Crank Arms

BMX race crank arms: length, spindle and stiffness

The crank arm is the lever between the rider's foot and the front teeth. Its length sets how fast the legs can turn at the start, its stiffness sets how much of that energy actually reaches the chain. On a thirty-second sprint ridden standing up, both are felt from the first pedal stroke.

Three-piece, two-piece, one-piece

BMX race runs almost exclusively on three-piece cranks: two independent arms and a through spindle, joined by splines and pinch bolts or by a spindle bolt. That build lets you choose length, spindle diameter and tooth count freely. One-piece cranks, made from a single bent steel bar, only survive on entry-level bikes: heavy, flexible and short on options. Two-piece sets remain marginal in competition.

Which length for which category

• Micro and 18": 127 to 140 mm, so young legs can spin quickly without a dead spot.
• Mini: 140 to 150 mm.
• Junior and Expert: 145 to 165 mm, the most versatile range.
• Expert XL: 165 to 170 mm.
• Pro, Pro XL and Cruiser: 170 to 180 mm, for torque out of the gate.

Length is measured from the centre of the bottom bracket spindle to the centre of the pedal hole. If you are torn between two sizes, the shorter one favours cadence and pedalling through a berm; the longer one gives torque but brings the foot closer to the ground.

19, 22 or 24 mm spindle

Spindle diameter drives stiffness and dictates the bearings to order. The 19 mm remains the lightest and suits the younger categories perfectly; the 22 mm is the dominant compromise in race; the 24 mm brings maximum stiffness, often sought by Pro and Cruiser riders. The spindle must match the bearings in your bottom bracket, with a length suited to the 68 or 73 mm shell width.

Materials and manufacture

Forged and then machined 7075 aluminium is the benchmark: a good stiffness-to-weight ratio and excellent fatigue life. 6061 turns up on training ranges. Carbon, kept for the top of the range, saves more weight and damps vibration, but copes poorly with side impacts against a gate or a rail. Chromoly survives on a few models where toughness matters more than weight.

Compatibility to check

Three points before ordering: the bottom bracket standard of the frame, how the front teeth attach — spline drive sprocket or chainring on a spider — and the pedal thread, which is 9/16" x 20 on every three-piece crank against 1/2" on a one-piece. Finally, allow for the spacers needed to set the chain line.

The brands available

Inspyre and Chase cover the full range, from 127 mm for children to 180 mm for Pro. Box Components offers spindle and arm sets consistent with its bottom brackets. Tangent puts the effort into machining and finish. Supercross and Excess provide solid references for day-to-day training at a more measured price.

Servicing and inspection

Check the torque on the pinch bolts after the first few sessions, then regularly: an arm allowed to work loose damages its splines for good. Inspect the arms visually after a crash — the start of a crack near the pedal hole or the spindle means replacement, with no discussion.

The BMX RACING advice

We look at rider height and category before recommending a length: on a young rider, going from 165 to 155 mm often changes the gate start more than a gearing change would. Tell us your bottom bracket standard and we check the spindle, bearings and spacers as a set. Dispatched within 24 to 72 hours, you can spread the cost over 2 or 3 payments.