Editors Note: This article was written by Art’s Cyclery web content editor Brett Murphy, who uses his mechanical engineering background to explain the latest industry advances and breakdown component design. The original post can be found here.
Standing on the pedals, sweat dripping off the tip of your nose, you look ahead and see the road disappear around the next corner. It seems like the climb will never end — a purgatory that continues to tease you with thoughts of escape. Finally reaching the top, you take a short break, catching your breath and drinking water before continuing down the other side. The descent makes the climb worth it, carving through the trees, throwing your knee into the next corner, the bike is your ticket to freedom, the dedication of hundreds of engineers allowing for this moment in time. You are reminded of this with every gear change and every click of the freehub body as it skips across the pawls. Or maybe you don’t think about your freehub. Most people don’t.
Indeed, the rear hub on a bicycle is one of those seldom understood components. Everyone uses one, but few realize what’s going on inside. We expect it to just work, which, for the most part it does, because a lot of time and engineering goes into the design and construction of freehubs. Without that effort, cycling would be an entirely different experience.
There are several different assemblies that make up a complete rear hub. We’ll start from the inside and work our way out. Running between the dropouts, and serving as the connection point between the frame and the wheel is the axle assembly. Rotating around the axle assembly is the hub shell. Lastly, the freehub mounts onto the main hub shell and contains a ratcheting mechanism that engages and disengages under pedaling and coasting scenarios. Without the freehub, the pedals and wheel would rotate together.
A Little Historical Perspective
Prior to the 1980’s, bicycles used a screw-on freewheel assembly where the gear cassette attached directly to the hub shell. Instead of a modern system that incorporates the ratchet into the hub, the freewheel gear cluster itself contained an integrated ratcheting mechanism. Freewheels became less popular as the years progressed because the drive-side bearing is located in the freewheel, and as more sprockets are added (for more gear combinations) it pushes the bearing further from the support. This increased bearing distance from the support results in a larger magnitude of flexing stress in the axle, which can bend or even break the axle. This design setback as well as the inability to change out gearing without changing the whole system led bicycle companies to look for a better-designed rear wheel. In the late 1980s, Shimano introduced the freehub and cassette design, which became the new standard in bikes with multiple rear gears. The figure below from Sheldon Brown shows an example of a freehub compared to a freewheel. Notice that on a freewheel the gear cassette threads onto the hub and on the freehub the gear cassette slides onto the splines of the freehub body.
Why should you care about the number of engagement points, something often touted in high-quality hubs and ignored in lower end versions? Because your riding experience is greatly affected by the quality of your freehub. Beyond precision bearings and tight tolerance machining, the number of engagement points will be the most noticeable aspect of a particular hub. The difference between an 18-tooth star ratchet and a 72-tooth will be quite noticeable. With 18 points of engagement spread over 360 degrees of rotation, a hub, and thus your pedals, will move 20 degrees before it reengages and begins rotating the wheel after coasting.
In contrast, a 72-tooth Chris King hub has only 5 degrees of movement in the same scenario. Although you may have never noticed, this 15-degree difference in engagement can really change how the bike feels. It removes that slight play in the pedals before reengagement and gives the rider a positive feeling of almost instant acceleration and connection to the road. This can be especially important in mountain biking and cyclocross, where you’re more likely to encounter situations where you slow way down, then try to regain speed quickly.
A single pawl as it engages with the ratchet (click to enlarge). Photo courtesy Art’s Cyclery
Freehub Body Design
The two common freehub designs are the pawl and ratchet and the star ratchet. In the pawl and ratchet design, the ratchet spins with the rim, and the pawl is fixed with the rotation of the cassette. When the rider pedals forwards, the pawls engage the ratchet, and they both spin together. When the freehub is spinning more slowly than the wheel, the pawl disengages and the assembly spins. The picture to the right shows a single pawl as it engages with the ratchet. In real applications, there would be more than one pawl engaging with the ratchet.
The clicking sound that you hear coming from your rear wheel when coasting is the pawl running over the ridges of the ratchet. The spring on the backside of the pawl pushes outwards so that when the rider begins pedaling, the pawls “catch” on the ratchet teeth. Engagement can be increased by increasing the number of teeth in the ratchet, or by offsetting some of the pawls, so that not all pawls engage at the same time. Adding pawls can increase complexity, cost and weight.
Smaller teeth can create more problems later on — tighter tolerances mean that issues arise more quickly with smaller teeth in high mileage systems with a lot of wear. The number of pawls varies between companies and models, and the type of spring used to hold the pawls in place varies as well. Some hub pawls are held in place with coil springs, while others use leaf springs, and still others use circular springs.