Introduction
Hub gears, where all the gear ratios are contained in a hub shell and the bike has a single chainring and rear sprocket, have a few advantages over derailleur gears:
- The chain can be made to run completely straight at all times
- The mechanism is protected from contamination and wear due to dirt
- Maintenance is usually easier because there are fewer external parts to clean
- The mechanism is quieter than a derailleur setup
- Gears can be changed when the bike is stationary – no need to struggle away in a high gear after an unexpected stop
The downsides, and why more bikes don’t use them, balance these out:
- A hub geared setup is virtually always heavier; the small size of the parts means they have to be made solidly, from hard and heavy steel
- There is usually a material efficiency loss, although this is completely negligible for most 3-speed hubs, especially as a derailleur’s chain tensioner also adds a lot of drag
- Wheel removal is slightly more complicated because of the need to disconnect a control cable
- Generally, horizontal dropouts are required on the bike to allow for chain tensioning, and most new bikes have vertical dropouts. A chain tensioner can be used but is clunky and reduces efficiency.
- Hub gears are torque-changing machines and the dropouts must resist torque in all gears except direct drive. Normally this is a modest load and can be done with anti-rotation washers that fit the dropout slots. More ambitious hub gears may need special dropouts or a separate torque arm to the chainstay.
Manufacturers
Historically, the big manufacturers were Sturmey-Archer, SRAM (who took over Sachs of Germany) and Shimano. SRAM stopped supplying hub gears and spares in 2017, leaving just two main suppliers. A niche manufacturer is Rohloff of Germany, who produce one hub gear, the 14-speed Speedhub.
How they work
Hub gears work on the epicyclic principle. An epicyclic gear has a central sun gear attached to the axle and a gear ring (or annulus) with internal teeth around it. Filling the space between the two are a number of planet gears, kept at a fixed spacing by a planet cage. The number of teeth on the planet gears is irrelevant to the ratio provided; their size is determined by the space available. The basic ratio of an epicyclic is (teeth on sun+teeth on annulus)/(teeth on annulus). For the most common 3-speed hub, the sun has 20 teeth and the annulus 60, so the ratio is 1.333.
This ratio is used in different ways by locking different parts of the mechanism to the input drive from the rear sprocket and the output to the hub shell (which then turns the spokes and the rim).
Connecting the input to the planet cage and connecting the output to the gear ring gives a speed increase of 1.333 times the sprocket speed.
Connecting the input to the gear ring and connecting the output to the planet cage gives a speed decrease of 1/1.333 = 0.75 times the sprocket speed.
The input can also be connected directly to the hub shell, giving direct drive. This has the highest efficiency; the epicyclic gears still turn in this arrangement, but losses are extremely small as they are not transmitting power.
The selection of different gears is effected by sliding dog clutches, sprung pawls or roller clutches (the last two of which can be “overrun” when not required). Conventional automatic gearboxes for cars use a similar arrangement, although the locking in those is most often done with brake bands and multi-plate clutches, which allow a more gentle takeup of drive.
Hubs with more than three speeds vary in design but most will use multiple epicyclic stages where drive is cascaded from one to the other or where only one is used at a time, by locking a sun to the axle and allowing the unused suns to rotate freely.
The theoretical efficiency loss of a perfect epicyclic stage is 3%, so a cascade of three stages in a poor hub design would lose 9.3% of the input power – quite detectable by the rider. The shaping of gear teeth is rarely optimal and there are additional frictional losses in the multiple shaft bearings, so losses are greater than this. Nevertheless, a suitably run-in and adjusted Sturmey-Archer AW with oil lubrication approaches the theoretical limit and there is no detectable drag in any gear. Remember that the jockey wheels of a rear derailleur also lose a few per cent efficiency, and a hub gear doesn’t have jockey wheels.
The hubs
Sturmey-Archer AW
The most common hub gear, with a 1.333 ratio. Steel shell. Older UK-made hubs use oil lubrication, which can sometimes make a mess of your rear wheel but is more efficient and saves any need to open the hub up for regular service, since any wear particles generally leak out with the excess oil. Later Taiwan-made hubs use grease lubrication and are generally less efficient and more problematic. A redesign (N.I.G. or “No Intermediate Gear”) adds an extra set of pawls in the driver to prevent a “no drive” position between gears 2 and 3. Unfortunately the pawl actuator tends to rattle or become bent. Grease lubrication means regular service is required, as dirt and wear particles build up inside the hub.
I like the older oil-lubricated AW and it is especially nice to use on a bike with full-size 700c wheels. I don’t like the modern one.
Sturmey-Archer SRF3 (aka Brompton BSR)
Basically a N.I.G. AW in a nice alloy shell, which ironically weighs no less than the old steel shell. I don’t like it much. It gets very sticky with age, and a five year-old hub is like pedalling through treacle if it hasn’t been serviced; a grim job involving a total stripdown, solvent clean and relube with special sticky brown grease. The hub is also less well-sealed against wet weather than an old AW, as it uses a plastic ball retainer under the sprocket instead of the old metal labyrinth seal.
