Hub Engagement Designs – Part 2

In Part 1 we covered some of the engagement mechanisms theory. Whilst these concepts are not too excited to dive into, they are important to understand the practical implementations of various designs that we cover below.

In the classical pawl and toothed ring design the rotational force is transferred with the help of several pawls. In general, three to six pawls are used. Each one carries a substantial part of the total load. But there are other designs, where the distribution of the rotational force is more balanced. The two widely known are of DT Swiss and Chris King. In mid-90’s DT Swiss acquired a technology and since then made it one of the famous designs known as the “Star Ratchet”. It is available in several variants for road and MTB, ranging from 18 to 54 teeth. The main difference compared with the pawls, is that the system has two parts, often referred as rings.

Each ring has the same number of teeth, 54 in case of the top-level DT Star Ratchet. Because of large number of identical teeth on the both rings, the load is distributed evenly between them. Each tooth deals with relatively small load, opposite to that one the pawls. The point of difference of the ratchet-based designs is that the number of teeth on both rings, or plates, is exactly the same and that number is substantially higher than the number of pawls of the classic design.

The time proved it is a very reliably design. There are number of manufacturers with the engagement system inspired by DT’s Star Ratchet – Absolute Black, ACSE, ACROS, Erase, Extralite, Mavic, Soul Kozak and Syntace.

Chris King’s “Ring Drive” system went even further, driving up the number of teeth to 72 on both rings. Reaching theoretical 5 degrees of the engagement. It has a very interesting feature that allows it to operate reliably even with so many teeth. This feature is known as the “Helical Splines” and it provides an additional force that keeps both rings locked together when the system is engaged. Simply put – the higher the load, the stronger the system engages. Chris King inspired two other recent designs – Trailmech Vortex and Shimano Scylence. The first design also uses the concept of the stronger engagement under higher load. It is achieved with the help of the toothed rings, having an unusual conical shape and the specific teeth profile. There are several versions, with number of teeth ranging from 50 to 60, for theoretical 7.2 to 6 degrees of the engagement, respectively.

Pictured below are ratchet rings of DT Swiss, Chris King and Trailmech (left to right).

The Scylence extends the ratchet-based system design to employ the helical splines to retract the toothed rings in the disengaged mode. Shimano states that it has theoretical 7.6 degrees of the engagement. The difference it’s near soundless operations whilst coasting. Whilst many of the quick to engage hubs feature distinctive sound in the coasting mode. Chris King even dubbed a specific name to refer to that sound – “The Angry Bee”.

At the same time complete soundless is not something new in the engagement designs. These designs were around for years. For example, long ago discontinued Shimano’s Nexave – also known as the “Silent Clutch”, True Precision Components Stealth MTB hubs or Zipp’s hubs with Axial Clutch for road. Zipp’s system also has two ring plates. Although only one of them has teeth, whilst the other features special openings to mesh with the teeth when engaged. Zipp’s Axial Clutch has 10 degrees of the engagement.

The more recent entrant to soundless hubs space is Onyx Racing Hubs. Whilst Zipp’s Axial Clutch, Shimano’s Scylence and Onyx are soundless, or near soundless in case of Scylence, they are completely different designs. Onyx contrary to the Zipp and Shimano is, in fact, similar with one of True Precision. It has no teeth whatsoever and uses friction forces to lock the system in the engaged mode. Sometimes these systems are called one-way bearings. Onyx refers to it as the “Sprag Clutch”. What it also a distinctive feature of the friction-based designs found in True Precision and Onyx products, is that they have near 0 degrees of engagement. They truly engage instantly. Whilst 0 is the theoretical figure, for all practical purposes they are quicker to engage that any other engagement designs out there.

In addition to the above, reliability and weight, two other primary factors that influenced the evolution of various designs. If not for them, chances are that we still be pretty much OK with the proven pawls. With aluminum being the primary material used in majority of road and MTB hubs, often the pawls seats experience fatigue, which may lead to the freehub failure. Though, the notion of the load level, that hub deals with is application specific. For the road the drivetrain gearing option and ranges remain relatively stable. At the same time for MTB with the swift shift forwards 1x drivetrains and now with e-MTBs and the ever-increasing ranges of cassettes, the level of forces, under which the engagement system must operate reliably has grown.

When steel or titanium are used for freehub equipped with pawls, the reliability is better, but the weight must to give in. Ratchet-based systems distribute the power load internally in a more balanced way. Roughly, the load that each individual tooth carries in a ratchet-based system, is 10 to 20 times less, then for each pawl tooth, provided that applied force remains the same. It is because there are so much more teeth, engaged at the same time. These systems, typically, exhibit longer operating life, whilst remaining light enough for even the most demanding cycling disciplines.

Friction-based system design, due to the nature of how it transfers the rotational force, excludes pretty much all materials but steel for the core of the mechanism. There is a noticeable weight gap between hubs using it, and those, that are built around other designs.

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