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.
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.
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.