he chain tension on a fixed gear is quite critical, and is regulated by moving the rear axle back and forth in the fork ends. If the chain is too tight, the drive train will bind, perhaps only at one angle of the pedals (chainwheels are not usually perfectly concentric). It should be tight as it can be without binding. If the chain is too loose, it can fall off, which is quite dangerous on a fixed gear.

hen your install the rear wheel, there are basically three things you need to adjust simultaneously:

• The wheel needs to be straight. This basically means that the tire needs to be centered between the frame's chainstays. If you get it centered between the chainstays, it is properly aligned.
• The chain tension needs to be correct. (See previous section )
• The axle nuts or quick release skewer need to be tight. Note: if you have a nutted axle, it is vitally important that the threads be properly lubricated with grease or oil. You should also have grease or oil on the contact surface where the axle nut presses agains the washer that contacts the frame.

Some folks who are used to derailer bikes find it frustrating, especially with a nutted hub. This is usually because they don't know the technique of "walking" the wheel back and forth in the fork ends.
Start by installing the wheel at approximately the correct position and tightening the axle nuts. They don't need to be super tight at this stage, but should more than finger tight. Check the chain tension and wheel alignment.
Most likely, the chain will be a bit loose, but perhaps the wheel is correctly aligned. Loosen one of the axle nuts and push the tire to the side so that the loose side of the axle moves to the rear, then tighten the axle nut you loosened.
Now the chain tension should be better, but the wheel is no longer centered between the chainstays. Loosen the other axle nut and re-center the wheel in the frame. This will actually tighten the chain a little bit more.
The key is to keep one or the other of the axle nuts tight at all times, and "walk" the wheel forward and back.
This takes a bit of practice and getting used to how much axle movement is needed to adjust a given amount of chain droop, but it isn't really hard as long as you keep one side secured at all times. Note, this technique doesn't work with a quick release hub, but those are generally easier anyway.