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or die try'n
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Discussion Starter · #1 ·
Here's a hot tip for those looking for a nice double crank setup on a 29er platform. Since we all know that the gearing on a 29er should really be reduced by 10% for the size of the wheels, we have two options. Make the rear cassette bigger (hence the new 36t versions), or make the front rings smaller ( hence the new doubles in the 27/39 range. Here was my solution since I was looking for a double. I purpose bought the Shimano M771 version XT crankset new for $175 on sale. It comes with the 26/36/48 rings. Ditch the 48 and run the 26/36. The 36 is more than I ever need on the mountains, and the 26 is fine for me with a 34 in the rear.

Here is the dilemma that had to be solved. The 771 spindle is 6.5mm wider than the 770. The BB is the same as the 770, so there is a plastic 6.5mm spacer located on the ring side of the spindle to account for the added width. Problem is that on the majority of our 29er frames, this offsets the right crank arm 6.5mm out farther than the left, and no you can't center it with the BB shims on a 68.

Call Shimano and order an extra 6.5mm plastic spacer. Remove the original spacer and grind or sand or file the two spacers down to approx. 3.25mm each. The spacers also hold the crank arm seals, so keep the seal in the right one as new and reinstall on the spindle after you file. On the left side, remove the seal from the crank arm and place it in the other new spacer. Now after you slide the spindle into the BB, you add the other spacer with the seal toward the BB, and then the crank arm as usual.

There, a centered crank arm crankset. And to boot you have a slightly wider pedaling platform, which can be an asset on those out of saddle power climbs or that single speed you like to blow past the roadies on.


To the Top
You May be Fast
But Coming Down
I'll make you Last:cryin:
 

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Carbon & Ti rule
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getjohn said:
Here's a hot tip for those looking for a nice double crank setup on a 29er platform. Since we all know that the gearing on a 29er should really be reduced by 10% for the size of the wheels, we have two options. Make the rear cassette bigger (hence the new 36t versions), or make the front rings smaller ( hence the new doubles in the 27/39 range. Here was my solution since I was looking for a double. I purpose bought the Shimano M771 version XT crankset new for $175 on sale. It comes with the 26/36/48 rings. Ditch the 48 and run the 26/36. The 36 is more than I ever need on the mountains, and the 26 is fine for me with a 34 in the rear.

Here is the dilemma that had to be solved. The 771 spindle is 6.5mm wider than the 770. The BB is the same as the 770, so there is a plastic 6.5mm spacer located on the ring side of the spindle to account for the added width. Problem is that on the majority of our 29er frames, this offsets the right crank arm 6.5mm out farther than the left, and no you can't center it with the BB shims on a 68.

Call Shimano and order an extra 6.5mm plastic spacer. Remove the original spacer and grind or sand or file the two spacers down to approx. 3.25mm each. The spacers also hold the crank arm seals, so keep the seal in the right one as new and reinstall on the spindle after you file. On the left side, remove the seal from the crank arm and place it in the other new spacer. Now after you slide the spindle into the BB, you add the other spacer with the seal toward the BB, and then the crank arm as usual.

There, a centered crank arm crankset. And to boot you have a slightly wider pedaling platform, which can be an asset on those out of saddle power climbs or that single speed you like to blow past the roadies on.

To the Top
You May be Fast
But Coming Down
I'll make you Last:cryin:
People have been using that type of setup for years,Now it's as cheap & better, tidier easyer to just fit SLX double of Sram X7,X9
 

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bikenut316 said:
I may be missing the point but can't I just use a 770 crank and add the aftermarket rings I like.
If you have the XT crank....you can by the 771 26/36 rings....or any after market rings.
Or...you can by SLX cranks ( which the arms are actually lighter than XT) in the 26/36 set up or put on after market rings.
I bought the 771 crank.....but it is way too wide....and sold it. Even the 770 cranks has a too wide Q factor.
 

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Or just wait until Shimano gets off its a$$ is puts out an XT double chainring. I am not that patient, so I am running a double set-up with the 770 XT cranks with Specialties TA rings. Works great.
 

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Rider and Wrench
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the mayor said:
If you have the XT crank....you can by the 771 26/36 rings....or any after market rings.
Or...you can by SLX cranks ( which the arms are actually lighter than XT) in the 26/36 set up or put on after market rings.
I bought the 771 crank.....but it is way too wide....and sold it. Even the 770 cranks has a too wide Q factor.
To boot there was just a comparison done on a bunch of cranksets in either WMB or MBUK a few issues back and their testing found the SLX was also stiffer than the XT...

So I guess lots of people would really rather ride the SLX.... lighter (a whole 3-5grms for the arms but the stock steel rings are not as light as XT), stiffer and cheaper, how often do those words end up in the same statement in bike land?
 

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the mayor said:
If you have the XT crank....you can by the 771 26/36 rings....or any after market rings.
Or...you can by SLX cranks ( which the arms are actually lighter than XT) in the 26/36 set up or put on after market rings.
I bought the 771 crank.....but it is way too wide....and sold it. Even the 770 cranks has a too wide Q factor.
Just curious if you have tried this as it appears to me that the middle chainrings on the XT 771, XT 770 and various SLX cranks may not be as interchangeable as we wish they were.

I wore out the 36t I had on my XT 760 (I found the 36t at a bike swap meet) and I'm looking for a good replacement (and looking to save a few clams if possible too). I will use steel if I can find one that fits well enough.

Check out the pics below:

770


771


SLX
 

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Rider and Wrench
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jeffj said:
Just curious if you have tried this as it appears to me that the middle chainrings on the XT 771, XT 770 and various SLX cranks may not be as interchangeable as we wish they were.

