Inside Internal-Gear Hubs

Internal-gear hubs – also known as “planetary gears” – were first devised around 1890 in England, and have been produced in half a dozen countries ever since. Sealed within a hub, they are less susceptible to damage than derailleur gears, so it’s not uncommon to find units half a century old still in service.

Although they were once popular in North America, the peculiar evolution of cycling from pleasurable transportation to rigorous sport here has meant that for three-quarters of a century, hub gears have comprised a minuscule part of the market.

However, with renewed interest in low-maintenance bicycles suitable for commuting, a resurgence of hub gears is making its way to a bike shop near you. They are a sturdy, fuss-free way of providing gearing options for urban hills and windy days and can even help add joy and convenience into your daily travels.

But what goes on inside those mysterious, large hubs at the center of your rear wheel?

On the axle of an internal-gear hub resides the “sun gear,” around which the “planet gears” revolve. The planetary gears mesh with the sun gear at the center along with a toothed ring on the outside. As your wheel turns, the planetaries and outer ring are always turning as well. Connected to these are two sets of pawls that transmit driving forces to the hub shell as you pedal, one set (driven by the outer ring) turning faster than the other (driven by the planetaries).

In order to shift gears, a clever little device is attached to the control cable from your shifter and slides back and forth, disengaging one or the other set of pawls. This movement results in either the high or the low gear. In a third position, it allows the sprocket to drive the hub shell directly, giving the middle gear.

The result of these moving pieces is the famous three-speed hub. The range of gears is determined by the relative sizes of the sun and planetary gears within the hub.

While three speeds are often enough for flat cities, riders who take on hills and longer distances often want more. To add more gears, for example in a five-speed hub, you have two sets of planetaries of different sizes. Since one gear of each set is direct-drive, that makes five, not six gears as you might expect. You will find hubs with up to 14 speeds, but all are variations of the three-speed.

There is an exception to this design: the NuVinci, which uses nesting cones and big steel ball bearings to provide a stepless movement throughout its range. By twisting the grip shifter, you’re not moving internal gears but sliding cones for an infinite choice of speeds.

The world of internal-gear hubs may sound complex, but due to their sealed nature and near-maintenance-free operation, you may never need to know what’s going on to keep you moving. While internal-gear systems are heavier than derailleur systems, the conveniences they provide, such as the option of using a full chaincase to keep your hems out of a greasy chain, are gaining popularity with transportation cyclists across North America.

Richard Risemberg is a daily cyclist in Los Angeles. He designs bike-friendly clothing, writes for numerous publications, and butts into bike politics regularly. bicyclefixation.com @bicyclefixation