13/03/2001
Understanding the Fixed-Gear Hub: A Deep Dive
The allure of the fixed-gear bicycle, often referred to as a 'fixie', lies in its simplicity and direct connection between rider and machine. Unlike geared bicycles, a fixed-gear bike has no freewheel mechanism. This means that when the rear wheel is spinning, the pedals are always spinning too. This direct drive system offers a unique riding experience, allowing for seamless transitions between pedalling, slowing down by resisting the pedal rotation, and even riding backwards. At the heart of this simplicity is the fixed-gear hub, a marvel of mechanical engineering that makes this direct connection possible.
This article will delve into the intricacies of how a fixed-gear hub works, guide you through the process of converting a standard bicycle into a fixed-gear machine, and provide essential information for a successful build. Whether you're a seasoned cyclist looking to explore a new riding style or a beginner contemplating your first fixie project, understanding the hub is paramount.
The Mechanics of the Fixed-Gear Hub
The fundamental difference between a standard freewheel hub and a fixed-gear hub lies in their internal mechanisms. A standard hub incorporates a freewheel, which allows the wheel to rotate independently of the pedals. This is achieved through a clutch mechanism, typically involving pawls and a ratchet ring. When you stop pedalling, the pawls disengage from the ratchet, allowing the hub shell (and thus the wheel) to spin freely while the cassette or freewheel body remains stationary. You'll hear that characteristic clicking sound as the pawls engage and disengage.
A fixed-gear hub, on the other hand, has its cog (or sprocket) directly threaded onto the hub shell. There are no pawls, no ratchets, and no freewheeling. The cog and the hub shell are one solid unit. This direct threading is the key to the 'fixed' nature of the drivetrain.
The Crucial Lockring
However, direct threading alone presents a potential issue: unscrewing. When a rider slows down by resisting the pedal rotation, or even during aggressive riding, the force applied to the cog can cause it to unscrew from the hub. To prevent this, fixed-gear hubs feature a secondary threaded section, smaller in diameter and reverse-threaded, onto which a lockring is fitted. This lockring screws on in the opposite direction to the cog. As the cog attempts to unscrew, it tightens against the lockring, effectively locking it in place and preventing the cog from coming loose.
This lockring is a critical safety feature. Without it, the cog could detach from the hub, leading to a loss of control and a potentially dangerous situation. Many dedicated fixed-gear hubs have threading for both a cog and a lockring on the drive side. Some also have threading on the non-drive side, allowing for the possibility of running a second cog or a freewheel for versatility, although this is less common for pure fixed-gear setups.
Converting to Fixed Gear: A Step-by-Step Guide
Building a fixed-gear bike often involves converting an existing bicycle, and older road bikes are a popular choice due to their robust frames and availability. Here’s a breakdown of the conversion process:
1. Choosing a Donor Bike
Older 10-speed road bikes with steel lugged frames are excellent candidates. Look for bikes with horizontal dropouts, which allow for easier chain tension adjustment. While 27-inch wheels are common on older bikes, replacement tires and rims are still readily available, so you don't necessarily need to upgrade immediately.
2. Removing Unnecessary Components
The beauty of a fixed-gear bike is its simplicity. You'll need to remove:
- Shifters and Cables: No longer needed as you’ll have only one gear.
- Derailleur(s): Front and rear derailleurs are redundant.
- Chain: Replace with a new one of appropriate length, especially if the old one is worn.
- Front Chainring: You’ll typically use only one chainring.
3. Essential Maintenance Checks
Before embarking on the conversion, it’s wise to service the core components:
- Brake Pads: Check and replace if worn.
- Brake Cables: Inspect and replace if frayed or stiff.
- Tires and Tubes: Essential for safe riding.
- Bearings: Service or replace headset, bottom bracket, and wheel hub bearings for smooth operation.
4. The Wheel Decision: New vs. Old
This is a crucial fork in the road:
Reusing an Old Freewheel Rear Wheel:
- Pros: Lowest cost option.
- Cons: Requires removing the freewheel and fitting a track cog. The biggest challenge is redishing the wheel to align the new cog with the front chainring for a straight chainline. This process can be complex and may require specialized tools or a bike shop's assistance. There’s also a risk of damaging spokes or nipples during the process, potentially costing more in repairs than a new wheel. Crucially, a freewheel hub lacks the necessary threading for a lockring, making it unsafe for true fixed-gear riding where back-pedal resistance is used.
Buying a New Fixed-Gear Specific Wheel:
- Pros: Simplest and safest option. These hubs are designed with the correct threading for both a cog and a lockring. They are typically new, straight, and properly dished.
- Cons: Higher initial cost.
