How Planetary Gearing Works Inside a Hub

A planetary gear set gives different ratios by changing which part drives and which part is driven. Nothing slides sideways, and the chain never moves. That's how a sealed hub can offer several gears.

The four parts

Wikipedia's article on epicyclic gearing describes "a sun gear (yellow), planet gears (blue), and a carrier (green) inside a ring gear (red)"¹. In plain terms:

• The sun gear sits in the middle.
• Planet gears mesh with the sun and run around it.
• The carrier is the frame that holds the planet gears.
• The ring gear, also called the annulus, wraps around the outside with teeth on the inside.

In a bicycle hub, the sun gear "is mounted solidly to the axle and is thus fixed relative to the bicycle frame"². It doesn't spin.

How one set gives three ratios

Think of a car's gearbox, but folded into a ring. You pick which part is held still and which part you push on. The output turns at a different speed each time. Wikipedia's general rule is that by holding the carrier, the ring gear or the sun gear still, "three different gear ratios can be realized"¹. A bicycle hub keeps the sun gear fixed and swaps the other two roles around. In a simple three-speed hub, the source describes it like this²:

• Low gear: the sprocket drives the ring gear, and the carrier drives the hub. That's a gear reduction.
• Middle gear: the sprocket drives the hub directly.
• High gear: the sprocket drives the carrier, and the ring gear drives the hub. That's a gear increase.

Shifting changes which part is connected to which, inside the shell. The chain stays on its one sprocket.

Simple and complex hubs

Wikipedia says the simplest hub gear model uses "one set of planetary gears with four planets"³. That's the layout described above.

More speeds come from linking more than one set. One 14-speed hub is described as having "three planetary gear series, connected one after the other"⁴. An 11-speed hub from another maker is described as having "four stepped planetary series" offering up to eleven speeds⁵. A stepped planet is a planet gear with two sizes of teeth on one shaft. Each extra set adds options, and together they multiply into more ratios. The page Gear Ratio Range Explained shows what those numbers mean.

The axle takes a twisting load

Wikipedia says the axle of a hub gear carries torque in every gear except direct drive, "so must be securely braced against rotation"². The page on one 14-speed hub talks about a "torque reaction that must be countered"⁴.

For a buyer, this means the frame, dropouts (the slots the axle sits in) and any torque arm matter. Look at how the maker's manual says the axle should be held. Follow the manual for your own hub and frame.