Public Service Announcement: Shimano TL-CN42 chain checker wear limit

[wafter]

For those who don't know this is one of the better-regarded chain wear checking tools on the market.

This is because it measures between the equivalent parts of the rollers - i.e. on the same side of their respective pins, rather than opposite sides as you'd get by measuring with an internal or external caliper.

The tool's approach is different to most on the market and makes it more accurate at assessing actual chain elongation (the most important metric when assessing whether a chain requires replacement) without the wear between the roller and side plates skewing results and suggesting more wear than is actually present..

Anyway, pretty much the only downside to these go / no-go tools is the uncertainty over what point they consider the chain excessively worn (the typical standards being 0.5 or 0.75% depending on the drivetrain in question and who you ask).


I recently measured mine (as best as possible with a the depth facility on a vernier) suggesting the limit faces on the tool are around 127.62mm apart - thus allowing for up to 0.62mm of elongation over the 127.0mm pitch of ten links / five link-pairs - which of course is around 0.5%.

So, for reference these tools are more on the conservative side and correlate largely with the advice to replace chains at 0.5% wear as is often encountered on newer groupsets with more speeds.

 

[chriswoody]

I have one of these chain checkers, but have always been a little unclear as to what it's exactly showing me. As you know, I run 1x drivetrains and like to change the chain before it starts to wear the cassette. I'd assumed that if it drops into the chain, then it was worn to the point of also having worn the cassette.

So if I understand you correctly, then you are saying that according to your measurements, when it just drops into the chain then it's showing 0.5% of wear? So if I wait to change the chain until that point, I will hopefully have incurred no wear on the cassette? If that's the case, then 0.75% of wear is actually past the point that the checker drops into the chain?

It's interesting stuff and if I can wrap my head around it I can maximise my chain use. I do find these 1x cassettes do last a crazy long time, but if I could understand chain wear better, then I could eek it out even further.

 

[lostinthought]

This is very true. Only some chain wear checkers actually measure wear alone. I've given up complaining about those crappy tools that compound wear and roller clearance, nobody seems interested in the subtlety of the problem..!

I've always been happy to use a steel rule to establish chain wear, I find a sixteenth of an inch elongation over a foot easy to identify (even as my eyesight gets worse), at which point I replace the chain.

There is A LOT on this subject at pardo.net, if you want a deep dive!

http://www.pardo.net/bike/pic/fail-004/000.html

 

[wafter]

I have one of these chain checkers, but have always been a little unclear as to what it's exactly showing me. As you know, I run 1x drivetrains and like to change the chain before it starts to wear the cassette. I'd assumed that if it drops into the chain, then it was worn to the point of also having worn the cassette.

So if I understand you correctly, then you are saying that according to your measurements, when it just drops into the chain then it's showing 0.5% of wear? So if I wait to change the chain until that point, I will hopefully have incurred no wear on the cassette? If that's the case, then 0.75% of wear is actually past the point that the checker drops into the chain?

It's interesting stuff and if I can wrap my head around it I can maximise my chain use. I do find these 1x cassettes do last a crazy long time, but if I could understand chain wear better, then I could eek it out even further.

Yes; you're correct regarding its use:

Snap sprung end "1" (RHS of the image in my OP) between a pair of rollers, then gently rest the other end of the tool "2" on the chain - ensuring it's on a roller rather than the side plate.

If it remains proud of the chain (again, as shown in the OP) this suggests chain elongation is less than 0.5%. If the single tooth on end "2" disappears completely into the link however (so that the long edges of the tool are parallel with the chain / the lower one is sitting flat againtst the chain) then the chain is beyond 0.5% worn and should be replaced.

Unfortunately the cassette wear thing isn't binary. It will always wear to an extent, but chain elongation makes this significantly worse as it pushes the contact point of the roller forward on the tooth so presumably causes it to engage at increasingly less-desirable angles while moving the wear point further towards the centre of the tooth too.

I get the impression that this accelerated cassette wear is probably exponential with respect to chain elongation - i.e. getting disproportionately worse as the chain pitch increases. For example you might get three chains per cassette if you replace them at 0.5% wear, but only one per cassette if you let it run to 1.0% wear.

If you want to prolong cassette life there seems merit to running multiple chains at once rather than in series so that the elongation of the chains grows slowly, together over their collective lifespan so that the cassette sees one long progressive change from 0-0.5% chain elongation, rather than three shorter successive exposures of 0-0.5%... if this makes sense.

Generally speaking if you want to prolong drivetrain life w*x is highly recommended, while IMO having three chains on rotation would suit this application as you can treat two at the same time while running the third; reducing the number of necessary operations whilst always having one in reserve. The only down-sides to this are the initial investment, "inertia" if you decide you want to switch to a different chain and the need to cover a lot of miles to get the value out of them...

