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Internal Gear Hub Theory

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by John "Compound Planetary" Allen
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What is this article about?

This article will describe how internal-gear hubs work, starting with three-speed hubs and working up to more complex hubs. The article will go on to describe design trends and make recommendations.

General remarks

Internal-gear hubs have been used on bicycles since before 1900. Four companies build them today. SRAM and Sturmey-Archer have a heritage dating back to the first decade of the 20th century. Shimano has manufactured internal-gear hubs since 1957 and Rohloff is a relatively new company with a technically very advanced hub.

Hub gears are generally somewhat heavier and/or less versatile than derailer gears. On the other hand, hub gears can be more rugged and immune to contamination, and can shift when the bicycle is stopped – of special importance in urban stop-and go riding. Hub gears have offered indexed (detented, “click”) shifting since their first appearance. Some even offer a reverse gear, useful on a tricycle, or fixed (non-freewheeling) gearing, preferred by many cyclists for winter riding and training, or just because they like it.

Hub gears, unlike derailer systems, accommodate available full chain cases, greatly increasing the life of the chain and preventing soiling of clothing.

Shifter mechanisms

Many internal-gear hubs use one or two rods which slide lengthwise inside the hollow axle, each engaging an axle key which protrudes from a slot in the axle inside the hub and moves elements of the hub along the axle lengthwise. The rod is connected to a shifter cable by a pullchain or a bellcrank mechanism at the end of the axle. Sturmey-Archer calls the rod and chain assembly an indicator spindle.

A Sturmey-Archer indicator spindle

Sturmey-archer indicator spindle

Many newer hubs instead route the cable around a pulley inboard of the right dropout. Rotation of a sleeve that encircles the axle engages or retracts a pawl or ratchet, and/or operates a cam that shifts elements lengthwise along the axle. With this rotary shifting mechanism, the cable attachment is better protected.

The Sachs Elan hub used rotary shifting, controlling tilting of shifter rods which encircle the axle, allowing pawls to engage the axle as shown in the photo below, or to slip past.

Sach Elan pawl engaging axle

The traditional control for an internal-gear hub is a trigger assembly with a clamp band which can be fitted to any handlebar. Some hubs use twist-grip controls which fit only flat handlebars; drop bars have a slightly larger diameter. Aftermarket solutions are available for some hubs, and do-it-yourself solutions are possible.

Sturmey-Archer trigger control

Sturmey-archer trigger control

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Planetary gearing

All common hub gears use planetary gearing, which in its simplest form comprises

Cutaway of Sturmey-Archer 3-speed hub gear.

Cutaway of Sturmey-Archer 3-speed hub

Advantages

Simple planetary gearing offers some important advantages over ordinary spur gearing:

Limitations

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Common 3-speed hubs: Sturmey-Archer, SRAM, Shimano

The common three-speed hubs from Sturmey-Archer, SRAM and Shimano have a single-stage, simple planetary system, with the sun gear locked to the axle. Shifter parts direct power through the planetary system in either direction for step-up and step-down ratios, or lock the drive sprocket to the hub shell for the middle, unity ratio.

Both Sturmey-Archer and SRAM offer three-speed hubs with splines to fit a Shimano-standard cassette, for multi-speed hybrid gearing. These hubs eliminate the need for a front derailer and are especially suitable for small-wheel bicycles: the step-up top gear avoids the need for an oversize chainwheel.

SRAM DualDrive 3-speed internal hub gear with multi-speed cassette
SRAM Dualdrive hub

Compound planetary gearing

A planetary system may have stepped (compound) planet gears. A compound planet gear is two or more gears, side by side, made in one piece so they turn at the same speed. Each section has a different number of teeth. One set of teeth engages the ring gear and a sun gear. A different sun gear engages each additional section of the planet gears. Locking one another sun gear to the axle achieves different ratios, possibly including ratios outside the range achievable with simple planetary gearing.

A stepped planet gear
Stepped planet gear

Sturmey-Archer and Sachs/SRAM 5 speeds.

Sturmey-Archer and Sachs/SRAM have made several models of 5-speed hub using single-stage compound planetary gearing. Low, middle (unity) and high ranges are selected as in the three-speed hubs, but the low and high ranges offer two speeds each.

The small end of each planet gear engages the gear ring and one sun gear. The large end engages a second, smaller sun gear. With the larger sun locked to the axle, the hub operates as a simple planetary system. Locking the smaller sun gear to the axle selects a narrower range.

Early Sturmey-Archer and SRAM 5-speed hubs used a shifter rod and cable on the right side for the decrease-unity-increase function, and another on the left for the wide-narrow function. More-recent 5-speed hubs use a single shifter and cable, simpler for the user but requiring a more complicated connection to the internal mechanism.

