The rear hub is the most common location for gears on a bicycle. Hubs which accommodate gears, may have gears internally, however more common is external gears. Those that do have external gears will have the gears mounted in one of two ways:
- Via a screw on freewheel
- Via a freehub body
The first is very popular globally and often overlooked in the UK where it is less common. It is the cheaper system of the two and it locates the gears and freewheeling mechanism together on a unit that is entirely removable from the wheel. The wheel itself simply comes with a thread and the freewheel just screws on, if the sprockets are worn or the freewheeling mechanism becomes defective, that whole unit requires replacement.
The freehub in the UK is the more popular design. It comes with a number of benefits:
- It moves the bearing under greatest load outboard and closer to the frame dropout, increasing stiffness and strength.
- It allows the use of additional load bearing bearings – most commonly four in a cartridge bearing based rear hub – conversely a freewheel based rear hub normally has only two bearings. So the freehub system is much stronger – especially on longer OLNs – historically the freewheel system was based on 126mm spacing, it becomes more problematic on 130mm+ as the lever on the axle from the bearing loading point to the frame becomes extended.
- The freehub itself should be straightforward to remove and service itself. It can be replaced as an independent part. However some freehubs are very difficult to service.
- It allows a broader range of gear. Freewheels are rarely used with more than 7 sprockets and will commonly only offer a range of for example 14-28 and sprockets below 13t are extremely rare and require a design change. In contrast a cassette can provide a range of up to 10-51.
- Freehubs allow for more gears – the largest currently is 13 – although this is rare. However, 12 speed is relatively common and 8 would be considered unusually low, where freewheels are rarely seen beyond 8 speed.
Of the freehub bodies out there, there are a number of different types, different types have different benefits but mainly are simply a matter of compatibility with the appropriate groupset you run on your bicycle. Commonly if you have a groupset, that groupset will have a cassette and that cassette will require a specific freehub body – you have to match the two up, some wheels can have their freehub bodies changed, sometimes there are compatibility issues where a hub cannot be run with a certain groupset. For example Shimano hubs can never be run with Campagnolo groupsets.
DCR hubs can be run with all the main freehub types here but not the outliers. So let’s go through the main ones first:
Shimano HG:
There are three versions of this freehub body. It is HG for hyperglide which was an innovation Shimano made to improve shifting. There is one thick and one thick spline on this to ensure specific cassette sprocket alignment – sprocket 1 for example would have specific teeth shaping to encourage easy chain transition to sprocket 2, if the two were misaligned this would cease to function properly. If the sprockets were independent of one another (which they often are), the shape of the freehub is there to ensure they are installed in the correct position.
The splines themselves are shallow and the shell of the freehub fairly thin. The shallow splines here have historically caused some issues with ‘bite’. Bite is where the freehub is marked, sometimes badly by the cassette, the cassette alignment is compromised and can become difficult to remove and reinstall. Cassette lockrings should be done up to 40nm of torque and ideally would be on a spider design. However lockrings are regularly fitted with less than this and more economical cassettes tend to be made of steel which bites freehubs more.
Some companies for this reason choose to make the Shimano freehub out of steel which is heavier but resists bite. Some make it out of titanium which is heavier than aluminium but lighter than steel but more expensive than both. Some choose to make it out of aluminium which is the lightest option but is prone to bite. For those companies that make freehubs out of aluminium, some of them use an anti-bite system of some form to cut back on this bite. DCR HG freehubs are alloy with anti-bite systems, the heaviest duty hubs can have a steel freehub instead which also comes with an additional bearing.
The HG splining length has been extended twice over the years. It’s first incarnation was as a 7 speed freehub body. That was made longer to allow 8 speed cassettes to be fitted. These freehubs are still in use today in the MTB arena as even Shimano 11s MTB cassettes fit on this splining. However, on the road when Shimano went over to 11 speed they made the freehub 1.8mm longer still. The use of spacers allows reverse compatibility. All DCR hubs are Shimano road 11s compatible and come with a spacer for running 8/9/10 speed cassettes. A 7 speed cassette is still compatible with these freehubs with additional spacers. When fitting a spacer, fit it first with the cassette second. This allows the final sprocket of the cassette to still correctly lock with the serrations on the lockring.
Fit spacers are generally supplied with cassettes, these are far thinner spacers and are there to compensate for potential disparities in cassette sizes. If your lockring can be tightened up appropriately and secure the cassette well without these spacers, it is better to omit them to allow the maximum thread contact between lockring and freehub. However sometimes the lockring can be fully tightened and the cassette appears still to be slightly loose, if this is the case, a fit spacer is appropriate. All spacers should be installed before the cassette to allow the lockring to tighten directly against the cassette.
