What freewheel mechanism designs are used in bicycles?

Question

What are the various freehub mechanism designs that have been designed and or put into production? (doesn't need to have been made necessarily!)

A good post will contain:

• A description of the design, ideally with images or technical drawings
• Companies that have used the design or who created it
• Advantages and disadvantages
• Original design/incremental improvements that have occurred

Designs known so far (those with a post are linked):

• Standard Pawl and Ratchet (and variants)
• Star Ratchet (DT Swiss/Chris King)
• Sprag Clutch (Onyx)
• Roller bearing clutch (true precision stealth, shimano silent)
• 'Cam plate design' (American Classic) possibly a combination of pawl/ratchet and star ratchet.

I'll be going through and trying to fill in for designs that I know of, but feel free to post an answer if I haven't already. Also feel free to post an answer for a design not listed here! And provide additional info for already posted designs!

Answer 1

Standard Pawl and Ratchet Design

This design is by far the most common on bicycle freehubs, and as such the least expensive to employ.

Mechanism

In it's simplest form, this design consists of a surface that is toothed and a pawl (which is a lever that engages the toothed surface and only allows movement in one direction).

In a bicycle this design is typically inverted, the pawls are on the freehub body and they engage a toothed surface on the interior wall of the hub.

a basic animation of the mechanism in action can be found here. In this design, some tension must be applied to the pawls to constantly force them outward into the body of the hub. (This is also where the buzz or click comes from in this design, from the pawls being forced over the teeth on the ratchet surface) This is usually accomplished by leaf springs, small coil springs, and/or a metal ring.

this design utilizes leaf springs

coil springs

technical drawing of a mechanism utilizing a metal ring

Advantages and Disadvantages

Advantages

• Common and inexpensive
• Easy to service

Disadvantages

• Lower degree of (quicker) engagement limited by physical constraints. A lower degree of engagement requires smaller teeth contact surfaces and pawls which become weaker/can wear more quickly. (see below for some clever workarounds)
• Prone to failure due to weakening springs or grease thickening with cold/age ("nothing happens when I pedal")
• In it's most basic design, relatively high degree of (slow) engagement
• Prone to wear as teeth and pawls are constantly in contact

Variations

Many designs will use more than one engagement tooth per pawl, allowing smaller teeth and more points of engagement (lower degree of engagement). These small teeth are more prone to wear. Halo supadrive employs 120 points of engagement for 3 degree engagement

Halo Supadrive utilizing 120 points of engagement

SRAM's Double Time design utilizes four pawls in opposed pairs. The pairs are offset slightly, causing only one pair to engage at any time, but gives their 26 tooth ratchet 52 points of engagement for 7 degree engagement.

Answer 2

American Classic Cam Plate Design

The full name is "Six Pawl Cam Actuated Engagement System." This design is one of the more complicated ones, but according to American Classic provides a stronger freehub with relatively low resistance and causes all 6 double tooth pawls to engage simultaneously with high precision.

Mechanism

There are several parts to the engagement of this mechanism, listed in order of actuation from the chain/cog:

• Wire spring in freehub body engages the..
• Cam plate inside hub which engages the..
• Pawls in the hub body which lock the freehub to the hub body

The wire spring rests on the hub side of the freehub body inside a circumferential track:

This wire moves along the teeth of the cam plate as the wheel freewheels. If power is applied to the pedals, this wire forces a clockwise movement of the cam plate

shown with one pawl removed and pawls in 'engaged' position

This clockwise movement forces the pawls to move inward and engage the freehub body, providing engagement.

This video below shows the action of the cam plate, the screwdriver is acting as the wire spring. This is another helpful video in which designer Bill Shook explains the mechanism. At ~0:33 you can see the whole mechanism in action via cutaway.

Once the freehub moves more slowly than the wheel, the freehub body itself moves the pawls back into their retracted state as well as the cam plate.

Advantages and Disadvantages

Advantages

• Low resistance, the only things in contact are the cam plate and wire spring
• As these are the only parts in constant contact, replacement parts are minimal
• Sturdy, as the contact surfaces are relatively large
• Lighter? particularly for it's strength. Part of the design goals was to make the freehub body/contact surface and pawls out of aluminum, although I don't have any numbers here

Disadvantages

• Whether or not this relates to production cost, these are only available on Am Classic wheels and these wheels are generally expensive, so therefore obtaining a wheelset with this design is expensive.
• Relatively low engagement - 24 points means 360/24 = 15 degree engagement

Answer 3

Star Ratchet and variations

This design in it's simplest form is used by DT Swiss. This design incorporates easily replaceable ratchet plates that offer the added benefit of every engagement point transferring torque. A more complicated variation is what is used in Chris King hubs.

