The four main bike axle standards are: quick-release (5 mm rod with cam lever, road bikes pre-2018, hybrids); thru-axle 12 mm (modern road and gravel disc-brake bikes, also some MTB rear); thru-axle 15 mm (modern MTB front + some gravel); and Boost 110/148 (wider thru-axle spacing for modern MTB, allowing shorter chainstays and better tire … Read more
Bike dropouts (rear) and fork ends (front) are the slots in the frame and fork where the wheel axles attach. Modern bikes use closed thru-axle dropouts (a circular hole the axle threads through); older bikes use open quick-release dropouts (a slot the axle drops into). Other variants: vertical dropouts (axle slides straight in, common on … Read more
A thru-axle is a 12mm or 15mm diameter bolt that runs all the way through the wheel hub and threads into the dropout, while a quick-release uses a thinner 5mm rod with a cam-action lever clamping the wheel into open dropouts. Thru-axles are stiffer (no flex under hard braking), more secure (won’t pop out), and … Read more
A quick-release skewer is the lever-and-rod mechanism that holds a bicycle wheel in the dropouts — you flip the lever open to remove the wheel and close it to clamp it tight. Modern bikes increasingly use thru-axles instead (a bolt-on system that’s stiffer and more secure), but quick-release is still standard on road bikes under … Read more