· By Austin Duggan

All about hub engagement

Hubs with high points of engagement have a distinctive sound. But the noise doesn't tell you all that much. What are points of engagement, and why do they matter? To get the full story, we need to distinguish between engagement points, phases, and degrees. 

What is "engagement", anyway?

When you pedal the cranks of your bicycle and the rear wheel spins, the hub of that wheel is engaged. When the cranks are at rest but the rear wheel continues to spin, your hub is disengaged, and the wheel is spinning "free" from its connection to the cranks. 

This capacity to engage and disengage at will requires a mechanism either internal to the hub, or attached to it. The name "free wheel" is reserved for one of the first mechanisms to allow a rider to coast without pedaling, invented in 1869. But there are all sorts of other engagement/disengagement mechanisms on the market; freehubs, sprags, star ratchets, etc. In any of these mechanisms, the "points of engagement" refer to one of the following:

Individual points of engagement - the total number of points during the 360° free rotation of a hub that the hub engagement mechanism(s) can engage.
Grouped points of engagement - the total number of times during the 360° free rotation of a hub that a group or groups of the hub engagement mechanism(s) can engage.
Unfortunately, hub manufacturers rarely distinguish when advertising "points of engagement" between whether they're referencing individual points or grouped points. As we'll see below, this distinction makes a big difference.

How to count engagement in freehubs

By far, the most common engagement mechanism on the market is a freehub. So that's what we'll focus on for the rest of this article. Every freehub consists of a ratchet ring with teeth, and a cassette or driver that has pawls and springs. To determine the individual or grouped points of engagement in a freehub, just use the following equations:
Individual P.O.E. = Ratchet teeth x individual number of pawls
Grouped P.O.E. = Ratchet teeth x groups of pawls
The same equation applies to freewheels. The calculation for individual p.o.e. is described in this video:

Phases of engagement and degrees of engagement

The pawls of a freehub (or freewheel) can engage all at the same time as a single group, or groups of pawls can instead engage in a sequence. This is what determines phases of engagagement. 

Phase of engagement - a group of hub engagement mechanisms (e.g. pawls) that engage at the same time.

A hub that has pawls that all engage at the same time as a single group has one phase of engagement. A hub that has two groups of pawls, each of which engage at a different time has two phases of engagement, and so on. You can see this demonstrated in the next video. Phases of engagement are important because they decrease the slack that you sometimes feel when you try to pedal your bike after coasting for a while, a.k.a. "degree of engagement".

Degree of engagement (a.k.a "engagement angle") the maximum degree of slack that exists from one point of engagement to the next.

To determine the degree of engagement, you divide 360° by the number of ratchet teeth, and then divide that by the phases of engagement. 

D.O.E. = 360° ÷ ratchet teeth ÷ engagement phases

For example, in the following video, there are two hubs, each of which have 4 pawls and 24 teeth. That means that each hub has 96 individual points of engagement. However, the first hub only has one engagement phase. That means the first hub has up to 15° of slack (360° ÷ 24 ÷ 1 = 15°). By contrast, the second hub has two engagement phases. That cuts the degree of engagement down to 7.5° (360 ÷ 24 ÷ 2 = 7.5°). 

Do points of engagement matter?

All else being equal, a hub that has a higher degree of engagement will be more responsive when beginning to pedal after a period of rest, and a hub that has fewer engagement phases with a higher number of pawls will be able to handle more torque. So, a hub with higher points of engagement will be more responsive, or able to handle more torque. Each are good things on a bicycle. 

Responsiveness is especially important. It's jarring to have too much slack in the hub engagement when you begin to crank. With no resistance, your weight falls on the cranks in a way that can make you slip a pedal or lose your balance when the hub finally engages. Also, the torque that you transfer with your weight falling against the cranks in this way hits the engagement mechanism much more aggressively. It's akin to popping a clutch while revving the engine in a car, and can sometimes lead to the hub skipping an engagement or two.

Granted, there can be a bit of a tradeoff when designing a hub between torque capacity and responsiveness. Fortunately, hub designers are able to compensate for torque loss when they increase the degree of engagement. One method is to make the pawls bigger - you can see this when comparing the one phase hub to the two phase hub in the video above. Another is to use higher strength materials with tight tolerances.

Are there drawbacks to high engagement freehubs?

Sure. But they aren't that severe. Increasing the points of engagement in a hub tends to increase the drag when coasting, the overall cost of the hub, and also high engagement hubs tend to be really noisy. However, the increased drag and cost for higher engagement hubs isn't necessarily all that much. Also, most people seem to like noisy hubs. It's certainly a good way to keep your presence known to pedestrians and other bikers. So, all things considered, aiming for a higher engagement hub is best for most riders. 

How high can you go?

Pretty high! The new Chimera hub (the one on the right in the very first video) has 36 ratchet teeth and six pawls with 2 phases. That gives us 216 individual points of engagement (or 72 group engagements) and only 5° of slack with plenty of torque capacity. You can check it out here:

If your design and machining is good enough, you can go even higher. The Industry Nine Hydra hub has a 115 tooth ratchet and six pawls. That's 690 points of engagement!

Note, though, that this entire discussion has centered on the most common hub design: the freehub. There are also hubs on the market that instead utilize a sprag. Sprags have nearly instant engagement with far less drag than freehubs. That's why the we chose sprags for the Chimera clutch and transmission system. But that discussion merits a post all by itself. 

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