Inside a freehub body: What makes the clicks and angry bees?
The freehub of any geared bike is something that is easy to take for granted. As it stands, the only attention that it normally receives depends upon just how much noise it makes. There is, however, more to the function of a freehub than making a sound, and in this article, Matt Wikstrom unpacks the design of the modern freehub to explain the importance of points of engagement to a rider’s effort.
The buzz of a freewheeling bike is perhaps the most joyful sound in the world. It ushers in those carefree moments when the rider is able to stop pedalling for a while and enjoy the surroundings and/or the speed of the bike.
Every hub seems to have its own voice when it’s freewheeling. Some are loud, even obnoxious, while others speak softly. It makes for a second conversation of sorts, rising and falling with the speed and effort of any bunch of cyclists.
All of that chatter can be traced to the rear hub, and specifically, the freehub body that contains the drive mechanism for the sprockets. In the past, this was wholly contained within a sprocket set that was screwed onto the rear hub, but as the number of sprockets increased from seven to eight and nine, it was replaced by a cassette system popularised by Shimano during the ‘80s.
Now, every 10/11/12-speed hub has a cassette freehub, and while all perform the same function, there are important differences within and without. The most obvious of these concerns the size and shape of the freehub body, because it dictates sprocket compatibility. As such, it is one aspect that every rider must contend with, and thankfully, the rules of compatibility are easy to understand.
The inner workings of a freehub are likely to be less familiar, and perhaps a little intimidating, too. The rhythmic tick of the mechanism suggests the complexity of a clock, and while springs are normally involved, there are not a lot of moving parts. Indeed, most freehubs are quite simple, and in general terms, there are just three types, starting with the standard ratchet that dominates today’s market.
When freehubs started replacing freewheels during the ‘80s and ‘90s, the ratchet mechanism that long served single- and multi-speed freewheels was transferred to the hub. Shimano created a self-contained (and unserviceable) unit that simply bolted onto the hub, but as other manufacturers tackled the design, many opted to incorporate parts of the ratchet into the body of the hub, and this has become a common strategy.
Regardless of the approach, these ratchets comprise at least two spring-loaded pawls that are surround by a drive ring. The teeth on the drive ring are directional, so that it is only possible for the pawls to engage with the drive ring in one direction. Each pawl engages with one tooth on the drive ring — like a foot on a step — to lock the freehub body into place and drive the rear wheel.
When the freehub body is driven in the opposite direction, the pawls can’t engage with the teeth of the drive ring, so it is able to spin freely. The pawls click up and down over the teeth, which is what produces the buzz of the freehub.
The frequency of that buzz depends on two things: firstly, how quickly the wheel is rotating, and secondly, the number of teeth in the drive ring. More teeth means that the pawls will click more often, producing a richer (or angrier) buzz from the rear wheel.
The pitch and volume of the freehub is influenced by the number of pawls, the stiffness of the pawl springs, the choice of construction materials, and to a lesser extent, the amount/type of lubrication present. Importantly, none of these features, including the drive ring, can be adjusted to change the sound or angle of engagement of the freehub, though extra lube can dull the buzzing somewhat.
William Hügi developed an alternative ratchet design during the early ‘90s where he abandoned pawls in favour of two spring-loaded ratchet rings. Teeth were cut into the opposing faces of each ring to create an axial (rather than radial) drive mechanism for the hub. Importantly, every tooth of the ratchet is engaged when the wheel is being driven, which is quite distinct from a standard ratchet.
Hügi’s star ratchet system was first adopted by DT Swiss in 1995 and soon after, the two companies merged. The patented mechanism has been a hallmark of DT’s high end hubs ever since, though other companies, such as Bontrager, Giant, and Roval have licensed the design.
DT’s Ratchet System is not the only design that can be found in today’s market. Chris King developed and patented his own take on this strategy in 1996, adding a helical spline to bring the rings together. There is also Mavic’s Instant Drive 360, Zipp’s Axial Clutch, which depends upon magnets rather than springs, and Shimano’s Scylence ratchet, which disengages completely to provide silent coasting.
Once again, the frequency of clicking from each of these ratchets (except Scylence, for obvious reasons) depends upon the number of teeth on the drive rings, and in most cases, this cannot be adjusted. There is one important exception: DT’s star ratchet rings are available with a choice of 18, 36 or 54 teeth, so that the amount of buzzing (and the angle of engagement, see below) can be varied by swapping one set of rings for another.
Roller and sprag clutches
Not all freehub mechanisms are built around a ratchet. Instead, a roller clutch can be used to grasp a drive-shaft in the hub. This design offers very quick engagement as well as silent coasting, however the requisite hardware is much heavier than any ratchet, which limits its applications.