Sturmey-Archer and SRAM 7- and 9-speeds.

Sturmey-Archer and SRAM 7-speeds (both now discontinued) are very similar to the 5-speeds, except with 3-part stepped planet gears. The ring gear engages the middle set of teeth of the planet gears.

The SRAM i-Motion 9-speed also is similar, with 4-part stepped planet gears; the ring gear engages the second-smallest set of teeth of the planet gears.

With all of these hubs, the smallest section of the planet gears must have enough teeth for acceptable strength and wear life, limiting the hub’s range. The i-Motion 9 hub has the widest range of any such hub, 3.504 to 1. It has a rather large, 72-tooth ring gear, so that the smallest planet gear section can still have 12 teeth. This hub is therefore bulkier and heavier than the others. It has been described as a “cannonball” by one mechanic entrusted with building it into a wheel! SRAM offers a version with a carbon-fiber shell, achieving some weight reduction.

Multi-stage planetary gearing

Multi-stage planetary gearing offers more flexibility than single-stage gearing, at some expense in efficiency.

All three elements rotating

In one approach, all three elements of a planetary system are in motion: the sun gear is turned forward or backward by a second planetary system. Several mid-20th century narrow-ratio Sturmey-Archer hubs used this approach to achieve ratios near unity. Intended to appeal to racers and avid recreational cyclists, these hubs had only three or four ratios, and lost out in the market to derailers, which could offer narrow steps, but also, more ratios. The very elaborate Sachs Elan 12-speed hub from 1995 also had all three elements rotating.

Stages in series; the Shimano Nexus 7-speed

More than one planetary stage may also be used in series, overcoming limitations of single-stage planetary gearing -- a common approach with newer hubs.

The first common hub to use stages in series was the Shimano Nexus 7-speed.

This hub uses two stages. The first stage gears down, and the second, up. Each stage has two-part compound planet gears and two sun gears, offering two different ratios, or a unity ratio. In the middle three of the hub’s seven gears, power passes through both stages, and so the hub does not use the unity ratios of both stages at once; rather, it achieves a near-unity ratio by gearing down and then up through two ratios that are nearly complementary, avoiding an awkward shift which could allow te hub to freewheel forward.

The Nexus 7-speed was the first common hub with rotary shifting. Though often praised for its smoothness of shifting, its protected cable connection and its even progression of ratios, the Nexus 7-speed has been criticized for low efficiency in the middle gears, and for short wear life due to the small diameter of the sun-gear pawls.

Stages that only gear up or down, and unity ratio

Only to gear up or gear down (besides unity ratio) is usual with 2-speed hubs, some of which shift when the cyclist backpedals – making them especially suitable for take-apart and folding bicycles where a cable would pose a complication. But also, Sturmey-Archer once made, and now (2010) has again introduced, a three-speed fixed-gear (non-freewheeling) hub. The fixed-gear hubs only gear down, because the planet cage, which drives the hub shell, must be permanently attached to it.

The Shimano Nexus 4-speed

The Shimano Nexus 4-speed hub has a unity ratio and three increase ratios. It followed the Shimano 7-speed into the market, but was withdrawn after a few years. It uses three sun gears, each meshing with a different set of teeth of the compound planet gears. If no sun gear is locked to the axle, the hub operates in its lowest, direct-drive unity ratio. Each of the other sun gears selects a different higher ratio. The pawls that drive the hub shell in the step-up ratios override those for the unity ratio. Like the 7-speed, the 4-speed has a rotary cable connection.

The Shimano Nexus 8-speed

The Shimano 8-speed is very similar to the 4-speed, but has a preliminary stage which either operates at unity ratio or gears down, doubling the number of speeds. This hub’s sequence of ratios is rather uneven; the steps from first to second and fifth to sixth are wider than the others, due probably to the difficulty of achieving a ratio near unity with a single stage. Efficiency is highest in the fifth gear, which is unity ratio.

The Sturmey-Archer 8-speed

The Sturmey-Archer 8-speed hub has three simple planetary systems, each with a different increase ratio. The hub achieves 8 speeds by using 1, 2 or all 3 of the planetary systems in series, or bypassing them all for the unity ratio. Consequently, the lowest gear is most efficient and the top gear, which uses all three planetary systems, least efficient. On a conventional bicycle, this hub must use an unusually-large sprocket and a small chainwheel, but on the other hand, this hub is especially well-suited to a bicycle with small wheels.

