Chain Tensioners

BMX chain tensioners: keeping the axle put on the gate

On a BMX race frame with horizontal dropouts, nothing mechanically stops the rear wheel creeping forward when the rider puts full power down on the gate. A tensioner creates an adjustable stop behind the axle: the chain keeps its tension, the wheel stays centred, and the axle nuts no longer have to absorb all that torque on their own.

What a tensioner actually does

It performs two inseparable jobs. The first is holding chain tension: too slack and the chain slaps and can jump off the chainring or sprocket on the first bump; too tight and it loads the hub and bottom bracket bearings and costs efficiency. The second is alignment: by setting both tensioners the same number of turns, you bring the wheel exactly into line with the frame, which avoids brake pad rub and angled wear on the teeth.

The types of tensioner

• Push-screw tensioner: a plate sits around the axle and a screw bears on the back of the dropout. The most common design, adjustable a quarter turn at a time.
• Wrap-around plate tensioner: the plate cups the dropout on both faces, spreading the load and protecting the frame paint.
• Frame-integrated tensioner: some frames have a screw housed in the dropout itself. Check this before buying, as an external tensioner is then unnecessary.

Dimensions and compatibility

Three measurements to check before ordering. Axle diameter first: 10 mm on the vast majority of BMX race rear hubs, 14 mm on freestyle-type hubs and some Cruisers. Dropout thickness and shape next, because a thick aluminium dropout will not take every plate. Slot length last, which determines how much adjustment travel you have. Tensioners are often sold in pairs, and while a single one on the drive side is enough on some builds, two is the safer way to guarantee alignment.

Setting tension step by step

Slacken the axle nuts without removing them, push the wheel forward, then turn both tensioners alternately a quarter turn at a time. Correct tension leaves roughly a centimetre of vertical movement at the midpoint of the top run: the chain should neither slap nor sing when you turn the cranks by hand. Check the wheel is centred between the chainstays, then torque the axle nuts. Re-check after the first session, because a new chain always settles a little.

When a tensioner is no longer the answer

If the wheel keeps slipping despite a correct setting, look elsewhere: tired axle nuts, missing serrated washers, a dropout scored by a previous slip, or a BMX chain stretched beyond what any adjustment can recover. A chain whose links show side-to-side play needs replacing, and you should check the condition of the chainring teeth at the same time.

The tensioner brands

Box Components and Tangent machine light aluminium tensioners in colours that match complete builds. Insight offers simple, affordable references suited to young riders' bikes. Elevn makes neatly finished wrap-around plates. Steel versions remain a safe bet for heavy training use.

The BMX RACING advice

Tell us your frame brand and rear axle diameter and we will check the plate suits your dropout shape before we ship. Our BMX sprockets and the rest of our drivetrain parts complete the build. Dispatched within 24 to 72 hours and you can spread the cost over 2 or 3 payments with Alma.