2004 Burner and 2005 XTR cranks, spacer or no spacer?- Mtbr.com
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  1. #1

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    2004 Burner and 2005 XTR cranks, spacer or no spacer?

    I am a little new at assembling a full bike from scratch.....Therefore, I am a little confused on the crankset/bb spacer issue. I have a 2004 Burner and am putting a 2005 XTR 180mm cranks on it using the XTR bb. Do I need spacers or not? I have two other Turners and have never had issues with chain suck so I do not really want the wider chainline to help free up a stuck chain. I want the most effecient chainline....Can someone simplify this issue for me? What does Turner reccomend (if they ever have)? Thanks for the help...

    Greyeye

  2. #2
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    The old spacer question rears its head again. Here's the scoop Greyeye - Shimano specs the Hollowtech II external bearing cranks for one spacer on the driveside when you have a 73mm BB shell, as is the case on all the Turners. This results in a 50mm chainline. Many riders prefer a 47.5mm chainline as optimal for a 135mm rear hub spacing since in theory the 47.5mm set up puts the middle cog and middle chainring in a straight line (zero deflection). Everything is relative however. The reason for going to a 50mm CL is to accomodate the 73mm BB shell rather than the more common 68mm shell. Why does a bike maker like Turner go with a wider BB shell? Typically it's to accomodate a fatter seat tube. The fatter tube requires the 34.9mm front derailleur and as a result the inner and outer limits of the derailleur travel are moved futher outboard from the centerline of the bike. Hence, a 50mm chainline is an option to permit better shifting on the front derailleur. It's all inter-related. The downside to a 50mm CL is that the Q factor will be altered (you'll have a wider pedal spacing relative to feet, knees and hips) and you loose the "perfect" middle cog middle ring alignment. There is no requirement that you opt for the 50mm line, assuming you don't have issues with front derailleur performance, crank arms or heels hitting the chainstays, or other spacing problems. Personally I just go with the spacer and run the 50mm line - it's really virtually unnoticeable and I found the derailleur adjustments were slightly easier (I had more leeway). I found a benefit in resolving chain jam between the yoke and the chainring when I did chainsuck on the O2. The actual amount of deflection in the chainline going from 47.5 to 50mm is only 1/3 of a degree so I don't think it's much of an issue for anyone other than Lance. Plus it's all related to where you spend you time cranking - if you run small chainring and closer to the middle of the cogs, your line is actually "straighter" at 50mm, same with middle chainring and lower end of the cogset. In otherwords if you tend to stay out of the lowest cogs and run a more agressive gear ratio, the 50mm CL can actually result in less deflection. If you want to try the 47.5mm go for it. Unless you experience the spacing or adjustment issues mentioned above, there's no prohibition. If you eliminat the driveside spacer, you need to add a spacer to the non-driveside to maintain the proper distance between the bearings..
    Last edited by cutthroat; 04-01-2005 at 09:03 AM.
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    Very well said Cutthroat.

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    I have heard people suggest putting a spacer on the non-drive side if you eliminate the driveside spacer - I don't understand why you would do this.


    You have to use one spacer as the crank arms must be pressed onto the bb bearing.No spacer would leave a gap and the splines do not allow the crank arms to take up the play with out a spacer.

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    Quote Originally Posted by CragRat
    I have heard people suggest putting a spacer on the non-drive side if you eliminate the driveside spacer - I don't understand why you would do this.


    You have to use one spacer as the crank arms must be pressed onto the bb bearing.No spacer would leave a gap and the splines do not allow the crank arms to take up the play with out a spacer.
    OK - mystery solved - thanks for the clarification CragRat. I still wonder about the overall effect on the distance of the two crank arms from the centerline though.
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    On my Burner I use no spacer on the drive side. I had one incidence of chainsuck so far. Hopefully it won't an issue. Now, as far as putting a spacer on the non drive side, I can't see how you can omit it due to the fact that a gap would be present between the crank and the BB.

    As far as alignment of the cranks go, I have a 2005 Stumphumper Pro which uses two spacers on the drive side and one on the non drive side. Wouldn't that cause the cranks on the drive side to stick out 2.5mm further? I think the cranks would sit out further on the side you decide to put the spacer on. Am I wrong?

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    Quote Originally Posted by Unknownrash
    On my Burner I use no spacer on the drive side. I had one incidence of chainsuck so far. Hopefully it won't an issue. Now, as far as putting a spacer on the non drive side, I can't see how you can omit it due to the fact that a gap would be present between the crank and the BB.

    As far as alignment of the cranks go, I have a 2005 Stumphumper Pro which uses two spacers on the drive side and one on the non drive side. Wouldn't that cause the cranks on the drive side to stick out 2.5mm further? I think the cranks would sit out further on the side you decide to put the spacer on. Am I wrong?
    If the Stumpy has a 68mm BB shell, then I think you are incorrect. The bearing cups on the Hollowtech II cranks are designed to sit 75.5mm apart. On a 68mm shell, this requires three spacers: two 2.5mm spacers on the drive side and one spacer on the non-drive side. A 73mm shell adds 5mm to the 68mm width, hence the need for only one spacer on the driveside. I am assuming, but don't know for certain, that Shimano sets this up so that each crank arm is spaced an equal distance from the true centerline of the BB shell (37.75mm on each side) to achieve Shimano's design spec of a 50mm Chain line. My question is if we assume a 73mm shell, by putting one spacer on the non-driveside and eliminating the drive side spacer, does this alter the balanced alignment and result in the left crankarm sitting 40.25mm left of the BB centerline and the right crankarm sitting 35.35mm right of BB centerline? My guess is that it does, but whether this is a significant effect, I can't say. The result is a 47.5mm chainline, but more offset on the non-drive side crank arm. Sounds like a question for the drivetrain forum.
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  8. #8

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    Quote Originally Posted by Unknownrash
    On my Burner I use no spacer on the drive side. I had one incidence of chainsuck so far. Hopefully it won't an issue. Now, as far as putting a spacer on the non drive side, I can't see how you can omit it due to the fact that a gap would be present between the crank and the BB.

    As far as alignment of the cranks go, I have a 2005 Stumphumper Pro which uses two spacers on the drive side and one on the non drive side. Wouldn't that cause the cranks on the drive side to stick out 2.5mm further? I think the cranks would sit out further on the side you decide to put the spacer on. Am I wrong?
    Thats because specialized uses 68mm BB shells. If you don't put in the two additional spacers on each side you'll have a 5mm gap between the crank arms and BB cups. You'll also crush the plastic sleeve inside the shell.

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