How a Belt Drive Works: Belt, Front Cog and Rear Cog

What are the main parts?

There are three that you'll hear named again and again: the front cog, the belt and the rear cog.

The front cog is bolted to the crank, which is the arm your pedal is fixed to. Some makers call it a chainring, the name used on chain bikes. The rear cog sits on the back wheel's hub. The belt loops around both.

The same cycling site that describes the belt's moulded nylon teeth and carbon fibre cords also says it's paired with stainless steel rear cogs and durable alloy front chainrings.¹ That's one description of one common setup. Your bike's manual will list the exact parts on your bike.

How does the belt move the wheel?

You push a pedal. The crank turns the front cog. The cog's teeth push into the belt's teeth, so the belt moves. The belt's teeth then push on the rear cog, and the rear cog turns the wheel.

That's the same job a chain does. The difference is that a belt is a flat, toothed band. The teeth mesh with grooves on the cogs rather than a chain's links wrapping around pins.

The cycling site says the belt is reinforced by carbon fibre tensile cords.¹

Where do the gears come from?

A belt can't be shifted across a stack of cogs the way a chain is. Wikipedia's page says derailleurs don't work with belts, and that a hub gear is the answer when you want more than one ratio.²

Wikipedia describes hub gears as a way to shift gear ratios on drivetrains that don't suit external derailleurs, belt drives among them.³ The gears sit inside the rear hub, and the belt turns a single rear cog that drives the hub. The same cycling site says belts rely on some internally geared hubs and gearboxes.¹

Some bikes have no gears at all and use a single speed hub. The belt works the same way either way.

Why does alignment matter?

Because a badly lined-up belt wears faster. Wikipedia says the front and rear pulleys or sprockets must be well aligned to avoid excessive friction and wear.²

"Aligned" means the front and rear cogs sit in the same flat plane, so the belt runs straight between them. If they're out of line, the belt rubs on one side of a cog.

Your manual will say whether anything on your model can be adjusted, and how.

What's a belt snubber?

You may see the word in a parts list. The cycling site says a snubber helps stop the belt riding up and off the rear sprocket under high load, or when tension is slightly low.¹ It sits near the rear cog.

The site recommends one for a particular hub setup. That's advice for that case, not a rule for every bike. Check your own bike's manual.

Why is belt tension talked about so much?

Compared with a chain, belts typically run at a higher tension.² Wikipedia adds that high belt tensions can reduce the life expectancy of the bearings in the bottom bracket and the rear hub.² The page on belt tension explained covers how makers say it's checked.

For how the frame makes room for the belt, see why belt drive frames have a split or opening.