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Freehub and freehub body: function, compatibilities and conversion

How does the freehub on a bike work? What should you pay attention to when converting shifters &derailleurs, which freehub is compatible with which models?

The freehub is nowadays (almost) unthinkable without the bicycle, as it makes life easier for many bikers. Some also appreciate it for its individual sound, which in some cases can even replace the bicycle bell. Incidentally, the bicycle freehub, as we know it to this day, was invented by Ernst Sachs in 1889 in one of the German bicycle metropolises: Schweinfurt. We explain here how the freehub works on your bike, what you should pay attention to when converting your gears and which freehub is compatible with which gear.

The freehub: Functional principle and application

A freehub is a clutch that acts in only one direction of rotation. On a bike, it decouples the chain and crank from the engaged rear wheel, so the crank arms don't keep spinning when you roll without pedalling. The rear wheel is therefore able to overtake the stationary crank in speed, which is why freewheels are somewhat old-fashionedly called "overrunning clutches".
Of course there are also bikes without freehubs. They're called fixies, and they're not exclusive to urban bike messengers. Track cycling is also mostly fix-geared. Fixies have a rigid connection between the rear wheel hub and crankset. As long as the rear wheel is turning, the cranks are also turning. As a rider, this means you have to counter with your legs, also known as "holding against." This requires strength and coordination and increases braking distances. It's almost un-ridable off-road and with a suspension bike, and at the very least challenging in everyday riding and when using the bike lane.
Therefore, the freehub is a pretty ingenious invention for us bikers. As with all innovations, there was wild criticism in the early days of using freehubs--that it would lead to accidents because people would forget how to ride a bike properly, that you would lose control of the bike--but those voices have been subdued for about 100 years.

A central element on the bike: You can't see much of the freehub

A central element on the bike: You can't see much of the freehub

Feel it work every meter that you do not pedal while riding.

Feel it work every meter that you do not pedal while riding.

Pawl vs. Ratchet

Freehubs on bicycles are usually designed either with so-called pawls or as toothed disc freehubs. In a pawl freehub, form fit is produced with the help of coil springs. Two or more spring supports (pawls) are installed counter-clockwise in the freehub. When you pedal, they “lock” and the wheel turns. If you do not pedal, the pawls do not lock and the wheel runs freely. There are different operating principles as to how many pawls a freehub has, or how many of them can engage at the same time. The finer the ratchet of the pawl engagement, the smaller the engagement or disengagement angle. The idle travel of the crank decreases, and after rolling you have traction almost immediately again. This is especially important for time trial, mountain and gravel bikers. On a road or triathlon bike, where you are aiming for constant pedal rotation anyway, this is a less decisive factor. In this instance, fewer but larger pawls offer higher robustness and resistance against your leg force.
The second important design principle is the ratchet drive. DT Swiss and Shimano rely on ratchet drive in particular, as it promises low maintenance and a high degree of mechanical resilience. Following the expiration of a DT-Swiss patent in 2019, more and more ratchet designs are now coming onto the market. Here, two toothed pulleys are able to mesh with each other during pedalling. Unlike ratchet freewheels, toothed pulleys engage all teeth simultaneously. This ensures a very large contact area and makes freehubs very robust even with high force inputs. Small release angles, however, are not often possible.

There are two basic design principles for freewheeling: There is the pawl freehub.

There are two basic design principles for freewheeling: There is the pawl freehub.

And there's the ratchet freehub.

And there's the ratchet freehub.

With the pawl freehub, small, spring-loaded pawls provide the frictional connection or freewheel.

With the pawl freehub, small, spring-loaded pawls provide the frictional connection or freewheel.

These pawls engage in a grid inside the hub and thus transfer the pedalling force to the wheel.

These pawls engage in a grid inside the hub and thus transfer the pedalling force to the wheel.

With ratchet freehubs, two ratchets mesh with each other.

With ratchet freehubs, two ratchets mesh with each other.

