HUB ELEMENTS – INTRODUCTION
This section presents specific hub elements which can also be found – in different design – in many other hubs. There are some elements to be comprised in every geared hub, e.g. at least one planetary gear train, a hub shell, etc. but there are also elements which are not built-in everywhere, e.g. a driver, a coaster brake, etc. Not every potential hub element is described, but the elements of the Sram Spectro P5 are choosen as an exemplar of a contemporary hub frequently to be found. Similar elements can be implemented very differently in other hubs.
Most of the hubs contain a sprocket as drive member but there are also constructions with pulley for a belt drive to be found. Moreover you’ll find combinations, which – similar to a racing bike – use a gear rim instead of a single sprocket. The sprocket is driven by chain from the chain wheel and it has mostly between 16 and 24 teeth, the Spectro P5 has often times 21 teeth. Contemporary sprockets matching for most of the hubs are designed identical at its bore diameter (dimension about 35mm) comprising three lugs meshing with three corresponding grooves of the drive member when joined to it thus transmitting the torque. They are locked by circlip and washer. In some historic hubs the sprockets have a female thread and are screwed on, others have an internal toothing and are locked by a nut. There also exist some hubs with significantly bigger sprockets due to its larger designed shifting unit. The sprocket of the Fichtel & Sachs Elan has 26 teeth with an inner diameter of 80mm.
In some hubs the sprocket is placed directly on a gear component (e.g. in the Sturmey-Archer X-8RF8 this is the planet carrier of the first gear train). Various hubs contain a driver holding the sprocket and distributing the power flow to the different gear components by a clutch. In the Spectro P5 the driver is a bushing with internal splines including a sliding clutch (radiating clutch). This clutch can be joined selectively with the planet carrier or the ring gear. In the range of the sprocket the driver contains the grooves as mentioned before, apart from that it contains two bearing seats. With an internal ball ring it is supported on a fixed bearing cone of the hub axle and with an external ball ring the hub shell is supported on it. Various hubs contain a driver designed in this way. In some hubs the driver contains internal pawls meshing with a gear component which can be deactivated by a cam plate or similar (e.g. Shimano Nexus Inter 7). Some hubs also contain a driver with both forward directed and backward directed pair of pawls meshing with a toothing within the ring gear which can be deactivated in pairs. The backward directed pawls cause high braking force when backpedaling. Many historic Sturmey-Archer hubs contain a claw-like driver with radiating clutch.
The hub shell is ball bearing mounted in all hubs both on the drive side and the brake side, additionally a ball bearing is integrated in the drive member. The Spectro P5 – like most of the other hubs as well – doesn’t contain standard ball bearings but ball rings revolving between bearing seats. Thus the driver contains a small bearing seat and a small ball ring revolving on the seat of the fixed bearing cone. A large ball ring placed on the second bearing seat of the driver supports the drive side of the hub shell. Finally there is a third, medium-sized ball ring placed between the bearing seat of the brake arm and the brake side seat of shell. All other gear elements are mostly not ball bearing mounted but run within oil or grease – apart from some special hubs with needle bearings within the gearwheels (e.g. Rohloff Speedhub). Some historic hubs contain additional bearings, e.g., the Fichtel & Sachs Model 29 has a built-in needle roller and cage assembly between the planet carrier and the drive cone. In Sturmey-Archer hubs the ball ring within a bearing ring is always screwed in the hub shell on the sprocket side.
The axle of a hub presents the ‘base frame’ of the gearing and it is always screwed non-rotatable to the bike frame. In simple hubs the sun gear is firmly mounted to the axle or sun and axle are produced from one part. The Spectro P5 contains a dual stepped planetary gear train, i.e. two suns rotate on the axle with one sun being freely rotatable and the other one being fixed. In many hubs the axle contains some components of the clutches in order to connect the different gear members with each other. The Spectro P5 has a hollow drilled axle including two indexing pins, springs and sliding blocks which protrude from two elongated holes bothsided. Via the sliding blocks the sun gears get fixed or released and a radiating clutch gets moved within the driver. The axle has a small toothing for fixing the sun gears when they mesh with it. This pretty simple design is implemented in various hubs. In some other hubs the sun gears get fixed in another way, e.g. by a shift drum situated around the axle, to be found within the Shimano Nexus Inter 7 or Inter 4. In that case the axle has small cams in the area of the suns and combined with the shift drum this allows specific sun gears to be fixed or released at specific rotation angles. This can be called an axle unit, because some components of the internal clutch are already integrated in the axle and a cam plate, springs, etc. which regulates the external clutch. The axle unit of the Shimano Nexus Inter 8 is even more complex, since the axle contains small extendible pawls.