This is very true. Only some chain wear checkers actually measure wear alone. I've given up complaining about those crappy tools that compound wear and roller clearance, nobody seems interested in the subtlety of the problem..!

I've always been happy to use a steel rule to establish chain wear, I find a sixteenth of an inch elongation over a foot easy to identify (even as my eyesight gets worse), at which point I replace the chain.

There is A LOT on this subject at pardo.net, if you want a deep dive!

http://www.pardo.net/bike/pic/fail-004/000.html

Yes, it seems to be a problem that most people either don't understand or don't want to spend the time trying to.

I like the simplicity of a steel rule, however am always put off by the fact that you're still eyeballing stuff to an extent. I've had seemingly reliable results with a steel tape over the whole length of the chain, although this requires a degree of certainty that it's fully straight / all links are pulled tight.

Really I think either a properly made caliper-type affair would be the way to, or something like the Shimano tool that has an arc at the end rather than a flat face so that the extent to which it drops in can illustrate the amount of wear. Not sure how wear within / subsequent position of the rollers might affect accuracy in this area, though.

Thanks for the link - will give that a look when I get a moment


EDIT: As it happens having seen that page in the link before I was remembering some of the content as I typed the above - a great resource that really seems to explain the wear and measuring processes very well!

 

[chriswoody]

The problem I have with the Shimano tool, is that whilst on the surface it's incredibly easy to use and understand, I don't really understand it! You see, when the end drops into the chain like the photo below shows on my gravel bike, then it's clear that my chain has reached the end of it's life according to the gauge. What I'm unclear on, is how worn the chain actually is.

So conventional wisdom would have it, that when the chain is at 0.5% wear, then the cassette will have worn, but not by much. So not only do I not have to replace the cassette, I can run a brand new chain on the old cassette with no issues. At 0.75%, not only is the chain worn, but the cassette will be too and there will be no chance of running a new chain on the old cassette. Well, that's at least as far as I'm aware.

So on the photo above, it's clear my chain needs replacing, but how worn is it? Up till now I've assumed that when the gauge sits in the chain as per the photo, then it's at 0.75%. Normally I would never let my chain get this worn and will often aim for replacing it when the gauge is about halfway between the picture in the OP and my photo. Could it be though that I'm wrong and my chain is currently at 0.5%?

 

[wafter]

The problem I have with the Shimano tool, is that whilst on the surface it's incredibly easy to use and understand, I don't really understand it! You see, when the end drops into the chain like the photo below shows on my gravel bike, then it's clear that my chain has reached the end of it's life according to the gauge. What I'm unclear on, is how worn the chain actually is.

View attachment 760244

So conventional wisdom would have it, that when the chain is at 0.5% wear, then the cassette will have worn, but not by much. So not only do I not have to replace the cassette, I can run a brand new chain on the old cassette with no issues. At 0.75%, not only is the chain worn, but the cassette will be too and there will be no chance of running a new chain on the old cassette. Well, that's at least as far as I'm aware.

So on the photo above, it's clear my chain needs replacing, but how worn is it? Up till now I've assumed that when the gauge sits in the chain as per the photo, then it's at 0.75%. Normally I would never let my chain get this worn and will often aim for replacing it when the gauge is about halfway between the picture in the OP and my photo. Could it be though that I'm wrong and my chain is currently at 0.5%?

All the gauge tells you in the image that you've posted is that it's elongated / worn by 0.5% or more. The gauge is intended to be a simple no / go tool so I'm not sure you can really read much into "partial" results. Check the chain regularly and as soon as it drops in the chain's toast and needs replacing.

I'd also be wary of attempting to draw direct correlations between measured chain wear and cassette wear. One influences the other but they wear independently and there's no linear relationship. Pushing the limits of chain wear will accelerate cassette wear, but exceeding some arbitrary figure won't automatically mean that your cassette is toast; only increase the chances that it will be due to the accelerated wear. The chain wear tool can't tell you when your cassette needs replacing.

So.... my approach would be to keep at least one spare chain and cassette, check the chain regularly (I'll be aiming for every time it needs to come off for a w*x, so at intervals of no more than 300-350 miles), keep replacing chains when they hit 0.5% and change the cassette once it starts skipping - which apparently often happens when a new chain is fitted, if the cassette's worn.. so hopefully shouldn't occur when you're in the middle of nowhere, and even if it did you've probably got plenty of other sprockets on the back to choose from to get you home

I think this is the generally accepted protocol - I don't know of any tools to accurately check cassette wear or any advice that stipulates when they should be replaced other than "when it's fooked"