One annoying myth in wide circulation in the fixed wheel world is that you need a fix/free or “flip-flop” hub if you want to put a fixed cog on one side of the hub and singlespeed freewheel on the other.

“Flip-flop” is a generic term to denote any hub that can have drivetrain components attached to both sides but doesn’t describe what kind of component you can attach to either side of the hub. Before the fixie craze was mainly used to refer to BMX hubs.  Most competition track hubs have traditionally been “single fixed”. The term “flip-flop” has become very widely misused to refer exclusively to a fix/free track hub.

The threading for a track cog and a freewheel is exactly the same thread pitch–1.370″ X 24 tpi (threads per inch). Which means you can attach a freewheel to any hub threaded for a track cog.

So if you have a fixed/free hub, here are your options.

• Singlespeed freewheel on one side, nothing on the other
• Track cog and lockring on one side, nothing on the other
• Track cog and lockring on one side, singlespeed freewheel on the other

On the other hand, if you have a fixed/fixed threaded hub, here are your options.

• Singlespeed freewheel on one side, nothing on the other
• Track cog and lockring on one side, nothing on the other
• Track cog and lockring on one side, singlespeed freewheel on the other
• Track cog and lockrings on both sides
• Singlespeed freewheels on both sides

A fixed/fixed hub is much more versatile.

But won’t you have less thread engagement on a singlepseed freewheel mounted to the side of a hub threaded for a track cog?

This is a good question.  The answer is yes, but it doesn’t matter.  The freewheel will have enough engagement to function perfectly.  I’ve never seen a hub or freewheel ruined by this use.

Is there any reason to prefer a fix/free hub?

If you understand the threading and options correctly, I can only think of one fairly ridiculous situation to which other solutions would be preferable that buying a less versatile hub.  The only benefit to the “free” side is that the extra threads allow the insertion of spacers behind the freewheel to correct a too-wide front chainline.  A single chainring road bike (whether a singlespeed, fixed wheel, coaster, or most internal gear hub) should have a chainline of 40.5-42mm.  Let’s say your front chainline was wider than that and you wanted to put 5mm of spacers behind a freewheel to get a straight chainline.  In this case all the extra threads on the “free” side of a fixed/free hub would be a benefit and allow you to get a 47mm chainline by inserting 5mm of spacers between the hub shoulder and freewheel.  But of course doing so would make the other side of the hub unusable, because any cog you theaded on the fixed side will have ~42mm chainline.

So if you’re using a bike with an incorrectly set up front chainline measurement, for some reason don’t want to correct your front chainline problem, and only want to use a singlespeed freewheel then a fix/free hub might be the ticket.