In the case of ratchet freehubs, the contact surface is significantly larger and the freehub is very robust even with a large amount of input force.

In the case of ratchet freehubs, the contact surface is significantly larger and the freehub is very robust even with a large amount of input force.

During conversion, the freehub body counts: How to find the right freehub

While choosing the right freehub is important when buying a new bike, a rear hub or a system wheelset, the conversion, upgrade or tuning of the freehub body (also known as "driver body" or also "rotor") plays the main role. This is because the actual freehub is now structurally part of the rear hub, and well-protected from dirt on the inside. In the past, the freehub was in the sprocket set - also called a freewheel. The modern freehub body sits on top of the freehub and makes a multi-tooth connection to the cassette, where your leg power is transferred via the chain. If you want to tune or rebuild your drivetrain, you need to pay special attention to the freehub body. Many hubs are compatible with multiple mounting standards (see our table below) via different freehub bodies. However, there is not a suitable freehub body for every hub standard that currently exists. To put it another way: we'll probably have to wait a little longer for a Shimano Dura Ace rear hub with a freehub body for an XD-compatible cassette from SRAM.

Freehub body and cassette: What you have to pay attention to during conversion

Your "contact" for the right freehub body is the hub manufacturer. Most offer conversion kits for different cassette mounting standards, or drivetrain manufacturers. If you convert your drivetrain within the product range of the same manufacturer, for example from Shimano ten- to eleven-speed or from SRAM eleven- to twelve-speed, you will usually not have to replace the freehub body. The decisive factor, however, is the freehub compatibility of the desired cassette. This is why you can filter all cassettes in our shop by "freehub compatibility".

In short: You are looking for a freehub body that a) fits your rear hub and b) is suitable for the desired cassette.

When converting, you must ensure that the freehub body and cassette fit together.

When converting, you must ensure that the freehub body and cassette fit together. © bc GmbH & Co. KG

Conversion: best without tools

How exactly the freehub body is converted varies between hub manufacturers. Many manufacturers like Hope, DT Swiss, SRAM or Shimano allow a tool-free conversion. You can find out ehether this is also possible for your hub in the operating manual or from our service team.

Materials: aluminium, steel, titanium

Freehub bodies are offered in various materials. Aluminium freehubs, for example, are quite light, but when used with (cassettes made of) single sprockets, they are susceptible to dents from kicks. With many cassettes, therefore, almost all sprockets sit on a carrier with full freehub width. Steel freehubs are more robust, but heavier. High-quality hubs sometimes come with freehub bodies made of titanium, which combine low weight and robustness.

3 materials are used for the freehub body (from left to right): aluminium, titanium or steel.

3 materials are used for the freehub body (from left to right): aluminium, titanium or steel. © bc GmbH

Overview: The most common mounting standards

To help you keep perspective while shopping, we have briefly summarised and explained the most common mounting standards and cassette compatibilities. If you are in a hurry, jump directly to our overview table.

Shimano (Hyperglide) MTB

The Shimano Hyperglide MTB standard is suitable for 8- / 9- / 10- and 11-speed mountain bike cassettes from Shimano and various third-party manufacturers, where the smallest sprocket is eleven teeth or larger (including SRAM, but not SRAM XD or XDR).
Attention: Up to max. 10-speed versions can also be used with Shimano road-compatible cassettes.

Shimano (Hyperglide) Road

The Shimano Hyperglide Road standard is suitable for 8- / 9- / 10- and 11-speed road cassettes from Shimano and various third-party manufacturers (including SRAM, but not SRAM XD or XDR) where the smallest sprocket is eleven teeth or larger. Spacers are required for the use of Shimano 8- to 10-speed road or 11-speed MTB cassettes.
Attention: Up to max. 10-speed versions can also be used with Shimano road-compatible cassettes.

