Bicycle Chain-stretch :
Norms and Measurement
see Main Article : Chain-Suck in a NUTSHELL
by Jonathan Levy
... to main article

WHAT IS IT ?        &       DOES IT MATTER ?
As already explained, bicycle chains do not actually stretch ; their joints wear internally causing overall elongation of the chain when under tension. Even though this wear may be roughly the same at each chain joint, it manifests as stretch between every alternate set of roller-pairs.

Why is one bothered about chain-stretch ? The greater the elongation of the chain, the more it over-loads individual teeth of your cassette and chain-rings - they can then be worn/damaged very fast. Worn chain-rings cause chain-suck, and worn cassette teeth cause chain-skipping ...and replacing them costs you a lot extra. These components should last through 3-to-6 chains, if the chains are maintained well and replaced timeously.

NORMS AND REPLACEMENT CRITERIA
So, why not just swop out your chain on a basis of time or distance ? If you're riding in identical conditions, this may work ...but you may be in for a big surprise with all your cogs worn out prematurely ! Conditions affecting chain wear can vary dramatically from muddy MTB trails (where chains may last less than 500 km) to clean road conditions (where chains have lasted 10 000+ km). And don't imagine that your particular MTB or road condtions remain static - they won't - they vary with season, rain, and when you forget chain cleaning or lubing. Intuitively knowing accumulated chain wear isn't actually feasible   ...but its easy enough to measure.

Norms when new : On new chains, the pitch for single links should be exactly 0.50" (ie link-pairs 1.00"). Thus 12 link-pairs on a new (or unstretched) chain, should measure exactly 12.00" = 304.8mm, but there are always manufacturing inaccuracies. On a range of brand new chains, I measured "stretch" to be in the range of   -0.10% to +0.15% (measured using a long steel ruler over 48 link-pairs).

Replacement Criteria : For normal maintenance (chain-suck not an issue), chains should be replaced at about +0.5% overall "stretch". This +0.5% = 1/16" = 1.5mm over 12 link-pairs.

By +1.0% stretch (1/8" or 3.0mm over 12 link-pairs), chains will have done significant damage to both the chain-rings and rear sprockets. Then when you install a new (or low-stretch) chain, it will inevitably skip over the worn teeth on the rear sprockets during high-load pedalling ; this is very disconcerting and its impact loading will do further damage and will do it quickly.

If you have installed a new granny-ring or flipped the ring to use its unworn faces, for the purpose of alleviating chain-suck, then replace the chain preferably ; and definitely if its stretch is greater than +0.3%.

HOW TO MEASURE CHAIN-STRETCH
Conventional Method : For measuring chain "stretch" properly over 12 link-pairs (ie 24 links or 12") using a steel rule or tape measure, see Mike Tierney's website (which has clear explanatory photos).

An alternative is to measure over 48 link-pairs. Remove the chain from the bike which is easy if you use an SRAM type "power-link" or any other of these universal type links ; clean the chain to remove any dirt packed into its joints which could affect the measurement ; hang the chain up with a small weight at the bottom ; use a long steel rule to measure the total pin-to-pin distance (see Tierney's photos - but over 48 link-pairs). The +0.5% stretch will now be 1/4" or 6.1mm which can be measured more easily and accurately over the longer chain length.

Fast and Simple Method : Another very simple method I use to assess chain wear is to have three chains always hanging next to each other (full-length) from the same thin steel rod in my garage ; they only needed to be measured the first time. One chain is badly worn at +1.0% wear, the next is normally worn at +0.5% wear, and the nearest is brand new. As soon as I've removed my current chain from the bike (in a jiffy with its "power-link"), I hang it on the same rod - with an instant view of its "stretch". Typically, chains on most bikes have a few more than 100 single links (50 link-pairs) ; when a chain has stretched 0.5%, its lower end will be about half a link longer than the new chain - then put the new chain on the bike, and buy a replacement new chain.

Measuring Gauges : Park Tools (and others) make gauges for measuring chain stretch. However, I have experienced very inaccurate results using it, when compared directly with the more accurate methods mentioned above. I also consider that Park's recommendation of a +1.0% stretch allowance is far too great (although it is noted that this type of gauge often reads +1.0% when the real stretch is actually +0.5%).

I also repeated my investigation into "stretch" of brand new chains on the same chains indicated in "Norms when new", using a borrowed Park Tool. This showed "stretch" of +0.20% to +0.40% (using a method to interpret the Park reading). So the Park Tool is conservative, but unneccesarily and wastefully so if the +0.5% criterion is used for discarding chains. Park's own criterion of +1.0% gets around this to some degree but leaves a wrong understanding of the issue. There are fundamental geometric reasons why this device (and other generics based on the same idea) will over-measure, and the degree of over-measurement gets worse as the chain wears. I don't know if any generics have some way of mitigating the problem and providing more accurate measurements, but it would appear not.

  Copyright Jonathan Levy, 2000. All rights reserved.
  email Jonathan Levy - must copy email address by hand
: Visitors