Wrong results measuring a bike chain

Question

I measured the wear on a new Shimano 10-speed HG-95 chain by measuring the length of a certain number of links with a caliper like this (the jaws of the caliper are inside the chain): But the results don't make any sense to me.

When measuring 11 links I got a length of 132.7 millimetres which should be equivalent to a length of 120.6 mm on the more common 10 links used as a reference. Obviously this is way too long, as a new chain should be 119.5 millimetres long.

I got another caliper and measured 7 and 8 links (because it isn't long enough for more links) and got a length of 116.9 and 118 millimetres which is too short.

I switched back to the first caliper and got decent results when measuring 9 links (about 119.2 mm) but now I am very confused and don't know what I am doing wrong, because as far as I know my calipers are at least decently accurate.

Answer 1

First, 10 links is not 119,5 mm. The pins on the chain are half an inch apart. Counting 10 pins would give you 5 inches, or 12,7 cm (1 inch = 25,4 mm).

Where are your measurement reference points? Usually one would measure from the center of one pin to the center of the next reference link. You can use the edges of the links if you find that easier, but I don't find the accuracy of my method to be a problem. When a chain is (sufficiently) worn, you will be able to see it even if you eyeball the reference points.

See the image below (excuse the dirty background). The five links of the brand new lower (thicker, single-speed) chain are almost exactly 5 inches long (lens distortion, camera angle etc. may skew the measurement, but I assure you the chains are aligned and the caliper shows 5" exactly):

Note that the green and brown lines capture 5 inches of the chain, the blue line on top only captures 4 links. Sorry if this is confusing.

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NB: The problem I've had is that measuring just a handful of links gives iffy results. Maybe if the chain is tensioned and on the bike, this works well, but laying them flat on a table, I can never get a nice result. I couldn't tell from my own image, nor from the 5 inches of chain right in front of me, that the top (narrower, 10-speed) chain pictured, is used, and warrants replacement. Thankfully, while the relative error stays constant, the absolute error increases with the length of the chain. Pictured below is this same 10-speed chain over a brand new chain, but after 10 inches (I did my best to keep them aligned at the reference link, ten inches to the left):

I personally don't trust cheap chain measuring tools to be able to measure this with any sort of repeatability. What chances are there for a stamped piece of metal to be accurate to within 1% at 127 mm? I compare the chain with a brand new one, across the 114 links supplied, which will show you immediately and without a doubt whether your old chain is stretched. I also don't see the value in spending $20 or more on a more expensive tool given that a new chain will cost me less.

Answer 2

If you are using calipers, then you need to either eyeball against a reference point, or butt the jaws against points that are not on-center and then measure back to center and subtract the difference. Either method seems likely to introduce inaccuracies. There are dedicated chain-measuring tools (for example) for this purpose.

Answer 3

Usually when measuring chains by eyesight, you measure 24 half-links or 12 full-links, requiring an inch ruler that has 1/16" marks and is longer than 12 inches. If using an exactly 12 inch long ruler (these are most common), then you have to extrapolate with a worn chain as the ruler ends at 12" and a worn chain can be 12"+1"/16.

Working with millimeters is harder. The problem with calipers is that they usually end at about 14 centimeters or so, and 6 full links would be too long for a caliper so you have to use 5 full links. That's 127mm when unworn and 127.63mm when worn.

A caliper can perfectly well measure to an accuracy of 0.1mm, but only when you measure not by eyesight but by feeling. Chains are measured by eyesight and if measuring by eyesight there easily can be 0.3mm error. This error would be 50% of the difference between new and fully worn.

Measuring halfway between full links is not recommended, as every other half-link wears in a different way, so only full link measurements are accurate.

I have found 24" rulers to be too long to conveniently store and 12" rulers to require extrapolation by eye. Finding 13" rulers which would be optimal for chain measurement is practically impossible. So I use good quality chainwear tools such as Park Tool CC-4, Shimano TL-CN40, Shimano TL-CN41, Shimano TL-CN42 or Pedro's Chain Checker Plus II. Most chainwear tools are crap because they do the measurement at different sides of chain rollers so they measure pin wear and roller clearance at the same time, being sensitive to roller diameter as well. Good tools only measure pin wear, not measuring roller clearance at all (roller clearance doesn't matter here), and are insensitive to roller diameter, because they do the measurement at the same side.

I don't think that with normal eyesight it's possible to correctly identify chain wear state with a millimeter ruler, or a caliper, or any measurement shorter than 12 inches. With 10 inch measurement for example, the 1/16" mark would be 0.625% wear which could be arguably a little too much, although it might be barely acceptable if not desiring maximum cassette life.