Shimano Micro Spline

With a shift to 12-speed MTB drivetrains, Shimano has reintroduced the Micro Spline standard. It allows the smallest ten-tooth sprocket to be used, and is compatible with 12-speed MTB cassettes from Shimano as well as some third-party suppliers. Shimano licenses the use of Micro Spline technology to other rear hub or cassette manufacturers. This process ensures consistently high quality, but results in a relatively low number of options during the introductory phase of new technology.
Attention: Cannot be used with SRAM XD and XDR-compatible cassettes.

SRAM XD

The XD standard was brought on by SRAM with the introduction of the 11-speed MTB drivetrains, which, as an open standard, allow the use of cassettes with ten-tooth sprockets or smaller (e.g. e*thirteen nine-tooth sprocket). It is compatible with 11- and 12-speed cassettes from SRAM and various third-party suppliers.
Attention: Cannot be used with Shimano compatible cassettes. Cannot be used with SRAM XDR cassettes (12-speed Road).

SRAM XDR

In contrast to SRAM XD, the XDR standard has a 1.85 mm larger intake. It is designed to use cassettes with a smallest sprocket of ten teeth or less and is compatible with SRAM road 12-speed cassettes. When using a 1.85 mm spacer, it is retroactively compatible with SRAM XD cassettes (see above).
Attention: Cannot be used with Shimano compatible cassettes.

Campagnolo

There are two generations of freehubs from the legendary Italian manufacturer: One for 8-speed, the other for 9- / 10- / 11- and 12-speed road bike cassettes from Campagnolo and various third-party suppliers.
Attention: Cannot be used with Shimano or SRAM-compatible cassettes.

Singlespeed/Fixie

For singlespeed and trial bikes, there are special hubs with narrow freehub bodies, which usually offer enough space to adjust the chainline with spacers. The narrow freehub allows for wider standing hub flanges, which enable the construction of more symmetrical and stiffer wheels. However, it limits the flexibility, because with such rear hubs a subsequent conversion to gears with cassette is not possible. On the other hand, you can convert freehub bodies for Shimano Road, MTB and SRAM (not XD or XDR) with appropriate spacers and singlespeed sprockets to single-speed operation. Singlespeed sprockets have a wide base to protect the freehub body.
On track bikes and fixies, the sprocket is screwed directly onto the hub body and secured with a counter-rotating ring. This prevents the sprocket from unscrewing itself from the hub without a freehub. With high quality track hubs such as those from Miche, the sprocket is placed on a carrier ring, making it easy to change ratios.
Occasionally, there are also singlespeed hubs with threading. For this type of construction, you need a sprocket with integrated freehub, i.e. a freehub pinion.

Table: Overview of compatibilities

What type of free-hub? Which cassettes fit from the profile? Which cassettes fit from the number of sprockets? Is a spacer needed?
Shimano (Hyperglide) MTB Shimano, SRAM (except XD/XDR) and third-party (SunRace, OneUp Components, etc.)  8-/9-/10-speed cassettes (Road and MTB), additionally Shimano 11-speed MTB cassettes  
Shimano (Hyperglide) Road Shimano, SRAM (except XD/XDR) and third-party (Rotor, Miche, etc.) 11-speed Road 8-/9-/10-speed Road require spacers; 11-speed MTB with spacers yes. 1.85 mm spacer for 8-/9-speed cassette; 1.85 AND 1 mm spacer for 10-speed cassette
Shimano 12-speed Micro Spline Shimano 12-speed (so far only MTB)  
SRAM XD SRAM and various third-party suppliers (e*thirteen, KCNC, etc.) 11-/ 12-speed cassette  
SRAM XDR  SRAM 12-speed cassettes (Road) yes. 1.85 mm spacer for SRAM XD cassettes.
Campagnolo 7-/8-speed  Campagnolo 7-/8-speed cassette  
Campagnolo 9-/10-/11-/12-speed Campagnolo Ultra-Drive 9-/10-/11-/12-speed Ultra-Drive cassettes  
Campagnolo N3W Campagnolo N3W 13-speed, with the adapter kit (78166) also fit 11- & 12-speed cassettes from Campagnolo  

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