How it Works

The Velotegra hub is planetary transmission hub, but it has a different kind of planetary geometry than conventional transmission hubs.

Sprocket Hub 6

 

The bevel benefit

The design employs a bevel planetary geometry rather than a standard planetary geometry. The figure below is a schematic showing a standard planetary unit (left) and the equivalent bevel planetary unit (right).

Bevel benefit 4

As can be seen, the bevel planetary configuration allows much larger planet gears because the width of the hub provides the needed space. The larger size allows these planet gears to be designed as sprockets, which, in turn, mesh with roller teeth on corresponding ring gear elements. This reduces the meshing friction of the gear train and increases the allowable torque handled by the hub. The gear ratio is obtained by the angle of mounting of the planet elements, rather than by their size. Thus the size of the planet elements can be same for all ratios, maintaining the space needed for sprockets and roller teeth for every gear.

Gear Changes

The bevel planet gears do not change their angle to change the gear ratio. A specific planet gear cannot change its angle because the geometry of each planet gear is unique to its designated angle of mounting. Instead, gears are changed by mounting opposed planet gear pairs at different angles around the circumference of the planet gear holder, as shown in the figure below, and selecting which pair is “active” in the gear train.

Multi angle 4

The gear pair is selected by holding the ring gear of that pair stationary, and allowing the remaining planet gears to run free. In this way only the designated planet gear pair will drive the output gear. This results in the following gear shifting sequence, as illustrated in the figure below:

First gear: Gear shift slider (A) is in its lowest position, allowing all the ring gears to turn freely. Therefore no drive is transmitted through the bevel planetary gear train and the hub shell (F) is driven directly by the main shaft (B) through the freewheel (C). This gives a 1:1 ratio.

Second gear: The gear shift slider (A) is positioned to prevent the rotation of the first ring gear (R1). When the main shaft (B) is turned the planet gears (G) turn the output gear (E) at a higher rate of speed than the main shaft (B), while the other planet gears and associated ring gears turn freely. The output gear (E) therefore drives the hub shell (F) through freewheel (D) while freewheel (C) over-runs.

Third gear: The gear shift slider (A) is positioned to prevent the rotation of the second ring gear (R2). When the main shaft (B) is turned the planet gears (H) turn the output gear (E) at a higher rate of speed than the main shaft (B), while the other planet gears and associated ring gears turn freely. The output gear (E) therefore drives the hub shell (F) through freewheel (D) while freewheel (C) over-runs.

Fourth gear: The gear shift slider (A) is positioned to prevent the rotation of the third ring gear (R3). When the main shaft (B) is turned the planet gears (J) turn the output gear (E) at a higher rate of speed than the main shaft (B), while the other planet gears and associated ring gears turn freely. The output gear (E) therefore drives the hub shell (F) through freewheel (D) while freewheel (C) over-runs.

An advantage of this configuration is that the ring gears are stationary under load, and only turn when not loaded. This simplifies the bearing design for the ring gears.

Four speeds 6

The adaptable hollow shaft

To provide adaptability between regular chain-drive and direct-drive, the main shaft (B) is hollow, as shown below. For chain-drive, a threaded axle (K) with bearings (L) is installed in the hollow shaft so that the hub can be mounted on the bike with the threaded axle. A sprocket (M) is then fixed to the exterior of the main shaft on the right side, to provide drive to the main shaft. For direct-drive, the threaded axle is replaced with a crank axle (N), securely fitted within the hollow region of the main shaft. There is no sprocket mounted, but instead bearings (P) are mounted to the exterior of each end of the main shaft. The hub is then mounted to the bike by these bearings.

Hollow shaft 6

The stationary mounting surface

The bevel planetary configuration evenly spaces each of the fundamental elements of the hub transmission at distinct locations along the axial length of the hub. The ring gear elements are all on the right side of the hub, the planet gear holder and planet gears are in the central region, and the output gear and hub shell freewheel are on the left side. Unlike some hub designs, these elements do not interchange their respective functions: the ring gear elements are the only elements held stationary; the planet gear holder is the only driven element; and the output gear never functions as a stationary or input element. This simplifies the gear shifting mechanism, and places all the stationary elements on one side of the hub. This facilitates mono-blade mounting by providing a large stationary surface on one side of the hub (the right side). Conveniently, this is the same side as the gear shift selector, simplifying the gear shift cable configuration.

The hub can be mono-blade mounted for chain-drive or direct-drive, as illustrated below. For more details of the four mounting and drive combinations available, see the specific web pages.

monoblade 4