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Bicycle Technology: Hub Gearing



Guide to Bicycle Technology (article index)

Although derailleur gearing is used on most bicycles sold these days, the other method of gearing also bears some mention. Instead of moving the chain sideways between different sprockets and chainrings, this system uses an epicyclic gear incorporated in the rear hub.

The advantages of hub gearing include uncomplicated shifting, fewer external parts to get damaged, and the possibility to enclose the chain. All these add up to enough reasons to choose this system for a wind-and- weather utility bike, as indeed it is in countries like Holland, where the bicycle is to this day seen as a means of transport, as well as in England, where hub gearing remains the choice of many bicycle commuters. Fig. 11.1 shows the typical parts that make up a hub gearing system with — in this case — three speeds. Systems with 2, 4 and 5 speeds are also available. A shifter on the handlebars operates a mechanism connected to the pull rod in the hub via a bowden cable.

Hub gears are available from at least three manufacturers: Sturmey-Archer, Fichtel and Sachs and Shimano. If you ever encounter one that says SunTour, it is in reality a Sturmey-Archer hub. There was a brief period when Sturmey-Archer built 3-speeds with coaster brake for Fichtel and Sachs, which explains the difference with other hubs sold under the same name.

11.1. Hub gearing system components

11.2. Fichtel and Sachs Pentasport: five gears spread over a wide range. Available in versions with coaster brake and with drum brake.

The Epicyclic Gear Principle

The guts of a typical 3-speed hub are shown in Fig. 11.3. All internal hub gears used these days are based on the epicyclic, or planet, gearing principle illustrated in Fig. 11.4. This section will explain the principle by which these systems operate.

11.3. Internals of a typical three-speed hub. This is Sturmey-Archer’s most popular AW model.

11.4. The epicyclic gearing principle

The rear wheel axle holds a fixed central, so-called sun gear. At some distance from this sun gear, on the inside of the hub shell, is an inward facing annular gear. The space between these two gears is bridged by a set of mutually connected small gear wheels, referred to as planet gears. The teeth of the planet gears mesh on the one side with the central sun gear, on the other side with the annular gear.

The axle is hollow and contains a clutch mechanism that is operated by the pull rod and connecting certain parts of the system. The sprocket, or cog (which is the term I shall use throughout this section to prevent some confusion in the abbreviations used in the formula that follow, even if sprocket is used in the rest of the guide) driven by the bicycle’s chain can be connected by this same clutch mechanism to particular parts of the system, while a freewheel mechanism sees to it that the wheel can rotate forward while the cog is stationary.

In the normal gear position, the clutch is set so that the entire planet system is by-passed: the cog is connected via the freewheel to the wheel hub. This way, the hub turns just as fast as the cog and there are no mechanical losses — it corresponds to what used to be an overdrive on older cars. Expressed in a formula, the wheel rotation speed in this gear is:

Vn = Vc

where:

Vn = wheel rotation speed in normal gear

Vc = cog rotation speed.

When the high gear is selected, the clutch mechanism connects the cog with the cage on which the planet gears are mounted, while the hub shell is connected with the annular gear. Consequently, the planet gears turn forward as they move together around the sun wheel and drive the annular gear. The speed with which the annular gear — and consequently the whole wheel — turns can be determined as follows:

Vh = Vc x (Ta + Ts)

where:

Vh = wheel rotation speed in high gear

Vc = cog rotation speed

Ta = number of teeth of annular gear

Ts = number of teeth of sun gear.

In the low gear position, the clutch mechanism connects the cog with the annular gear, while the wheel hub shell is connected with the cage holding the planet gears. Consequently, the planet gears turn backwards and drive the hub shell more slowly than the speed of the cog. The wheel rotating speed will be:

Vl = Vc x (Ts / Ta)

where:

Vl = wheel rotation speed in low gear

Vc = cog rotation speed

Ta = number of teeth of annular gear

Ts = number of teeth of sun gear

Combining the three formulas for the individual wheel rotation speeds, the ratio between the three gears is:

Vl / Vn / Vh = (Ta - Ts) / (Ta) / (Ta - Ts)

Hub Gear Progression

Readers familiar with mathematics will recognize the above relationship as a geometric progression. That means that the steps between bigger gears are also bigger than those between lower gears. As was explained in Section 10, this contradicts the requirements for an ergonomically correct gearing progression. It is more desirable to have steps that become proportionally smaller as the gears get higher. This is the disadvantage of hub gearing, although it should not be over looked that few riders actually use hub gears quite so critically that their limitations bother them.

11.5. Complete braking and gearing system for touristic bikes from Sturmey-Archer It includes aluminum hubs, five- speed gearing and a choice of shifters.

11.6. Gear progression compared: Hub gear progression vs. Derailleur gear progression. The hub gears do not follow the optimum curve as closely as can be achieved with derailleur gears — even though most unsophisticated riders may never notice.