Sturmey-Archer SRC3
An SRF3 with a coaster brake. Better than the SRF3 as the N.I.G. design is done differently. Still needs a regular stripdown and three types of grease when rebuilding (the coaster brake part needs an extremely temperature-resistant grease – I used Ceratec brake grease). I commuted with one of these for many years, with two-yearly rebuilds, and it was entirely satisfactory.
Sturmey-Archer AM
A “medium ratio” cousin of the AW, this went out of production in the 1950s but is a truly marvellous hub. A closer ratio is achieved by using compound (stepped) planets where one end engages the gear ring and the other engages the sun (giving the effect of a much bigger gear ring). It is still a single-stage hub, with excellent efficiency. The ratios are about half as large as for the AW (1.155), so in flattish country you have three very usable gears. The only downsides are that most are in 40-hole shells (sometimes steel, sometimes a very thin alloy) and they use a fiddly two-part indicator rod that tends to unscrew itself in use. No problem: the mechanism screws straight into an AW or SRF3 shell, and you can convert it to use an AW one-piece toggle chain with a few easily-available new parts. In fact, most of an AM can be swapped out for current production AW parts except the epicyclic gears themselves.
Sturmey-Archer FW
The four-speed wide-ratio hub used on most F-frame Moultons. Not made since 1969. There are two suns which can be alternately locked to the axle, and stepped planets to engage whichever sun is in use. Gearing is closer than an AW, and reasonably good for general riding. First gear (or “super-low”, as SA would have it) can be tricky to engage, as you are overcoming a lot of springs to lock the second sun gear to the axle. The shifters tend to wear out in this position due to the high cable tension rounding off the 1st gear detent. They can be sharpened up on a grinder, if you have the time and patience. I’m not keen on the FW: it’s a clever design but needs a lot of mechanical sympathy to use, and spares are very scarce now.
Sturmey-Archer ASC
A 3-speed fixed gear. Never sold in huge numbers, and if you ride one, you’ll see why. In theory a fixie that makes getting up and down hills easier is a great idea, but changing gear requires your legs to instantaneously change speed too! To make the hub work, there are no-drive positions between all gears, and significant backlash (play) when the hub is in a gear. This feels like riding with a very slack chain. First gear requires huge cable tension (an ASC is a hack of an FW) and is prone to slipping out of gear, just as you are straining to get up a hill. It sells for big money and the unique ASC shifter sells for even more than the hub, but it’s best avoided. If you must try one and can’t find a shifter, you can modify an FW shifter to work by grinding the 1st gear position deeper on the pawl plate. It’s a fiddly job and yes, I have done it. The best thing about the ASC is that it’s better than the…
Sturmey-Archer S3X
SEX – get it? A modern version of the ASC. Looks nice, has a choice of nice shifters (which don’t work with an old ASC),is otherwise not great. There is even more backlash than in the ASC, it is poorly-sealed, there are huge no-drive positions, it’s inefficient and noisy in 1st and 2nd gears, and it’s just as prone to slip out of 1st gear as is the ASC. The design is also poor, with the sun gears being locked by very small dogs on a sliding axle key. Mine looked worn after 200 miles. The best thing about it is that it is unusually easy to strip down.
Shimano Nexus 8
An 8-speed gear aimed at utility bikes. Usually comes with a roller brake, which is a kind of drum brake that runs in grease. I quite like it. The range is good enough for most terrain and the ratios are reasonably close. It is very, very heavy and there is a detectable loss of efficiency in some gears. Taking the rear wheel out is an odyssey best not attempted at the roadside; get Marathon Plus tyres to avoid punctures. The main disadvantage of these is that they run in grease and no-one can overhaul one fully due to its complexity. Even a Shimano dealer will just dunk the internals in a refresher oil, which you could do yourself. They are fairly cheap so you could do the same dunking operation yourself (the internal unit is easy enough to remove) and buy a new one if anything goes wrong.
There are various models available. Some have more ball or needle bearings inside for a small* efficiency increase, and are worth seeking out.
Sachs/SRAM Spectro T3
A jewel-like little hub fitted to Bromptons in the mid-2000s while Sturmey-Archer was moving to Taiwan. Ratios are slightly wider than an AW/SRF3/BSR, but not by much. Always greased, but the original SRAM grease for them is no longer made. SA grease would be OK. The hub is a bit draggy due to tiny gears but it is fairly easy to rebuild. Sealing is nowhere near as good as on SA hubs. The main problem with these now is parts availability – most of the remaining spare parts stock is in Germany, and the retailers with stock won’t ship to the UK since Brexit.
Sachs Duomatic R102
A two-speed hub where a slight kickback on the pedals swaps from one ratio to the other, and pedalling back further engages a coaster brake. Reliable and beautifully made, these were used on the Moulton Stowaway. Otherwise, it has the same pros and cons as the Spectro T3 hub. These, peculiarly, use two types of grease AND oil lubrication. I imagine it all mixes together in the end!
Sachs Duomatic R2110
A redesign of the R102. I have rebuilt one of these but not actually ridden it – sorry! It has plastic components and is not as robust. You won’t see many around as they weren’t made in huge numbers.
*Sturmey-Archer tried ball bearings for the planet gears about 100 years ago and concluded that they made no real difference. However, that was a simple 3-speed running in oil. A modern hub gear with grease lubrication and multiple stages is likely to benefit more.
Moultons and other endangered species 