Freehub bodies act to transfer the power applied to the chain to the rear wheel to drive it farward. They engage in one direction but not the other, this allows a cyclist to stop pedaling but still allow the rear wheel to rotate which is commonly called "coasting" or "freewheeling". The freehub body also acts as a mounting interface allowing a cassette or sprocket to be installed onto a wheel hub.
There are many different freehub body to hub interfaces, many of which look similar but are not cross compatible. Please refer to related component schematics or small parts diagrams for your components specific options.
Some common freehub engagement systems include pawls & springs, ratcheting rings and clutches.
Pawls & Springs:
A pawl could be best described as a small tooth that slots into notches in something called an engagement ring. One pawl (tooth) isn't strong or reliable enough for most use cases so freehub bodies will use anywhere from two to nine pawls. In order for the pawls to engage and disengage they require some kind of spring to move them into place. Each pawl could have its own individual spring or there could be a single spring that wraps around and actuates all the pawls. Although all pawls perform the same basic function there are many different shapes and sizes which makes cross compatability extremely limited. Engagement rings and pawls are machined to slot together in perfect unicine to transfer the power from the pedals to rotating the wheel. Engagement rings contain all the notches in which the teeth engage or slot into, without the engagement ring the pawls would have nothing to bite or hold onto resulting in pedaling but the wheel not turning.
A ring drive system consisting of one or two rings, each with a series of teeth. When these rings are put together the teeth interface with each other establishing a very solid connection. The teeth on both rings are sloped in a certain direction so they can freely slide over each other one way or lock together when moving the other way. Each ratchet ring needs to have pressure applied from behind to push them together to they'll engage and rotate the wheel. Some form of spring is generally used to perform this function but there has also been some experimentation with magnets. As for engagement, the more teeth that each ring has the faster they will engage into each other since there is a smaller gap between teeth. It is worth noting, as the tooth count increases they also get smaller and smaller teeth are more fragile making them easier to break. One of the greatest benefits of ring drive systems is that since all the teeth are attached to the same component they move together as one piece so 100% of the teeth will engage simultaneously. Compare this to a Pawl & Spring system where each pawl is on its own; there is no guarentee that all the pawls will engage at the same time. If only one of three pawls engages when power is applied to the pedals there is a high likelyhood that the pawl or engagement ring will break since all the power is channeled through a single part.
Instead of using teeth, clutches use friction to engage. In order to increase engagement strength the amount of friction needs to be increased which directly relates to more surface area. A ratcheting clutch is different from many peoples understanding of a clutch in a vehicle in that it engages in one direction but not the other.
Hub manufacturer ONYX utilizes small pawl looking parts called sprags. These sprags are aligned in two seperate rows to increase surface area (friction) around a smooth axle. When the axle rotates clockwise they all engage, gripping the axle with significant force causing the wheel to rotate. When the axle rotates counter clockwise the sprags just glide over the smooth surface of the axle allowing the wheel to rotate unobstructed.
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