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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Last weekend I was pedaling around and stopped to visit an old friend, recently retired, who has lots of bicycles and accessories. Took the opportunity to check my chain with a gadget that directly reads the "stretch", from 0.25 to 1.0 percent. I was dismayed to see readings of 0.9 or 1.0. At that level there's usually consequential damage to the sprockets, causing the (new) replacement chain to skip at inopportune times. On my previous bicycle I once had to replace the 7- or 8-speed rear cassette when that happened.
Before the end of the day I had procured a new chain and, for the first time, a one-piece "go,no-go" wear limit gauge. Removed the old chain before trying the new gauge on it, or even checking its pitch with a plain metal ruler. Hung it up next to the new one, expecting a photo opportunity. Here are the top and bottom ends. Worn chain on the left and new one on the right:
That is _not_ enough stretch to be a problem, IMHO. Would any bicycle experts out there care to agree or disagree? My new limit gauge has fixed pegs at 0.5% and 0.75%. The 0.5 did not "go" with the new chain, and almost did not go with the old chain. Casually applying a metal ruler, the stretch in 12 inches was hard to measure -- much less than 1/16 inch, which would be 0.5%. The photographs show old longer than new by, at most, 0.2 pitches in 112 pitches. I don't think weights or springs to increase the tension would substantially change the result.
So I wasted some time on premature maintenance, based on a single instrument reading. Later I found this magnificent illustrated article on the subject. It's got references to the late Jobst Brandt and Sheldon Brown. But while calling gauges "wrong" for including a roller-wear component, it fails to quantify that term, and to mention that roller wear becomes less significant as the gauge length increases.
The basic pin pitch is 20 in exactly 254 mm, for the benefit of people without an inch ruler.
[edit] Here's another curiosity. Many mechanical chain references mention that new chains begin with a pitch slightly greater than nominal. Allegedly because of the design clearance between pins and bushings. Why can't that be compensated by stamping plates with holes closer together?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
After reading that article about chain gauging, I measured my new 1-piece gauge (a Park Tool CC-3.2) and am pretty happy with it. At the end with concave contact surfaces, the curvature is too much for 5/16" dowels (or chain rollers) but OK for 19/64". With access to a digital height gauge, I measured the spans of both pegs from their respective concave contacts. The peg labeled 0.5 will "go" in spaces of 6.733" or greater. The peg labeled 0.75 needs a space of at least 6.751". The difference between those values, 18 thousandths of an inch, is 0.26% of the 14-pitch (7 inch) gauge length. The pegs will match chains with 0.5 and 0.75% pitch increases, if (worn roller diameter - roller backlash) is 0.302". I think the diameter of new rollers is supposed to be 7.75 mm (0.305") so the gauge allowance seems reasonable. My worn chain has roller diameters of about 0.301 and a presently unknown amount of roller backlash. So the gauge's 0.5 peg should "go" when the excess chain pitch is slightly less than 0.5%. Better to replace chain too soon than too late.
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False positive from bicycle chain stretchmeter