With the gear wheel sizes used on virtually all 3-speed hub gears available today, the sun gear has the same number of teeth as the planet gears, while the annular gear has three times as many. Consequently, the progression is 67%, 100%, 133% — in other words, the low gear is 25% lower than the normal gear, while the high gear is 33.3% higher. The range of gears can be described by the ratio between the extreme gears: the highest is 133 / 75 = 1.78 times as high as the lowest. If we assume a chainring with 48 teeth and a cog with 19 teeth, the normal gear will have a development of 5.40m (a 69-Inch gear). The low gear will have a development of 0.75 x 5.40m = 4. 10m (a 52-inch gear), while the high gear will have a development of 1.33 x 5.40m = 7.10m (an 89-inch gear). On a 2-speed system, the low gear would be missing, meaning that the available gears have developments of 7.10m and 5.40m (gear numbers of 89 and 69 inches), respectively.

On 5-speed systems, which incorporate a double planet gearing system, the progression is as follows: 67%, 78%, 100%, 122%, 150%. Consequently, the individual gears, assuming the same set-up with 48-tooth chainring arid 19-tooth cog, would be as follows: 3.60, 4.20, 5.40, 6.60, 8.10. Thus, these gears do not lie quite as far apart as is the case on the 3-speed, making this a more suitable arrangement with adequate range: the highest gear is 2.24 times as high as the lowest.

11.7. Sturmey-Archer AW three-speed hub with steel hub shell

Ergonomically, the disadvantage of the geometric progression applies here too, but it should not be over looked that in reality derailleur gearing usually does not allow perfect progression either. Fig. 11.6 shows a comparison between the two gearing systems, which is clearly not all that unfavorable for the 5-speed hub. On the other hand, the mechanical losses are greater in the extreme gears — on the Sturmey-Archer 5-speed, I actually measured so much more resistance in the lowest gear, that it merely slows you down, without making the load any easier.

The examples listed above were based on a 48-tooth chainring and a 19-tooth cog. Although the ratio between the gears remains the same whatever cog and chainring are selected, the whole range can be made higher or lower to match the user’s preference. This is done by installing a cog (or a chainring) with a different number of teeth: either a larger chainring or a smaller cog to increase all gears, a smaller chainring or a bigger cog to reduce them.

Hub Gear Controls

11.8. Three-speed shifter

Generally, the three-speed hub is operated by means of a handlebar shifter, which operates a little chain or hinge connected with the clutch mechanism in the hub via a flexible cable that runs over rollers or guides. Fig. 11.8 illustrates a typical 3-speed gear shifter. Five- speed hubs may either be controlled by means of a double lever or (less predictable, though more popular) by means of a single lever as illustrated in Fig. 11.9.

11.9. Single-lever 5-speed shifter

The hub axle is hollow and carries the operating rod on which the clutch mechanism is held. On most models, the clutch rod is attached to a little chain, while other models connect it with a hinge mechanism screwed on in the location of the axle nut. The cable is attached to the little chain or the hinge by means of a cable adjuster which serves to correct the adjustment of the gears. When shifting towards a lower gear, the pull rod is pulled further out of the chain and sets the clutch in the appropriate position. The two-speed gear, used mainly for portable bicycles, works without external controls: it is operated by pedaling back.

The simplest adjusting mechanism is the one used by Fichtel and Sachs. In conjunction with the mechanical superiority of the gears themselves, this is a good reason to choose this make in preference to all others — certainly now that this manufacturer has finally introduced models without a coaster brake.

Hub Gear Maintenance

Whenever hub gearing does not work properly, it is generally not due to the mechanism itself, but rather to the controls. Slipped cable guides or pinched cables are the most frequent causes of control problems. Consequently, these points should be checked before at tempting to adjust the mechanism.

Most models made by Fichtel and Sachs are lubricated for life and only break down when the hub overheats during prolonged braking with the built-in coaster brake. In that case, the hub should be disassembled and the bearings repacked with the manufacturer’s special grease. Most other models are equipped with an oil nipple and should be lubricated with 10 drops of light oil once every three months or whenever the hub appears not to run or shift smoothly. On three-speed models, the coaster brake can simply be eliminated by dismantling the unit and removing the sectioned, cylindrical brake mantle with the brake cone shown in the illustration of the cross section.

11.10. The heart of a Sturmey-Archer three-speed hub mechanism.

To date, Sturmey-Archer’s models with built-in drum brakes are not equipped with a seal that separates the brake from the gears. The result is that when the bike lies on its left side, the oil enters the brake drum where it ruins the brake pads. Consequently, the manufacturer delivers them unlubricated — make sure you lubricate such a hub before use. Of course, bikes sometimes do lean over to the left, and during transport the cyclist often has no control over what happens. My value judgment is that this displays a manufacturer’s disregard for the consumer’s needs (or perhaps it is merely a sign of technical incompetence).

