One of the uncertainties that come up if you want to assemble a fixie bike yourself by restoring an old road bike or directly by buying a track frame is how to mount a fixed or freewheel cog to the back wheel.
Well, for the fixed gear cog, you need to be sure that the rear hub is compatible. To find out about this, you need to check if the hub has two threads with different diametres on at least one of the sides. One thread is for the cog and one for the lockring. In the picture of a flip-flop hub (meaning on one side you can mount a fixed gear and on the other a freewheel cog) you’ll see that at one of the sides the threads have two different heights:
If you look at the left part of the upper photograph of the hub, the fixed gear cog is mounted on the thread on the inner side and is threaded clockwise, the lockring, however, is mounted in the opposite direction. It is easy to see where to put the cog and where the lockring fits, because the lockring doesn’t fit on the bigger thread and the fixed gear only fits on the larger thread.
You can also see that the fixed gear cog is not the same on both sides. On one side, it is completely flat while on the other, the cog looks like this picture:
The lockring, that is the ring that holds the cog and prevents it from spinning off the hub, is mounted to the thread with the smaller diameter and in the opposite direction as the fixed gear cog.
To mount both rings, you need at least a tool to tighten them really well. You have to use a lot of force and tighten until it is not possible to tighten any more.
The chain on the tool will be used to tighten the cog onto the hub, the other part of the tool, shaped like a croissant, is used to tighten the lockring.
If we don’t tighten the cog properly, it can loosen and if that happens, you could even fall off the bike. But the most frequent case is that the cog sort of “eats” the thread of the hub, leaving it unusable. The majority of the manufacturers don’t guarantee for hubs if they have been damaged following bad assembling or insufficient maintenance.
The freewheel cog is really easy to mount. You have to be sure to mount it in the right direction and on the right side. Even if the freewheel cog can be mounted on both sides of a flip-flop hub, like the one of the photo, it is best to put on the widest diameter threading. Look at this picture, you’ll see that it has grooves next to the letters.
Those grooves serve in the future to extract the cog using a special tool, that’s why you’ll have to mount it with the grooves facing outwards. To tighten the freewheel cog, you don’t need a specific tool, because it can be tightened with the hands as far as possible. The pedal movement will help with the final adjustment.
We hope that these tips will help you for the assembly. And finally, a video from our Youtube Channel, where one of our mechanics explains how to install a fixed gear cog: