Dual motor bot spinning in circles

I have a sumo bot built from scratch and here is what I am using to power it:
*) Pololu Dual VNH5019 Motor Driver Shield
*) Two Pololu 20.12V HP 25mm motors (20.4:1)

These motors are mounted on the front of the bot and “pull” it. I have used a whole series of different free wheels in the back to support it. I’ve used several caster wheels and omni wheels. The problem is if there is a minor difference in drag of the two wheels (even just the caster cases not mounted in exactly the same direction) the robot runs in a curve and finally spins in circles. If the nicer caster wheels are nice and clean and I oil the bearings if works for a little while but then starts turning, and gets worse until it just spins in circles. One motor simply stops turning altogether. I really don’t know what to do. Does the motor driver have some logic that it may send all the power to one motor under some specific condition? It almost seems that way. I’m working on rigging up a tank tread set up and I don’t know if that will alleviate the situation or not. I’m really at a loss as what to do next. Thanks for any advice in advance.


The motor driver shield does not have any internal logic like that. Can you tell me more about the issue you are having? What are the motor indicator LEDs on the VNH5019 shield doing, and do they match what the motors are doing? Do the gearmotors behave the same way if you swap them? Can you send a video showing the behavior?