Mountain Bike Chain Too Loose. Chains often loosen when a bike does not have a derailleur. IF YOUR CHAIN CAN'T PASS BOTH TESTS: Avoid these gear combinations–or replace cogs and chainrings.
Generally, look for loose rocks, deep sand, water, wet roots, logs and other cyclists, hikers, and animals.. A wrap of electrical tape to enlarge the valve at the rim should work too. IF YOUR CHAIN CAN'T PASS BOTH TESTS: Avoid these gear combinations–or replace cogs and chainrings.
Place the stiff link into the chain tool, on the slots closest to the turning handle.
The cause may differ from bike to bike because the mechanics vary.
If it is sagging or much looser than that, you need to tighten that chain up. The structure of a mountain bike may be more complex than the structure of a regular bike, but the chain changing process is similar. If your bike chain keeps loosening up, you need to find the cause of the problem and address it before riding again.