Condensed Answer: Technically, it’s possible to install a freewheel onto the larger threads of a fixed-gear rear hub.
However, the freewheel’s thread is about two times wider than the large thread on the fixed hub.
Thus, 50% of the freewheel’s threads will not engage, and one can argue that the system won’t be as secure as possible.
A safer choice would be to get a flip-flop hub with a dedicated freewheel side.
Characteristics of True Fixed-Gear Hubs
A true fixed-gear hub has stepped-down threading on both sides consisting of a large thread that’s close to the hub’s flange and a smaller thread on the outer side.
The large thread is designed for the fixed cog and runs clockwise.
The smaller thread is for the lockring and runs anti-clockwise (reverse threading).
The purpose of this engineering is to prevent the fixed cog from untightening in all situations.
When the rider is pedaling forward, the chain is dragging/spinning the cog in a clockwise direction and thus tightening it against the hub even more.
However, if the rider back-pedals, the motion applies pressure in the opposite direction (anti-clockwise) and creates an opportunity for the cog to untighten itself.
Since back-pedaling is one of the ways to slow down a fixed-gear bike, this problem is addressed by using a lockring with a reverse thread.
When tightened the lockring squeezes the cog. Subsequently, when the cog tries to untighten itself during back-pedaling it spins anti-clockwise and pushes against the lockring.
But since the lockring has a reverse thread, the cog’s untightening motion tightens the lockring. And if the lockring is tight, there’s no way for the cog to “escape”.
Installing a Freewheel On a Fixed-gear Hub
The only way to install a freewheel cog on a fixed-gear hub is to screw it onto the larger thread of the hub. To tighten it maximally, you will need a chain whip.
However, the thread on the freewheel is about 10mm wide. Meanwhile, the large thread on the hub is approximately 5mm. Thus, 5mm of the freewheel will not be supported.
A professional mechanic might refuse to make this conversion because the freewheel may untighten itself.
That said, some people do the conversion this way without experiencing any problems.
Of course, no one can say with 100% certainty whether you will be one of them.
FAQ: Does the freewheel need a lockring?
A freewheel doesn’t necessitate the use of a lockring because the back-pedaling motion does not untighten it. Instead, the freewheel spins backward thanks to a ratcheting mechanism.
Flip-flop hubs have different threading on both sides. One side is designed for a fixed cog and therefore has a big and a large thread.
The other is built for a freewheel and has wide threading that will fully support a freewheel, not just 50% of it which is the case for fixed/fixed hubs.
Flip-flop hubs allow you to quickly and safely switch between a fixed-gear drivetrain and one with a freewheel. Or in other words, the user can run the bike as a fixie or a single-speed machine.
Flip-flop Hubs vs. Fixed Hubs
The advantages and disadvantages of Fixed/Fixed Hubs
- The user can run two different fixed cogs on both sides. This is beneficial for fixed-gear riders who want to have a lower gear.
A flip-flop hub will not allow you to safely run two fixed cogs because one side of the threading is designed for a freewheel and thus one cannot install a reverse-threaded lockring.
That said, some people run fixed cogs on hubs designed for freewheels by relying on a lockring with normal threading.
The negative of this approach is that the lockring and the fixed cog can unscrew when the rider backpedals. If the rider is relying solely on back-pedaling for stopping, an accident may take place. For that reason, it’s not recommended to use this technique without brakes.
- If the user runs a freewheel, it won’t be fully supported by the hub as explained above.
The advantages and disadvantages of Flip-flop hubs
- Safe installation of a fixed-cog on one side and a freewheel on the other.
- Flip-flop hubs do not allow a safe installation of a fixed cog on both sides due to the aforementioned reasons.
In conclusion, a flip-flop hub provides more versatility and will be a better choice for most cyclists. A fixed/fixed hub, however, is better for cyclists who only ride fixed but would also like to have a lower gear.
Summary: What You Need To Know
- A freewheel can be installed on a fixed-gear hub, but it won’t fully thread onto the hub because fixed-gear hubs have “stepped-down” threading. Thus, 50% of the freewheel won’t be supported.
Many people have done this procedure without experiencing failure. Still, one has to be informed of the problem before making a decision.
- A flip-flop hub has enough threading on one side to fully support a freewheel. However, flip-flop hubs cannot safely accept a fixed cog on both sides whereas fixed/fixed hubs can. Thus, die-hard fixed fans who want to run two different gear ratios while still riding a fixed drivetrain will need a fixed/fixed hub.