Will Shimano XT 24T 3x10 chainring work on 2x10??

[LouisS]

Want to change my Shimano XT 38/26 crank to 38/24 configuration. I am aware that I'll have to buy a new 38 chainring as well (38/24 AM) as appose to the current (38/26 AK). I mistakenly bought a 24T (3x10) xt chainring. Will it work with the (38/24 AM) 2x10 configuration? Some bike shops says it will, others says it won't??

[popcorn_skollie]

Want to change my Shimano XT 38/26 crank to 38/24 configuration. I am aware that I'll have to buy a new 38 chainring as well (38/24 AM) as appose to the current (38/26 AK). I mistakenly bought a 24T (3x10) xt chainring. Will it work with the (38/24 AM) 2x10 configuration? Some bike shops says it will, others says it won't??

 

That was probably one of the most confusing things I've ever read. Lets try and clear things up shall we. Lets forget about tooth count for the moment and focus on the important bit. BCD. It means bolt circle diametre and its what matters when fitting a chainring to your crank 

http://www.onebikesworld.co.uk/wp-content/uploads/2013/01/chainring_bcd.gif

See the 4 bolt holes on your chainring? If you draw an imaginary circle through them the diametre of that circle would be your BCD. Its what determines whether a chainring would fit to a crank. 

 

Shimano has only recently changed things a bit with the release of their new groups.

But before they hit the shelves and our bikes. Shimano's BCD whether its a single, double or triple crank has always been the same 104/64.

 

The BCD of your middle and large rings are 104mm.

They screw into the same set of bolt holes opposite sides of your crank.

 

The BCD of your small ring is 64mm

This one screws onto the inside of your crank on a seperate set of holes.

 

So when you buying rings for your crank. Make sure its the right bcd's

This is especially helpful when deciding to buy chainrings made buy aftermarket companies. But since you not doing that lets move on.

 

Its not so much of a concern when buying shimano deore, slx, xt or xtr rings for a shimano deore, slx, xt or xtr crank, but it can be an issue when buying shimano alvio or acera rings for a deore, slx, xt or xtr crank. Particularly the small ring. Despite them ALL using the same 64mm bcd on the small ring. Lower specced rings (Alvio and Acera) don't allow the same clearence inside the ring required by cranks which use external BB's (bottom bracket) That bit that keeps the cranks spinning in your frame. Deore through to XTR all use external BB's. Alvio and Acera don't. There are exceptions but its best to blanket things for now so you have a better understanding.

 

As for your 2 big rings. The middle and the large. They both use 104 bcd. The middle ring screws onto the inside of the crank and the big one screws onto the same set of holes on the outside of your crank. You could change a triple crank to a double simply by removing your big ring. Yep its that simple. Again. There are exceptons and rules to thins but its all trivial when trying to explain the basics.

 

Now onto tooth count. Once you understand the whole BCD thing. Then its only logical to assume that rings of a certain BCD are limited to a certain range of teeth per ring.

The more teeth you have on the ring. The bigger the ring is. The BCD's are always the same between the 2 shimano standards (104/64) But the tooth count per ring have their own range. I might be off slightly but here are the standards off the top of my head.

 

Shimano chainrings with 64 bcd - 20 to 28 toothed chainrings (the small one)

Shimano chainrings with 104 bcd - 30 to 38 toothed chainrings (the middle one)

Shimano chainrings with 104 bcd - 40 - 48 toothed chainrings (the big one)

 

So it doesn't matter matter if you bought a ring from a triple crank to fit onto a double crank as long as the BCD's are the same which I'm sure they are.

 

You also do not need to buy another 38 tooth for the same reason.

 

Someone who knows better might be able to more accurately fine tune what I said.

But I think it more or less puts things into perspective so you understand.