But lubrication is not the only aspect of hub gear maintenance. Here is a list of frequent causes of improper shifting with their respective solutions:

- Cable (seemingly) stretched: adjust.

- Hub, adjusting mechanism, cable or handle does not run freely: lubricate.

- Hub defective: overhaul or replace.

- Control part (shifter, cable, cable guide, hinge mechanism) defective: repair or replace.

Most often, the cause is one of the first two points, so it can simply be corrected by means of adjustment or lubrication. On the three-speed, the adjustment is generally correct when the cable is taut, but not under tension, when the shift lever is set for the high gear. However, the procedure is slightly different for each of the various makes arid models, as will be described individually below. When the system is operated via a little chain protruding from the axle (Sturmey-Archer and Fichtel and Sachs 3-speeds), this chain must be perfectly aligned with the cable — that may mean that it is not fully turned in but backed off a little.

Adjusting Sturmey-Archer 3-Speed

On these hubs, the cable is connected to a tiny chain that comes out of the (special) RH axle nut. The correct adjustment can be checked based on the alignment as seen through a viewing port in this nut. Generally, no tools are required, although a pair of pliers may be needed if the adjuster is too tight.

Procedure:

1. Check whether the cable, its guides and stops, and the shifter all operate correctly and the stops and guides are attached properly.

2. If the hub has not been lubricated for more than three months, first lubricate it through the oil hole, then turn the cranks several times in each gear with the wheel lifted off the ground.

3. Place the shift lever in the normal gear position (N or 2), while rotating the cranks at least half a turn.

4. Check the situation through the viewing port in the special RH axle nut. The hub is correctly adjusted if the shoulder on the internal pin to which the chain is connected is exactly aligned with the end of the wheel axle (move the shift lever back and forth a little to check if necessary).

5. If necessary, adjust the cable adjuster: loosen the locknut, turn the internally threaded bushing relative to the threaded pin, then hold in position while tightening the locknut.

6. Check and re-adjust if necessary.

Adjusting Fichtel and Sachs 3-Speed

11.11. Adjusting detail for Sturmey Archer three-speed.

Since about 1975, these hubs no longer have a neutral position because they are usually sold with a built-in coaster brake (braking would then become impossible when the hub is accidentally in neutral). Consequently, the problem here is never evidenced by loss of trans mission, but merely by the fact that a different gear than the one selected remains engaged. The adjustment is done in the high gear, and no tools are required.

Procedure:

1. Establish whether the cable, its guides and stops, and the shifter all operate correctly and the stops and guides are not loose. Correct if necessary.

2. Place the shift lever in the high gear position (H or 3), while turning the cranks at least half a revolution.

3. Adjust the special cable adjuster, which is simply clamped on a serrated rod: push the clip in, slide it up or down the serrated rod held in the other hand until the cable is just taut but not under tension, and let go of the clip.

4. Check and re-adjust if necessary.

Adjusting Shimano 3-Speed

11.12. Shimano 3-speed adjusting detail

On this hub, which is available with and without coaster brake, the controls are carried via a ‘bell crank’ hinge mechanism on the LH side of the hub. This hinge mechanism has an integral locknut and must be positioned so that the pivoting movement is fully aligned with the cable. To achieve this, you may have to loosen the locknut a little, adjust the bell crank in the right position, and then hold it there while tightening the locknut. Generally, no tools will be required for the rest of the work.

Procedure:

1. Check whether the cable, its guides and stops, the bell crank, and the shifter all operate correctly and the stops, guides and bell crank are properly attached. Correct if necessary.

2. Place the shift lever in the high gear position (N or 2), while turning the cranks at least half a revolution.

3. If the gears are correctly adjusted, the letter N (or the number 2) should now be completely visible in the window in the bell crank.

4. Adjust the cable adjuster attached to the bell crank until the number is visible in the window.

5. Check each of the gears in turn, while turning the cranks with the wheel lifted, and re-adjust if necessary.

Five-Speed Hubs

11.13. Sachs-Huret gruppo with hub gearing and hub brakes — intended for recreational use.

These units, which include a double set of planet gears, each with its own controls, are available from Sturmey Archer and Fichtel and Sachs. The former exists in versions with and without drum brake, while the latter, which is mechanically far superior, is available with a coaster brake or with a drum brake.

Nowadays, most 5-speeds are not operated by means of separate shift levers, but by a combination lever that controls the two cables used to shift. Although this may seem simpler to use, it is considerably more prone to trouble, and by no means easier to adjust. The main problem is that these shifters are made of a relatively soft fiber-reinforced plastic which deforms too easily, especially at the point where the cable nipple is held. A lot of problems are prevented by taking the unit apart early in its life and applying a generous amount of bearing grease to everything that moves — even though this is not generally considered necessary for the material in question.

