Old 12-09-20, 10:43 AM
  #80  
2_i 
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Originally Posted by John_E
I don't think I get what you are saying here. These tools are designed to measure elongation without the addition of roller wear. They assume that roller wear is uniform among all rollers hence when the front and mid rollers are pushed the same direction the roller wear is canceled out.
Another advantage is this design can measure wear for chains with different roller sizes.
You are right that the wear cancels when it is uniform and the measurement is from the same side of the roller. However, the roller wear and diameter differences contribution get suppressed when a measurement is made over a number of links, no matter how these contributions, when different from pin-to-pin elongation, enter. Let us take the roller diameter differences mentioned before, of 0.002". I took a popular Park Tool checker and its hooks are 4.5" apart. We get variation in the apparent elongation from roller diameter difference of 0.002/4.5=0.04%. It is not completely negligible, but when deciding on a 0.7% elongation it nearly is. I do not have TL-42CN, but have TL-41CN. Though the latter may be nominally provide more faithful results than simple checkers, it is a pain to use. When I need to go over several bikes and check the chains at different sample locations along the circumference, I go with the simple Park Tool and it is good enough for me.
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