We will explore what the hub’s engagement is, its function and qualities. Things that we are, as riders, care the most. What POE is and what it means in the real life. As it is often used when discussing various hub engagement designs.
Table of Contents
1. Hub engagement explained
Majority of the bicycles come equipped with an engagement system. It is almost always an integral part of the rear wheel. To be precise – of its rear hub. There are some exceptions – like track, or fixed gear hubs and trial bikes. The latter uses the engagement system not as part of the hub, but of the bicycle frame. The system there is an integral part of a bottom bracket unit.
The engagement system’s purpose is to briefly disconnect the drivetrain from the wheel. That is when a rider stops pedaling, the wheel continues to roll. This mode is also known as “coasting” or “freewheeling”. It enables a one-way power transmission from the cranks to the wheel. One can call it a one-way “clutch”. The one-way means that the rotation motion always goes from the cranks to the wheel. And never the other way around. All engagement systems seamlessly switch between engaged and disengaged mode. It disengages when the speed of rotation or direction of the wheel and of the drive unit do not match. It is either the wheel rotates faster than the cranks and so the drive unit it. Or when they rotate in an opposite direction.
The drive unit is often referred to as freehub, rotor or just driver. It mounts the sprocket or cassette and receives the rotational motion from the cranks.
2. Difference between freehub design and engagement design
Freehub’s design is not the same as the engagement design. Indeed, these two have different meaning.
One of freehub’s functions is support of cassette or sprocket attachment. Most of the time, it is about the support of a drivetrain standard. These are proprietary manufacturer standards. Such as Shimano’s HG and Micro Spline. SRAM’s XD & XDR. Or Campagnolo’s N3W and Classic. That is what freehub design is about.
Yet the hub engagement design deals with the power transfer from the cranks to the wheel. These two designs merge within in a single part – the freehub. Still, we distinguish the two. Usually, the engagement system comprises of other hub’s components. And not only of the freehub as such. Except for threaded freewheels.
3. Hub engagement design qualities
What are qualities of the hub’s engagement? For most of us that would be efficiency, reliability, and weight. All three are interdependent, and the gain in one aspect often comes with the trade-off in another. Let us review them one by one.
How quickly the hub engages? This is time, or distance that crank’s arm travels before it pulls the wheel in motion with it. It is about efficiency of the system. The longer it takes for the system to switch into the engaged mode – the less efficient it is. And thus, more of the rider’s power diverts. As it does not contribute to the primary purpose – rotating the wheel.
Another point of efficiency of the system is what are the internal loses, such as losses to friction. For road bikes the internal losses are a significant factor. Whilst the speed of the engagement is less important, compared with MTB. An off-road terrain demands frequent changes between gears and between pedaling and freewheeling.
4. Hub’s POE – what is it?
One way to measure how quickly system engages is to relate it with the angle of the circle. This is the angle that cranks travels before the wheel follows its motion. It is not a fixed and precise measure, though. Gear ratios affect this angle in a broad range. Hub manufacturers state a parameter, often named as Points of Engagements or POE. The POE number gives the measure in degrees of an angle with the simple formula. It equals to 360° divided by the POE number. For example, Chris King’s R45 road hub has POE of 45. This gives the theoretical 8° degrees of an angle for the engagement speed.
On practice it will differ as the gear ratios come into play. Thus, there is an effective degree that a rider experience. That is what we “feel” on the cranks. Here are two further examples with R45 hub.
A road gear ratio with 50T chainring and 11T sprocket. The effective degree equals to the theoretical 8° divided by 4.54. The latter is the gear ratio of 50 divided by 11. This gives as the effective degree of 1.76.
Now, let us take 28T chainring and 45T sprocket. This gives the gear ratio of 0.62. Common to MTB. The effective degree becomes 12.86. Quite a difference from 1.76, is it not?
5. POE in real life
To compare these two numbers, we need to add a perspective by adding another hub. The one with POE of 18. Which translates into the theoretical 20 degrees. Using the same gear ratio of 0.62, the effective angle becomes 32.12 degrees. About three times larger. And about the same proportion to POE of 45 vs POE of 18.
What is we convert it into the distance that the average 175 mm crank’s arm travels? The related distance is 56 mm. This is large enough to notice. It will feel very different from the mere 22 mm of the system with Chris King’s R45 hub.
These examples show that higher POE hubs make a big difference when the gear ratio is less than 1. Such us when taking on the steep off-road climbs.
These are the theoretical aspects of hub’s engagement. Its function, the meaning and implication of POE. They become helpful when evaluating various existing designs. Among them are the ratchet-and-pawl, the “Star Ratchet” and its derivatives and the famous “Ring Drive”. And of course, our own Vortex system.
Moreover, there are two more qualities of the engagement design: reliability and weight. Read on.