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  1. #1
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    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline

    Just about to preorder the new M8000 XT group.

    What is the Diff between the 48.8 and the 51.8 Chainline option when purchasing an M8000 XT crank?
    Is the 51.8 for the new Plus size hubs ?

    thanks

  2. #2
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    The difference is 3 mm.

  3. #3
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    51.8 is for Boost 148 rear hubs.

    The model number is M8000-B1

  4. #4
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    Don't believe the industry, get the 48.8 chainline option, you will get better chainline on all cogs in all rings. I run a standard 50mm chainline M760 crank with 150x12 rear 9spd and no problems with bad angle what so ever.
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  5. #5
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    anyone have any experience with one of shimanos front rings? hows the chain retention on the 1x?

  6. #6
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    Note that the 48.8 and 51.8 chainline is for the 2x11 cranks, measured as average (= in between the two chainrings). The 1x11 is just like the XTR's chainline 50.4mm, measured in the middle of the outer chainring.

    Seems like only SRAM has a chainline of 49mm on their 1x11 cranks. Race Face Next SL is 51mm.

  7. #7
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    Hye there,

    Got a question about m8000 crank.. will it fit the old 10speed xt groupset?

  8. #8
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    Is the Axle-Spindle length different on the FC-M9020-B1"Boost" cranks with +3mm chainline wider than the standard crank? It has to be +6mm wider, right?

    I have a Trek Top Fuel with Boost hubs. The guy I bought it from replaced the OEM Sram 1X11 with Shimano Di2 2X11. I believe it has the standard XTR 2x11FC-M9020 crank with 48.8mm chainline. FWIW, it shifts fine as is.

    However, I am wondering about that's going on in there. Should I replace the crank with a FC-M9020-B2 "Boost" +3mm crank? Does this crank have a wider spindle? It seems like it must. But in the Shimano exploded spec cartoon below, it shows the only difference to be the two 3mm spacers. It does not show a different spindle.

    I don't understand how I could add the two +3mm Y1PW98070 spacers to my current crank. That would not work on a standard 48.4mm chainline crank, right? After I installed the spacers, the spindle wouldn't be wide enough to reach the crank arms. So this makes me think the spindle (i.e. the axle) of the FC-M9020-B1 must be +6mm wider >> +3mm on each side. Does anybody know for sure?

    And if I get this wider crank, do I need to add the two 3mm Y1PW98070 spacers? No, right? The extra width of the Boost bottom bracket must take up the extra +6mm width? The two spacers would only be used to run the Boost crank on a normal non-Boost bike, right?

    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline-slide1.jpg

  9. #9
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    Why in the hell would you want a worse chainline? That is what you'll get if you get a BOOST specific crankset. Are there any issues with the current crankset? Is the chainline way off high or low on the cassette? Does the chain rub the tyre when in the small ring and biggest cog? If the answer to these questions is NO, then you're wasting your $$ to buy a new crankset.

    Quote Originally Posted by Scott In MD View Post
    Is the Axle-Spindle length different on the FC-M9020-B1"Boost" cranks with +3mm chainline wider than the standard crank? It has to be +6mm wider, right?

    I have a Trek Top Fuel with Boost hubs. The guy I bought it from replaced the OEM Sram 1X11 with Shimano Di2 2X11. I believe it has the standard XTR 2x11FC-M9020 crank with 48.8mm chainline. FWIW, it shifts fine as is.
    One day your life will flash before your eyes, will it be worth watching??

  10. #10
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    The spindle for boost cranks and non boost cranks is the same. The chainrings (for example those that attach to a separate spider) are moved outwards by 3mm to make the chainline work with the offset on the rear part of the bike. Not really sure if those 3mm will make a difference as to use non boost cranks on 'boosted' frames, but, as far as I understand the spindle in itself is the same. Now, I believe Shimano's don't have a spider (such as Race Face or SRAM) and the chainrings attach to a fixed metal part on the cranks, therefore, I would use/buy a boost crank for a boost frame. Price difference is not that much anyways and when every part is selected for compatibility, everything works better.

  11. #11
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    "However, I am wondering about what's going on in there."

    That's why.

    (The wider chain line makes a lot of sense when the hub is wider. And the wider hub means there's more dish, so the wheel is stiffer laterally. And the tires are wider, so they have more air in them, and more traction. All that, too.)

    But I dig that you are not afraid to lay it out there, bro. PosRep for you.

  12. #12
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    Look at the diagrams pictured. The cranks work just like fat bike cranks. The BB housing on the frame is the smaller size (non-boost 63/73 or 100 on fat bikes). Then the crank spindle and spacers are made wider to accommodate a wider rear.

    However with boost rear, using standard cranks is actually how it should be done. This boost crankset stuff is total crap.

