Read this article in connection with the lead article on chainline, It covers chainline measurement, and adjustment of the front chainline (at the crankset). This article covers the adjustment of the rear chainline of a fixed sprocket or single-speed freewheel.

The word "chainline" refers to how straight the chain runs between the front and rear sprockets. Ideally, both sprockets should be in the same plane, so that there is no sideward motion or stress to the chain. This constitutes "perfect chainline". This article covers issues specific to bicycles without derailers.

I have measured a selection of track sprockets, single freewheels and hubs with respect to chainline (units are millimeters.) The "Chainline" column is the distance from the mounting shoulder to the centerline of the sprocket teeth.

To figure the actual chainline for a given combination, add the number from the "chainline" column of the sprocket or freewheel to the corresponding "chainline" column of the hub chosen.

Chainline may be adjusted at the rear wheel by rearranging spacers on the rear axle, and with a steel frame, also by re-spacing the rear dropouts to allow use of a longer or shorter rear axle. Adjustable hubs use conventional threaded axles, so you can increase the OLD spacing by removing the locknuts and adding spacer washers.

If you add equal thicknesses to both sides, the chainline is unaffected, since it's measured from the middle outward.

If you add more spacers to one side, or move them from one side to the other, you can change the chainline, but this will render a double-sided hub unusable on one side or the other, since you'll be increasing the chainline on one side while decreasing it on the other -- unless you also use a double chainring in front. Moving spacers from one side to the other also will disturb the alignment of a disc brake.

• Axle Re-Spacing/Re-Dishing

  This is the usual way to go for converting bikes from a multi-speed derailer setup to a single-speed fixed-gear or freewheel drive.

  The spacers are located just behind the outer locknuts, between the locknuts and the cones (or the bearing mounting nuts, if your hub uses cartridge bearings.) You will need cone wrenches and a general understanding of hub bearing adjustment to do this job.

  Assuming the rear hub's spacing already matches your frame's spacing, you can take spacer washers from one side of the hub and move them to the other side, thus shifting the entire hub from side to side. In the case of a hub originally meant for a multi-speed freewheel, there is usually a rather thick spacer on the right (freewheel) side of the hub. You can replace this with a stack of thinner spacer washers that add up to the same overall thickness, then distribute the washers from side to side as needed to get the chainline where you need it to be.

  Once you have done this, you'll need to re-dish the wheel by adjusting the spokes. This is a bit of trouble, but, in the case of multi-to-single-speed conversion, the re-dishing actually results in making the wheel much stronger than it was.

  Also see my article about frame spacing.

• Sprocket Dishing

  Most internal-gear hubs have "dished" sprockets that can be installed either dished in, or out, providing a choice of two different chainlines.

  Fixed-gear sprockets are usually intended to be installed with the flange facing inward, so the teeth are outboard. However, in many cases, it is possible to flip a fixed-gear sprocket over if you need to move the rear chainline to the left.

• Spacing Washers

  Sturmey-Archer internal-gear hubs generally have two 1/16" (1.6 mm) spacer washers next to the sprocket. These can be arranged in various ways, along with the dishing of the sprocket, to optimize chainline. Shimano makes the same diameter of washers in 1 mm thickness, for use with a 10-speed cassette. These are metal washers. Plastic washers that fit between the sprockets on a Shimano cassette have internal lugs that get in the way, and they compress too easily to be suitable for use with a threaded sprocket.