Condensed Answer: While it’s technically possible to install a fixed cog on a freewheel hub, the cog will not be as secure as possible due to the absence of a reverse-threaded lockring. Consequently, the cog may untighten itself when the rider backpedals.
How Does a Fixed Hub Operate?
To understand why it’s dangerous to install a fixed cog on a freewheel hub, one first has to become acquainted with the operation of a fixed-gear hub.
The image above showcases a classic fixed-gear hub with stepped-down threading on both sides.
The purpose of the stepped-down threading is to make the bike safe to ride by preventing the fixed cog from untightening.
Here’s how that works:
The fixed cog is installed onto the larger thread (the one closer to the hub’s flange). The large thread is right-handed and thus the cog is tightened in a clockwise direction.
When the rider is pedaling forward, the chain is also spinning the cog in a clockwise direction and therefore tightening it to the hub.
However, when the rider back-pedals, the chain spins the cog anti-clockwise. Meanwhile, the rear wheel wants to continue spinning forward due to inertia. As a result, it becomes possible for the cog to untighten itself.
To prevent this outcome, a lockring with a reverse thread is used to secure the cog. The lockring goes onto the small thread on the hub.
When the rider backpedals, the pressure on the cog is anti-clockwise. If the cog gets loose and tries to rotate, it will spin the lockring too. But since the lockring is reverse-threaded, the untightening motion of the cog tightens the lockring. As a result, the cog is always secure.
How Does a Freewheel Hub Operate?
Unlike fixed hubs, freewheel hubs have leveled threading. This is necessary because freewheels are always wider than fixed cogs regardless of the number of speeds. Thus, the freewheel requires a longer thread to be fully secured.
If one uses a freewheel on a fixed hub, about 50% of the freewheel will be left non-supported (read more).
The freewheel hub does not need a separate thread for a lockring because back-pedaling cannot untighten a freewheel. Unlike a fixed cog, the teeth of the freewheel will simply rotate anti-clockwise when the rider back-pedals. The rotation of the teeth is independent of the body thanks to a ratcheting mechanism.
The Problems with Installing a Fixed Cog On a Freewheel Hub
If you install a fixed cog on a freehub designed for a multi-speed freewheel, the position of the cog may put the chain too much to the left or right. As a result, the chain will become crooked when looked at from the top.
This phenomenon is known as chain crossing and has two major negatives – more stress on the drivetrain and reduced pedaling efficiency.
The main issue that one faces when installing a fixed cog on a freewheel hub is the stability of the freewheel.
Since there isn’t a separate thread for a lockring, one has to rely on another solution.
To fight this problem, some people screw the cog onto the freewheel hub and then secure it by threading a lockring from an old-school bottom bracket.
While this improvised lockring will definitely increase the stability of the cog, it has a major downside – it’s not reverse-threaded. Thus, when the cog rotates anti-clockwise, it can untighten itself and the lockring.
For that reason, this practice is not recommended, especially if the cyclist is riding brakeless and relies solely on back-pedaling to slow down. If the cog gets loose, the rider will lose the ability to brake. Hence why this method requires the use of a brake.
Why Not a Flip-Flop Hub?
A flip-flop hub has one side designed for a fixed cog (stepped down threading) and another built to accommodate a freewheel (wide, normal threading). The user can switch between the two by flipping the wheel.
Summary: What You Need To Know
- Fixed-gear hubs have stepped down threading that allows the installation of a fixed cog secured by a reverse-threaded lockring preventing unwinding of the cog during back-pedaling.
- Freewheel hubs, on the other hand, have normal, wide threads without a stepped-down portion because freewheels are wider and do not need extra security for safe back-pedaling.
- A fixed cog can be installed on a freewheel hub and then secured with a lockring from an old bottom bracket. While this method can work fairly effectively, the fixed cog may still unwind because the lockring will not be reverse-threaded.
If that happens, the rider will lose the ability to brake by resisting the rotation of the pedals. If one chooses this route, a brake will be needed for safety reasons.