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Hello I was looking to upgrade my 2011 Specialized Stumpjumper Hardtail comp to a 1x drivetrain. 11 speed Shimano deore M5100 caught my eye with its low price and wide range (max 51 tooth cassette). But can i run a 51 tooth cassette on a 26er? I'm afraid the derailleur in the lowest gear would hang so low it would interfere with the tire or the rim. Thanks.

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  • Why do you even consider this? Converting a hardtail to 1× is generally a downgrade, unless you're replacing a low-quality and/or worn out 3× with a top-end new 1× (but then replacing it with a good 2× would be an even better upgrade, though those are unfortunately getting more and more difficult to obtain). Only on full-sus bikes does 1× play out its advantages, but especially on a 26" bike even a good 1× will probably leave you spinning out on flat asphalt or gravel. Commented Sep 6, 2022 at 19:58
  • @leftaroundabout 1x lets you go up to around 35km/h with an cadence of 90rpm, that's not sustainable for a long time on flat asphalt. Even less on gravel. If you want to do this often, then a MTB is the wrong bike for you, a road/gravel bike would be better.
    – airace3
    Commented Sep 7, 2022 at 7:48
  • @airace3 an MTB with 2× is the right bike (certainly for me) because it works great on both asphalt and proper offroad. Of course this requires relatively light-running tyres and riding in a somewhat aero position. Sure, my gravel bike makes the latter a little bit easier, but on the flip side it's much worse on rooty off-road than my hardtail MTB. I don't sustain 35 km/h for a long time with either bike, but I do like to go faster than that on shorter rides, and on longer tours I like the ability to ride a tailwind at low cadence. Commented Sep 7, 2022 at 16:44
  • @leftaroundabout I have a 3x10 with 24-33-44 gears in the front and 11-36 cassete. I like the range but on rougher terrain the chain slaps alot and sometimes even falls. From my understanding, I would need a 1x derraileur with a clutch to fix this. Or do the 2x deraileurs with a clutch work just as well?
    – KarelPrdel
    Commented Sep 7, 2022 at 16:52
  • @KarelPrdel yes, older 3× are noisy but the clutched 2×11 on my hardtail works almost as reliably as the 1×12 on my enduro bike. Only on extremely rough descents, the chain sometimes comes off (because it's not a narrow-wide chainring), but also only when riding so fast that it's plain painful without rear suspension. And even when the chain does come off, shifting once back-and-forth with the front derailleur is usually enough to get it back on. Really, I think the widespread opinion “1× is better” just stems from the fact that people compare old 3× with new 1×, so it's not a fair match. Commented Sep 7, 2022 at 17:07

2 Answers 2

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It's totally doable to use these large cassettes on 26" wheels. Some high end children's MTBs still use 26" wheels and are sold using such cassettes. For example COMMENCAL Meta HT JR 26" with Sram Eagle 11-50 or Scott Ransom 600 26" using the deore M5100 11-51 cassette and derailleur.

While modern 29ers with the same gear range typically use 30 or 32 tooth chainrings, you should opt for 34 - 36 tooth on 26" to get a similar speed range. The smaller diameter acts like an overall gear reduction.

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The wheel diameter is not related to the maximum cog size on the cassette.

Instead the restrictions are:

  • The maximum tooth count in the derailleur's specifications
  • How far over the maximum you can push it
  • Whether you have a hanger-extender installed
  • The maximum capacity in your deralleur's specs
  • The difference between your front chainrings (if more than one)

So a boring 11-32 cassette in a 1x system needs a derailleur with a capacity of 21 or more.

If we select the 1x derailleur from the M5100 groupset as per https://bike.shimano.com/en-EU/product/component/deore-m5100/RD-M5100-SGS.html it has specifications of

  • Capacity 40 - the length of chain it can gobble up is some number of links equivalent to 40 teeth on the cogs (which is not 40 links, more like ~20) This is the difference between fully stretched out and folded up so the chain almost touches.

Compare those specs with the double derailleur at https://bike.shimano.com/en-EU/product/component/deore-m5100/RD-M5120-SGS.html and you see it gets more complex.

If you're hoping to use your existing derailleur, look up the published specs and compare.

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    Line 1 - I suppose if the cassette was large enough that it hit the ground, or made the derailleur hang low enough to hit the ground, then that's a limitation based on diameter.
    – Criggie
    Commented Sep 5, 2022 at 21:15

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