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My MTB is a Lapierre X-Flow 512 2012. It has a Shimano RD-M980-SGS 10v rear derailleur which has a total capacity of 41t (see official specs). But it comes with a 11-36 cassette and a 24-42 crankset (see bike specs).

I can calculate the capacity of my bike: (36-11)+(42-24) = 43t.

So the max capacity of the rear derailleur is exceeded by 2t, which I find strange for a stock bike as Lapierre is a pretty serious brand. But everything works well and never had any issue so far.

  • Is that a common thing that bike manufacturers ignore the max capacities of rear derailleurs?
  • What are the potential (good or bad) effects of exceeding the max capacity?
  • If I increase again that capacity to 45t by mounting a 44t chainring (which can have a chainguard) instead of the original 42t chainring, do I risk breaking anything?
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  • Along with the other very good answers here, you only need to use the full total capacity of the derailer if you are in the small small combo which is usually a gearing you'd avoid anyway on a 3x chainring set up – GageMartin Nov 13 '20 at 15:18
  • I agree, never cross the chain, however what I did not know is that my front derailleur may work poorly with a 44T ring + 32T middle ring – Gabriel Hautclocq Nov 13 '20 at 16:24
  • @GageMartin - small-small at worse gets rattly and chains drop. Large-Large is where things start to break. – mattnz Nov 13 '20 at 21:36
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This relates to the lack of 'full groupsets' fitted to nearly every MTB sold today.

The spec of your bike is largely 'Deore XT M780', but you have an 'XTR M980' rear derailleur.

This is something seen on many many new bikes - a higher spec RD and lower spec elsewhere

The first and most important thing to note here is that XTR M980, XT M773, SLX M6663, and Deore M593 are the same generation (the first Shimano MTB 10-speed) and likely to be very similar.

While M980 is specced 41t, the RD-M773 introduced the same year was specced 43t, as indeed where all the other SGS 10-speed RDs except XTR.

If you look at the original XTR M980 spec, it was 44/32/22 plus 11-34 or 11-36 M980 cassette.

One observation I'd make here is that the 9-speed generation had 11-34 max cassette size, and 10-speed went to 11-36. XTR is always developed first, with the new 10-speed technology, and it's possible that the 11-36 cassette was a later addition, since it doesn't make any sense for the specs to say 41t capacity when the other specs add up to 43.

Note here that there are two M980 RDs, long-cage (GS) and super-long cage (SGS) (you have linked to M981, which is different though maybe not in important ways for this question)

According to the M980 tech doc, the inner plate for SGS is Y5XC10000

https://si.shimano.com/api/publish/storage/pdf/en/ev/RD-M980/EV-RD-M980-3085B.pdf

this was also used on the replacement model, M981. This replacement added direct mount for compatible framesets; it's just a removable b-link and doesn't affect the derailleur design

https://si.shimano.com/api/publish/storage/pdf/en/ev/RD-M981/EV-RD-M981-3429.pdf

According to this the exact same derailleur cage pattern is used on M781, which is specced for 43t.

Since M780/M781 and M980/M981 use the exact same cage (just with different materials), it follows that there is no difference in the cage capacity.

The use of different slant angles in the derailleur itself can result in a different capacity; to verify this definitively you'd need to compare an M98x series RD with a lower-series contemporary 10-speed RD.

However in my opinion it is vanishingly unlikely that they would use the same derailleur cage on XTR and XT of the same generation but change the derailleur to add 'extra capacity'.

The most reasonable conclusion is both derailleurs are exactly the same in function/capacity.

Since 'total capacity' is mostly a reflection of what Shimano sells, there's no particular reason to suspect it represents 'maximum capacity' - they are not going to test their RD with some random wide-range front crankset. So it might work perfectly, but Shimano don't care about such things and won't certify it. My guess is that since your RD is likely 43t capacity already, you would have no RD issue.

However your FD will work poorly since it has grooves in it for the middle ring, so unless you can find some FD designed for these gaps, then it won't work well. Of course if you change all three rings from 42/32/24, to 44/34/26, then I'd expect that to work well. But note that Shimano rings are matched - a 44t ring from a 44/34 triple is different from a 44t ring from a 44/32 triple - only the small ring is interchangeable - you'd also need to have a middle ring that matches to a given size lower for good shifting performance.

Looking at the existing 10-speed chainsets

https://docs.google.com/spreadsheets/d/1HdcNgmQKFA-lAqQJ3lXQ8SwA4tblAKiOXe9XYJJHYN8/edit#gid=375920416

the obvious switch from FC-M780 42/32/24 is FC-T781 44/32/24; i.e. the 44t chainring fromt that, and then FD-T780-6, which has the correct 12t top-middle jump, and 66-69 degree chainstay angle (FD-M780 is 10t top-middle)

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  • Thanks for this very detailed answer. Please note that for either FC-M780 and FC-T781, they both use the Y1MM98130 part for the 32T middle ring, as seen from official shimano documentation. That's why I think I only need to change the 44T ring. – Gabriel Hautclocq Nov 13 '20 at 16:10
  • But I thought I wouldn't need to change my front derailleur. I was wrong, it seems that Shimano has a specific derailleur for the T780 series, and it doesn't exist with E-mount. So maybe I will not change that 42T ring. – Gabriel Hautclocq Nov 13 '20 at 16:17
  • XTR is Shimano is top range. Its possible that at the extremes the shifting performance is not acceptable to Shimano for its XTR brand, but is acceptable for lower gear sets, so they lower the capacity on XTR. – mattnz Nov 13 '20 at 21:41
  • As a follow-up, I tried to put the Shimano 44T ring + integrated bash guard and it replaces the 42T nicely. I had to move the front derailleur up a little but it works very well and I didn't need to replace it. Gear changes are flawless. Of course I will not use it with the big cassette rings. I'm glad I did the change. – Gabriel Hautclocq Feb 17 at 11:05
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A common observation is that Shimano sets conservative limits to capacities of its derailleur products. Many people, including myself, run bigger than allowed cassettes with no problems. Additionally, there are aftermarket adapters available that are designed to help exceed the nominal capacity of certain road and MTB derailleurs.

Is that a common thing that bike manufacturers ignore the max capacities of rear derailleurs?

I do not know how common this can be. In any case it is the bike manufacturer's liability to provide a whole package (the bike) that is safe. They ought to test all chosen parts to work nice together, or else face frequent recalls.

What are the potential (good or bad) effects of exceeding the max capacity?

In the worst case, a too stretched (or too compressed) derailleur may start touching other components it is not supposed to touch, which will lead to its destruction. In a better scenario it has its internal components subject to increased mechanical stresses, resulting in increased wear. Finally, the thing may simply refuse to shift to the cog that is outside its range at all, or would take a long time to shift.

Good effect: you can have a wider gearing range on your bike without having to buy a new derailleur.

If I increase again that capacity to 45t by mounting a 44t chainring (which can have a chainguard) instead of the original 42t chainring, do I risk breaking anything?

It really depends on how tight your derailleur is now at its extremes, and how you ride it. If you do not intend to ride with badly cross-chained combination 44 front - 36 back, your derailleur will never "know" that you've gotten a bigger front chainring.

Do not forget to get a longer (+2 halflinks) chain.

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    Anecdotally, it does seem like exceeding Shimano's RD capacity by 2 (i.e. in Lapierre's stock configuration) is rarely a problem. – Weiwen Ng Nov 13 '20 at 14:21
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    Thanks Grigory, your answer was useful; however thelawnet answer was more detailed and actually helped me to choose what I should do, so I chose it as the answer. – Gabriel Hautclocq Nov 13 '20 at 16:21

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