Sturmey-Archer 8-speed hub, with three gearing stages (right to left in the illustration)

Sturmey-archer 8-speed hub

The first model of this hub offered had a total range of 3.05. The current (2010) version has a total range of 3.24, and steps of approximately 14% except for the outermost steps of 30%. A second-lowest ratio closer to unity would require a small sun gear that would reduce reliability of the hub. Shifting is by locking none, 1, 2 or all 3 of the sun gears to the axle. When a sun gear is unlocked, the stage operates at unity ratio. When a sun gear is locked to the axle, the stage gears up, overriding the pawls used for the unity ratio. This hub uses a rotary shifter-cable attachment inside the right-side dropout.

No-holds barred design: the Elan and the Rohloff Speedhub

The Sachs Elan, no longer available, was the first internal-gear hub which tried to rival modern derailer gearing. It is an interesting case and has its own page on this site.

Sachs Elan 12-speed hub

Sach Elan hub

The currently-available Rohloff Speedhub has 14 speeds, with even steps of 13% each and an overall range of more than 5 to 1. It is marketed as a replacement for a wide-range derailer system, competitive in terms of weight and efficiency.

Cross-section of Rohloff 14-speed hub

Cross-section of Rohloff hub

This hub has three planetary stages, from left to right in the illustration above. The first and second stages have two-step compound gearing with identical tooth counts. The first stage only gears down and the second stage, up. Seven of the nine possible gearing combinations are used; the two unused combinations would provide unity ratios but would transmit power through both sets of gears.

The third stage has its input at the sun gear and output through the planet cage, transposing the entire range downward to double the number of available ratios.

The Rohloff Speedhub is the most versatile, and by far, the most expensive internal hub available as of this writing. It revives desirable features of the classic internal-gear hubs – oil lubrication, weight-optimized construction – but it prices itself out of the urban-cycling market. Rohloff engineering applied to a hub with 7 somewhat more widely-spaced speeds would be an interesting prospect.

The Shimano 11-speed

As of late 2010, Shimano is offering an 11-speed hub. It is oil-lubricated. Examination of drawings and drive ratios shows its construction to be similar to that of the 8-speed hub, with a first stage that can gear down or operate at unity ratio, and another stage that gears up. The middle gear, however, is not at unity ratio. The first stage has helical-cut gears. No sample was available for inspection at the time of this writing.

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Trends

The general trend among internal-gear hubs as of 2010 has been toward greater complexity and expense, though simple 3-speed hubs still are available.

The classic Sturmey-Archer 3-speed hub was very economical if the owner oiled it occasionally and kept the cable adjusted. Oil washed out and soiled the shell, but kept the internals clean. This hub could be serviced quite easily with widely-available and inexpensive parts. Some 5-speed models also were reliable, as long as the owner replaced the stock shifters with a pair of Sturmey-Archer trigger controls. Sturmey-Archer hubs could freewheel forward if the cable was misadjusted -- see comments by Jobst Brandt. That problem is resolved with current hubs of all brands.

Current 3-speeds from Sturmey-Archer, SRAM and Shimano are simple and rugged, as are SRAM 3, 5, 7 and 9 speeds -- and the 9-speed avoids the vulnerable clickbox shifter attachment of the other SRAM hubs. In my experience, Shimano internal hubs have been less reliable than the others. The early 3- speeds could not withstand hard pedaling. The 7-speed would sometimes skip forward after a couple of years’ use. The 8-speed improves over the 7-speed, but is rather complicated.

These days, all internal-gear hubs except the Rohloff and the Shimano 11-speed are grease-lubricated and lack an oil port. Grease is imperative with hubs that include a coaster brake. With most hubs, it is still possible to add oil in one way or another without disassembling the hub -- see advice here. All in all, the trend has been toward less user maintenance, at the cost of more frequent service intervals and vulnerability to water contamination. Many users will simply run a hub until it malfunctions.

Several other trends decrease serviceability compared with the classic three-speeds

Efficiency

High efficiency is important in a bicycle drivetrain. Let’s consider the factors affecting efficiency in a simple planetary system. We’ll use the example of the common Sturmey-Archer 3-speed hub, with 20-tooth sun and planet gears and a 60-tooth ring gear.

I'll give only one example here; others may be developed similarly. Consider a single turn of the driver (the input), resulting in 1 1/3 turns of the hub shell, in the top gear.

The driver is connected to the planet cage. As the driver makes one turn, each planet gear makes two turns as it circles the stationary sun gear, but only one turn on its axle – because the axle is rotating forward with the planet cage; also, only one turn relative to the sun gear, because it is circling the sun gear in the same direction it is turning.

The planet gears advance the 60-tooth ring gear by 20 teeth.

