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Chainline on Bicycles with Derailers
Translations of this article (older version): German German flag
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Sheldon Brown photo
by Sheldon "Multi-Range" Brown
revised and expanded by John "Cross-Chain" Allen
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Chainline Illustration

Read this article in connection with the main article on chainline. That article covers chainline measurement, and adjustment of the front chainline (at the crankset). This article covers the adjustment of the rear chainline (at the cassette or 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".

In the case of derailer geared bicycles, the chainline is not perfect in most gears. Chainline mismatch can cause the chain to rub against the side of an outer, larger chainring when engaged with a smaller one, and can cause problems with shifting, especially with the front derailer. The worse the chainline, the worse the mechanical efficiency of the drive train, though research has shown the loss to be minor, at least with modern, flexible chains.

"Correct" chainline for a derailer system is a matter of opinion, and depends on the intended use of the bicycle. There are three "simple" answers to the question of what constitutes proper chainline:

  1. One view is that the middle of the cluster should line up with the middle chainring (or half-way between the two, in the case of a double).
  2. From the parts manufacturers' point of view, the chainline depends on the diameter of the seat tube where the front derailer mounts. For fatter seat tubes where the front derailer is farther to the right, derailer manufacturers want the chainline to be farther to the right also, because their main priority is shifting performance, and their front derailers have an optimal chainline with respect to the edge of the seat tube. This is particularly a concern in the case of bicycles with indexed front shifting.

From the rider's point of view, chainline is partly dependent on how you are going to use your gears. For instance, consider a road triple vs. a mountain-bike triple:

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Chainline Standards:

Application Dimension Notes
Road Double 43.5 Shimano spec, measured to the midpoint between the rings.
With typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm.
Road Triple 45 Shimano spec, measured to the middle ring.
MTB Triple 47.5-50 mm Shimano spec, measured to the middle ring.
47.5 preferred, but for frames with oversized seat tubes, the longer dimension may be needed, because the fat tube places the derailer mechanism farther to the right.

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Chainline Adjustment-Rear, on Bicycles with Derailers

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 adding spacer washers under the locknuts. If the OLD is only a few mm greater, you can almost always use the same axle; otherwise, you may have to replace the axle, especially a quick-release axle.

You will need to adjust the chainline when replacing a multi-speed freewheel or cassette body with a wider one. Then you will also have to re-dish the rear wheel, unless you are going to a a wider OLD and can add the same number of spacers on both sides. The wider OLD is better, as it will maintain the same front chainline, and the rear wheel will be stronger. A cassette with 8 or more sprockets should be used only with an OLD of 130 mm or greater.

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.

Moving spacers from one side to the other will disturb the alignment between a disc brake rotor and caliper. Even if you leave the same width of spacers on the left side, re-spacing a frame will tilt the caliper slightly and cause a slight lateral misalignment. Avoid these modifications unless the brake allows adjustments to compensate.

axel-set photo

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Sprocket-to-Dropout Spacing

Sprocket-to droput spacingModern 8-through 10-speed cassettes are intended for use with a 130 or 135 mm dropout spacing. 7-speed cassettes and freewheels are a few mm narrower and are intended for use with a 126 mm spacing. With any of these, a wider dropout spacing will reduce dishing and increase the chainline, possibly improving the alignment with a single chainwheel or where the outer chainring is used with all the sprockets -- see Sheldon's advice on multi-range gearing.

Racing teams need to maintain compatibility among wheels for quick replacement. Bicycle rental agencies and touring groups may have the same concern. Other than that, the closer you can get the sprocket cluster to the right dropout, the better. The closest possible adjustment without interference can prevent chain jams, strengthen the rear wheel, and in some cases allow for an additional sprocket. This is usually only a matter of a few mm. The chain may curve a bit to the right of its normal position during a shift, so do check that there is enough room between the smallest sprocket and the dropout for smooth shifting. Avoid creating a space between the smallest sprocket and the dropout which is just wide enough for the chain to fall into and jam.

A second choice is for the space between the smallest sprocket and right dropout to be wide enough so the chain can fall onto the axle assembly. Then the bicycle will freewheel forward, but the chain will not jam. The rear wheel will be dished deper, and so weaker, and there is a greater risk of bending the axle.

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Sprocket-to-Spoke Spacing

If the chain can fall between the largest sprocket and the spokes, it can jam, spokes can be damaged or broken, the rear derailer pretzeled, the dropout bent or broken -- not a happy situation, which can be prevented by a spoke protector disk -- or, if you find the cost in weight and coolness unbearable -- by careful derailer adjustment.

Hubs leave enough space so the rear derailer will still clear the spokes when the smallest possible inner sprocket is used. With a larger sprocket, the spokes are farther away, making room for a spoke protector disk -- or for one more sprocket.

8-speed cassette on 7-speed body, using dished inner sprocket

cassette with dished sprocket

A dished sprocket lets you use a variation on Sheldon's "8 of 9 on 7" trick to place all 8 of 8 sprockets on a 7-speed cassette body, and reduce the dishing of the rear wheel on a frame with unchangeable 126 or 130 mm dropout spacing. You could even have a 9 of 9 or 10 of 10 by bolting the largest sprocket to the next smaller one.

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Thanks to John Dacey, Marten Gerritsen and Nilay Kothari for some of these data.

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Articles by Sheldon Brown and Others

Reports of the demise of this Web site are greatly exaggerated! We at thank Harris Cyclery for its support over the years. Harris Cyclery has closed, but we keep going. Keep visiting the site for new and updated articles, and news about possible new affilations.

Copyright © 1998, 2008 Sheldon Brown,
2011 John Allen

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