Sram XX Chain ring gap to chain stay?- Mtbr.com
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  1. #1
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    Sram XX Chain ring gap to chain stay?

    Hi,
    I've just fitted a 28/42 XX crank to my Ibis Mojo. I used a GXP BB and put spacers either side. The problem is the large chain ring is very, very close to the chain stay. I moved the non drive side spacer to the other side to increase the gap. Now I'm a bit concerned that by doing this I've put the whole chain line out and also not sure if you should put both spacers on the same side. Has anybody else had similar issues?

  2. #2
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    Spacing is very narrow, which is normal : the middle 32T ring of your triple crank is replaced by a 42T ... I have about a 1-2mm gap.

  3. #3
    Cuánto pesa?
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    Bottom Bracket Shims
    Allows fine adjustment of chainline by shimming cup away from frame...
    Silver: ( 1 / 1.5 and 2mm ) spacers are sold in bags of 20...$11.00 per bag.
    Black: ( 2.5 / 1.8 and 0.7mm ) are sold individually...$3.00 per shim.


  4. #4
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    New question here. 1-2mm from chain stay?

    I had probably about that gap between large chain ring and the chain stay. Is that enough have you had any problems? I read that there should be significant gap to allow for some frame flex.

  5. #5
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    Shimming the driveside bottom bracket cup will not change the chainline with GXP cranksets. The design of a GXP crankset means the chainline is set by the NON-driveside bottom bracket cup which is trapped between the non drive crankarm and a lip on the spindle. To increase the gap between chainring and chainstay remove all spacers from the non-driveside bottom bracket cup. It sounds like you've already done this. Once all spacers are removed the chainline can't be increased anymore without facing the non-driveside of your bottom bracket shell.

  6. #6
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    Quote Originally Posted by aussie_yeti
    Shimming the driveside bottom bracket cup will not change the chainline with GXP cranksets. The design of a GXP crankset means the chainline is set by the NON-driveside bottom bracket cup which is trapped between the non drive crankarm and a lip on the spindle. To increase the gap between chainring and chainstay remove all spacers from the non-driveside bottom bracket cup. It sounds like you've already done this. Once all spacers are removed the chainline can't be increased anymore without facing the non-driveside of your bottom bracket shell.

    Can you elaborate? What is unique about the GXP setup? To increase the shim thickness on the drive side means that (assuming we're not dealing with an idiot here) he's decreasing the non-drive shim thickness by the same amount. Otherwise, he'll reach bearing preload long before he gets the crank arm tight on the spindle. The crank either falls off 1/4 mile down the road or it won't spin. I've assembled a bunch of these and I still can't wrap my head around what you're saying. It seems like semantics to me... both the drive and non-drive sides settle in on the BB cup, and you pull spacers from one side to the other to change chainline. Same as any other external cup crank/bb.

  7. #7
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    Quote Originally Posted by eric1115
    both the drive and non-drive sides settle in on the BB cup, and you pull spacers from one side to the other to change chainline. Same as any other external cup crank/bb.
    GXP is the exception to this. Yes on the pulling spacers side to side to an extent (only the NDS matters). No on both the crankarms settling down on the BB cups.

    Assuming you either have a 68mm BB shell with two 2.5mm spacers installed or a 73mm BB shell with zero 2.5mm installed as per factory instructions. With the crankset installed ready to ride have a look where the spindle passes into the DS bearing cup. You'll see a 2.5mm gap between BB cup and crankarm where the spindle is visible. The crankarm doesn't actually settle down against the BB cup. This is to allow the use of an E-Type front derailleur or BB mounted chainguide with a 73mm BB shell. Every wonder why all the other guys use three 2.5mm spacers in their setup and GXP only uses two? Try adding an extra 2.5mm spacer to what SRAM recommends to the DS. Q-factor and chainline will not change but that gap will disappear and the crankset will still work perfectly.

    Have a look at the spindle of a GXP crankset. All other designs maintain a 24mm diameter along their length where as a GXP crankset steps down to a smaller diameter near the NDS bearing. All other designs have an internal bearing diameter of 24mm for both sides of their bottom bracket whereas a GXP bottom bracket has a smaller diameter in their NDS cup (or uses a shim to reduce it from 24mm such as a GXP compatible BB from Hope).

    Bearing preload is not maintained by the two crankarms tightening down on the BB cups. Rather, the lip on the spindle and the NDS crankarm sandwich the inner race of the NDS bearing to eliminate side to side play. The DS bearing doesn't have any preload on it at all, rather it just floats on the spindle.

    This also means that the chainline is altered by the position of the NDS bearing in relation to the frame. If you add spacers to the NDS you decrease the chainline, if you remove spacers from the NDS you increase the chainline. Once the NDS BB cup is installed straight against the frame with no spacers you can't increase the chainline anymore...therefore on a frame with a 73mm BB shell the stock installation setup is the widest chainline possible.

    This is the issue the OP will have if his frame has a 73mm BB shell. He won't be able to increase his chainline to get more chainstay clearance without facing the NDS of his frame.

  8. #8
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    Quote Originally Posted by aussie_yeti
    With the crankset installed ready to ride have a look where the spindle passes into the DS bearing cup. You'll see a 2.5mm gap between BB cup and crankarm where the spindle is visible. The crankarm doesn't actually settle down against the BB cup. This is to allow the use of an E-Type front derailleur or BB mounted chainguide with a 73mm BB shell. Every wonder why all the other guys use three 2.5mm spacers in their setup and GXP only uses two? Try adding an extra 2.5mm spacer to what SRAM recommends to the DS. Q-factor and chainline will not change but that gap will disappear and the crankset will still work perfectly.
    .
    You just hit on the thing that's been bugging me - the DS spider not sitting flush to the BB cup, which based on the completely rainy, gritty race I just did, I need to overhaul the bearings (2nd ride ). Will give the third spacer a go to see if that helps the situation, but given that SRAM didn't send spacers with the BB in the first placed, that was my first source of aggravation. I wish SRAM would highlight this in their installation manual and videos - it really seems strange to not have the cranks not touching the BB cup.

  9. #9
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    To answer your question about 2 spacer being on one side, I just had my XX drivetrain installed last weekend. Brought it a reputable LBS. They put both spacer on the DS. So far no problem with shifting. My bike is a 05 SC Superlight.
    Attached Thumbnails Attached Thumbnails Sram XX Chain ring gap to chain stay?-img464.jpg  


  10. #10
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    that will mean your NDS crankarm is 2.5mm closer to your chainstay, your DS crankarm is 2.5mm further away from your chainstay and you chainline is 2.5mm wider. there are two reasons for doing this.

    - to increase clearance between chainrings/DS crankarm and chainstays
    - to stop the chain rubbing on the big ring when you're in the little ring and the smaller cassette cogs

    seeing as your chainstays look pretty symmetrical it'd be pointless increasing clearance to DS crankarm as the NDS clearance would decrease. we can therefore eliminate the DS crankarm clearance hypothesis.

    if it looks like you've got heaps of clearance (>4mm) between chainstays and chainrings i'd swap one spacer (or both if clearance >6.5mm) to the NDS so it makes the chainline better in the big/big combo. if you swap both spacers to the NDS you'll almost certainly get rubbing on the big ring when in little/little combo's. you'll have to decide if you're happy to avoid those gears or have a slightly wider chainline in big/big and have those gears available for use.

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