Chain Tension on One Speed Bikes
This article will address the adjusting chain tension on bicycle with a single sprocket in the rear and a single sprocket in front. It is assumed there is no derailleur or other chain tensioning devise. For derailleur chain sizing see chain sizing.
The rear wheel on a two sprocket bikes can be moved forward and back to allow for adjustment to chain tension. Generally, the chain should be set tight enough so that it does not come off during use. Too loose a chain setting will fall off either the front or rear sprocket. Too tight a chain setting will bind the hub and bottom bracket bearings. To set chain tension, the frame must have rear frame dropouts with slots that are somewhat horizontal. If the slot is vertical, the wheel cannot be moved relative to the front sprocket to change chain tension.
The rear wheel should be aligned straight in the frame. If the wheel is off to either left or right side, the rear sprocket will not be running straight to the front sprocket. To change the tension, loosen the axle nuts and move the wheel forward or back. If the bike uses a coaster brake, loosen the brake arm clamp bolt.
IMPORTANT: The brake arm clamp bolt must be fully secured before the bike is used.
As a rule of thumb there should be about one-half inch (12mm) movement of the chain up and down at point half way between the front and rear sprockets. Push the chain downward and then pull it upward in the middle. Note the travel.
After setting chain tension, pedal the bike in the repair stand and check for any tight and loose positions as the crank arms turn. It is common for sprockets to be out of round. This will result in the chain being tighter in some points of its rotation. After setting chain tension, pedal the bike in a repair stand and check the tension all the way through the crank arm rotation. If necessary, readjust so there is only 1/4 inch (6mm) movement at the tightest point.
To test the tension adjustment, pedal the bike and push sideways on the chain at a point in between the front and rear sprockets. The chain should not derailleur from the front rings. Repeat the test by pushing sideways on the chain on the lower section. The chain will make a rattling sound, but it should not derail. If the chain comes off, increase tension by moving the wheel further back.
Secure rear axle nuts to 300 to 400 inch-pounds. This is approximately 60 pounds of effort if you are holding a wrench six-inches from the axle.
If the bike uses a coaster brake, make sure the coaster brake arm is fixed to the bike and the coaster arm is secure.