Can I change my large chainring without changing the entire crank

Question

I am running a 2x10 Deore M4100 set up on my current bike. My crankset is 36/26 T and my cassette is 11-42T.

I want to make this bike faster, and I want to know if I can just replace the upper chainring on the crank to a 38T or larger?. I was looking at the chainring 38T for the Shimano FC-M600-2 because it seems to be compatible with my existing crankset but I am unsure. Trying to avoid changing the entire crankset if possible.

I have 2 questions. Is it compatible? will it work with the lower chainring 26T [38/26T] or do I have to change that to a 28T to fit [38/28T]?

Answer 1

This is one of those questions with some different permutations of what you could do and what the compromises might be.

FC-M600-2 appears to be a 4-bolt 104/64 symmetrical crank, which is not what you need. Also, it generally will not work to take a ring for a crank that's made to mount on the outer spider position, where the bolt counterbore is on the outside of the ring, and adapt it to one of the newer style doubles like FC-M4100-2, where the large ring mounts on the inside of the spider. In some cases you might be able to make that work just by machining or grinding your own counterbores, but beyond that there's the question of will the ring-to-ring spacing still be right even though you're using a spider contact surface that wasn't intended. In some cases that surface might not even be flat. I haven't experimented much with that and don't have all the answers to it, but suspect it won't go well because the design of the two kinds of cranks and rings are quite different.

The bolt pattern for the 38t ring (WP-Y1WD98030) from FC-M6000-2 is the same as what you need. That ring isn't listed as compatible by Shimano with your crank and has fancy 3d contouring around the mounting holes, so it may interfere with the spider, but that may be a problem that could be fixed with a dremel or file if it doesn't just go on. I suspect that's likely true. If you did that and it goes on, you're left with two other potential issues:

• The shift aids on it are intended for a 28, not a 26, so the shifting will probably be laggy. I don't think that's a problem you can get around with any existing 10-speed 38t ring for Shimano 96mm asymmetrical double cranks.
• Your front derailleur capacity is 10t (doesn't want more than a 10-tooth jump between rings), which means it won't be usable at some point in the small/small combinations. That is a problem you can solve by getting an older 10-speed double mountain front derailleur. A lot of the earlier Shimano 2x10 mountain ones have capacity of 12 or 14, say from the 2010-2012 era. FD-M785 is an example. You can look up this data in the archives on si.shimano.com, and then go on an ebay goose chase to find one. I couldn't figure out a new example of a specimen like this that's actually available - there's probably technically one that's still made as a repair part, but I couldn't discern what it is, and there may not even be one.

If keeping the range the same and just moving it higher is acceptable, then getting the 28/38 set for FC-M6000-2 and doing the above mods to the mounting area if needed is likely to be a simpler answer, and should work without issue with your current FD. Make sure you've got the frame clearance to do it; many more recent bikes play every trick possible to get their tire clearance, and will be unforgiving about this. The same is true for going to a 38 by itself.

Note that there's usually a tooth offset difference between Shimano 10 and 11 chainrings. If you find an 11-speed ring that appears to do what you need, that's another thing you would potentially need to deal with, although it might be a problem that could be solved with micro shims under the small ring.

Answer 2

Welcome to Bicycle Stack Exchange. The FC-M4100 crankset is a 4-bolt having a 96/64mm bolt circle diameter (B.C.D.). Looking at the exploded view of the M4100 and referring to the parts list at the bottom, one can see there are several interchangable parts with the M7000, M8000, and M5100 cranks but chainrings are not among them save the M8000's small chain ring. This lack of direct compatibility is likely due to the slightly different shapes of the spiders (the portion of the right crankarm that the chainrings bolt on to). Typically there is a little extra material that stands in the way of another models chainring fitting flush on the spider. In the past with other models its possible to grind material off the chainring to make it fit. The material removal is quite minimal and irrelevant to structural integrity, but advising someone to grind on a brand new chainwheel is dubious at best. At any rate, generally speaking, a chainwheel that is 4 bolt with a 96mm BCD, should be compatible with the crank but there are other considerations.

First, you need to determine if a larger chainring on the bike will have adequate clearance at the chainstay on the right side. Many MTBs these days have wider, 148mm over lock nut (O.L.D.) or "boost" spread if the rear dropouts. This results in a wider spread of the forward part of the chainstay which limits the possible size of a large chainring. The bike manufacturer will have that information in it's user manual or complete list of specs. So determine the allowable size of chainring to avoid interference with the frame. At least 3mm between chainstay and chainring teeth is necessary.

Second, the front derailleur, the FD-M4100 is spec'd to handle a maximum size chainring if 36 teeth and works optimally when the difference in tooth count between large and small ring is 10 teeth. Putting on a larger chainring will create a need for a different front derailleur that will accommodate more teeth. Most all MTB double front derailleurs (designed to use with 2x cranksets) can accommodate a max chainring of 38 teeth, so that's an issue for a new ring. The selection of FD-M6000 front ders is the closest model in terms of quality and level of hierarchy to your current and can accommodate a 38 tooth large ring.

While the problem isn't insurmountable, increasing the gearing by going with a larger chainwheel presents problems that may not be overcome without different parts, adding to the cost. Then one must evaluate whether the 6-10% (38t-40t) rise in gear ratio is worth the additional cost. (Comparison of the amount of increase in the ratios 36/11, 38/11, & 40/11) This translates to an increase of 3mph at 90rpms between 36 and 40 teeth. Figures obtained by plugging them into Bike Calc website found here.