Hub Gears Versus Derailleur Gears: The Mechanical Differences

The big difference is where the gear change happens. A derailleur system moves the chain across a stack of sprockets in the open. A hub gear changes ratio inside a sealed hub, and the chain stays put.

Shifting

On a derailleur bike the chain has to be moving for a shift to work. Wikipedia describes a hub gear as able to change gear "when the rear wheel is not rotating"¹. That helps at stop lights and in stop-start traffic.

The reverse needs care too. The same source says it "can be hard or impossible to select another gear while pedalling, because a release of pressure is required"¹. For e-bikes and motor assist, follow your own maker's manual. See Shifting a Hub Gear on an E-Bike.

Gear spacing

Many derailleur bikes have two shifters. A hub gear usually has one, and the ratios don't overlap. Wikipedia says that with a hub there is "generally only a single shifter to operate and there are no overlapping gear ratios"¹. One 14-speed example is described as having "14 equally spaced, sequential, non-overlapping gear ratios operated by a single twistgrip"².

Exposed or sealed

A rear derailleur "is positioned in the path of the bottom, slack portion of chain"³. It hangs low, close to the road and the dirt. A hub gear keeps its gears in the shell.

Wikipedia adds that on a derailleur, "the chain will only touch the cage while shifting" when the unit is properly adjusted³. It's a reminder that a derailleur needs setting up and checking.

The chain's path

With one rear sprocket, the chain runs in a straight line. Wikipedia says this "single chainline allows for a full chain enclosure chain guard"¹. A chain guard keeps grit off the chain and clothing off the grease.

The same source adds that on bikes with fixed chain-lines, "no chain tensioner is required"¹. A tensioner is a sprung arm that takes up slack.

A rear derailleur does two jobs. Wikipedia says it "moves the chain between rear sprockets while taking up chain slack"³.

Trade-offs in plain terms

That last row matters when you fix a flat. Wikipedia says the rear wheel "cannot be completely separated from a bicycle with hub gears without also disconnecting the gear cable"¹. On an e-bike, motor and brake leads may add more steps, so the maker's manual comes first.

What to ask a seller

• Who makes the hub, and can you read its manual?
• How does that manual describe wheel removal?
• What does it say about shifting while the pedals are under load?

Those answers tell you more than a speed count on a spec sheet.