H-Base Drivetrain Turns

My team has an h-base drivetrain with four wheels, two on either side of the h-base. Each wheel is powered by a single motor, which we have connected to a sprocket train with a 2:1 gear ratio. We recently switched all of our motors from torque to high speed and noticed that our entire drivetrain turns left after the power to the motors is cut (we have the two wheels on the left side of the drivetrain connected to one joystick and the two on the right connected to another). Upon discovering this issue, we immediately looked for friction, but were unable to find any major source of friction. We then replaced all of the wheels and found that the one wheel did have more friction than the others so we replaced the bearing surfaces. All wheels are able to rotate freely when spun by hand, but our drivetrain continues to turn after the power is cut. Any suggestions as to what could be causing our drivetrain to turn?

As much as you try to eliminate friction, it may be virtually impossible to make sure that the friction on one side is exactly the same as the other side. Also, there is enough variability in the motors that two motors might not coast exactly the same when power is off. Also, if the weight of your robot is not perfectly balanced from side to side that will effect the friction on one side relative to the other. There are a lot of variables to consider. These are just a few.

Thank you. We will most likely take a look at the motors next and see if any of them run quite a bit longer or shorter than the others. As you have already stated, there is quite a bit of deviation between motors and it may just be the difference in the running time that is causing the one side to run longer than the other. At the moment, the weight of our robot is pretty well balanced on both sides, so I don’t believe it is friction caused by weight.

We had a similar problem with our drive and it turned out one of the motors labeled for HS was a torque motor. After continued driving, the internal gearing of the torque motor was destroyed and caused the side of the robot to slow drastically. We looked for friction and could not find a source, similarly to what your team has done, and it was only when we did a complete tear-down/redesign and switched the way the motors were set up (geared together vs direct drive) that we found the problem. IMO it would be a safe bet to go ahead and make sure those internal gears are the proper ones for HS.