What Is an Internal Gear Hub?

An internal gear hub is a gearbox built into the middle of the rear wheel. The gears that change your ratio are inside the hub shell. You won't see a row of sprockets. You won't see a derailleur either, which is the sprung arm that pushes a chain between sprockets.

What's inside

Wikipedia describes the system as one "implemented with planetary or epicyclic gears"¹. Those are small gears that circle a central gear, a bit like planets around a sun. The page How Planetary Gearing Works Inside a Hub covers the idea step by step.

The same source says the gears and lubricants are sealed in the hub shell. That's "in contrast with derailleur gears where the gears and mechanism are exposed to the elements"¹. Wikipedia also says that one long-established hub maker uses planetary gear trains in all its hubs, from simple to complex².

What you see from outside

Look at the rear wheel. You'll find one sprocket, not a stack. A chain runs from the pedals to that one sprocket. A cable usually runs from the shifter on the handlebar to the hub. The maker's manual for your hub says exactly how yours is shifted.

The shifter is often a twist grip. One maker's page describes it plainly: "Rotate up to shift up, down to shift down"³.

What changes for the rider

Shifting can happen when the wheel isn't turning. Wikipedia notes that hub gears "can change gear ratios when the rear wheel is not rotating"¹. A derailleur can't do this, because it needs the chain to move.

There's a cost. The same source says hub gears "tend to be heavier than derailleur systems with equivalent gearing characteristics"¹. The extra weight also sits in the rear wheel.

Where you'll find them

Gear hubs aren't new. Wikipedia says patents for epicyclic hubs date from the mid-1880s, and that European city bikes are now commonly fitted with 7-speed gear hubs¹. Many folding bikes use 3-speed hubs, according to the same page¹.

Not every e-bike has one. Plenty use a derailleur. The two are compared in Hub Gears Versus Derailleur Gears.

One fitting detail

Wikipedia says hub gears are "generally incompatible with quick release mechanisms/skewer axles"¹. In other words, the wheel is usually held on with nuts, and the frame has to suit that. Check how the wheel is fixed before you assume you can take it off roadside.

Questions worth asking before you spend money

• Which hub is fitted, and does the maker publish a manual for it?
• How does the hub shift, and what does the manual say about it?
• What does that manual say about oil, if anything? See Hub Gear Oil and Service Intervals.
• Which motor is on the bike, and does the hub maker say anything about the pairing? See Hub Gears With a Mid-Drive Motor.
• How wide is the range? See Gear Ratio Range Explained.

Ask to read the manuals for the hub and the motor, not only the sales sheet. If a seller can't name the hub, that's worth knowing before you hand over any money.