PLANETARY GEAR TRAIN
A planetary gear train is the heart of every geared hub, in the Spectro P5 it is a single stepped planetary gear train providing two hill gears (speed reducing ratio when the ring gear is driven), two fast gears (speed increasing ratio when the planet carrier is driven) and one direct gear.
The ring gear has a short toothing in its rear area which allows it to be coupled with the radiating clutch. Various Sturmey-Archer hubs share the same design but they have an internal wedge profile instead. Furthermore, pawls can be seen meshing with a corresponding toothing within the hub shell (freewheel). Ring gears as input member AND output member (with pawls) are often to be found in geared hubs. The pawls are often implemented with a spring ring in Fichtel & Sachs hubs. In Sturmey-Archer hubs, however, there are bigger pawls rotatably mounted in a pin.
The planet carrier comprises stepped planets supported on both sides, because an axle mounted on one side would bend too much. Some hubs also contain planet wheels mounted on one side, but only with simple gear trains. The planet carrier merges into the brake screw which is needed for the coaster brake version. The two staged planets are produced from one part, some hubs containing triple stepped planets use two pluggable parts. In some historic hubs the planet carrier also contains an internal toothing in which the sun gear could be inserted for achieving a direct gear. Planet carriers as input member AND output member (with pawls) are often to be found in geared hubs as well. In the Spectro P5 the pawls of the planet carrier are installed in the braking cone which co-rotates with the planet carrier. In many Sturmey-Archer hubs the planet carriers have directly installed pawls – similar to the ring gear – since often there’s no coaster brake.
There are several grooves in the suns which can be moved by the sliding block in a way that one sun remains rotatable or gets fixed within the axle toothing. This principle – to insert a sun with internal toothing to an external toothing of the axle – can be found in various hubs. But there are also other interesting principles – if hubs are not shifted with a rotary shifter instead of a toggle chain: The sun gears of the Nexus Inter 4 or Inter 7 e.g. have small internal pawls revolving around a shift drum. Depending on the rotation angle of the drum the pawls climb over the cams of shaft and drum (sun released) or they get locked (fixed sun).
Let’s define external clutches as clutches which join gear members within a gear train to gear members outside the gear train. In the Spectro P5 this is the radiating clutch which can be moved within the driver in order to get connected with the planet carrier or the ring gear. The driver is internally splined in order to transfer torque, allowing to transfer the rotation of the sprocket to the radiating clutch. The clutch can mesh with the planet carrier by its radiating outline and allows meshing its small toothing to mesh with the toothing of the ring gear. The radiating clutch can be moved by pressing the external indexing pin (pipe) against a sliding block which moves a spring loaded ring towards the clutch. The external clutch allows selecting between hill gear, direct gear and fast gear and the internal clutch allows selecting within two hill gears or two fast gears by fixing the corresponding suns.
Fast gears: The external indexing pin is pushed in completely and the radiating clutch is joined to the jaws of the planet carrier with its radiating outline. The ring gear is the output and drives the hub shell by its pawls. The pawls of the planet carrier rotate as well but the get ‘overtaken’ by the toothing within the hub shell, thus they cannot transfer any torque and you can hear the distinctive fast ‘klacking’.
Direct gear: The external indexing pin is pushed in mid-position causing the radiating clutch to release the planet carrier and connecting the clutch with the toothing inside the ring gear. The pawls of the ring gear now revolve the hub shell as fast as it is driven by the driver, the gear train is bypassed and the pawls of the planet carrier is overtaken as well without transferring any torque. The klacking gets slower.
Hill gears: The external indexing pin is not pushed, the radiating clutch is pushed to the sprocket side by a spring thus also pushing the ring gear. The pawls of the ring gear get disengaged with the toothing of the hub shell and cannot transfer any torque any more. Now the ring gear is driven and the slower rotating planet carrier is the output, i.e. the hub shell is driven by the pawls of the brake cone in the toothing of the brake side.
