Tightening a BMX Chain Tensioner

Remove the rear wheel for maintenance by first loosening the axle nuts. These are most commonly 15mm, but in a few cases may be 5/8-inch. Undo the nuts by turning them counterclockwise. Loosen the chain tensioners to allow the wheel to move forward in the frame. Back off the locking nut, then turn the adjusting screw to let the wheel move forward. When there's enough slack in the chain, slip it off the front chain ring and slide the wheel out of the frame.

Install the wheel by sliding the chain back onto the cog on the wheel. Put the wheel into the frame and install the chain on the front chain ring. Rotate the chain tensioners into position. Pull on the wheel to put some tension on the chain and tighten the axle nuts by hand, not with a wrench. Ensure that the wheel is centered in the chain stays by checking the alignment near the bottom bracket. There should be an equal gap on either side of the tire.

Begin the final chain adjustment by tightening the chain tensioners evenly. Adjust them alternately to keep the wheel centered. The axle nuts should be tight enough to hold the wheel, but not so tight as to prevent the axle from moving. When the chain exhibits 1/2-inch of up and down motion at the center point between the rear cog and the chain ring, tighten the axle nuts firmly with a wrench. Turn the pedals slowly while checking the chain tension. Chain rings are often slightly eccentric, so there will be a point of maximum chain tension. Note where this occurs, and if necessary, loosen the axle nuts and re-adjust the tensioners to achieve 1/4-inch motion at the chain's center point.

Tighten the lock nuts on the chain tensioners. Check that the wheel is still centered in the frame. If the bike is equipped with a coaster brake, be sure to re-attach the coaster brake arm to the frame bracket.