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I have to replace my chain and gears on my 28'' cross-country mountain bike and I like to use the opportunity to make some modifications. I've tried to read up on this and I already learned a lot from previous questions here, but nothing fits quite right or the answers are too old to be relevant in part searches now.

Currently I'm using a Shimano XT drivetrain with a 10speed 11-34 rear set and a 26-36-48 crankset. The shifters are just Deore. The bike and parts are roughly 8-10 years old.

I'm rarely using the lowest gear but I find myself wanting for a higher one.

I figured the newer hubs with 10t smallest gear need new wheel hubs because of their small diameter, so this might be not an option. The 48t seems to be already the largest chainring offered by Shimano in the XT range.

Is there anything I can do to get a >50t chainring on that bike while keeping the front and read XT derailleurs? I'm changing rear cassete, chain and chainrings anyway. Any other manufacturer that offers compatible equipment? Is there a Shimano road series that works with XT? Can I mix-and-match chainwheels from MTB and road series on my existing crank? Would it work with a new crankset? I think the bearings are worn anyway...

Any help much appreciated..

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    Some confusion in your question. I think you are talking about a cross-country MTB, not a cyclocross bike, right? Also, you say you want lower gear ratios, but a larger chainring gives you higher gear ratio
    – Argenti Apparatus
    Apr 3, 2020 at 23:24
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    Thanks. This always confuses me. I edited the question.
    – Chaos_99
    Apr 4, 2020 at 6:49
  • when discussing gears I sometimes find it good to over-describe. So a Low gear is a Slow gear is an "Easier" gear and the chain is on a Bigger cog at the back and/or a smaller chainring at the front. Conversely a High gear is a Fast gear is a "Harder" gear with the chain on the small cogs at the back and the big chainring at the front.
    – Criggie ♦
    Apr 4, 2020 at 9:32
  • Out of curiosity, where are you riding that requires a 50t chainring? Primarily on the road?
    – Argenti Apparatus
    Apr 4, 2020 at 11:33
  • Usually bike paths or service roads with all kinds of pavement or fine gravel/compacted dirt. I ride with slow cadence, but I could still deliver more force with a higher gear.
    – Chaos_99
    Apr 5, 2020 at 11:34

3 Answers 3

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Wanting a bigger ring indicates you'd like a higher gear, not a lower one. Low gears are your climbing gears.

Part of the answer depends on which front derailleur you have. Front derailleurs have a profile to their outer cage that needs to match well with the profile of a given chainring size in order to perform well and not cause derailment. If you have a hybrid or trekking type triple FD whose spec sheet lists compatibility up to 48t, then pushing up to 50 might work okay, but it might also be pretty mediocre. If you want to really go nuts, some success can be had be grinding the shape different, but it's easy for this to go south.

I don't think that 50t 104mm BCD chainrings for derailleur use exist. Narrow-wides in 50 and 52 do exist if you wanted to 1x it. There are numerous questions here about how to determine if you want to do that.

If you wanted to spend some money, you could easily do what you want here by getting a road shifter, front derailleur, and crank.

Many bikes with the sort of gearing you've got will encounter clearance problems with the chainrings if you go any bigger.

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  • I'd rather not go for replacement of the front shifter, derailleur and crank because of the price. But if I do: will I get into more compatibility problems because of the chain? When looking at Deore XT cassettes I found that are pretty picky on what chain they allow. Will this be compatibel with the road cranksets?
    – Chaos_99
    Apr 4, 2020 at 7:22
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    Replacing your chain is for the most part a given when you're putting on a bigger ring. That said, yes there can be an on-paper mismatch of chain compatibilities when you do this with Shimano parts. There are some 10s road triples (FC-4703) that do want the same HG-X chain as your cassette.
    – Nathan Knutson
    Apr 4, 2020 at 15:06
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Specialites TA makes a 50 tooth 9-speed chainring for MTB cranks. Since the inner width of all derailleur chains is the same, it is probable that the outer ring from 9-speed series does not cause problems. If you install one, it's on your own risk. The clearance problems affect mostly inner and middle rings, the outer one is probably fine. If your front derailleur works with 48 tooth ring, it is very probable that it works also with 50 teeth.

If you don't need the lowest gear ratios, with 10-speed Shimano you can just use a road cassette instead. Using a narrower range cassette would also solve any derailleur capacity issues. With 11 speed the freehub bodies are different width, so this would not work.

50/11 is a huge gear, though, and its probably better to learn to spin. I have installed that 50 tooth ring on my bike so that I could use cassettes with 12t or 13t smallest cog.

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I'm guessing that what you could do is look for 3rd party larger rings that fit on the XT crank. Beware you may not have enough clearance between the drive side chainstay and larger chainrings.

You won't be able to increase the size of just the largest chainring from 48 to 50 or more. On a triple chainring drivetrain the rear derailleur will not have enough capacity (difference between sprocket teeth + difference between chainring teeth) to accomodate more difference between the chainring sizes. Also there is a max difference between adjacent rings to be able to shift properly.

Update:

It occured to me that if you have a 68mm threaded bottom bracket shell (MTBs tended to have 73mm shells but some retained the 68mm width) and 130mm rear dropout spacing you could fit a road groupset. The 10 speed Tiagra 4700 group has flat-bar shifters in its line up.

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  • I found some specs for the XT front detailleur (although I don't know if this is the same for all model years) of 12t between gears and 22t full capacity. This fits my original crankset of 26-36-48. So I was looking for a 28-38-50, which I of course didn't find in Shimanos model range.
    – Chaos_99
    Apr 4, 2020 at 6:56
  • If the OP has use for higher gear than 48/11 on mtb, they probably don't need the low end of the cassette.
    – ojs
    Apr 4, 2020 at 7:28
  • @ojs If the OP has use for higher gear than 48/11 on MTB the question is why do they need an MTB?
    – Argenti Apparatus
    Apr 4, 2020 at 11:23
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    I have no idea, but the overall vibe of the question is that they want to modify existing bike for their needs, not get a new one. Whether that makes sense or not is a different question, but replacing worn out drivetrain consumables with a set that has more suitable ratios sounds sensible to me.
    – ojs
    Apr 4, 2020 at 12:47
  • @argenti Surfaces here are rough, my ride style is rough, I sometimes go off-road, this often needs to be packed and tow a trailer, I don't have the space for a second bike (nor am I willing to shell out the money for it). So a MTB frame is more universal and robust.
    – Chaos_99
    Apr 5, 2020 at 11:42

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