There are no teeth in a roller clutch, hence the silent coasting. Instead, a ring of rollers slide onto a drive shaft, each one like a chock under a wheel, to wedge the freehub against the hub as it is engaged. A spring encourages each roller to retract when coasting, however there is a risk of jamming under heavy loads unless hardened steel is used throughout the clutch.
A sprag clutch is built upon the same basic principle as a roller clutch, however it makes use of asymmetrical “sprags” that rock rather than slide onto the drive shaft of the hub. Once again, each sprag behaves as a wedge to lock the freehub body against the drive shaft until the load is removed, at which point a spring coaxes the sprags to release the shaft for coasting.
Aside from silent coasting and near-instant engagement, roller/sprag clutch promises less drag than a ratchet when freewheeling. In some instances, that might mean a bit of free speed, but it can also prevent auto-rotation of large sprockets that can create significant backlash for the chain and cranks when coasting.
As attractive as these benefits may be, roller/sprag clutches are rare in today’s market. Shimano introduced a roller clutch (dubbed “Silent Clutch”) to a few of its MTB hubs for a short period starting in the ’90s, however the only place it can be found today is in its Alfine and Nexus internally-geared hubs. True Precision Components, by contrast, has been building its MTB and BMX hubs with roller clutches for almost two decades, while Onyx has been championing the merits of a sprag clutch over the last few years. Any question about the reliability of these clutches seems to have disappeared, so the only drawback really seems to be the extra weight of a clutch.
Angle of engagement
There is more to appreciate about the freehub mechanism of a rear hub than simply the amount of clicking that it makes or how loud that swarm of bees might be. Every hub offers a specific angle of engagement, which is a measure of how far the freehub must rotate before the drive ring or shaft is engaged.
For a ratchet, this angle primarily depends upon the number of teeth present in the drive ring, as shown in Figure 1. In this example, a freehub with 18 points of engagement requires 20° of rotation to move the pawls from one tooth of the drive ring to the next; doubling the number of teeth on the drive ring halves the angle of engagement to 10°.
Clearly, adding even more teeth to the drive ring will further decrease the angle of engagement, but there is a limit to this number. At some point, the teeth will simply become too small to contend with the amount of torque that can be applied to the freehub.
It is possible to increase the number of points of engagement without changing the number of teeth in the drive ring. This is achieved by adding an extra set of pawls and then offsetting them to reduce the angle of engagement. This is how Industry Nine is able to create 120 points of engagement for a 60T drive ring, while Kappius and Profile take this notion a step further with three or four sets of offset pawls to create over 200 points of engagement.
Roller and sprag clutches are able to grasp the drive shaft of the hub at any point of rotation, making for infinite points of engagement. Some rotation of the freehub is required before the rollers/sprags engage the drive shaft, but this will vary, increasing with the amount of load on the wheel. Thus, the angle of engagement for these hubs cannot be defined (or measured) in the same way as a ratchet-driven hub, but it can be considered near-instantaneous.
Increasing points of engagement will always add to the expense of a hub, but there is another, and far more relevant, distinction to be made. Road hubs tend to have a relatively high angle of engagement, while high end MTB hubs strive for much lower angles of engagement (Table 1). There is a good reason for this, because there is a difference in the gear ratios that each group of riders use, and this has a big effect on the amount backlash for the cranks.
Backlash and gear ratios
The angle of engagement of a hub creates a lag in the drive-train of the bike that can be felt any time a rider resumes pedalling after coasting (or back-pedalling). As the freehub body and sprockets rotate to engage the drive mechanism of the hub, so too will the cranks. Sometimes this backlash is barely noticeable, while at others, it can frustrate a rider, especially when they are struggling to regain momentum in challenging conditions.
A hub with a low angle of engagement will always produce less backlash than a hub with a high angle of engagement, however, the final result will vary, depending upon the gearing of the bike (Figure 2). When the sprocket is the same size as the chainring, then the hub and crank will share the same angle of engagement. As the size of the chainring increases, though, the cranks will rotate to a lesser degree, while larger sprocket sizes will have the opposite effect.
For example, when a hub that has 20° of engagement is combined with a chainring that is four times larger than the sprocket (e.g. 52 x 13), then that will reduce the angle of engagement for the cranks to 5°. For 170mm cranks, that is equivalent to 15mm of backlash, which is likely to go unnoticed. If the same hub is then combined with a chainring that is half the size of the sprocket (e.g. 25 x 50), then that will increase the angle of engagement for the cranks to 40°, and a set of 170mm cranks will rotate a much more noticeable 120mm.
