This submenu presents some mechanisms for gear shifting in more detail. The clutches are divided into two categories. Internal clutches are all shiftable mechanisms for coupling gear members within the same gear train. How will sun gears get fixed or released? How to get sun gears coupled with the planet carrier? External clutches are all shiftable mechanisms for coupling gear members within the same gear train with gear members outside the gear train. This will be explained in the next section. Both sections will be a feast for the eyes for friends of fine mechanics, as promised before. Only some examples out of the multitude of geared hubs all over the world can be presented. Have fun!


This explains the mechanisms to couple sun gears with the axle. Coupling a single sun with the axle leads the axle to transfer the counter-forces which are needed to drive the bike. It is similar to using a cordless screwdriver: we have to hold the handle of the screwdriver firmly in order to apply the counter-force resulting from the screwing procedure. Driving the ring gear of a simple gear train for example (hill gear) leads the planet carrier to drive the hub shell. The torque at the planet carrier tries to turn the sun gear backwards when loaded – imagine a loaded planet carrier is like retarding it by hand. This imaginary retarding leads the gear train to a quasi – stationary gear, in which sun gear and ring gear have different directions of rotation, as you know. It would be sufficient to fix the sun gear in backward-direction when driving the ring gear. It could remain turnable in forward-direction or be fixed in both directions, of course. Driving the planet carrier of a simple gear train (fast gear) leads the ring gear to drive the hub shell. The torque at the ring gear tries to turn the sun gear forward when being loaded. It would be sufficient to fix the sun gear in a forward direction when driving the planet carrier. It could remain turnable in a backward-direction or be fixed in both directions, of course. In hubs with a shift drum e.g., sun gears get only fixed in the direction in which the torque of the load is applied, they remain rotatable in the opposite direction.

Shimano Nexus 8 internal clutch
Shimano Nexus 8 external clutch
Sram i-motion 9 driver

Sliding block, axle toothing and teethed sun

Locking a specific sun rotationally or releasing it via pushing it with a sliding block into an attached toothing of the axle is a relatively simple way for coupling sun gears. Thus, the double stepped planetary gear train of the Sram Spectro P5 was shifted, as already explained in the section Internal clutch within Hub elements. The predecessor models Pentasport were almost built identically. Many historic hubs applied that basic principle in different variations. In the Fichtel & Sachs Doppeltorpedo the sun gear’s teeth could be pulled into an internally toothed conus which was screwed and countered on the axle at the sprocket side. The large spur-cut sun gear of the Fichtel & Sachs Model 29 allowed it to be pulled into a spur-cut conus which was attached to the axle. The small sun, however, was internally toothed at its collar, which could have been engaged with a short axle toothing with round teeth. Model 53, also named ‘Dicke Berta’ applied the same principle for both of its sun gears. The Sturmey-Archer 4-speed hub FW has a sawtooth shaped spur toothing and the sun gear’s teeth within the 5-speed hub S5 are chamfered on both sides – probably in order to achieve a smoother coupling process. All models shared the same basic idea to couple the sun with an axle toothing.

Sachs Model 29
Sturmey-Archer FW
Sachs Double Torpedo

Sliding block and grooved sun gears

Sturmey-Archer and Fichtel & Sachs made several 5-speed hubs from 1966 on using an axle which was toothed for grooved sun gears for a certain period of time. These hubs mostly differed in the type of activation of the sun gears. The corresponding sliding block was moved by a second toggle chain, or with toggle chain and push rod or with two rods. From 1993 to 1999 they launched the S5 Sprinter version with one toggle chain only. The sun gears are internally grooved – as in some other designs – but there is no axle toothing any more. The corresponding sun gets rotationally fixed when a sliding block is moved into its groove thus locking the rotary motion. The S5 sprinter hub contained two opposite long sliding blocks with cams protruding from the axle keeping a fixed distance when the toggle chain was pulled. The right sliding lock additionally moved the radiating clutch within the driver. This arrangement allowed Sturmey-Archer to shift five speeds by pulling the chain incrementally.

Sturmey-Archer S5 blocks
Sturmey-Archer S5 sun gears
Sturmey-Archer S5 clutches

The right block locked the right sun gear when the chain was unstressed and the left block was still outside of the small sun (freely rotatable). The radiating clutch was meshing with the planet carrier resulting in the most speed increasing ratio, the 5th speed. After actuating the toggle chain the left sliding block was pulled into the small sun (fixed now) and the right sliding block left the large sun (freely rotatable) leading to the 4th speed with smaller speed increasing ratio. When pulling the chain a bit further the right block caused the radiating clutch to disengage with the planet carrier. The ring gear now was driven directly by pawls inside the driver – the 3rd speed with 1:1 ratio. The pawls of the ring gear were deactivated when pulling the chain any further and the hub shell now was driven by the slower rotating planet carrier. The left sliding block was still within the small sun locking it leading to a small speed reducing ratio – the second speed. When finally pulling the toggle chain completely the left sliding block snapped in the grooves of the large sun and released the small one. In this way the highest speed reducing ratio – speed 1 – was achieved.