For a reliable and safe fixed-gear setup, investing in a dedicated fixed-gear rear wheel is highly recommended.
5. Removing the Freewheel (If Reusing Old Wheel)
You’ll need a specific freewheel removal tool that matches your existing freewheel. Once removed, you’ll fit a track cog, ensuring it’s screwed on tightly.
6. Achieving the Perfect Chainline
A straight chainline is vital for efficient power transfer and to prevent premature wear on the chain and cogs. The chain should run in a perfectly straight line from the front chainring to the rear sprocket. Even a few millimeters off can cause noise and damage.
Measuring Chainline:
| Measurement | Description |
|---|---|
| Dropout to Sprocket Center | Measure from the inside of the dropout to the center of the sprocket (fully threaded onto the hub). |
| Dropout Width | Measure the distance between the inside faces of the rear dropouts. |
| Centerline to Sprocket | (Dropout Width / 2) - Dropout to Sprocket Center |
| Centerline to Chainring | Measure from the bike’s centerline to the center of the chainring. |
Ideally, these two centerline measurements should be within 1mm of each other.
7. Respacing and Redishing
Respacing the Axle: To achieve the correct chainline, you’ll need to move axle spacers. For solid axles, you’ll loosen the cone locknuts and adjust the cones and spacers to re-center the axle. Ensure equal axle protrusion on both sides. For hollow axles (quick-release), you’ll need to ensure the axle is centered and that enough axle protrudes for the quick-release skewer to clamp securely. If the dropout spacing changes significantly, you might need to shorten the axle or use fewer spacers on the quick-release skewer.
Redishing the Wheel: This involves adjusting spoke tension to move the rim laterally, centering it within the frame relative to the new sprocket position. It’s a delicate process that requires a spoke wrench. You’ll gradually tighten spokes on one side (e.g., the left side if moving the rim left) and potentially loosen spokes on the other. It’s advisable to have spare spokes and nipples of the correct length on hand, as old spokes can break or nipples can strip during this process.
8. Sprocket and Chainring Selection
Choosing your gear ratio is a personal decision based on your fitness, riding style, and terrain. A common starting point is around 46-48 teeth on the chainring and 16-18 teeth on the sprocket. It's often recommended to experiment with a couple of different sprocket sizes to find what suits you best. Remember, you can't coast, so a gear that’s too high will make climbing difficult, while a gear that’s too low will lead to excessive spinning on descents.
Important Note: Always use high-quality track sprockets that are fully threaded across their width. Cheaper sprockets with limited threading can strip the hub’s threads. Use thread locker and a lockring for added security.
9. Setting New Chain Length
With the wheel roughly in place and the chain around the chainring and sprocket, determine the correct chain length. Aim for a setup where the axle sits near the middle of the dropout slots when the chain is taut. This allows for future chain stretch. Use a chain tool to break the chain and a master link (if applicable) to join the ends.
10. Final Touches
Ensure your brakes are functioning correctly, especially if you plan to use them. Re-tape your handlebars for a comfortable grip.
Common Fixed Gear Conversion Issues and FAQ
Q: Can I convert a bike with vertical dropouts?
A: While possible, it's much more difficult. Vertical dropouts don't allow for chain tension adjustment, requiring the use of a chain tensioner, which is generally avoided in purist fixed-gear builds. Horizontal dropouts offer much more flexibility.
Q: My chain is making noise. What could be wrong?
A: This is almost always a chainline issue. Ensure your chainring and sprocket are perfectly aligned. Check Sheldon Brown's excellent resources on chainline for detailed guidance.
Q: Do I need a lockring?
A: Absolutely, yes! If you intend to resist pedal rotation for slowing down, a lockring is a non-negotiable safety component to prevent the cog from unscrewing.
Q: What if my old wheel’s hub is worn out?
A: If the hub bearings are shot or the hub shell is damaged, reusing the old wheel becomes impractical and potentially unsafe. This is another strong reason to opt for a new fixed-gear specific wheel.
Q: How tight should the cog and lockring be?
A: The cog should be screwed on as tightly as possible by hand, then tightened further with a chain whip and potentially a spanner. The lockring should also be tightened very securely, often using a specific lockring tool or a bottom bracket cup tool.
Conclusion
The fixed-gear hub is a simple yet ingenious component that defines the riding experience of a fixie. Understanding its mechanics, particularly the role of the lockring, is crucial for a safe and enjoyable build. While converting an old bike can be a rewarding and cost-effective project, always prioritize safety and functionality. For many, the investment in a dedicated fixed-gear wheelset is the most straightforward path to a reliable and exhilarating fixed-gear bicycle. Happy building!
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