I wore out the 36t I had on my XT 760 (I found the 36t at a bike swap meet) and I'm looking for a good replacement (and looking to save a few clams if possible too). I will use steel if I can find one that fits well enough.

Check out the pics below:

770


771


SLX
There is never a "never" when it comes to Shimano but for the past few years they have played fairly nice with the 104BCD stuff, the only notable issue is combining an XTR Granny ring with other 64mm granny rings- it has a slightly different offset than std 64mm rings. I actually came across a 26T granny as well, making it very easy to create your own 2X "trail" crankset as it is called by shimano in the XTR dress-

Here is a thread I posted a while back that has some good references on it-

https://forums.mtbr.com/showthread.php?t=672753

I ended up going to a new crankset since this whole ordeal and will eventually get around to listing the xtra 24T, 26T, 36T etc..
 

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or die try'n
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Discussion Starter · #10 ·
Don't forget the chainline factor. On XT 770, and 771, and SLX, the chainline is 50mm to the center of the middle ring. The spider of the 771 is different than the 770 and SLX. It is wider to account for the 6.5 offset spacer. When you split the spacer and center the crank the chainline now becomes 47mm. This puts both the 26 and 36 gears closer the the center of the bike for a better chainline of the gears I run most and produce the most torque in. The chainline is not as critical when I run the 36/11 gear as it is when I run a 36/34.
Also, one of my points was that if you are going to buy a crank, this is a nice way to go because the 771 INCLUDES the 26 and 36 rings. The SLX is 22/36, and the 770 is 22/32.
 

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SLX double with Middleburn rings, fairly light, fairly cheap.
 

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Rings is rings on the XT.....no tricks needed.
A shims ( very thin washers...so no one gets all confused) that any good bike shop should have ( you needed them shims in the old days) will allow you to use after market grannys on XTR.

And I don't remember the granny being any closer to center on the 771....chain line was the same......but it had a long spindle and wider Q facter
 

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or die try'n
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Discussion Starter · #13 ·
The chainline is 50mm on a stock 771, but remember the right crankarm is also offset to the right by 6.5mm, so in reality your crank arms are NOT centered on most of our bike frames. It is only when you split the plastic spacer that the chainline gets altered because you are moving the right arm, spider, rings and spindle 3mm to the left. On one of my bikes I chose to run a 32 tooth M770 ring in the middle slot and the 36 tooth M771 ring in the outer position. Having the chainline 3mm inward while running two rings in these positions does make a big difference in my opinion. I also like the wider footprint of the 771 crankset due to the longer spindle, makes for better torque on out of saddle climbs + I am not a spinner.
 

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getjohn, can you post some pics of how your 771 double setup looks in real life on the bike? I have exact same chainset and wish to try it double also. Thanks
 

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XT 770 cranks should take just about any ring out there. I've personally uses slx, race face and another. However, the 770 middle (and I assume outer) will not work on at least some other cranksets because of the reversed curve in the picture another person posted. I tried to mount one on an FSA K-Force but it wouldn't fit.

Sent from my AT100 using Tapatalk
 

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or die try'n
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Discussion Starter · #17 ·
Update

I think this forum has lost touch with its original intentions. There are a lot of great ways to convert to a double drive train format.

Point #1 of this forum was to indicate that if you only bought one crankset, the Xt 771 came with the gears needed to convert to double without any extra cost. I am sure Middleburn and others make great gears in alternate sizes for the SLX and XT cranks, but they don't come with them so you investing another 50 bucks on top of the crank.

Point # 2 is that the chainline can be a factor for some double setups. The stock 665, 770, and 771 all have a 50mm center ring. But on the 771 the spider and the right crank arm sticks out farther than the left, so they are not centered. If you center the crank by splitting the spacers the new chain line is 47mm. This is nicer for those who use this crank to run a single speed center ring only, or for those who are running the center and outer rings only because it puts the chain in better alignment for the gears. If you are running the inner and middle rings you may have interference problems with the inner gear hitting the frame on a centered 771 crankset.

Point #3 is that there are some people who would like a wider pedal stance and crank arms. The 771 offers that by 7mm or so. It makes for better side leverage on a single speed or steep out of saddle climbs.

I ran my geared bike with a 26/36 and had to cut spacers off an old ring because the crank ring bolts were too long.

/Users/MacJohn/Pictures/Johns iPhoto Library/Previews/2012/01/04/20120104-175603/IMG_0451.
 

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getjohn, thanks for the image now it's more clear to me. I have 771 chainset already factory installed on my bike. It's a hybrid bike which I converted to 29er. So I guess I don't need any extra spacers, just to remove the outer chainring... How about front shifter? Mine is 3 position XT..is it necessary to exchange that to double or I only need to adjust front derailleur travel not to go too much outside? Thanks again!

And one more thing why your front derailleur is so high? is it impossible to move it down near 36 chainring?
 

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or die try'n
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Discussion Starter · #19 ·
I set up my double front dérailleur using low and middle and set the high limit screw to stop it from going into high. I did not adjust the derailleur height on the seat stay. With the top 48 tooth chainring removed the chain will occasionally want to shift past middle and fall onto the crank arm. I have seen people grind away the teeth of a larger chainring and keep it in place as a make shift bash guard or pants protector on travel bikes. This will also keep the chain from jumping off. Make the bash guard about 1/4" diameter larger than the middle ring overall. Hopefully we will see some nice 2x derailleurs come into market at a reasonable price. The 3x derailleurs have too large of a cage and will hit a lot of frames if you try to lower them on the seat stays for better alignment with just 2 gears.
 

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OK, today I took off the outer chainring and seems that bolts are too long fort just one chainring - like it is in your picture. I can not tighten them properly to crankarm. How you solved that? bought a shorter nut?
 
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