    Ignore wider chainline options, use the standard cranksets, standard chain lines on boost rear bikes.

    Come back and thank all of us that are telling you this later when everything just works instead of the headaches you find in other threads.

    Sent from my XT1565 using Tapatalk

  13. #13
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    I thought it has to be a wider spindle (wider axle) too, but the exploded schematic doesn't show two different versions of the drive-side integrated spindle. Only one.

    So the Boost axle (aka Spindle) must be +6mm wider on the Shimano Boost crank?

    Running a standard 2-by set up with 36-26 front chainrings, the inboard ring could stand to be about 1mm further outboard ... when I trim the front derailleur all the way inboard, there needs to be a little more clearance between the inboard side of the chain and the front derailleur cage. So I may keep my eye open for a new spindle. I've already got the spacers.

    I'm pretty happy with 1X11 ... this 2X11 electronic seems like overkill. I haven't ridden the bike all that much yet. So Ima' go ride now and will post some impressions.

    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline-img_2789.jpg

  14. #14
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    [QUOTE=tigris99;12621955] This boost crankset stuff is total crap.

    /QUOTE]

    Yeah, I always knew those engineers at Shimano were a bunch of posers.

  15. #15
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    No, they want more parts to sell so they make you believe Y u need a boost crankset to run a boost rear. No engineering involved, not hard to add 6mm total to the spacers. Make in spindle, make non-drive side arm able to accommodate both (isn't hard if you look at previous shimano cranks). Or simply have a longer spindle.

    But boost DOES NOT need a special crankset. 1x chainline will be better with regular spacing.

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  16. #16
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    I think I agree with tigris. Shimano chainlines got notoriously wide maybe 10-12 years ago. I also think its why, now with the even wider cranks, you have riders complaining about chains walking down when back pedaling.

    3mm wider out back would seem to line things up just about right - if you stayed with a non-boost crankset.

    Then again, it does seem that Shimano knows what they are doing.

  17. #17
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    Fun ride with a lot of climbing today .... but wow, 2X11 Di2 is a solution searching for a problem. I'm pretty set on one-by. How do I convert my 2X11 36-26 to a 1X11 30. Just need one new chainring, right? Does it mount on the outer or the inner?

  18. #18
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    You don't get a choice on mounting on a 2x crank. Have to match BCD. I don't think 30t comes in the option to fit on the inner so it's just replace the outer.

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  19. #19
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    OK, did some homework on Boost. The Q-factor and spindle (axle) length is the same, but the chainrings mount on a different spider and move outboard 3mm. Or the chainrings mount on the same spider but require a wider spindle (axle) with spacers insider the spider rather than outside. Either way the pedals are in the same place but the chain ring is not.

    Yes, a standard axle will work, especially for a 1X, as long as the chainring is not so large in diameter (i.e. 36 tooth versus 32 tooth) that it rubs the frame. And a 2X standard crank set may not work, because on a Boost +3mm frame the front derailleur mounting bracket also moves out 3mm, so getting the front derailleur to travel inboard enough to center over the little ring is difficult, or on some frames, not possible. But who cares? 2X sucks.

    Sure you get shorter chain stays, but I'm not all that hyped about shorter chain stays. I do like the bigger tires and more dish angle though.

    IMO the big advantage of boost is the extra dish angle on the rear and front wheels. The ratio of some length to flange width in same on a Boost 29er as on a 135mm 26er. So, in theory, you get the stiffness of a 26 wheel with all the other advantages of bigger hoops, with only a slight (virtually zero) weight penalty.

    VitalMTB has some good info on Boost.

    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline-max_2_up_chainline_657929.jpg

  20. #20
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    Sorry, yeah, bit passionate about all these standards the industry keeps releasing without first asking if they're need and if they actually improve on already existing standards. Don't get me wrong, BOOST axle spacing is definitely a step in the right direction,it's what 142 should have been, an actual improvement over 135, but 150 already exists and works with standard cranks.

    Quote Originally Posted by Scott In MD View Post
    "However, I am wondering about what's going on in there." That's why.
    (The wider chain line makes a lot of sense when the hub is wider. And the wider hub means there's more dish, so the wheel is stiffer laterally. And the tires are wider, so they have more air in them, and more traction. All that, too.)
    But I dig that you are not afraid to lay it out there, bro. PosRep for you.
    Yup, 50-51mm chainline came along and was and still is horrible for 135/142 spacing, chain angle to the largest cog on the middle position is so extreme compared to years ago when chainline was 45-46.5mm and that's why I've had absolutely no issues using said 50-51m chainline cranks with a 150x12 rear setup.
    Quote Originally Posted by MikerJ
    I think I agree with tigris. Shimano chainlines got notoriously wide maybe 10-12 years ago. I also think its why, now with the even wider cranks, you have riders complaining about chains walking down when back pedaling.

