Published on July 29th, 2014 | by Brett
Science Behind the Magic | Freehub Design
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 down 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.
The rear hub on a bicycle is one of those seldom understood components. Everyone uses one, but few realize what is 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; let’s 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 respectively. Without the freehub, the pedals and wheel would always rotate together.
Prior to the 1980’s, bicycles used a screw-on freewheel assembly, in which 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. The further bearing distance from the support results in a large magnitude 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 we 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. A 72-tooth Chris King hub will only see 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.
Freehub Body Design
There are two common freehub designs; the pawl and ratchet design and the star ratchet design. 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 left 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 can hear 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 the 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. A few different pawl designs can be compared in the gallery below.
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The other type of freehub is the star ratchet design. This design uses two ratchets that are pushed together using springs. The ratchets are able to spin freely in one direction but not in the other. DT Swiss has patented their star ratchet system and Chris King hubs use a type of precision star ratchet as well. Star ratchets distribute loads through all points of engagement simultaneously. In pawl designs, there can be a lot of engagement points—the Hope Pro2 Evo Rear Hub has 40 points of engagement on the ratchet—however loading is still distributed through the four pawls. One system is not necessarily better than the other; high loading through fewer points in the system is just one factor engineers have to consider during the design process. Star ratchets can vary from 18 teeth to 72 teeth, such as in Chris King’s hubs. The picture below shows a disassembled DT Swiss hub with the ratchets exposed.
Forces and Loading
Hub shells and freehubs are required to endure considerable forces of varying magnitudes and directions. The free body diagram (engineering term not to be confused with freehub body) below illustrates the different forces that are exerted on the hub. A free body diagram is a two-dimensional drawing that shows the magnitude and direction of forces on an object.
Engineers have to consider all of these forces, and how they affect the material in a freehub body. Considerations must be made for extreme conditions. What if a six-foot rider, weighing 210 pounds, hits a four-foot drop while riding a full suspension bike?
There are methods to evaluate how each force will affect the hub. When evaluating forces on components for engineering purposes, there is one rule to always remember with static systems—all the forces must sum to zero. If the forces don’t sum to zero, then the result is a dynamic system that is moving in the direction of the larger magnitude force.
Applying this concept to the diagram, we can sum forces in the X (horizontal) and Y (vertical) directions separately to simplify things. First, in the x-direction the chain will be pulling on the drive side of the hub. In order to counteract this force, the frame dropouts need to push on the hub in an equal and opposite direction, resulting in dropout reaction x. Therefore, if we know the load a rider will put on the chain, then we can predict how that load will be transmitted into the cassette, hub, axle, and ultimately the frame. Loads vary depending on conditions, but a chain could easily transmit 1000 newtons (225 pounds force) of load into the freehub body. This force will then have to be distributed amongst the pawls and ratchets. If the body is of a 4-pawl variety, then this scenario would see over 250 newtons (55 pounds force) being loaded onto about one square centimeter of pawl contact area.
Different materials will react differently to these loads. This data can be used to ensure that the material properties and thicknesses will be able to endure loading without failure both now and after repetitive loading years later. The same logic can be applied to the rest of the forces. It really is amazing how many loads are being exerted on such a small component, which is one reason so much engineering goes into these components, ultimately causing designs to change the more we learn about materials, loads, and production processes.
Science Behind the Magic delves into the inner workings of your two-wheeled steed. Web Content Editor, Brett Murphy, uses his mechanical engineering background to explain the latest industry advances and breakdown component design.
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