GoPiGo pulls to right

My GoPiPo pulls to the right when it is supposed to go straight forward. I have tried rubber bands (suggested in an earlier posting for the same issue), but this does not help. Also, the black encoder wheel on the right side falls off regularly. Any tips for either issue?

For the encoder wheel, I had to try a couple times with the paper “shim” to get it hold in place. Might just take a couple of tries.

For the pulling one direction or the other, I went through this procedure to “trim the motors” and got mine to go fairly much in a straight line.

Hope this helps.

Both of your suggestions were spot-on Darrell. Thank you very much for the rapid help.

Thanks Darrell and thanks Andreas!