Extreme curve?

[Anthony Galea]

So yesterday during our unbag time, we decided to test our autonomous that should drive straight over the defenses, and the drive is set for 70% forward. However, when we tested it for the first time on the field, instead of driving straight, it curved extremely left, crashing into the drawbridge instead of going over the moat. We went into teleop mode and made sure the shifters were set both to low gear just to be sure, then we tested it again, and it still drove to the left. We have greased our gearboxes, and have both belts set to proper tension. We did notice that the shifter coupler (interfacing the piston with the shifter) on the right side (the faster side) is rotating with the shaft, but the left side was not. What could possibly be causing this huge curve, and how would we fix it? We don't have time to buy encoders and have them shipped to us, as we have a week 1 event. Thank you for your help!