So, all in all, for each turn of the driver, the gearing involves gear tooth contact losses due to:

Properly-formed and aligned teeth of spur gears are in rolling contact, without sliding.

In addition, there is the drag on the bearings.

The gears and bearings, then, add some drag compared with a single-speed system, but on the other hand, an internal-gear hub does not drag from the chain's running over derailer pulleys or from chain misalignment. Research has shown internal hubs to be somewhat less efficient than derailer gearing, but they make up for that in urban riding with their ease and speed of shifting. The direct-drive middle gear of an internal hub should in theory be very efficient, because power is transmitted directly to the hub shell. In practice, this gear does not always prove to be much more efficient than the others, for reasons as of yet undetermined.

Older internal hubs, as well as the Rohloff and the Shimano 11--speed, use light machine oil for lubrication, keeping drag low. Many newer hubs may have more than one gearing stage, and use grease, making them less efficient.

A comprehensive study of internal-gear hub efficiency was conducted by Dr. Chester Kyle and Frank Berto, and published in the journal Human Power, issue #52. It makes an interesting comparison with the examination of derailer-gearing efficiency in issues 50 and 51. Efficiency increases as chain tension or applied power increases. Kyle and Berto show lower efficiency than manufacturers' claims – though still impressive – for the Rohloff hub. For derailer gearing, the results in issue 50, showing lower efficiency for smaller sprockets, are overturned in issue 51, when recalculated to apply to the same power, at the same wheel rpm.

Making good gearing choices

Wide-range, narrow-step derailer gearing (or the Rohloff hub – at a high price) makes bicycling more accessible in steep terrain and to weaker riders. But for most urban utility use, a hub gear, if properly chosen and set up, has advantages that well outweigh those of derailer gears.

The single-speed bicycle and the classic, overgeared three-speed were the mainstay of urban cycling for decades, but they limited the utility of bicycling. It is no coincidence that urban cycling has been most popular in cities that are very flat – Amsterdam, Copenhagen, Berlin.

Even in such cities, single-speeds and overgeared three-speeds are unable to accelerate smartly or to adapt to changes in wind, baggage and cyclist strength. Typical utility bicycles also have often had only a coaster brake, and stopping distance twice that of a bicycle with a front brake. Such bicycles may legally still be sold new in the USA and the Netherlands, though not in other countries of the European Union. The coaster brake also impedes quick restarting by making backpedaling impossible, and it overheats on long downhill runs.

I understand that dual handbrakes, usual for decades in the U.K., France, the USA and Canada, are now capturing an increasing share of the market in northern Europe. Sometimes the rear brake is a disk, drum or roller brake integrated with an internal-gear hub – and unlike a rim brake, immune to wet weather.

In my opinion, a three-speed with the top gear in the 70- to 80-inch range (5.6 to 6.4 meters development) is the minimum for smart performance in urban cycling. I think that the preferred gearing sweet spot in most urban areas has 5 to 8 speeds.

The newer Sturmey-Archer 5-speed hubs have a single shifting system, and the latest generation, like the Shimano hubs, has rotary shifters. The recently-discontinued SRAM 7-speed hub, though its internal mechanism is reliable, has the protruding and vulnerable plastic “clickbox” shifter assembly. To get rotary shifting on a SRAM hub, you have to go to 9 speeds, with the attendant increased expense, complication, weight and theft potential.

A very hilly city such as San Francisco demands a wider range than a hub gear (other than the Rohloff or the Shimano 11-speed) can offer. Hybrid gearing or an all-derailer system is less expensive than these hubs, though more .trouble to maintain.

Weight also figures into the choice of a hub. Light weight of a matters much more when a bicycle is being carried than when it is being ridden, and is especially important in folding bicycles, which are more often carried.

With its three-speed and five-speed hubs in aluminum alloy shells, Sturmey-Archer still maintains the tradition of weight optimization that it established in the mid-twentieth century. Though SRAM’s five-speed and (now discontinued) seven-speed hubs are much like Sturmey-Archer hubs internally, they are sold with heavy steel shells. Shimano stands somewhere in between. None of the hubs with more than seven speeds is very light, though the Sturmey-Archer is the lightest.

My own bicycles have from one to 30 speeds, but for urban cycling, I am still riding an old Sturmey-Archer 5-speed hub. Feeling the need for a lower gear range to pull a trailer up the hill to my home, I am building up a wheel with the SRAM 7-speed model that includes a drum brake. This version has an aluminum alloy shell, and even with the brake, weighs only very slightly more than the brakeless version with a steel shell. I would not be using this discontinued hub if I did not have a spare on hand.

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