    3mm wider out back would seem to line things up just about right - if you stayed with a non-boost crankset.
    Then again, it does seem that Shimano knows what they are doing.
    Yes,it's that simple,several after market manufacturers offer 30t rings in all the BCD configurations, just to name one, my fav Blackspire offer their NW Snaggletooth in 86-96 & 104.

    Quote Originally Posted by Scott in MD
    Fun ride with a lot of climbing today .... but wow, 2X11 Di2 is a solution searching for a problem. I'm pretty set on one-by. How do I convert my 2X11 36-26 to a 1X11 30. Just need one new chainring, right? Does it mount on the outer or the inner?
    150x12 rear on Banshee Phantom

    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline-150mm-rear.jpg
    One day your life will flash before your eyes, will it be worth watching??

  21. #21
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    I'm thinking of picking up a 4iii or Stages power meter, and trying to be forward-thinking about the whole thing...

    If I get a FC-M8000 crankset now for my 142 bike(s), and pick up a boost bike running a FC-M8000-B1 "Boost" crankset, the left arms should be identical, correct? Looking at all the pics I've seen online, the left side arms are simply labled FC-M8000...

    (I realize that they're a shimano HTII crank, so they'll definitely bolt up)

  22. #22
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    XT M8000 Cranks. Explain diff btwn 48.8 and 51.8 chainline

    Normal chain line and Boost hubs are fine. You guys are right.

    1X: if you are running 11speed, unless you don't have adequate clearance between chain rings and your frame, no need to go to a special +3mm chain ring. The centerline of an 11-speed cassette is at 47mm from the centerline of the bike, so the 48mm chain line on a standard crank is actually a little better than the 51mm chain line Boost crank. I'm not sure what will happen with 12-speed. For this setup he Boost crank will probably be more suitable.

    2X: If you are running 2X, and you can cross chain on front/little/left and to rear/little/right without rubbing the chain on anything (especially on the empty big ring) there is no need to go to a special +3mm chain ring. Since the PLC "brain" won't even let you cross chain in Di2, this is a non-issue for me.

    I know this from studying the good work that Wolf Tooth did on the subject, from reading you guys' helpful posts above, and also from riding my bike with a "normal" Shimano hub and a Boost hub.


    Wolf Tooth artlcdle is here
    http://www.wolftoothcomponents.com/p...and-chainrings



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    Last edited by Scott In MD; 05-13-2016 at 06:54 AM.

  23. #23
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    the chainline for the new slx crankset is listed at 48.8 here...will they have a slightly better chainline than the xt crankset or will they be the same? any other benefits in waiting for the six crankset to come out?
    Shimano SLX M7000 Single 11sp Crankset | Chain Reaction Cycles

  24. #24
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    Thanks god I found this post. I was about to spend my hard earned money to boost xtr cranks or flight to japan in order to get the ****ing spacer (Y1PV20000) or kit (Y1PW98070) cause I cant find them on SA or Usa shops.

    Correct me If I am wrong but using regular cranks on boost frame are OK right?

    I just swapped from 2x11 XT to 1x11 XTR on my SC5010 2015 (12x142). About to pull the trigger on a devinci troy 2016 only frame (12x148) and was digging in google to find any kit of conversion for the crank, but looks like I dont need anything but buy a new hub and BB to make it happen

    Edit: On my search I found boost (B1) and normal XTR cranks are just the same but with spacers on them. Anyway if there is anyneed to convert the regular cranks to boost just need that spacer (on 1x cranks ofcourse)

  25. #25
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    Geeezas, if you are running 1x11 XTR, and "normal" wheels (not plus) and a "normal" from chain ring (30, 32, or 34) that has plenty of room, then you are all good.


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  26. #26
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    Regarding chainline, are we sure the Shimano 1x rings aren't offset inboard compared to the 2x rings? On the M8000, with a 1x ring installed, there's no room to mount an inner ring without adding spacers.

  27. #27
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    I just removed the XT M8000 boost crankset and replaced it with an M9020 XTR crankset. The spindle on the XT M8000 was quite a bit longer than the XTR spindle. I did not measure it, but it looked to be around 6mm.

    I tried to use the Shimano spacer to bring out the XTR cranks out to the +3mm chainline, but with the shorter axle/spindle, there was not enough spindle sticking out on the left side to properly mount the crank arm. It was recessed pretty far into the crank splines; did not look like a good idea at all.

    I just said the hell with it and used the XTR crankset without the spacer. It now has a 50.4mm chainline instead of the 53.4mm chainline I had with the XT boost crankset.

    No clearance issues between the chain and the tire at the lowest gear on a 11 speed cassette, and no issues with the front sprocket hitting the chainstay.

