Belt Tension Explained

Why does tension matter?

Because a belt that's wrong in either direction causes trouble. One bike maker's support page says that with too little tension the belt will skip. Too much creates noise and is harder on bearings.¹

"Skip" means the belt jumps over the teeth of a cog. "Bearings" are the small rolling parts inside the hub and the pedal axle that let them spin.

Wikipedia adds that belts typically run at a higher tension than a chain. It also says that a high tension can shorten the life of the bearings at the pedals and in the rear hub.² So there's a narrow range that does the job without overloading anything.

What number is the right one?

There isn't one for every bike. The same page adds that the right tension varies with the model, the rider's weight and riding style.¹

For that maker's bikes, the page suggests aiming for a value between 35-50 Hz if you use a phone app, and adds that usually lower is better.¹ That's one maker's advice for its own models. Don't treat it as a rule for your bike. Your manual has the figure that applies.

What is a "Hz" reading?

It's a sound test. You pluck the belt like a guitar string, and a tighter belt makes a higher note. Hz, short for hertz, counts how many times the belt shakes each second.

The cycling site describes the app method. You hold the phone near the belt, pluck it lightly, and the app calculates an average frequency based on the sound.³

Are there other ways to check?

Yes. The bike maker's page also mentions a deflection test, which means pressing the belt and seeing how far it gives. The page says this is normally done by hanging a 10-pound weight from the middle of the belt. You then adjust to 1/4" to 3/4" of deflection, which depends on rider weight, model and riding style.¹ That is one maker's guidance for its own bikes. The page says you can imitate it with a finger, though that's rougher.

The cycling site also mentions dedicated tension gauges sold through bike shops.³ Which method your bike's maker prefers is stated in its manual. Use that one.

Does the tension change as the cranks turn?

A little. The cycling site says belt tension can vary slightly as the crank rotates. It advises multiple measurements at different crank positions, with the results averaged.³

If you're reading a number, turn the cranks a bit between readings. A single reading at one position can mislead you.

How is tension changed?

That depends on the frame. Wikipedia says tensioning can be done in all the same ways as a single-speed chain drive.² The cycling site names three frame designs: an eccentric bottom bracket, a sliding dropout and a horizontal dropout.³

Each one moves the pedals or the rear axle a little, so the distance between the cogs changes. The site says sliding dropouts provide around 20 mm of adjustment.³

Some bikes have a tensioner pulley instead. The adjuster on your bike is described in the manual. Follow the manual's steps. The cycling site says horizontal dropouts need the tension reset each time the wheel is removed. With sliding dropouts, the belt can be reinstalled without resetting tension.³

When should I get help?

If the belt skips, makes new noises or you can't reach the maker's figure, take the bike to a qualified shop. Skipping and noise are the symptoms the maker's page lists for tension that's off.¹ A shop can also look at the belt and cogs.

For routine upkeep, see how to look after the belt.