Originally Posted by
Gresp15C
Looks good to me. The 0.172 figure could vary from chain to chain, but you can always re-measure it each time you check a chain.
The sources of uncertainty in a chain measurement fall into two categories: Some are constant, no matter how many links you include, for example the "slop" of the rollers and typically the precision of the measurement. Some are proportional to the number of links, such as the overall elongation. For this reason, most of the things you don't want to measure become insignificant, the more links you include.
This is why all of the measurements and gages are longer than just one or a pair of links. Six inches makes sense for go-no-go gages and inexpensive calipers that typically come in a 6" length and are a convenient size to throw in a toolbox. Twelve inches makes sense for a ruler. Anything longer gets unwieldy and might require removing the chain.
One thing I always do is double check in the following way. When I think I'm ready to replace a chain, I hang the old and new chains side by side. Over the full length of a chain, the elongation becomes pretty obvious. I keep the published limits (0.5%, 0.75%, whatever) in mind, but I might also swap a chain at the moment when it happens to be convenient, such as getting the winter bikes ready for a snowstorm that's supposed to arrive soon. A benefit of being an amateur is that we can often make decisions based on what's convenient for our schedule, rather than having to get a bike on and off the stand ASAP so another bike can be brought in.
You are right about outer plate distance and that's why I measured it. I should have stated that 0.172 is for the 11 and 10 speed KMC chains I had, the 8 speed chain has ~0.165 distance between outer plates.
I will try to find a spot to hang almost new and measured %0.5 chains next to each other when I have the time. But there is another problem with looking at the whole chain. The chain unfortunately does not wear uniformly so even if the total wear is less than %0.5 it may be possible to have a section above that point and that begs the question does it matter to have a 5-6" section over %0.5 if the average is below that ?
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