Chain tugs (also called chain tensioners) are used to help maintain correct chain tension on singlespeed bikes with horizontal dropouts, most commonly BMX bikes.
They are needed because your chain will naturally stretch over time, causing slack to form which delays power pickup when you turn the pedal and will result in your drivetrain wearing faster.
On a bike with derailleur gears this isn’t an issue as the spring in the rear derailleur will take up any slack, but on a singlespeed bike such as a BMX you will need to periodically undo the axle bolts and reposition the rear wheel in order to maintain chain tension.
When you this, an adjustable chain tug on your drive side uses a threaded nut to prevent the axle being pulled back again in the direction of the cranks.
Which chain tug is right for you?
Lots of modern BMX riders don’t need chain tensioners, mounting their wheels in the ‘slammed’ position tight to the front of the dropout and adjusting tension by removing chain links (usually with half-link chains). However if you want to run your wheels further back in the dropouts and find yourself suffering from Slack Chain Syndrome, the only prescription is a chain tug. The advise is to start with a single tensioner on the drive side, and if that doesn’t cure it, use one on the other side too.
The ‘classic’ chain tug consists of a steel or aluminium washer with a centre hole equivalent to your axle diameter (and an outside diameter the same as that of the average peg), an end cap that fits over the back of the dropout, and a threaded bolts that connects the two. Other designs (the ‘push’ type chain tug) may feature nuts designed to fit into the front of the dropout, or you can get some models that can adapt between the two mounting positions.
When buying a chain tug make sure that the diameter of the washer fits your axle – the most common sizes are 14mm and 10mm.