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Mountain bike chainring combination question

Old 06-14-07, 12:50 PM
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Mountain bike chainring combination question

Hi, All!

I'm new here, but not to cycling. Still, I have a couple of mechanical questions that I haven't been able to find answers to, so I'm hoping somebodies here will have the answers.

I have a GF Cobia, and since I mostly use it for commuting in this relatively flat city, I've wanted to switch from the stock 42/36/22 chainrings to (an upgraded) Deore XT (FC-M761 series) 48/36/26 set. At the same time, I've upgraded to the Deore XT (RD-M761 series) front and rear derailers (from the stock Alivios).

Question #1:
Upon assembly, I find that the 26-tooth inner chainring only has about 1.5mm of clearance with the chainstay (everybody recommends at least 2mm to allow for chain and frame flex). So, will it be OK to keep the 22-tooth granny with the 32 middle and 48 outside rings? Both sets are spec'd for the XT front derailer, but I can't find any info about mixing the manufacturer's combinations. Is 48/36/22 a bad combo?

Question #2:
So, I'm reading the installation sheet that Shimano provides, and it says, "The tooth number marking on the largest chainring should face outward, and the markings on the intermediate and smallest chainrings should face inward."

That's clear enough, except for the fact that this here largest chainring is quite obviously not designed to be installed that way. The chain drop prevention pin should face the inside of the right crank, right?

Well, if I install the largest chainring according to the directions, the 'chain drop prevention pin' is going to become the 'shifting-to-the-middle-chainring-prevention-pin', because it's positioned opposite the tooth number markings, not on the same side of the markings, as it would need to be if the tooth number markings should face outward.

Has anyone else encoutered this contradiction?


Thanks in advance!
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Old 06-14-07, 08:26 PM
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Q#2: Considering that the chain drop pin is a lot more important than etched/stamped numbers, I would ignore that.

Q#1:The 22 ring probably wouldn't do any harm if you swap out the 26 for the 22. It is not bad, but the 22 ring might be heavier because it sounds like it was on a lower end crankset. If you need it, then use it and if you find that you never use it then take it off. As long as it works mechanically, it can't be bad. If you are used to certain spin rates then it might take a little getting used to.
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Old 06-15-07, 10:00 AM
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Thanks, z415,

Yeah, the stock 22-tooth chainring is low-end... it's steel, and the XT FC-M761 rings are aluminum.

I'm thinking now that I'll just order the XT FC-M760 granny gear (Al, of course) from my LBD.

What I was most concerned about was the jump from the 36-tooth middle ring to the 22-tooth granny, because I'd read that 10- to 12-tooth jumps were OK, but that 12 was the limit.

I guess I'll just slap the old, steel, 22-tooth granny on there, try out the shifting, and if it works, I'll order the XT granny gear.

I want the 48-tooth large ring for the flats that I ride daily, but I occasionally do touring, and I'd like the smallest granny I can fit for the climbs in this country.
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Old 06-15-07, 11:49 AM
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Ba ba Ba Bump!

Any other opinions on this?

Is it too much of a jump from a 22-tooth granny to a 36-tooth mid-ring?

Well?
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Old 06-15-07, 05:04 PM
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Originally Posted by Halloween
The chain drop prevention pin should face the inside of the right crank, right?
No. It should face the crank arm (i.e. the outside of the crank), and be directly opposite it. The point is to catch the chain to keep it from jamming between the ring and the arm.

On question 1, try the new ring and if it hits you can shim the crank with a spacer.
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Old 06-16-07, 01:15 AM
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The only thing you'd need to worry about going to a bigger Big Ring and a smaller Little Ring would be chain wrap. Make sure your derailer can handle the increase in chain length you get by taking another 4 teeth out, and also make sure your chain isn't too big (from swapping out the 26) or too short (from swapping in the big ring -- this only matters if you try to re-use your current chain).

Either way, though, it seems like you ought to be able to fit the XT set up as it comes stock and evaluate whether there are any chain rub issues. If there are fix it. If not, well....it ain't broke, so........
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Old 06-16-07, 10:43 AM
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Originally Posted by Sheldon Brown's
................................................Bicycle Glossary

Sometimes called a "crank arm", but this is redundant and inelegant. Call it a crank, or call it an arm, but please don't call it a "crank arm"
Originally Posted by DMF
No. It should face the crank arm (i.e. the outside of the crank), and be directly opposite it. The point is to catch the chain to keep it from jamming between the ring and the arm.
Hmmm...