Thus, high gear ratios reduce the amount of backlash for the crank, while low gear ratios exacerbate it, as shown in Figure 3. This occurs in a geometric fashion, and while there is no consensus on an acceptable amount of backlash, it is clear that riders that make use of low gear ratios (mountain bikers, for example) will be more susceptible to it. This is where a hub with a low angle of engagement (6° or less) has a lot to offer, especially if the bike is equipped with very low gear ratios (Figure 4).
The length of the cranks has a much smaller effect on backlash, as shown in Figure 5. Longer cranks will always create more backlash, but for those riders hoping to minimise backlash, there is more merit in switching to a hub with a lower angle of engagement than contemplating a shorter set of cranks. Nevertheless, it is not unusual for MTBers to make use of 175mm cranks, which will only add to the amount of backlash that they will have to contend with.
At face value, a hub with a low angle of engagement promises to be more efficient because it produces less backlash, and therefore, offers more immediate power transfer whenever a rider resumes pedalling. However, the amount of backlash that any hub produces is generally quite small, and arguably unnoticeable, until very low gear ratios are used.
As such, there isn’t much on offer for road cycling because riders tend to use high gear ratios with a high cadence. Moreover, the amount of time spent coasting is small, and there is no need to backpedal. Less backlash might provide an advantage when exiting a corner, however road cycling is rarely that technical, and finishing straights are normally quite long. A rider will be better off concentrating on positioning, cornering, drafting, braking, and aerodynamics than relying on a low angle of engagement to give them an edge during the dash to the finish line.
The advantages for off-road riding are much more tangible. MTBers are more likely to be using very low gear ratios and travelling at low speeds while trying negotiate rocks, roots, and drop-offs. Under these circumstances, rapid hub engagement can be crucial for conquering an obstacle or simply preventing a fall, though it is unlikely to overhaul a rider’s level of skill.
Gravel riders and bike-packers generally fall somewhere in between these two extremes, so there may be some instances where a hub with a low angle of engagement might prove useful. These moments will be fleeting, though, and it certainly won’t do anything to ease the effort of tackling a long and unrelenting grade.
Too much of a good thing?
Any ratchet suffers an amount of drag, and in the case of a rear hub, there is a concern that it will slow the rider. This is something that is easy to see when spinning a rear wheel in a stand, since a freewheeling ratchet will shorten the time it takes for a wheel to spin down. A ratchet that has more teeth and/or extra pawls (and a lower angle of engagement) will generally suffer more drag, especially if it is very loud. This is one of the reasons why road hubs favour ratchets with fewer points of engagement (though another is that it also helps minimise the weight of the hub).
In absolute terms, though, obsessing over the amount of freehub drag is an exercise in marginal gains. The weight and momentum of a rider, even at relatively slow speeds, is more than enough to overcome the amount of drag in any ratchet. Moreover, there are more gains up for grabs by choosing a fast tyre and assuming an aerodynamic position when freewheeling on a long descent than relying on a low-drag freehub for an edge.
The combination of a freehub with some drag and large (and heavy) sprockets can create some auto-rotation when a rider is coasting, resulting in chain slack and extra backlash when the rider resumes pedalling. This is something that can take a rider by surprise, especially at high speeds, and this is where a frictionless freehub (like a roller or sprag clutch, see above) will have something to offer.
Another parameter that influences the price of hubs and wheels
The angle of engagement affects the price of hubs and wheels in the same way as weight, so shoppers should expect to pay more for less. In this regard, a low angle of engagement can be considered a premium feature, but that does not mean all high-end hubs are equipped in this way. Rather, it is a feature that is largely restricted to premium off-road hubs and wheels.
In the case of ratchet-driven hubs, opting for a lower angle of engagement will generally add to the amount of buzzing it will make due to the presence of extra teeth in the mechanism. That extra ticking may not be louder, though, just more frequent. Other factors, such as the stiffness of the pawl springs and the materials used for construction also influence the volume of the freehub, so there is no easy way to anticipate just how loud any freehub will be.
For those that would rather coast like a ninja, a silent freehub is also a premium feature, and quite rare in the current market. This may change if Shimano’s Scylence mechanism trickles down from XTR to lower priced hubs and wheels, but at this stage, that remains to be seen. In addition, there is no indication if the new freehub design will be adopted by Shimano’s road hubs and wheels. That just leaves hubs with sprag/roller clutches from Onyx and True Precision.
Summary and final thoughts
The introduction of cassette freehubs during the ‘80s and ‘90s brought on the demise of screw-on freewheels and ushered in a new era of hub design. Ratchets were incorporated into the body of the rear hub, new designs emerged, and a variety of strategies were employed to save weight, reduce drag, improve serviceability, and/or lower the angle of engagement. That this also produced differences in the sound that the hub could make when a rider was coasting was simply a byproduct of all this development.