Mechanism - DT Swiss

1. end piece 2. bearing 3. thread ring 4. axle 5. conical spring 6. star ratchet 7. rotor body

The DT Swiss system consists of two ratchet opposed ratchet plates that reside in the hub body and the freehub. These are held against each other by a pair of conical springs. When the rider pedals, the teeth engage as the freehub ratchet plate is turned via the splines along its circumference. This transfers power to the ratchet plate in the hub, and this in turn transfers power to the hub via the splines along its circumference.

There is a very basic animation here that shows the action of the ratchet plates.

Upgrading and servicing

One of the biggest advantages of this system is the ease with which you can upgrade, replace, or service the ratchet plates. This is actually a tool-free repair! The most basic DT Swiss hubs include an 18t ratchet plate, offering 20 deg engagement. This can be upgraded to a 36t (10deg) or even a 54t plate (6.67deg). This can be done to lower end DT wheels, so it is possible to take a less expensive wheel and gain a much higher engagement if desired. This also allows replacement of all contact surfaces (aside from the splines within the hub which are unlikely to wear or strip) to prolong the life of the hubs.

Advantages and Disadvantages

Advantages

• All contact points are transferring torque, always
• Easily serviced and wear components easily replaced

Disadvantages

• High contact leading to higher wear between contact surfaces
• The plates are pushed together by a spring, not rider torque, so they are more likely to skip as more torque is applied (contrast with Am Classic and Chris King), although I haven't personally ever heard of this happening.
• Relatively low engagement for cost on many wheels

Variations - Chris King

This is again one of the more complicated freehub mechanism designs. At it's core, it is a star ratchet system. There are a couple key differences:

• Both ratchets are inside the hub, allowing them to be larger
• The drive plate (the one timed to the freehub body) is the one furthest from it
• the drive plate is sprung, while the other is fixed inside the freehub body
• the 'ring drive' mechanism which forces the two plates together under torque

1.spring 2.drive plate 3.fixed plate

left: drive plate, right: fixed plate

What truly sets this apart is the ring drive mechanism. This is accomplished via helical grooves on the freehub body, which engage the drive plate. As torque is applied to the freehub, this forces the two plates together. As more torque is applied, the plates are forced together more strongly, effectively eliminating slipping or skipping.

drive ring shown on top with helical grooves

Many people would probably say that Chris King hubs are overkill - they are extremely well made and make use of many bearings that should contribute to their longevity (and your mechanic's headaches). At 72 points of engagement (5 deg) they can engage about as fast as any hub out there. Also, because the plates are forced together by rider torque, the spring can be weaker are cause less drag/wear and the teeth can be smaller without stripping (faster engagement). The cost/benefit can be hard to justify for many people, but if it makes you feel nice to have a well made hub on your bike, it's hard to do better than Chris King.

Answer 4

Sprag Clutch

The sprag clutch has existed in industrial applications for some time: being used in motorcycles, helicopters, airplanes, automotive transmissions, and others. As far as I know, Onyx is the only company to bring it to bicycle hubs. Sprag clutches offer low rolling resistance and virtually instantaneous engagement. They are also reliable and durable, but this can come at the expense of weight and cost.

Mechanism

At first glance, a sprag clutch can look very similar to a cartridge bearing. The key difference is that instead of round ball bearings, there are cam shaped lobes between the races that allow movement in one direction and bind in the other. These are held in constant tension by a metal clip.

As this type of clutch has many industrial applications, there are a good number of helpful videos explaining how it functions such as this one and this one (skip to 37s to skip marketing).

Onyx incorporates this design in a fairly simple manner. The cam lobes rest between the hub shell and a machined surface that extends inside the hub from the freehub body.

Advantages and Disadvantages

Advantages

• 0 degree engagement (instantaneous)
• able to withstand high torques
• lowest friction of any freewheel system (according to Onyx and a study by Duke university)
• very low maintenance required

Disadvantages

• as always, cost
• weight - as the contact surfaces need to be steel for durability. Recently, they moved to a two part freehub body with an aluminum cog carrier to reduce weight (as pictured above)