Fixed-gear Bicycle - Conversion

Conversion

Many companies sell bicycle frames designed specifically for use with fixed-gear hubs. A fixed-gear or track-bike hub includes special threads for a lockring that tightens in the opposite (counter-clockwise) direction compared with the cog. This ensures that the cog cannot unscrew when the rider "backpedals" while braking.

For a variety of reasons, many cyclists choose to convert freewheel bicycles to fixed gear. Frames with horizontal dropouts are straightforward to convert, frames with vertical dropouts less so. One method is to simply replace the rear wheel with a wheel that has a track/fixed hub. Another is to use a hub designed for use with a threaded multi-speed freewheel. Such a hub only has the normal right-handed threads for the cog and not the reverse threads for the lockrings used on track/fixed hubs. The sprocket on a hub without a lockring may unscrew while back pedalling. Even if a bottom bracket lockring is threaded onto the hub, along with a track sprocket, because the bottom-bracket lockring is not reverse threaded, the possibility still exists that both the sprocket and locknut can unscrew. Therefore it is recommended to have both front and rear brakes on a fixed-gear bicycle using a converted freewheel hub in case the cog unscrews while back pedaling. It is also advisable to use a thread sealer for the cog and bottom bracket lockring. The rotafix (or "frame whipping") method may be helpful to securely install the sprocket.

Bicycles with vertical dropouts and no derailleur require some way to adjust chain tension. Most bicycles with horizontal dropouts can be tensioned by moving the wheel forward or backward in the dropouts. Bicycles with vertical dropouts can also be converted with some additional hardware. Possibilities include:

• An eccentric hub or bottom bracket allows the off center axle or bottom bracket spindle to pivot and change the chain tension.
• A Ghost or floating chainring is an additional chainring in the drive train between the driving chainring and sprocket. The top of the chain moves it forward at the same speed that the bottom of the chain moves it backwards, giving the appearance that it is floating in the chain.
• A magic gear—the right math can calculate a gearing ratio to fit a taut chain between the rear dropout and bottom bracket. Also, using a chain half link and slightly filing the dropouts to increase the width of the slot increases the chances of finding a magic gear. It is worth noting that the magic gear setup is controversial, due to inevitable chain stretch and subsequent slippage that can lead to serious injury.

Separate chain tensioning devices, such as the type that attaches to the dropout gear hanger (commonly used on single speed mountain bikes) cannot be used because they are damaged as soon as the lower part of the chain becomes tight.

Additional adjustments or modification may be needed to ensure a good chainline. The chain should run straight from the chainring to the sprocket, therefore both must be the same distance away from the bicycle's centerline. Matched groupsets of track components are normally designed to give a chainline of 42 mm, but conversions using road or mountain bike cranksets often use more chainline. Some hubs, such as White Industries' ENO, or the British Goldtec track hub, are better suited to this task as they have a chainline greater than standard. Failure to achieve good chainline, at best, leads to a noisy chain and increased wear, and at worst can throw the chain off the sprocket. This can result in rear wheel lockup and a wrecked frame if the chain falls between the rear sprocket and the spokes. Chainline can be adjusted in a number of ways, which may be used in combination with each other:

• Obtaining a bottom bracket with a different spindle length, to move the chainring inboard or outboard
• Choosing a bottom bracket with two lockrings, which gives fine adjustment of chainring position
• Respacing and redishing the rear wheel, where permitted by the hub design
• Placing thin spacers under the bottom bracket's right-hand cup (Sturmey-Archer make a suitable 1/16" spacer) to move the chainring outboard
• Placing thin spacers between the chainring and its stack bolts to move it inboard (if the chainring is on the inside of the crank spider) or outboard (if the ring is on the outside of the spider)
• Placing thin spacers between the hub shoulder and the cog- only recommended in the case of a freewheel-threaded hub, which has sufficiently deep threads for this