Adjusting Sturmey-Archer 5-Speed

On most versions of this hub, there is a bell crank mechanism on the LH side, while the RH side has the same nut with viewing port as found on the same manufacturer’s three-speed models. Verify whether the cable runs freely, the cable stops are firmly installed, and whether the bell crank and the control chain are exactly aligned with their respective cable sections. If the hub has not been lubricated in the last three months, do that first, after which it should be tried again. Once these points have been corrected, no tools are required as a rule.

Procedure:

A. On models with two separate shifters:

1. Place the LH shifter in the position that releases tension on the LH cable, while turning the cranks at least half a revolution with the wheel lifted off the ground.

2. If necessary, adjust it so that it is just taut but not under tension.

3. Place the RH shifter in the intermediate position, turning the cranks.

4. Shift the LH shifter into the position by which the cable is fully tensioned.

5. Check all gears. If any do not work, adjust the RH adjuster a little looser or tighter.

11.14. Sturmey-Archer five-speed adjusting detail

B. On models with a single shifter:

1. Select the fourth gear.

2. Adjust the RH mechanism as described for the same manufacturer’s 3-speed hub.

3. Place the shifter in the position for the fifth gear while turning the cranks half a revolution with the wheel raised off the ground.

4. The LH cable should now be tightened — adjust, if necessary.

5. Try out all the gears and re-adjust, if necessary.

Adjusting Fichtel and Sachs 5-Speed

This hub with its built-in coaster brake is always operated by means of a single shifter. No tools are needed for the adjustment, which has to be preceded by the usual check of the cables, shifter, guide and stops.

Procedure:

1. Select the fifth gear while turning the cranks for ward with the wheel raised off the ground.

2. Loosen the cable adjusting clips on both sides, so the cables on both sides of the hub are completely loose in this position.

3. Turn the cranks forward by at least one revolution with the wheel raised off the ground.

4. Put the cable adjusting clips on their respective pins so that the cables are just taut but not under-tension.

5. Select the first gear, while turning the cranks for ward with the wheel raised off the ground.

6. Check whether this gear engages properly.

7. If the first gear does not engage properly, return to the fourth gear and tighten the looser cables one notch, repeating steps 5 - 7 as necessary until the first gear works properly.

8. Check all the gears, and re-adjust if necessary.

Replacing Shifter or Cable

This has to be done if the problem cannot be solved by means of adjustment and lubrication. If the cause is clearly not here either, the hub gear mechanism itself is at fault, which will not be described in detail here. If you are faced with this predicament, it will be easier to replace the entire wheel complete with hub. You can then take your time disassembling the old hub to see whether you can find an obviously defective part.

When replacing shifter or cable, it should be noted that each are basically designed for the same maker’s hub. Just the same, there are adaptor pieces that allow the use of one manufacturer’s cable with another manufacturer’s adjusting mechanism. In the case of the three- speed, it is often possible to use non matching shifters.

11.16. Freeing cable nipple

Five-speeds can be operated with two separate three- speed shifters of any make instead of the original. You will need a screwdriver and a wrench.

Disassembly procedure:

1. Loosen the cable adjuster at the hub.

2. Pull the cable back towards the shifter until there is enough slack.

3. Pull the shift lever in as far as possible as shown in Fig. 11.16 (or, in the case of a 5-speed lever, open it up to gain access) and remove the nipple out of the recess in the shifter.

4. Pull the cable back out of the shifter and off the guides and stops.

5. Remove the part that has to be replaced (cable or shifter).

Installation procedure:

1. When replacing the lever, install it in the right position.

2. Pull the shift lever in as far as possible as shown in Fig. 11.16 (or, in the case of a 5-speed lever, open it up to gain access). Feed the nipple in and hook it in the recess in the shifter.

3. Hold the cable pulled taut and place it over the various guides and stops.

4. At the hub, attach it to the control chain or the bell crank.

5. Adjust the gears as described above.

Adjusting Hub Bearings

The gear hub’s ball bearings must be maintained just like those of other hubs. Actually, the adjustment procedure is quite easy once you have the manufacturer’s special hub wrench which is usually supplied with the bike or the hub. Fig. 11.18 shows the relevant part.

11.18. Bearing adjustment detail

Procedure:

1. Loosen the LH axle nut 2 - 3 turns (to the left).

2. Loosen the locknut 1 - 2 turns.

3. First tighten the adjusting nut for the cone fully, then back it off (to the left) by 1/4 turn.

4. Hold the adjusting nut in this position and tighten the locknut.

5. Tighten the axle nut, while keeping the wheel centrally positioned.

6. Check the bearing to make sure it is neither too loose or too tight, and re-adjust if necessary.



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