What we got here is failure to communicate...

Originally Posted by Halloween
The chain drop prevention pin should face the inside of the right crank, right?
When I say 'outside', I mean the side away from the frame, and when I say 'inside', I mean the side close to the frame.
When I say 'crank', I mean that long thing between the pedal and the bottom bracket spindle.
The right crank has a 'spider' that holds the chainrings.
On my bicycle, there is no possible way that the chain drop prevention pin could face the outside of the crank.

I'm aware that the point is to keep the chain from falling between the crank and the outer (away from the frame) chainring.
My point was that if I followed Shimano's instructions about the tooth number marking on the large chainring facing outward (away from the frame), the chain drop prevention pin would not be facing the crank, but would instead be facing the middle chainring, and would make shifting onto the middle chainring impossible. The "right?" was rhetorical.

To say it another way, the chain drop prevention pin should be between the right crank and the outer chainring, or pointing away from the frame, and not pointing toward the frame.


Originally Posted by DMF
On question 1, try the new ring and if it hits you can shim the crank with a spacer.
I already know that there's not enough clearance for the 26-tooth ring.
I'm not going to risk frame damage. I have the OEM Bontrager press-fit cranks with a square spindle BB, so can't shim, but thanks.


Originally Posted by kandnhome
Either way, though, it seems like you ought to be able to fit the XT set up as it comes stock and evaluate whether there are any chain rub issues. If there are fix it. If not, well....it ain't broke, so........
Everything fits on the crank spider, it's just that there isn't enough clearance between the 26-tooth granny and the chainstay.
Even the middle chainring is just barely 2mm clear of the chainstay. Looks like GF never intended the RD-M761 series to fit on the Cobia. The chainstays quickly flare away from the BB shell to allow clearance for big, fat, 29-inchers.
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Old 06-16-07, 11:23 AM
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Originally Posted by Halloween
Hmmm...
The chain drop prevention pin should face the inside of the right crank, right?

What we got here is failure to communicate...

When I say 'outside', I mean the side away from the frame, and when I say 'inside', I mean the side close to the frame.
This is why I'm always pushing for the use of the simple English terms "right" and "left." The chain-drop pin faces to the right.

If you prefer obscure abbreviations, that's also sometimes referred to as "DS." ;-)

Sheldon "<- Left - Right ->" Brown
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Old 06-16-07, 11:59 AM
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Originally Posted by Sheldon Brown

If you prefer obscure abbreviations, that's also sometimes referred to as "DS." ;-)
'Distaff Side' ?

"If a man speaks in the forest, and his wife is not there to hear him, is he still wrong?"



How about something more nautical?

'<- Port - Starboard ->'.
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Old 06-18-07, 08:57 AM
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Looks to me that the whole question was rhetorical.
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Old 06-18-07, 09:24 AM
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Longer bottom bracket.
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Old 06-18-07, 09:44 AM
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Originally Posted by DMF
Looks to me that the whole question was rhetorical.
M'kay...
Thanks for answering.



Both times.
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Old 06-18-07, 09:46 AM
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Originally Posted by maddmaxx
Longer bottom bracket.
Will not spec with the front derailer.
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Old 06-18-07, 10:25 AM
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If for example you have a 113mm bottom bracket now, a change to a 115 will space the rings 1mm futher out providing clearance at the frame. I'm sure that the high limit screw on the front derailleur will accomodate this. There are many combinations of small bottom bracket length changes that will do this for you.
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Old 06-18-07, 10:57 AM
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OK, maddmaxx, I'll check into that.

My BB shell is 73mm.


Maybe upgrading to the XT Hollowtech crankset and a new BB is the way to go.
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Old 06-18-07, 11:48 AM
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Check Harris Cyclery in Boston area as their BB parts list is extensive. You will need the axle length of yours and you can see what ranges exist slightly above yours exist.

I use the Deore 26/36/48 with octalink BB on mine, but the frame is Haro and I have no clearance problem so I use the recomended length.
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Old 06-18-07, 11:54 AM
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Well, I'm in Japan.

This is Shimano-land, so I'll just check with my LBS first.

He's been pretty good with everything I've wanted so far.
I just didn't know that I wanted a new BB until now.

I'd still like to use the 22-tooth granny if I can.
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