Campagnolo:
The Campagnolo freehub has historically always been made out of aluminium – although some companies make them out of steel. The freehub itself is thicker, with deeper splines, which means they are not prone to bite. However, it does restrict the inner diameter of the freehub. For this reason, bearing size is internally restricted. Chris King, for this reason, actually make aluminium Shimano freehubs but a thinner steel freehub for Campagnolo to allow larger bearings internally.
SRAM XD:
Both Campagnolo and Shimano HG freehub bodies provide a restriction in minimum sprocket size of 11t. On a traditional road cassette of, for example, 11-28 that was a perfectly acceptable solution. On a traditional MTB setup, historically you had a triple at the front, that was then changed in favour of a double for an assortment of reasons (simplicity, weight, function) and further still down to a 1x (one-by) setup – with just one chainring at the front. That 1x setup requires more range from the cassette. Cassettes are now being run up to 51t at the back.
Given the increasing pressure for range at the back, SRAM sought for breadth not just at the top of the cassette but at the bottom too. For if your largest sprocket goes from 46t-50t, you’ve gained 4 additional teeth but only just over 10 percent gearing increment. In contrast, if you drop down from 11t to 10t, you have only lost a single tooth but your gear has increased by around 9 percent. Ultimately, the range is limited by the axle and the need for a bearing between the freehub and the axle, however, SRAM’s XD driver placed the final sprocket beyond the end of the freehub and allows the use of a workable axle/bearing/freehub interface. The threads on the design are towards the back of the freehub so the bearing stance can also be maintained, unlike on the Shimano and Campagnolo freehubs where the threads require the bearing to be set behind them. There is also no problem with cassette bite because the whole cassette sits on an alloy spider which drops on at the back of the freehub.
SRAM XDR:
Just as Shimano needed a longer freehub to allow for longer road cassettes, SRAM sought to make the XD freehub longer for its application on the road. Just as with Shimano 11s freehub bodies you can retro fit them with shorter cassettes by fitting a spacer before the cassette. So you can run a SRAM mtb cassette on an XDR freehub by using a 1.85mm spacer.
Shimano Micro Spline:
This freehub has been the cause of some controversy. A couple of issues presented themselves with micro spline. First of all, why has Shimano invented another freehub when so many already exist on the market and why are they being so restrictive about licences for the freehub?
The micro spline freehub is a freehub designed to achieve similar results to the SRAM XD and XDR freehubs. Shimano were aware that to compete on range for their mountain bike groupsets they needed to run down to 10t. They also turned the volume on their cassette up one notch higher by going 10-51 rather than SRAM’s humble 10-50t, so their range is now 2% greater than SRAM’s. Once again they could claim the gear range crown.
Shimano say that using the XD(R) interface was restrictive. The splines on the XD(R) freehubs sit right at the back, requiring a long alloy spider and an expensive cassette to run. In contrast, the splines on the micro spline freehub run the whole length of the freehub, allowing the cassette to be made from three separate pieces and therefore could be more economical to produce and easier to roll out throughout the Shimano range – rather than only being applicable on the higher end groupsets. However, a drawback from this system is the micro spline freehub needed to be shorter to run down to 10t, by running the splines for the whole length and retaining the lockring threads internally at the front of the freehub, the bearing stance had to be restricted. So the SRAM XD(R) bodies allows a greater bearing stance and will building into a stiffer and stronger wheel.
Licencing was frustrating as Shimano initially only opened this up to their own hub and DT Swiss, however more licences soon followed and from 1st January 2020 the licence is open. For this reason, micro spline freehubs from a broad range of manufacturers are readily available. DCR hubs will all be micro spline compatible from 1st January 2020.
Rarer outliers
Shimano UG:
This predated Shimano’s HG setup, it has a threaded end and universally spaced splines as cassette alignment wasn’t critical. There is no advantage to this freehub, it is a historical legacy and if you wish to run a uniglide cassette you must have a uniglide freehub. Shimano did offer a temporary interim solution with UG/HG compatible freehub which had internal threading for HG cassette and the external threading required to fit a UG cassette. A UG cassette was fitted by rotating the final sprocket around a thread – no additional lockring was required.
Campagnolo 8 speed Gen 1 freehub:
This is Campagnolo’s first attempt at a freehub and it didn’t last long. It was never run beyond 8 speed and 8 speed cassettes can be purchased that are compatible with the modern Campagnolo freehub body, making the original redundant even if you do wish to run 8 speed Campagnolo. The splining on this is very similar to Shimano HG but without the thick and thin spline because there was no hyperglide style technology deployed in these cassettes.
Short block cassettes:
These have some modest popularity, these are essentially a small HG cassette. They can be used for single speed systems or for 6 speed cassettes. Their advantage (beyond novelty) is they can allow for wider hub flange spacing, making stiffer, stronger and more comfortable wheels. They can also allow for a very lightweight groupset as a 1x 6 speed cassette based system is very light.