A few threads recently referenced one or other article that shows how the popular chain checkers are inaccurate. I have the Park Tools CC-3.2 checker and up till recently, I have trusted this gadget to advise me when to replace my chain. So I decided to calibrate it and see how bad it really is. The best way to measure a chain is to remove from the bike, stretch it out under light tension and measure it with an accurate ruler against a brand new chain of the same type. The longer the ruler, the better. I have a 1m steel workshop ruler in my workshop at home but what a pain.....my chain checker is much more convenient so if I know how dishonest it is, I can still use it for a quick check. There are many articles on chain wear but I like this one the best : http://pardo.net/bike/pic/fail-004/000.html It has diagrams, photos and references to engineering texts and is well written. Summarizing this article : 1) it is only pin-to-pin wear that matters, this is what causes chain length to increase resulting in wear of cassette and chainring teeth. 2) roller wear is practically a non-issue because it does not affect the pin spacing. 3) all but one of the popular commercial chain checkers include roller wear in their measurement of chain length and thus tend to overestimate the actual extension in chain length. I tend to use the same type of chain all the time (Shimano CN-HG95 HG-X 10 spd) so I figured that if I calibrate my chain checker to this one type, some truth might emerge. I measured a worn chain that I replaced at 0.5% according to my checker. Pin to pin distance should be 12.7mm (1/2") on a new chain so 50 full links (101 pins) should measure 50" (1270mm). My "0.5%- according-to-Park-Tools" worn chain actually measured 50+1/16 inches (1272mm) across 101 pins, corresponding to only 0.125% wear. The chain currently on my bike is somewhere between the 0.5% and 0.75% mark on the chain checker and I measured this one at 0.25% wear. So theoretically both these "worn" chains still have useable life. I figure that my chain is still less than 0.5% stretched when my checker indicates 0.75%. so I am happy to use this checker up to its maximum for this type of chain. By replacing chains at "0.5%" as indicated by this checker I am probably changing chains at only 1/3 of their useable life and thus buying new chains three times more often than is necessary. Something to think about.....