[River Rat]

Your problem relates to the teeth difference between the chainrings and not whether its is 3 x or a 2 x. Shimano actuation on the front derailleur is happy with 12 teeth and SRAM 14 teeth because of their 1:1 actuation. But their is hope in that I have done 38/22 with SRAM but you spend a lot of time tuning the cable tension to get it right.

[Skylark]

Want to change my Shimano XT 38/26 crank to 38/24 configuration. I am aware that I'll have to buy a new 38 chainring as well (38/24 AM) as appose to the current (38/26 AK). I mistakenly bought a 24T (3x10) xt chainring. Will it work with the (38/24 AM) 2x10 configuration? Some bike shops says it will, others says it won't??

 

You can generally ignore all that BS about matching chainrings and "use only with" so and so chainring set, use only with Shimano Directional chain blah blah - I've mixed and matched chainrings no problem, perhaps you get marginally faster/crisper shifts with matched chainring sets but not much. Perhaps matched sets is more important with 3 speed cranks because for a fast seamless shift from the smallest to biggest ring you will need all the ramps lined up and working together on the top 2 chainrings.

 

Pretty sure that 24t chainring will work with the 38t, give it a try. 38t to 24t is perfectly possible just as long as its within the capacity of your FD. Found this online "24/38 as a stock combination now in both SLX and XT, and it works perfectly with the their 2x10 mechs"

 

I'm running a 2x10 Sram X9 crank with a 36t SLX 9sp chainring and a no name brand(that I can see) 24t small ring, with an XX 2 x 10 FD, shifts almost as good as the XX chainrings I had on there before. As long as the difference between the chainrings is within the capacity of the FD, most of them can cope with around 12-14 tooth difference between chainrings, then its should be fine.

 

Your problem relates to the teeth difference between the chainrings and not whether its is 3 x or a 2 x. Shimano actuation on the front derailleur is happy with 12 teeth and SRAM 14 teeth because of their 1:1 actuation. But their is hope in that I have done 38/22 with SRAM but you spend a lot of time tuning the cable tension to get it right.

 

That's not true with 10sp - Shimano & Sram use the identical actuation ratio on their FD, I'm runing my XX 2x10 FD with an XTR 10sp shifter, works 100%.

[JXV]

24t AM ring will work with the 38t AK outer ring. I've done it. The 38t AM ring works a little better because its shift ramps are optimised for the 24t. When your current 38t AK wears out then replace with the AM version if still running the 24t.

 

If you have a medium cage Shimano RD (GS in the name denotes med cage) then this has a max capacity of 39t - all of which is used by your 24-38 front and 11-36 cassette combo - leaving little or no capacity for chain growth if your bike is dual sus.

 

Check the chain growth aspect if applicable. This gear combo would be better with a long cage (SGS) rear derailer.

[splat]

I want to change my Shimano XT 38/26 crank to 38/24 configuration. I mistakenly bought a 24T (3x10) XT chainring. Will it work with the (38/24 AM) 2x10 configuration?

Yes

Mine works fine

[LouisS]

Thanks for all the help/replies and explanations!

 

It just confused me that Shimano makes a 24T XT 10S chain ring with a different part nr. for 2x10 and 3x10 configuration?? If I understand correctly, they then should theoretically be identical because they don't have shift ramps as would a 38T outer ring have specifically manufactured to run either with a 26 or 24T chain ring. 

[Skylark]

Indeed many people have faced the exact dilemma you have, I have seen it many times before, what look like identical parts with different part numbers and specific instructions that they must be used with only so and so part.

 

Eventually you develop enough confidence in your own common sense and you by and large can ignore these specifications. I also had to have a good think at one time how a small chain ring with no ramps could possibly be designed to only work with the matching chain ring, common sense prevailed and I mix n matched chain rings from then onwards.

 

Another helpful tip that's not widely known is the small chain ring typically wears faster than the big ring and because it normally has no ramps and barely any offset you can turn it around and use the unworn side of the teeth profiles to essentially now have, from the perspective of wear, a brand new chain ring.