    I am hoping the 50.4mm chainline with the boost hub will not cause any premature wear issues....
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  28. #28
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    I'll put you $$ that if you shift into your centre/6th gear and look, you'll actually see that the chain is about as perfectly lined up/straight and not at an angle, i.e. good chainline to run through all gears without extreme chain angle. Take a pic if you so wish and post for us All this BOOST and needing wider Chainline is an absolute of horse ****e to sell new cranksets and drivetrain parts, the ONLY reason to MAYBE use a wider chainline is if you are running true Fat tyres in the 3.5"> range to get chain to tyre clearance, that's it.

    Quote Originally Posted by BigRugger03 View Post
    I just removed the XT M8000 boost crankset and replaced it with an M9020 XTR crankset. The spindle on the XT M8000 was quite a bit longer than the XTR spindle. I did not measure it, but it looked to be around 6mm.........................I am hoping the 50.4mm chainline with the boost hub will not cause any premature wear issues....
    One day your life will flash before your eyes, will it be worth watching??

  29. #29
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    Quote Originally Posted by LyNx View Post
    I'll put you $$ that if you shift into your centre/6th gear and look, you'll actually see that the chain is about as perfectly lined up/straight and not at an angle, i.e. good chainline to run through all gears without extreme chain angle. Take a pic if you so wish and post for us All this BOOST and needing wider Chainline is an absolute of horse ****e to sell new cranksets and drivetrain parts, the ONLY reason to MAYBE use a wider chainline is if you are running true Fat tyres in the 3.5"> range to get chain to tyre clearance, that's it.
    Yes, I did that. It looked pretty damn good in 6th gear. I am now convinced the 53.4mm boost chainline is incorrect. Normal cranks are fine, and in my case where I am in low gears climbing a lot, it is even better that 53.4mm.
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  30. #30
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    Quote Originally Posted by BigRugger03 View Post
    I just removed the XT M8000 boost crankset and replaced it with an M9020 XTR crankset. The spindle on the XT M8000 was quite a bit longer than the XTR spindle. I did not measure it, but it looked to be around 6mm.

    I tried to use the Shimano spacer to bring out the XTR cranks out to the +3mm chainline, but with the shorter axle/spindle, there was not enough spindle sticking out on the left side to properly mount the crank arm. It was recessed pretty far into the crank splines; did not look like a good idea at all.

    I just said the hell with it and used the XTR crankset without the spacer. It now has a 50.4mm chainline instead of the 53.4mm chainline I had with the XT boost crankset.

    No clearance issues between the chain and the tire at the lowest gear on a 11 speed cassette, and no issues with the front sprocket hitting the chainstay.

    I am hoping the 50.4mm chainline with the boost hub will not cause any premature wear issues....
    I did the same thing with my Mojo 3 even though Ibis said it wouldn't work. They are looking at it as far as fitting a 2.8 tire possible.

    I'll never run plus, so don't care. I'm using a 1Up 32t Oval and it's tight, but works fine and has much better chain line than a boost crank would.

    To be fair, for those wanting more than a 32t ring, then yes they'll most likely need a boost crank.
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  31. #31
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    Quote Originally Posted by BigRugger03 View Post
    Yes, I did that. It looked pretty damn good in 6th gear. I am now convinced the 53.4mm boost chainline is incorrect. Normal cranks are fine, and in my case where I am in low gears climbing a lot, it is even better that 53.4mm.
    The issue is not chainline affecting shifting but rather chainstay and chainring clearance. I can't run a chainring bigger than 30t on a non-boosted crankset on my Yeti SB 5.5

  32. #32
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    the Shimano site says m8000 is a 50.4
    m7000 is a 50

    I'm trying to figure out what I want for my non-boost Yelli Screamy
    The old XT im using now is around 58 or some crap. It's awful
    Fairly positive i'm going with the m8000 and a shimmed Oneup ring. https://www.oneupcomponents.com/coll...000-chainrings

    At a 11-36 10sp XT cassette now. Will eventually be upgrading to an 11sp XT
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  33. #33
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    Quote Originally Posted by Krigloch View Post
    Fairly positive i'm going with the m8000 and a shimmed Oneup ring. https://www.oneupcomponents.com/coll...000-chainrings

    At a 11-36 10sp XT cassette now. Will eventually be upgrading to an 11sp XT
    That would work, I'm using a non boost 50.4 XTR crank with a 1 up oval on my boost frame. Much better chainline than a boost crank.
    OG Ripley v2

  34. #34
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    I run boost m8000 cranks on my 1x11 setup and don't seem to have any of these chainline issues that you guys are talking about. M8000 drivetrain 1x11, 11-42 on a Top Fuel 8.

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