How to Properly Adjust the Tension of Your Mountain Bike Chain
To adjust mountain bike chain tension, first shift the bike into the smallest chainring and smallest cog. Loosen the rear wheel axle bolts and slide the wheel forward until there is about half an inch of slack in the chain. Make sure the wheel is centered and straight before tightening the axle bolts. Check the chain tension by lifting up on the chain in the middle of the chain stay. It should not move more than 1-2 centimeters up and down. Adjust as needed and repeat until the tension is correct.