My left side is loose. What should I do to keep it from happening again?

 

If it is loose because the bolt worked loose, you MUST replace the lock-nut, preferably with an all-steel lock-nut, not the nylok type. Replace the washer under the bolt head with a new one, and be sure to bend the ears over the flats of the bolt. If the hole in the frame is wobbled oval, a new one should be welded in. If it is just barely loose, it may hold. This is true with any of the pre-concentric drive units with chain drive. For the concentric drive units, I haven't seen one loose yet so wouldn't know what to suggest.