In the same period Fichtel & Sachs launched its successful 7-speed hub Super 7. Grooved sun gears were shifted with the same principle of grooves and sliding block. Sachs didn’t use a single toggle chain, however, but applied two indexing pins for shifting sun gears and the radiating clutch independently by using a ‘clickbox’. The interior pin moved the sliding block for locking the sun gears alternately and releasing the others. The exterior pin (pipe) moved the radiating clutch independently.

Sachs Super 7 sliding block
Sachs Super 7 sun gears

Finally, the S7 hub from Sturmey-Archer should also be mentioned, which had a rotary gear selector and was actuated by a cable drum. It contained a triple-stepped planetary gear train similar to the Super 7. Sun gears and radiating clutch were not shifted by indexing pins independently but by means of two cam sleeves which converted the rotary motion of the cable drum into a lengthwise movement. The right sliding block was controlled by the interior cam sleeve and moved a left hand sliding block with cam via a spring. The left hand sliding block finally locked and released the three sun gears alternately when traveling backwards and forwards within the sun gears.

Sturmey-Archer S7 blocks
Sturmey-Archer S7 cam sleeve

Shift drum and sun gears with pawls

In 1992 Shimano launched a hub with very interesting innovations with its Nexus Inter 7: a rotary gear selector instead of toggle chains, indexing pins and clickbox were introduced. Sun gear clutches get activated or deactivated by a rotary motion of a shift drum at a specific angle. The gear structure is explained in Gear configuration and the section ‘Multiple stepped gear trains connected in series’. The axle of the Nexus Inter 7 contains protrusions (locking protrusions) in some places, where the pawls of the sun gears either will collide with (sun gear blocked, clutch activated) or will climb over (sun gear released, clutch deactivated). The pawls of the sun gears are split: the lateral small, cam-like part glides over the shift drum while turning the sun gear and it gets lifted or it remains clinging to the outside diameter of the axle – depending on the angular position of the shift drum. The other, larger part of the pawls climbs over the protrusions – when being lifted by the shift drum (sun freely rotatable, deactivated clutch) or it collides with the protrusions (sun gear blocked).

Shimano Nexus 7 axle protrusions
Shimano Nexus 7 shift drum
Shimano Nexus 7 pawl of sun gear

The shift drum is a swiveling sleeve with control surfaces for the ca-sides of the sun gear’s pawls. The control surfaces are slewably attached beside the protrusions of the axle and get ‘sensed’ by the cams of the sun gear’s pawls. These pawls are generally freely rotatable in the opposite direction – like all pawls – it’s shallow angle allows to climb over the shift drum as well as the protrusions without getting locked. The sun gear’s pawls in the Nexus Inter 7 must be lockable backwards in gear train 1 and forwards in gear train 2. This was achieved by installing the same pawls – in right-left-configuration, however – in the sun gears with different orientation and by accordingly contriving the shift drum.

Shimano Nexus 7 shift drum assembled
Shimano Nexus 7 pawl activated
Shimano Nexus 7 pawl deactivated

The Nexus Inter 4 – which is the second hub from Shimano with shift drum – is built up more simply: it contains a triple-stepped planetary gear train with speed increasing ratio only (driven planet carrier) and thus all pawls must be lockable forwards – in the same direction. Shimano applied simple spring rings there keeping the pawls of the sun gears under tension – in the Nexus Inter 7 many delicate and small torsion springs were installed.

Shimano Nexus Inter 4 sun gears
Shimano Nexus Inter 4 sun pawls
Shimano Nexus Inter 4 shift drum

Cam rods and sun gears with pawls

Between 1996 and 1999 Fichtel & Sachs released a 12-speed hub which was very exceptional in many respects. It was the largest and heaviest geared hub in the world by then – weighing 3,5kg! Its function is explained briefly in another section. Though the Elan was not that successful, unfortunately, it was technically pretty innovative – but also pretty complex. Five out of its six sun gears were controlled by cam rods – i.e. getting fixed or released – this principle has been specially patented by Sachs. Profiles / longitudinal slots are milled at the perimeter of the hub axle in which the cam rods are mounted slewably. The pawls ofthe corresponding sun gears can either climb over the remaining bars where the rods are inserted (inactive clutch, released sun) or they can rest on the bars (active clutch, fixed sun).

Sachs Elan shifting
Sachs Elan cam rods
Sachs Elan cam

The cam rods are shaped semi-circular in its lower area and roof-like in its upper area along their entire length. At the position of the sun gears, however, the upper area is shaped roof-like, but flat. All pawls of sun gears situated outside the area always glide over the cam shafts without touching the bars of the axle. The pawls always get lifted up there at the highest point of the roof-like profile gliding over the bar – regardless of the rotation angle of a cam shaft. Pawls of sun gears, however, touching the flat area of the cam shafts now can be shifted selectively by a swiveling movement of the cam rods. The flat area of a cam shaft turned clockwise acts like an ascending ramp causing the pawl to slide over the bar (released sun, inactive clutch). The flat area of a cam shaft turned anticlockwise acts like a decreasing ramp causing the pawl to get blocked at the bar (fixed sun, active clutch). For switching a sun gear it needs two opposing cam rods with a flat area at the profile at the position of the corresponding sun since every sun gears contains two opposing pawls.

