Center Lock
Center Lock is Shimano's splined rotor mount: the rotor slides onto the hub splines and is fixed with a single central lockring at 40 N·m. It's faster to fit and remove than 6-bolt and centers the rotor perfectly. There are two lockring types: internal (tightened with a cassette-type tool like FR-5) and external (with a bottom-bracket tool; required on 15/20 mm thru-axles). Although it's Shimano's, SRAM adopted it: it's an open standard.
If mounting the rotor took 20 seconds and a single nut, you have Center Lock. Fast, centered and increasingly common.
What it is and the two lockrings
Instead of 6 bolts, the rotor engages the hub splines and locks with a central lockring at 40 N·m. That removes the warping risk from uneven tension and speeds up swaps. Mind the lockring type: the internal one uses the cassette tool (won't fit thick axles), and the external one, using the BB tool, is required on 15/20 mm front thru-axle hubs.
Key data
What it fits
It fits only hubs with Center Lock splines (Shimano, SRAM and many DT Swiss). It accepts Center Lock rotors of any brand (it's an open standard) and, with a specific adapter, 6-bolt rotors. It won't fit 6-hole hubs.
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Frequently asked questions
What torque does the Center Lock lockring take?
40 N·m. It's high: you must really tighten it, even though the ratchet clicks; if left loose, the rotor wobbles and damages the splines.
What tool do I need?
Depends on the lockring: the internal one uses the cassette tool (FR-5); the external one uses the BB tool (16-notch), required on 15/20 mm front axles.
Do SRAM Center Lock rotors work on Shimano hubs?
Yes. The Center Lock spline is an open standard both brands use identically.