Mechanism behind chain wear phases Methods

As an experiment I started taking regular chain stretch measurements, from when I first put on a chain until it is worn out.  A Park CC-2 was used and each measurement was done in triplicate, then averaged to improve precision. Measured chain stretch was then plotted against the total riding time at the time of the chain stretch measurements (Figure 1). 

These measurements came under real world commuting conditions on a 2x10 speed drive train, with a constant pacing over the same mixed terrain (35% gravel, 65% asphalt). The chain was wiped clean, re-lubed and wiped every 50-100 km. A Shimano XT chain was used.

Question 1

In the right panel of Figure 1 (logarithmic), it is clear that there are two distinct wear rates/phases, a slower rate until a ride duration of roughly 40 hours, where a clear change in the pattern occurs.

Does anyone know the mechanisms driving the two different wear rates?

My suspicions is that the change relates to wearing through one material type into another, but it would be nice to get confirmation from people who know more.

Question 2

The wear pattern observed in real world testing (Figure 1; left panel) appears to be quite different from the patterns I have seen in posted laboratory testing  (Figure 2), which used a machine to run, dirty and lube the chain.  


  Note: Both axes in Figure 2 are plotted on a linear scale so it should be compared to Figure 1, left panel, which uses the same figure scaling.


In real world testing, once into the second wear phase, the wear rate appears to be quite linear when plotted on the linear time scale (Figure 1; left panel).  This differs from laboratory testing that showed a distinct curvature with duration for the entirety of the observation period (Figure 2).

Does anyone know the reason for the different observed patterns in wear for real world testing versus laboratory testing?



Figure 1.  Chain stretch plotted against duration over the course of the life of a single chain (Shimano XT).  Left panel has been plotted on the linear scale, while x-axis on the right panel displayed in a log10 scale. Shading indicated 95% confidence band. Horizontal line indicates suggested replacement point for 10 speed chains.



  Note: Patterns remain essentially unchanged if plotted against distance, instead of duration, due to constant pacing in the real world test.




Figure 2: Chain wear patterns from laboratory testing (accessed Feb 26, 2016 but now offline).



  Note: These results were found online.  I did not participate in this study and I do not have the original data.
My suspicions is that the change relates to wearing through one material type into another, but it would be nice to get confirmation from people who know more.


Another hypothesis is that as the pin-to-link mating surfaces wear, they create an increasing gap for the entry of dirt.  Once dirt gets between the parts, wear accelerates due to a "wet sanding" action. At first smaller particles are able to enter, then larger ones as the clearance increases.

Eventual displacement of the fairly heavy-bodied factory lubricant might be a factor.

Exaggerated diagrams of the idea follow. Here is the state of a new chain: the pin is firmly pressed into the link with no clearance:



The arrows indicate the direction on of the stress under load. Then as the link and pin wear, there must appear a gap opposite to the where the load presses the parts together:



The "game changes" at this point. Of course the dirt works its way around the pin as the linkage rotates, and passes through sections of the drive cycle where it is not under load.