[MrForbes]

It might help to know how much slower the other side is going. Maybe you could put some video of it driving, on youtube, and link here? There is usually a slight difference in speed, resulting in the robot steering slightly when commanded to go straight...simply because the motors are made to turn one direction, and they are a bit slower going backwards (forward is the side that has positive power on the red wire, negative on the black wire).

We don't know if what you are seeing is normal, because you didn't describe it in detail. It might be normal, it might be abnormal.

Some things to check...see if all the motors are mounted tightly, they should not wiggle in the gearbox. Check to see if it's harder to turn the wheels by hand (with the robot power OFF) on one side than the other.