Sachs Elan cam ring
Sachs Elan bearing plate
Sachs Elan driving pins

The mechanics for swiveling the cam rods is pretty complex and it’s better not to disassemble it. There is a rotatable cam ring for every sun gear and a cam lever touching its internal contour. Turning the cam ring causes the cam lever to swing in and out while it pushes the pins of a bearing plate off which allows the bearing plate to turn backwards and forwards. The hook-shaped ends of the cam rods are both situated between two pins of the bearing plate. Turning the bearing plate finally causes the hook – and the cam rod – to swivel back and forth.

Control sleeve and extendible control pawls

Since 2004 Shimano offered an interesting 8-speed hub which has many features in common with the Nexus Inter 7. It also contains two stepped planetary gear trains connected in series with a common planet carrier. The first gear train, however, is not shiftable, it only provides a speed reducing ratio and a direct gear (two speeds). The second (triple) stepped gear train provides three speed increasing ratios and a direct gear (four speeds). In this way 3×4=8 speeds are realized in the Nexus Inter 8. All sun gears of the second stepped planetary gear train are internally toothed appropriate to three extendible pawls on the axle – there are no pawls inside the sun gears. The pawls are loaded with spring force and can be specifically extended (meshing with the internal toothing, clutch activated) by a slewable control sleeve or be retracted (sun released, clutch inactive). The sleeve has control surfaces – similar to the Nexus Inter 7 – which can be ‘sensed’ by the cams of the pawls. Cut-outs within the control surfaces cause the cams to be released and the corresponding pawl to extend when being turned. There is only one single pawl applied for one sun gear – which is remarkable. For the biggest speed reducing gear ratio the second gear train is inactivated (deactivated gear) when all pawls are retracted – similar to the Inter 7. Thus speed 1 is achieved with a speed reducing ratio of gear train one. Not bad – Shimano’s guys get more and more likable.

Shimano Nexus 8 gear unit
Shimano Nexus 8 sun teeth
Shimano Nexus 8 control sleeve

Cam rings and slewable control pawls

Since 2008 a revised 8-speed hub from Sturmey-Archer is available as well, the structure was already described in another section. Its construction was made – believe it or not – pleasantly simple. Three retractable pawls with long shanks mounted within the axle can be actuated by three cam rings. The end of each shaft is angular provided with a spring loaded cam which is located within a ring with small internal cut-outs. This internal contour of the cam rings is designed in such way that the cam of the pawl snaps into the cut-outs when reaching the designated rotational angle (deflected pawl, fixed sun, active clutch) or it gets pressed inwards (retracted pawl, sun released, inactive clutch). A cable drum revolves the three cam rings synchronously and retracts or extends all control pawls. The sun gears are internally toothed similar to the Nexus Inter 8 with an adequate profile. There is only one extendible pawl for one sun gear as well.

Sturmey-Archer 8-speed control pawls
Sturmey-Archer 8-speed cam rings
Sturmey-Archer 8-speed activation


Some historic geared hubs allowed coupling a sun gear with the planet carrier. In this way all three gear members shared the same rotational speed, a direct gear was realized. The Fichtel & Sachs Doppeltorpedo allowed to either fix the sun (hill gear) or to connect it with the planet carrier (direct gear). The sun gear gets pushed into the planet carrier’s internal toothing using its standard gear toothing. The model 29 and the model 53, however, both contained a sun gear with additional internal toothing for being fixed at the axle. Some historic Sturmey-Archer hubs applied the same principle to push a sun gear into a planet carrier’s internal toothing, e.g. model T or Model TF, as well as model FN, model A, model N and model V. This coupling principle is very easy, however, it leads to considerable wear because the toothings hit each other during the coupling procedure. There exist other solutions to achieve a 1:1 gear ratio, of course, e.g. by using a deactivated gear as it is applied in the Nexus Inter 4 or other hubs, i.e. all sun gears are deactivated. This is explained in other sections.

Sachs doppel torpedo sun within toothing of planet carrier
Sachs doppel torpedo direct gear


The ‘miraculous hub’ Rohloff Speedhub 500/14 contains several clutches to couple sun gears with ring gears. More information about it will be provided at a later date.


The several times quoted Fichtel & Sachs Elan contains a clutch to couple the shared planet carrier 1/2 with the ring gear 2 (which is directly joined to the differential sun gear 3). To do this, three pawls pivoted in bolts with chamfered side surfaces are installed within the planet carrier meshing with the toothing of ring gear 2. The shifting unit controlling the cam rods also enables a shifting ring to move to and fro. When the shifting ring is retracted (activated clutch), the pawls mesh with the toothing of ring gear 2 and thus they achieve an interfacing to the planet carrier. When the shifting ring is extended it pushes the pawls inwards via their side surfaces disengaging the toothing. This causes the planet carrier and the ring gear to get disconnected.

Sachs Elan shifting ring extended
Sachs Elan toothing ring gear 2
Sachs Elan chamfered pawl