Chain Wear Measuring Tools

As of 2004/01, the commercial bicycle tools for measuring chain wear are inaccurate. This note explains why, and also how to build an accurate tool.

Chain Construction

The following is a schematic diagram of a roller chain. The outer plates are pressed on to pins, forming a single unit. The inner plates are pressed on to bushings, forming another unit. Pins rotate inside bushings, allowing the chain to flex. Rollers are a hoops which rotate around the bushings, allowing the chain to roll easily (with low friction) on and off the teeth of sprockets.

In some ``bushingless'' chains (also called ``bushless'' chains), the bushing is formed from the same piece of material as the inner plates. The following discussion applies to both bushing and ``bushingless'' chains.

Chain Wear

For the rest of this note, the term spacing refers to the distance between rollers. A chain has a nominal spacing. For example, new bicycle chains have 1/2-inch spacing, meaning the chain has a roller every 1/2 inch -- 0.500 inches. As the chain wears, the spacing increases. For example, a chain for a derailleur bicycle is worn out when the spacing has increased by 1%, giving an actual spacing (spacing) of 0.505 inches.

The spacing depends on the size of the chain parts and also on chain tolerances. For example, rollers do not fit tightly around the bushing, and bushings do not fit tightly around pins. Thus, the chain should be measured under modest tensions, and if roller-to-roller distance is measured, the rollers should be aligned so all tolerances are biased uniformly.

Note also that a conventional chain has uneven dimensions. the pin-to-pin distance is fixed by the outer plates. Thus, two pins on the same outer plate always have the same spacing, but two pins connected by bushings and inner plates will be longer by the amount of slack in the bushings at both ends of the inner plate that connects them.

Sprockets have teeth (cogs) and the chain has rollers which rest on the teeth. The sprocket also has a spacing, which is the nominal distance between the faces of the teeth. Since the teeth are arranged in a circle, the tips of the teeth are farther apart than the roots of the teeth. The higher the roller rides on the sprocket, the greater the spacing. Thus, while the sprocket has a nominal spacing, it actually has a range of spacings, from the root to the tip.

Sprockets are typically designed so the nominal spacing is the distance between the teeth at the ``roots''. As the chain wears, the spacing increases and the chain rides higher and higher on the teeth.

If the chain wears too far, it rides too high on the teeth and can damage the sprocket, and can also skip. Measuring chain wear is thus important so you can replace a worn chain before it causes further sprocket damage or makes the bike unridable or unsafe.

Note: the actual operation of chains on sprockets is more complex than is presented here. What is presented here is enough to describe chain wear, but is not enough to describe sprocket wear or the engagement of a chain and sprocket. For details, see a professional chains book, for example ``Chains for Power Transmission and Material Handling'' by the American Chain Association, published by Marcel Dekker, Incorporated, Copyright 1982.

Pin/Bushing Wear vs. Bushing/Roller Wear

Let us consider chain wear in more detail. Wear occurs where surfaces rub. One wear surface is where the pin and bushing rub. As the bushing wears, the pin is able to seat more deeply, causing the chain spacing to increase. Similarly, as the pin wears, the bushing is also able to seat more deeply, causing the spacing to increase even more.

Pin/Bushing wear is shown schematically by A the following figure. Compared to the unworn chain, B, note that each pin has a wear indentation and that the bushing is worn thinner where it contacts the worn pin. Note also that the pin-to-pin distance is increased.

A subtle feature of the wear is that the distance between pins 2 and 3 is unchanged, since that distance is held by the outer plates. However, the distance between 1 and 2 (and also between 3 and 4) is double the total wear on the pin and bushing, because the pin-to-pin distance on inner plates depends on wear to both pins and both bushings.

A second wear surface is where the bushing and roller rub. The outer surface of the bushing wears and the inner surface of the roller wears. Such wear causes rollers to get looser and looser, and also causes rollers to shift relative to the unworn chain. However, the spacing of the chain remains unchanged.

Bushing/Roller wear is shown schematically by C in the following figure. Compared to the unworn chain, every roller is offset slightly to the left, but the pin-to-pin distance is the same as the unworn chain B. That is, roller wear does not affect the chain spacing.

Measuring Wear

The important issue of chain wear is that the spacing changes, causing the chain to ride up on the sprocket teeth. Thus, it is important to measure pin/bushing wear. However, bushing/roller wear does not affect chain performance unless it becomes so severe that it affects structural integrity -- the bearing is worn away, the roller fractures, or the like. Thus, bushing/roller wear should not be included in the overall wear measurement.

The standard procedure for measuring chain wear is to hold a ruler against the chain. With 1/2-inch spacing chain, 24 links should measure 12 inches new; if 24 links measures 12-1/8 inches, the chain has worn about 1%.

