If you've gone out and got yourself a half decent mountain bike in the last few years, the chances are it will have been fitted with a freehub in place of the less advanced freewheel units which are found on more traditional bikes. While the basic purpose and principal – essentially using a ratchet system to allow the wheel to spin freely even when the rider has stopped pedalling – remains the same, the freehub achieves this more efficiently than its predecessor.
So what are the advantages? Well, when you're actually riding the bike you would be unlikely to notice much difference. However when you come to carry out essential maintenance, or if a problem develops, the advantage of these more sophisticated units become clear.
Even though a freewheel can spin independently of the power exerted by your pedalling, when you are doing so the torque you generate continues to impact on the component. This makes it much more difficult to loosen and remove the freewheel when you need to. A freehub, by contrast, is insulated from this pressure by an outer cylinder, and is fastened into place using a lockring which can be easily removed. A freehub also allows for easier replacement of individual gear sprockets, which will inevitably wear out sooner than the ratchet mechanism itself.
All very technical, so it's a good job Ed from DirtTV is here to give us an informative introduction into the finer points of how a good quality freehub should function. Have a look at the video, and next time you buy a bike, make sure it's fitted with one of these clever little gizmos.