Quick release (QR)
Quick release (QR) is the classic way to fit and remove the wheel tool-free. It uses a 5 mm skewer that passes through the hub's hollow axle and is clamped with a cam lever against the frame or fork's open dropouts. The hub axle ends measure 9 mm (front) and 10 mm (rear) —not to be confused with the 5 mm skewer—. It's light and convenient, but less rigid and repeatable than a thru-axle, which is why the rotor can rub after you remove the wheel.
It's the time-honored system: one lever and you're out. Still common on entry-level road and older bikes, though the thru-axle is replacing it.
What it is and how it clamps
The 5 mm skewer passes through the hub's hollow axle; closing the lever drives a cam that compresses the assembly against the open dropouts. It doesn't thread into the frame: it only clamps. That's why you can pull the wheel in seconds, but also why it offers less rigidity and doesn't guarantee the rotor returns to exactly the same spot.
Key data
What it fits
It fits only frames and forks with open dropouts: front 100 mm, rear 130 mm (road) or 135 mm (MTB/gravel). It won't fit thru-axle frames (closed dropouts) unless the hub is convertible with swappable endcaps.
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Frequently asked questions
Can I use a QR wheel on a thru-axle frame?
Not directly: the dropouts don't match (open vs closed threaded). Only if the hub is convertible with endcaps and a specific adapter axle.
Is the QR skewer 9 mm?
No. The skewer is 5 mm; the 9/10 mm is the diameter of the hub axle ends that rest in the frame.
Why is the thru-axle replacing QR?
Rigidity and safety: the thru-axle threads into the frame and repositions the rotor identically, with no risk of the wheel loosening.