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. A conventional planetary transmission unit (right) includes a shaft (in grey) 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 shown has a step-up ratio of 1 to 2.67 (input to output).
The planet gears of the bevel-planetary unit may be arranged horizontally in a stacked configuration (right) rather than angled (above). This gives the same ratio 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.
To gain more than one speed, more ring gears can be added. As illustrated at right 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.
The figure at left 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 these examples have 18 teeth, but the bevel-planetary unit has three times the tooth size of the conventional unit. This gives much stronger teeth and more torque capacity, even with only two planet gears, as illustrated.
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 (shown in the figure at left). The rollers reduce the internal friction and wear of the hub by replacing the sliding friction between the teeth with rolling friction.
