I purchased a kit 2011 grubee skyhawk gt5. I am having extreme issues with the rear sprocket. Every chain I put on falls off the rear sprocket. I have the sprocket centered and have it so it does not wobble. It has got me so frustrated. I really want to ride this bike for more than 2 minutes. So if anyone near springfield MA can help or know a post that can help please give me advice.