Single Speed Conversions: Fixed Gear Hubs

Here is a Fixed Gear Threaded Hub:

Small Flange Miche Fixed Gear Bicycle Hub

You Can See on the Right Hand Side Two Rows of Threading on the Hub 

The first row of threads is for a fixed gear sprocket (Silver, Below)
Silver Fixed Gear Sprocket and Black Fixed Gear Lock Ring
The second set of threads is for a Lock Ring (Black, Right Above)

A Fixed Gear Sprocket Needs to Be Locked Into Place with a lock ring in order to stay in place. Furthermore the lock ring has a different diameter to the sprocket, and it screws onto the hub the opposite direction to the sprocket - the latter significantly increasing the security of the system.

This is why the two sections of threading are needed on the hub.

Flip Flop Hub
There is a particular kind of Fixed Gear Hub known as a Flip Flop Hub.

This is simply a hub with fixed gear threading like that above on one side; and then it has standard freewheel threading on the other side.

Flip Flop Bicycle Hub on Gold SSC Premium Wheel
Flip Flop Hub: Two Section Fixed Gear Threading on the Left hand side; The right hand side has a Single Row of Standard Threading

We believe the name Flip Flop comes from the fact that since you can fit one freewheel and one fixed gear sprocket on the wheel you can simply 'flip' the wheel over any time you want to change riding style. The archetypal example of this kind of behaviour is someone on the way home after a tiring day who flips from fixed gear to freewheel in order to coast down a long hill home...

Things to Know about Fixed Gear Hubs:

  1. Necessary for Fully Secured Fixed Gear Riding: As noted above you need the second row or threads on a fixed gear hub in order to properly secure a fixed gear sprocket. With a properly fitted fixed gear sprocket and lock ring we believe you can reliably use your pedals as a rear brake and forego a rear hand brake without causing undue risk to yourself or violating the law. Fixed gear sprockets can be fitted to standard threaded wheels. But the security is not the same
  2. Three Different Configurations: Fixed Gear hubs may have:
    1. Single sided fixed gear threading: two step fixed gear threading on one side of the hub, no threading on the other side - as in the top image;
    2. Flip flop threading: fixed gear threading on one side, standard freewheel threading on the other as on the gold hub;
    3. 'Double Fixed Gear' threading: two step fixed gear threading on both sides!
  3. You can Fit a Freewheel on a Fixed Gear Hub: Freewheels and fixed gear cogs have the same diameter and gauge of thread. So you can fit a single speed freewheel onto the fixed gear sprocket section of a fixed gear threading if you want. No lock ring is needed for a Single Speed Freewheel. This means that on a Flip Flop hub you could actually just fit two freewheels if you want, with different gearings on them.