth operation of the gear.
Last use the trimmer on the transmitter to align the movement ....

Trim on the controller is not enough to align the behavior. The gearboxes are brand new and turn by hand equally fine.
Steve

Swap the left and right motor leads on the IBU board and see if the other side slows or not.
Most likely a board programed issue.

I have solved the problem. Initially I thought it was because one of the new motors had most of the copper strands of its solder connection broken - perhaps not getting enough current. So, I re-soldered the leads. Problem persisted. I put the original plastic gearboxes back in - problem gone. I put the new steel gearboxes in and ran them with no drive wheels attached - all was fine. I could see that the fake bolt heads on the drive wheels were rubbing against the hull. I could tell as I loosened the drive wheel hex screw that the drive wheel shifted a bit. What was happening is the drive shafts were just a hair too short. When I tightened the drive screw hex bolts, the drive wheel was dragging on the hull. So I put a small washer on top of the drive shafts. This stood the hubs off by a small amount, and eliminated the rubbing, and the performance issue went away.

TLDR; drive hub was rubbing against the hull causing drag.

Steve