The following shows a worn Regina CX-S and an unused Sachs chain. The Regina chain has the pin is aligned with the ruler at 23 inches. One foot away, an unworn chain would hvae the in aligned to the 11 inch mark. However, this chain is worn so the pin is about 3/32" away. (It looks less, but due to camera parallax distortion. Consider the links at 10" and 9", which are much further from the inch marks.)

Special Tools

Using a ruler can be error-prone because it is necessary to hold the ruler precisely and measure one end while making sure the other does not slip. For that reason, several companies have developed chain wear measuring tools. The advantage of a special-purpose tool is that it is faster to measure wear.

Unfortunately, all commercial bike chain wear tools as of 2004/01 measure both pin and roller wear. The usual approach is to spread several links of chain by pushing the rollers apart. However, roller wear is added to the measurement, even though it does not affect proper chain operation.

Thus, as shown in the following figure, an unworn chain with loose rollers adds Error to the measurement and thus gets reported as worn even though measuring with a ruler shows there is no spacing change.

Note also that when there is W% wear at a pin/bushing joint, that shows up in measurement as 2W% wear between inner-plate pins and 0% between outer-plate pins. The tools must, therefore, measure an even number of links (odd number of pins) in order to provide an accurate reading -- measuring an odd number of pins leaves it to chance whether you measure N inner and N+1 outer or N+1 inner and N outer. (The above picture shows odd links/even pins -- the wrong way -- due to space constraints on the drawing.)

Compensating For Roller Wear

In principle, it is possible to build commercial-style tool so is always conservative -- that is, it never reports a chain is good when in fact it is worn. However, doing so means that a chain which is only a little bit worn -- and thus good for more use -- is reported as worn.

Thus, if the tools is conservative, chains get replaced too soon, increasing chain costs. If the tool is not conservative, then a worn chain with slow bushing/roller wear will be reported as good, and so will be used too long, increasing drivetrain damage.

Park CC-3, Rohloff Caliber 2

At least two of the chain wear tools are ``go/no-go'' gagues, the Rohloff ``Caliber 2'' and the Park CC-3. The idea of a ``go/no-go'' gague is that the tool fits (``goes'') if wear is too great; it does not fit (``no-go'') if wear is minor. The following diagram shows these tools schematically. The ``heel'' is set on the roller, then the ``toe'' is moved down. The outer edge of the ``toe'' is made to be precisely the distance for ``too much'' wear. Thus, the ``toe'' only fits behind the roller when there is too much wear.

Proposed Alternative

An alternative is very nearly the same design, but with the ``toe'' and the ``heel'' on the same sides of the roller. Again, the ``toe'' is placed precisely at the distance of too much wear. Thus, the ``toe'' only fits over the roller when there is too much wear. By placing both heel and toe on the same sides, no roller wear error is added to the mesaurement.

A disadvantage of the alternative design is that the toe tends to pull the heel away from the roller. Thus, it is necessary to push the toe down only gently. In other words, the inaccurate design can be used quickly. In contrast, the alternative design requires developing some skill in its use. That somewhat reduces its benefit over the conventional design.

Speedtech CW-1089

A third design is the Speedtech CW-1089, which is no longer (2008) available. It uses a hook to fit over the chain and a series of calibrated holes that are matched against the pin to indicate wear. However, it relies on the distance between pin and roller, which may vary if rollers are a different size than the roller size used to callibrate the Speedtech.

Chain Wear Indicator Instructions

Pedal forward to tension chain for accurate measurement. Place chain wear indicator roller set tab on the chain roller. Pivot the tool until a chain pin aligns with a display hole.

Replcaement Recommendations

Bicycle Type Indicator Reading

High Performance Fair

General Performance Fair to Replace

Low Performance Replace

Determining Chain Type

Pass the chain through the width gauge slots on the top of the tool Choose the sdize that allows the cahin to pass through it having minimum pin clearnace.

Chain Wear

Chain wear occurs at the pin and around bearing pivot points as the chain travels around the sprockets. The wear on the pins and bearing surface is measured as elongation — the more the chain wears, the longer it becomes.

As chain wear increases, the pitch of the chain becomes greater than the pitch of the sprockets. causing fewer teeth to contact the sprocket teeth. This increases sprocket wear and decreases transmission efficiency. A worn chain can periodically catch on the tips of the rear wheel sprocket teeth, slipping over the sprocket before engaging. Slippage also occurs when a new chain is mated with a worn rear wheel sprocket.

SPEEDTECH INNOVATIONS
PO BOX 1377
MOUNTAIN VIEW, CA 94143

Thanks to Jobst Brandt for discussing chain wear, Regina CX-S photographs, and for reading and commenting on earlier drafts of this note. Thanks to Aaron Goss for describing the Speedtech.