The best mountain bike hubs are constructed from aluminum or carbon composite. While the front hub is simple in function, the rear is more complex, featuring a freehub body onto which the cassette attaches. It also houses the intricate spring-loaded ratchet-and-pawl mechanism which enables the wheel to spin when freewheeling and engage the transmission when pedaling. Bigger pawls will deal better with more torque (downhill/enduro) while on the opposite end finer pawls will result in a faster engagement and better power delivery (cross-country).
Some freehubs, like those from DT Swiss and Chris King use a ratchet system rather than pawls, which provides more simultaneous points of engagement.
The need to continually improve front and rear-end stiffness has led to the introduction of thicker, more stable thru-axles that bolt directly into the fork and drop-outs. Most newer bikes offer wider ‘Boost’ axle spacing, which uses a 110x15mm front axle with a 148x12mm rear thru-axle for added levels of stiffness.