Chainrings

BMX race chainrings: BCD, teeth and gearing

Changing chainring is the most common adjustment of a season: four or five bolts, ten minutes, and the character of the bike out of the gate changes with it. You still have to order the right part — and bolt circle diameter is the first filter, well before tooth count.

Chainring, spider and sprocket: who does what

The chainring is a toothed ring, drilled to bolt onto a spider, which is itself part of the crank arm or the spindle. That architecture lets you keep the spider and replace only the ring. A one-piece sprocket, by contrast, combines teeth and support in a single part: stiffer, but replaced in full to change gearing. Some chainrings also mount spiderless, bolted straight onto inserts in the crank arm.

BCD standards

BCD (bolt circle diameter) is the diameter of the circle passing through the centres of the fixing holes. In BMX race two values dominate: 110 mm on five arms, the most widespread, and 104 mm on four arms. Other patterns exist on certain proprietary spiders. Measure before ordering: on a five-arm spider, measure between two non-adjacent holes and refer to the spec sheet; on a four-arm spider, the distance between two opposite holes gives the BCD directly. A chainring with the wrong BCD will not mount, even with the right tooth count.

Tooth counts and choosing your gearing

The useful range runs from roughly 39 to 47 teeth. Paired with the rear sprocket, it sets the ratio: 41x15 gives 2.73, 44x16 gives 2.75, 45x16 climbs to 2.81. Front tooth count also affects efficiency and wear — a large ring spreads the load over more teeth in mesh and wears more slowly, at the cost of a few grams and a little ground clearance. Micro and Mini stay on short tooth counts, Expert riders sit mid-range, Pro and Cruiser higher up.

1/8" or 3/32" teeth

Tooth thickness has to match your chain. The 1/8" remains the track standard, strong and forgiving. The 3/32" lightens the assembly but requires a chain and a rear cog in the same format. A 1/8" tooth profile with a 3/32" chain leaves side play that derails; the reverse never seats cleanly in the tooth valleys.

Materials and machining

CNC-machined 7075-T6 aluminium dominates the category, usually heavily cut away and anodised. Tooth profiles are worked to ease engagement and limit noise, sometimes with a specific chamfer on the flanks. Training chainrings in 6061 weigh a little more but take gate contact well. A few carbon or two-material models exist at the very top of the range.

The brands available

Rennen has specialised in machined chainrings and covers the common BCDs across many tooth counts. Tangent offers proven references on five-arm builds. Inspyre supplies chainrings dedicated to its own cranks. Elevn plays on finishes and matching colours, Box Components on the price-to-durability ratio for training.

Wear and fitting

Tighten chainring bolts in a cross pattern, to the stated torque, with a low-strength threadlock: one loose bolt distorts the ring in a few motos. Then check alignment with the rear cog, correcting it with bottom bracket spacers. Pointed or hooked teeth signal advanced wear, usually alongside an elongated chain: replace both together.

The BMX RACING advice

Send us a photo of your spider with the make of your cranks and your current tooth count: we identify the BCD, the tooth format and the count to aim for, based on the rider category and the profile of the tracks you ride. Dispatched within 24 to 72 hours, you can spread the cost over 2 or 3 payments.

Before changing chainring, check what it actually moves in our BMX gearing calculations.