Rear Sprockets

BMX race rear sprockets: freewheel or cassette driver

At the rear there is only one tooth count, but two architectures that differ in weight, in the tooth counts available and in how they come apart. Choosing between them starts by looking at your hub: the hub imposes the solution, not the other way round.

The screw-on freewheel

This is the historic answer. The freewheel body contains both the teeth and the pawl mechanism, and screws onto the hub thread to a 1.375" x 24 standard. You fit it by hand, it tightens itself on the first pedal stroke, and it comes off with a dedicated removal tool — not always the same one from brand to brand. Common tooth counts run from 15 to 18. Upside: simplicity and price. Downside: the assembly is heavier and the whole body has to go when the teeth are worn.

The cassette driver cog

Here the pawl mechanism sits inside the hub, in a cassette driver, and the cog is nothing more than a splined ring held by a lockring — never bolted on. You can therefore change gearing by swapping an inexpensive part: the cassette driver covers the same 15 to 18 teeth as a freewheel, and it is what additionally opens up the small tooth counts, below 15 teeth on some builds, which a screw-on freewheel cannot do. This is the reference setup in competition: lighter and far easier to adjust between motos.

Tooth count, ratio and rollout

The rear sprocket is never chosen on its own: it forms a pair with the front teeth. A 44x16 gives 2.75, or 55.0 gear inches and 4.39 m of rollout on a 451 rim with a 20 x 1-1/8" tyre; the same 44x16 on a 406 with a 20x1.75 drops to 53.6 gear inches and 4.28 m. Always state the wheel format, otherwise the figure means nothing. Dropping one tooth at the rear lengthens the gear noticeably — 44x15 climbs to 2.93 — whereas it takes around three teeth at the front for the same effect. That is why most riders adjust gearing from the rear: the part is small, cheap and quick to swap in the pits. On a 24" Cruiser, remember to offset the larger wheel diameter with a bigger rear tooth count.

Engagement and mechanism

On a freewheel as on a cassette driver, the number of engagement points sets the dead angle before the drivetrain picks up again. The higher it is, the more immediate the pick-up coming off a jump or driving out of a berm. Some hubs use a pawl-free freewheel system with near-instant engagement, particularly valued out of the gate.

Compatibility to check

First establish whether your hub is threaded or fitted with a cassette driver, using the spec of your screw-on freewheels or of your hub. Then check the tooth format against your chain, 1/8" or 3/32", and its match with the front teeth, sprocket or chainring. Changing rear tooth count moves the wheel in the dropouts: allow for a half-link if you need to land the length exactly.

Materials, wear and replacement

Cassette cogs are machined from heat-treated steel for longevity, or from 7075 aluminium to save weight on the small sizes. Hooked teeth, a chain that skips under load or a sharp noise as you put the power back down all point to advanced wear. Replace the cog and the chain together at that stage: a new chain on worn teeth is ruined in a few motos.

The BMX RACING advice

Tell us your rear hub model and the gearing you currently ride: we tell you which tooth counts will actually fit and which one suits your category and your tracks. Plenty of riders travel to a race with two cogs — we help you pick the second. Dispatched within 24 to 72 hours, you can spread the cost over 2 or 3 payments.

One tooth at the rear is worth roughly three at the front: the full working is in our guide to BMX gear ratios and rollout.