Brake discs

BMX brake discs: diameters, fittings and wear

The rotor is the one part of a disc brake you replace as routinely as a brake pad. Buckled after a crash, contaminated by a spray of degreaser or simply down to its wear limit, it can be swapped on its own, provided you respect its diameter and its mounting standard.

One-piece or floating

A one-piece rotor is cut from a single sheet of stainless steel: tough, affordable and easy to true slightly by hand. A floating rotor pairs a steel braking track with an aluminium carrier, joined by rivets. It is lighter, sheds heat better and distorts less under heavy thermal load — a real benefit in Cruiser and on long straights.

140 or 160 mm: choosing a diameter

These two diameters cover almost all of BMX Race. The 140 mm rotor remains the standard on 20 inch bikes: light, tucked away inside the wheel, and easily enough for the weight of a race bike. The 160 mm adds leverage and heat-dissipating area, which makes sense in Pro, Pro XL and on 24 inch Cruisers. Be aware that the rotor diameter has to match what the caliper mount is designed for, if necessary through an adaptor.

IS 6-bolt or Centerlock

The IS 6-bolt fitting uses six M5 bolts, usually with a Torx T25 head, on a standardised 44 mm bolt circle. It is the most common interface and the easiest to sort out at the track. Centerlock instead splines the rotor onto the hub and locks it with a threaded ring: faster to fit, but it needs a specific tool. A Centerlock hub will take a 6-bolt rotor through an adaptor; the reverse does not exist.

Compatibility to check

• The rear hub interface: 6-bolt or Centerlock, to be confirmed on your wheels.
• The diameter specified by the brake kit and its caliper mount.
• The pad type: some very light rotors are rated for organic pads only, as sintered compounds overheat them.
• Thickness: most rotors are 1.8 mm when new, with a wear limit engraved at around 1.5 mm.

Wear, buckling and replacement

Check thickness with a vernier caliper, and check for buckling by eye, spinning the wheel in front of the caliper. Mild buckling can be corrected with a rotor truing fork; a clear deformation after a ground contact justifies a new rotor. A permanent squeal, bite that fades away or a blued, glazed surface all point to contamination or overheating: the track can be lightly abraded and cleaned with isopropyl alcohol, but pads soaked with oil have to be replaced.

The brands available

Shimano remains the reference for consistency and for the finish of its braking tracks. Promax offers simple, versatile rotors that are easy to replace. Box Components and Elevn aim their rotors squarely at racing, with lightweight cut-outs and good water clearance.

The BMX RACING advice

Before ordering, measure the rotor already on the bike and note the hub interface: that is the only way to avoid a mistake. Our disc brake kits state the diameter they ship with, and our disc brake mounts cover changes of diameter. Dispatched within 24 to 72 hours.