FLIP FLOP HUB

A flip-flop hub — sometimes written flip/flop, or called a double-sided or dual-sided track hub — is a rear hub with driver threads on both ends of the shell. You undo the wheel, turn it around, put it back in the dropouts, and you are driving a different sprocket.

Fixed/free versus fixed/fixed

There are two ways a flip-flop is normally set up, and they are not the same product decision.

Fixed/free puts a fixed cog on one side and a freewheel on the other. Flip to the freewheel side and you can coast. This is the common factory configuration on complete bikes, and it is what most people mean when they say "flip flop".

Fixed/fixed puts two fixed cogs of different sizes on the two sides — say 16T and 18T. You stay fixed either way and flip to change the gear: a taller gear for flat riding, a shorter one for hills. Both sides need a lockring.

The two threads, and why one of them is backwards

The fixed side of a track hub carries two concentric threads. The larger one takes the cog. The smaller one, cut just outboard of it, takes the lockring — and it is a left-hand (reverse) thread.

That reversal is the whole point. Under load the cog tightens onto its own thread, but when you resist the pedals to slow down, the force tries to unscrew it. The lockring, threaded the other way, tightens under exactly that condition. The two fight each other, which is what keeps the cog on.

The freewheel side is usually cut with the larger thread only, and no lockring thread. A freewheel does not need one — it only ever drives forwards, so it tightens itself.

Cog thread standard · Lockring thread standard · 120mm track spacing

What changes when you flip

Flipping is not free. Three things move, and two of them will bite you if you ignore them.

Gear. Obvious on fixed/fixed, easy to forget on fixed/free if the sprockets differ in size. Run the numbers before you commit to a hill — our gear inches calculator takes both.

Chain tension. Different sprocket sizes need different chain lengths, so a wheel that sits at one point in the dropouts on one side will need moving on the other. On a fixed/fixed with a two-tooth difference this is normal and expected.

Chainline. The two sides do not necessarily place the sprocket at the same distance from the frame centreline, particularly if you have spacers or a mixture of cog brands. A chainline that is fine on one side can be several millimetres out on the other, which on a fixed drivetrain raises the risk of throwing the chain. Measure both sides: chainline calculator.

Skid patches change too

If you skid, flipping changes how many contact patches you have — and it can change them dramatically. A 48/16 setup reduces to 3/1 and gives you a single patch; move to 48/17 and you get seventeen. Same wheel, same rider, one tooth of difference. Skid patch calculator.