How the Hub Works

The Velotegra hub is a planetary bicycle hub transmission, but unlike conventional bicycle hub transmissions it employs bevel gears. This bevel-planetary architecture has some unique benefits compared to conventional planetary units. These advantages and how the hub works are best understood by a comparison with conventional hubs.

Conventional planetary configuration

Figure 1 below shows a conventional planetary transmission unit. A shaft (in grey) is connected to a planet gear carrier (also in grey) which carries three planet gears (shown in silver). These planet gears mesh with a ring gear (in blue) which is prevented from rotating. Therefore the planet gears are forced to drive a sun gear (in red) as they move with the planet gear holder that turns with the shaft. This results in a step-up ratio, with the sun gear turning faster than the shaft. The example in figure 1 has a step-up ratio of 1 to 2.67 (input to output).

 

Standard planetary unit

Figure 1: Standard planetary gear unit

 

Equivalent bevel-planetary configuration

Figure 2 shows the equivalent bevel-planetary unit of the same ratio (where the same colours show the corresponding elements). It is immediately evident that the bevel-planetary layout allows larger diameter planet gears because they are not confined between the sun and ring gears. Instead the planet gears can spread across the width of the hub giving a larger diameter. In this way, the diameter of the planet gears is uncoupled from the gear ratio of the unit, allowing the planet gear diameter to be independently optimized to meet other design requirements, such as torque. A larger torque capacity can be achieved because the large planet gears allow the gear teeth to be larger for the same number of teeth. For example, the planet gears of both figure 1 and figure 2 have 18 teeth, but the bevel-planetary unit of figure 2 has three times the tooth size of the conventional unit of figure 1. This gives much stronger teeth and more torque capacity, even with only two planet gears, as illustrated.

 

Basic bevel planetary unit

Figure 2: Basic bevel planetary gear unit

 

Stacked planet gears

The planet gears of the bevel-planetary unit may be arranged horizontally in a stacked configuration rather than angled, as illustrated in figure 3. This gives the same ratio as the unit of figure 2, but with a simpler geometry. Other advantages of the bevel configuration are retained, such as the decoupling of the planet gear diameter from the gear ratio.

 

Bevel planetary stacked planet gears

Figure 3: Bevel planetary unit with stacked planet gears

 

Roller gears and sprockets

The large tooth size opens up another interesting design option – there is now enough space for regular gear teeth to be replaced with rollers meshing with sprocket-like teeth, as illustrated in figure 4. The rollers reduce the internal friction and wear of the hub by replacing the sliding friction between the teeth with rolling friction.

 

Bevel planetary stacked planet gears rollers

Figure 4: Bevel planetary unit with roller teeth and sprockets

 

Multiple gears and shifting

To gain more than one speed, more ring gears can be added. As illustrated in figure 5, a small ring gear is added that meshes with the lower planet gears of each planet gear stack. When this small ring gear is held stationary by a shifting mechanism (not shown) and the large ring gear is free to rotate, the gear ratio is a step-up ratio of 1:2 (input to output). When the small ring gear is released and the large ring gear held stationary by the shifting mechanism, the gear ratio reverts to 1:2.67 as before. More gears may be added by simply stacking more planet gears with corresponding ring gears. The gear ratio is then selected by the shifting mechanism holding the corresponding ring gear stationary and allowing all the remaining ring gears to rotate freely.

 

Bevel planetary stacked planet gears roller multiple rings

Figure 5: Bevel planetary unit with roller teeth and multiple ring gears

 

Other advantages of bevel-planetary architecture

The bevel-planetary configuration has other benefits also, including:

Higher gear ratios: The bevel-planetary configuration lends itself to higher gear ratios than conventional units. This means that the bike can have a smaller chainwheel and thus a more compact chain line. Higher ratios are also better suited to direct-drive where there is no intermediate chain to increase the overall ratio.

Step-up ratios of 1:2 in a single basic planetary stage: In a conventional planetary hub, a step-up ratio of 1:2 is impossible in a single basic planetary stage since it would require infinitely small planet gears. Even gear ratios close to 1:2 are impractical due to the planet gears being too small. No such restriction is present with a bevel-planetary configuration – a 1:2 step-up ratio is easily obtained, as shown in figure 5 when the lower ring gear is held stationary.

Improved gear contact ratio: The smoothness of meshing between two gears is dependent on the average number of teeth engaged at one time – the more teeth engaged, the smoother the load is transferred. This is known as the gear contact ratio; the higher this ratio the smoother the meshing of the gears. As can be seen from figure 1, in a conventional planetary hub transmission the contact ratio between the planet gears and the ring gear is generally very high (there are many teeth engaged at any given time). This is because the gear teeth approach each other at a shallow angle due to the inward-facing direction of the teeth of the ring gear. However the contact ratio between the planet gears and the sun gear is much lower – the same as an equivalent spur gear pair. With a bevel-planetary configuration, these contact ratio extremes are more evened-out throughout the transmission, resulting in a higher minimum contact ratio. This results in smoother meshing for the transmission as a whole.

Greater built-in clearances: Because the gear teeth are much larger, a larger clearance between meshing surfaces of the teeth can be built-in without any risk of the gear teeth skipping or marring. This has multiple advantages: it permits the hub to be manufactured with enough initial clearance to avoid any requirement of a break-in period to reach full mechanical efficiency; it avoids any binding of the gears in the hub due to high applied pedal loads; and it allows more generous manufacturing tolerances and therefore lower manufacturing costs.

Constant profile bevel gear teeth: Bevel gears normally require a tooth profile that varies radially from the center of the gear outwardly (across the face width of the teeth). However, when the gear pitch is large, the gear face can be small for the same applied load. This means that the variation in tooth profile is small across the tooth face. As a result, the rollers can be cylindrical in shape, rather than frustoconical, and the sprocket teeth can have a constant profile to match the cylindrical roller surface. This simplifies manufacturing and associated costs.