There are two main reasons why a fixed-gear bike would be “slipping” – lost chain tension or a loose rear cog.
Option 1– Lost Chain Tension
Description. The rear wheel of a fixed-gear bike moves forward and reduces the chain tension. As a result, the drivetrain starts to feel sluggish.
Explanation. During pedaling, the chain pulls the rear wheel forward. If the rear wheel isn’t secured, it moves forward inside the dropout. The movement shortens the distance between the rear cog and the chainring, and the chain tension has no choice but to decrease.
Option 2 – Untightened Rear Cog
Description. When stopping via the pedals or when performing a skid, the rear cog untightens, and consequently, the connection between the cranks and the rear wheel is no longer stable.
Explanation. The rear cog and the security locknut are either not properly installed or the locknut’s threads are stripped due to an improper previous installation.
Possible Sources of the Problem (Option 1)
1. Quick-release Skewers
If the fixed-gear bike is a converted road bike, chances are that the rear wheel comes with quick-release skewers.
The quick-release skewers often fail to provide the needed tension to prevent the wheel from moving forward when riding fixed. (Normally, quick-release skewers are found on bikes with gears that lower the torque reaching the rear wheel.)
Note: The issue is more likely to manifest when riding uphill due to the additionally generated torque.
If this is the culprit, the common solutions are:
- Inspect the skewers to see if they’re properly tightened. If you’re unsure about the procedure, it’s best to take the bicycle to a local bike shop for a proper demonstration.
For a video demonstration, check the clip below.
- Inspect the quick-release skewer and see if the quick-release nuts that come in contact with the dropouts are serrated. If not, you can replace the skewers with a model of higher quality that has that feature. The purpose of the serrations is to increase the friction between the quick-release skewers and the dropouts.
- It’s also possible to switch to hubs with solid bolt-on axles and track nuts. This is the classic setup found on track bikes because bolt-on hubs provide more clamping force and thus prevent movement of the wheel inside the dropout.
To increase the friction between the dropouts and the quick-release ends, some people even remove the paint on the outside of the dropouts. The downside of this approach is the increased chance of material corrosion in the area due to the absence of a protective layer. Truth be told, however, the quick-release ends will more than likely eat through the paint with time anyway.
2. Contaminated Area
Oil or grease on the axle, dropouts or the bolts could be reducing the friction in the area and causing the wheel to slide forward. (Sometimes excess oil accumulates when lubricating the chain.)
Clean the affected areas to rule out this issue.
3. Get a Set of Strong Chain Tugs
Chain tugs are a tool facilitating chain tensioning and preventing the wheel from moving forward. The vast majority of chain tugs are designed for rear-facing dropouts and are therefore non-compatible with road bikes converted to fixed-gear. (There are some models designed for forward-facing dropouts but those are rare and difficult to find.)
A good set of chain tugs will make it virtually impossible for the wheel to move forward in the dropout unwantedly.
Stay away from cheap and flimsy chain tugs as they aren’t durable and fail to provide sufficient tension.
The video below demonstrates how chain tugs are installed:
Possible Sources of the Problem (Option 2 – Untightened Rear Cog)
Another less common cause of fixed-gear slipping would be untightening of the rear cog. This problem usually manifests when using the pedals to stop or when performing the stunt known as skidding. If the rear cog isn’t tightened, the connection between it and the cranks becomes unstable too.
As a result, braking via the pedals is no longer a safe option.
If that’s the problem, the cog and the locknut securing it must be inspected.
In general, unwanted untightening of the rear cog is very rare on a standard fixed-gear bike.
The cog tightens onto the hub clockwise. Thus, the forward pedaling motion tightens the cog against the hub even more.
Additionally, a locknut is also added to the hub. The locknut threads on the hub counter-clockwise. This type of threading is a strategic choice. 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.
The job of the locknut is to prevent this outcome. 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.
That said, if the threads of the locknut are damaged/stripped, it will fail to do its job. Hence the need for additional examination.
The video below demonstrated a proper installation of fixed-gear cog and lockring:
A Note on Fixed-gear Conversions
In an attempt to save money, some people use freehubs for their fixed-gear conversions. The problem with freehubs is that they have the same threading all over the hub. Thus, one cannot use a locknut with a reverse thread to secure the cog.
Some people use a locknut from an old bottom bracket to secure the cog. While this method increases security, it doesn’t eliminate a potential untightening of the rear cog because the locknut has a normal thread and can therefore untighten together with the cog. To learn more about the topic, consider reading the dedicated post.