Let's Make Robots!

Wild Thumper Controller Left/Right speed way off


i'm using a Wild Thumper controller on a DFRobot 4wd chassis and motors.
I noticed if I set a certain speed, there's a lot of difference between the left and right wheels.

But it is not consistent. The speed tends to equal out after a few seconds.

And it is dependent on the direction. e.g.: going forward starts with left going faster than right, and with going backwards it starts with right going faster than left... weird....

 The effect is worse with higher speeds...

Any ideas?