Sheldon Brown's Bicycle Glossary Ch

Chain size is specified by pitch and width. The pitch is the distance between rollers (1/2" on all modern bicycle chain). The width is the internal width where the sprocket teeth fit in. Bicycle chain comes in two basic widths:

• 1/8" chain is used on most single-speed bicycles, and bicycles with internal gearing.

• 3/32" chain is used on derailer equipped bicycles that have more than 3 speeds.

Chains for derailer applications also come in various external widths. Newer clusters which have more sprockets use chain with thinner side plates and flush rivets.

For more information on chains, see my article on Chain Maintenance.

• The tradtional "hockey-stick" type chainguard extends from the seat stay forward and covers the upper run of chain, running down the front side of the chainwheel.

• The full gear case or chain case completely encloses the chain. This type of chainguard is, unfortunately, not currently available in the U.S., though they remain quite popular in the Netherlands.

• Chainwheel discs, unlike the above styles of chainguard, are usable on bicycles with derailer gearing and multiple chainwheels. They are primarily intended to prevent soiled clothing, since the front derailer tends to keep trouser cuffs from being snagged. Unfortunately, many chainwheel discs interfere with obtaining the best possible front derailer adjustment. Should the chain become derailed on a bike with a chainwheel disc, it may become seriously wedged between the disc and the large chainring. Chainwheel discs are rarely found on high quality bicycles.

In the case of derailer geared bicycles, the chainline is not perfect in most gears. The worse the chainline, the worse the mechanical efficienty of the drive train.

"Correct" chainline for a derailer system is a matter of opinion, and depends on the intended use of the bicycle. There are two "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.

• With a road triple, you mostly use the two bigger rings, the granny is only occasionally used, and only with the larger rear sprockets. For a road triple, having the cranks close-in is good, because it increases the usability of the big ring.

• With a mountain bike, the small ring is used much more often, and must often be used with fairly small sprockets to avoid having to shift the front under load. For mountain bikes, a more outboard chainwheel position is often preferable.

Chainline Measurement

Chainline is measured from the centerline of the frame to the center of the chain.

You can measure the front chainline directly with a simple ruler. Simply hold the ruler against the seat tube or down tube and measure the distance to the middle of the chainring teeth. In the case of triple chainwheel sets, measure to the middle chainring. In the case of doubles, measure to the halfway point between the two rings.

To measure rear chainline, the easiest way is to measure the distance from the inside of the rear fork end (or the outside of the axle locknut) to the middle of the sprocket. Double this, subtract it from the over-lock-nut dimension of the hub (or the frame spacing , which should be the same), then divide the result in half, and you have the rear chainline.

Fixed Sprockets and Single Freewheels

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 them mounting shoulder to the center line 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 corrresponding "chainline" column of the hub chosen.

(If you can provide dimensions for additional models, please

send the info to me!)

"Adjustable" means that the 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, you can change the chainline, but if you do this with a double-sided hub, you will render it unusable on one side or the other, since you'll be increasing the chainline on one side while decreasing it on the other.

For information on interchangeability, see: Bolt Circle Diameter.

This jams the crankset. Since you probably wouldn't have been shifting to the granny if you weren't already climbing, the sudden lock-up of the drive train deprives you of what little momentum you had, and you are very likely to stall and fall.

Chain suck is commonly caused by bent chainring teeth, dirty chains, or, occasionally, burrs on the teeth of new chainwheels.

Jonathan Levy has an extensive Web site about Chain Suck

For information on interchangeability, see: Bolt Circle Diameter.

A loose adjustment of the low-gear stop causes the chain to derail past the small chainring, but a tighter setting results in slow downshifting to the small ring.

In such cases, a good, if inelegant, solution is sometimes to install a chain deflector, an anti-derailment device that clamps to the seat tube. These products, such as the 3rd Eye Chain Watcher ® and the N-Gear Jump Stop ® set up a barrier preventing the chain from overshooting the small ring, no matter how loose the low-gear stop is set. This allows the low-gear stop to be set to allow the derailer to move farther inboard for faster, more precise shifting, even under some load. These devices can often save the day when extra-wide range gearing is used on a mountain bike or tandem.