A lot of hubs with 3, 5, or 7 speeds work that way, containing a similar designed external clutch. The planetary gear train is driven in both directions (bi-directional), with speed increasing ratio AND with speed reducing ratio when reversing the power flow. Some hubs work without an external clutch, e.g. the Shimano Nexus Inter 4 only contains internal clutches for the sun gears, and there is only a speed increasing ratio (unidirectional power flow). In many Fichtel & Sachs hubs the radiating clutch pulls the ring gear with its pawls out of the toothing of the shell thus achieving the hill gear. In Sturmey-Archer hubs, however, the pawls of the ring gear are deactivated when the radiating clutch presses them radially inwards out of the toothing of the shell.
Let’s define internal clutches as clutches which join gear members within a gear train to gear members within the same train, e.g. sun gear to the axle. In the Spectro P5 this is a sliding block to be actuated with the internal indexing pin in order to move sun gears against spring force. The displaced sun gets engaged with the axle toothing by its internal notches and thus it is fixed. In the stepped planetary gear train you achieve a new stationary gear ratio i12 which provides two different speeds increasing and two different speed reducing gear ratios instead of one. The different manufacturers have found some more interesting conceptual designs in order to fix or to release suns: e.g., in the Shimano Nexus Inter 7 you will find pawls within the sun gears which climb over the cams of the axle or get locked depending of the rotation angle of the shift drum. The Shimano Nexus Inter 8 uses the inverted principle: the sun gears are internally splined and revolve around controllable pawls situated on the axle. In the Sachs Elan slewable cam rods are installed in a notched axle instead of a shift drum thus causing the pawls of the suns either to climb over the notches (sun released) or to get locked against the bars of the axle (sun fixed).
A lot of hubs have two transmission outputs – as already mentioned in the section External clutch – some have only one output, but all outputs are always joined with the hub shell. To prevent the pedals from rotating permanently the power transmission to the hub shell is mostly provided by ‘free-wheels’. These clutches only allow transferring the torque in one direction. If the drive member rotates slower than the hub shell – i.e. the feet rest on the pedals or you pedal backward – the drive member is decoupled. The output (hub shell) can ‘overrun’ the input, that’s why these elements are also called ‘overrunning clutches’. There are some geared hubs without freewheels causing the pedals to rotate permanently, so-called ‘Fixed gear’ hubs, among them the historic Sturmey-Archer Model ‘ASC’, which is highly desired among the collectors. The Spectro P5 has pawls attached at the ring gear and at the brake cone which is driven by the planet carrier, accordingly the hub shell has two internal toothings fitting to the pawls. There are also hubs with only one output, e.g. the Sturmey-Archer X-8F8(W) has only one transmission output and therefor only one internal toothing within the shell. Some hubs have rollerramp clutches instead of pawls, small rollers are pressed radially outwards by an adequate profiled ring and drive on the unprofiled shell completely soundless. These rollerramp clutches are used in the Sachs Doppeltorpedo or the Shimano Nexus Inter 8. Some historic hubs, e.g. Sachs Model 29 also use a drive cone being pulled into a cone ring by a worm gear and thus driving the hub shell via the cone ring. This design is also completely soundless. A characteristic feature of Sturmey hubs is the screwed in ball ring at the driver side which also contains the internal toothing for the pawls of the ring gear.
The Spectro P5 contains a coaster brake according to the ‘brake screw’ design to be seen very often in coaster brake hubs. The planet carrier has an attached worm gear carrying a brake cone with pawls. The brake cone screws toward the planet carrier, gets revolved and transfers torque to the hub shell when pedaling in driving direction. While backpedaling the brake cone screws reverse and spreads the brake band which presses tangentially against the hub shell and brakes the bike. Coaster brakes also can be realized with the ‘spreader roller’ design operating similar to a rollerramp clutch already described. While backpedaling the rollers get pressed radially outwards spreading the brake band. In both designs the brake band is attached in the brake arm, unable to revolve, because it has to transfer torque via the brake arm to the bike frame.
The hub shell as the output member transfers the torque via the hub flanges to the back wheel. It is always ball bearing mounted on both sides and supplied with bearing seats, at the driver on the sprocket side and at the brake arm or the bearing respectively on the brake side. The shell of the Spectro P5 contains two internal toothings for both pawls of the ring gear and the planet carrier. The shell is stepped at the brake side since the internal gearing is smaller in size there. Most of the hubs of Sturmey-Archer are not stepped, but simply cylindric. Some historic hubs have built-in pawls instead of an internal toothing. In this case the toothing is performed in the planet carrier, e.g. in the Sturmey-Archer Model ‘K’. In some historic Sturmey-Archer hubs with two gear trains the ring gear is attached within the hub shell.