Saturday, May 28, 2011

Bicycle Chain Stretch Test & Results

Do bicycle chains get stretch marks? Will smearing cocoa butter on them be a step in preventive maintenance for future? I don't know, but hold that thought for a moment.

I, like many, am a fan of chains. For bikes, they present a technology that is  ubiquitous, economical, and proven to work almost seamlessly with external shifting systems. Belts are slowly staking their claim in the single speed road and mountain bike arena, however I have to be honest - show me a more simpler, self cleaning power transmission mechanism that doesn't load up shafts and bearings as much as a belt does, and I'll be sold on other ideas. 
".. the less stretch, the more responsive the bike becomes.."
But in cycling, as we all know, equilibrium is rare. Everything has to get scrutinized more thoroughly than a coroner would do a murder victim, from the pimple on our skin that's disturbing laminar air flow to the secret ingredients used to make those mundane Presta valves and you know, that's what keeps our world a bit interesting (or not).

Now if you just may recall, Wipperman was getting fancy in the recent past by testing a host of chains in order to rate them according to their wear rates. You can read that blog post here which described the test protocol, the results they came out with and so on.

Recently, I was told that the company commissioned a different test on a similar selection of chains to test for elongation under load. Tom Petrie of Cantitoe Road - a chain test data center - passed along some literature that said the following :

"Wippermann recently tested a number of popular 10-speed chains for stretch under load. For a reference point, each chain’s length was measured under a nominal load of 10 kg. Then each chain was measured under 75kg and 150 kgloads, and the results recorded. Not surprisingly, chains with cut-out plates and hollow pins stretched more than those with solid plates and pins. And, the chains that stretched least were the Wippermann’s Connex 10 series with solid plates and pins!

How much a chain stretches under load affects how quickly the load is transferred to the driven cog. The less stretch, the more responsive the bike becomes. And, less stretch means less energy is lost to stretching the chain! Especially in sprint, time trial, or hill climb events, reducing these losses is critical. 

Wippermann tested 31-link sections of chain. This is the average number of links under load between chain ring and cog. While the actual amount of stretch is small (from 1.10 to 2.15 mm) the differences are substantial. Among the various chains tested, the “stretchiest” stretched almost 100% more than Wippermann Connex!"

After testing, the data was cobbled up into a table to make sense of the results. They follow :

Summary of chain stretch test data

Elongation vs load plot

The document went on to make light of these  :

"In addition to raw material and proprietary heat-treating processes, the shape of Wippermann Connex outer plate is largely responsible for its resistance to stretch. Note that chains featuring elaborate side-plate cut-outs and hollow pins are the “stretchiest” while chains with solid plates and solid pins stretch less. But even solid-plate solid-pin chainswith sculpted “figure 8” outer plates stretch more than Wippermann Connex. The extra-strong rectangular outer plates on Connex chains contribute significantly to their resistance to stretch."

This stretching they're talking about should be nowhere big enough to cause a yielding in the chain material. So the material attains its original shape after unloading like a spring, and the real question then becomes - how will a 100 thou inch change in chain length in the worst case scenario affects overall power transmission efficiency? Is it any more significant than the normal vibrations introduced into the chain due to tensile load changes and sprocket tooth effect? Does the stretching get better or worsen in weaker chains when the chain is misaligned/cross chained? Finally in the big scheme of things, how will cyclic stretching/unstretching react with notorious elements like salt water? Could it possibly accelerate the failure of cut-out chains in those circumstances?

What do you think? While you sip your coffee, you may also be interested in glancing at a "shifting performance" study done on chains through Wippermann, the hardly surprising conclusion of which was that there is no observable correlation between a worn chain and shifting performance compared to a new one. You just may not want to break your bank over a chain.

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