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I was swapping the cranks and bottom bracket on my karate monkey from a Truvative stylo crankset (Mountain Double, GXP bottom bracket) to a Race Face Evolution DH (Mountain Triple, Race Face bottom bracket, It's Shimano Hollowtech II compatible I think).

However, the width (Q-factor?) of the Race Face triple crankset is wider than the Truvative Stylo. If I install it and tighten the bolt, it looks to be 5mm+ of exposed space on the crank spindle and the crank arms can slide side to side through the bottom bracket since the spindle is so long. I'm pretty sure my BB shell is 73mm (according to surly).

What do I need to do to compensate for this space? Spacers between BB Shell and BB Cup? How many spacers will I need, and of what size? Is there some common spec listed for the cranks that I should have been able to tell this from the start?

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FYI. I'm running this with a 1x9 setup, so I'm not overly concerned about running a perfect single speed chainline. –  Benzo Feb 12 '13 at 19:57
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2 Answers

up vote 8 down vote accepted

You're right, you want to space it between the shell and cup. You can put spacers on either side to get the chainline right. Most cranksets come with spacers (2.5 mm is probably the most useful size for you) but if yours didn't any LBS should have a few to sell you. Something like this: http://wheelsmfg.com/bottom-bracket-spacer.html (I don't know if these work with hollowtech BB's or not)

You'll probably want to get an assortment of sizes if you can, and use as few as possible to get things lined up (I assume this would help maximize BB stiffness, but really I just hate having excessive spacers - makes it more likely I'll lose one when disassembling/reassembling).

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I agree. Also watch out for spacing problems between the crank arms and the chainstays. A minimum of 5mm is required (more is better). Otherwise, you risk them touching when the frame is under a heavy load, like while climbing. I had problems with a Surly and VO crankset combination that no BB could undo. –  IAmNaN Feb 12 '13 at 21:25
    
@IAmNaN Is the surly that you have one of the "Fatties Fit Fine" ones that fits really wide tires like the Pugsley? I can see this easily becoming a problem with larger chainrings because the chainstays have to be so wide to accomodate the large tires. –  Kibbee Feb 12 '13 at 21:46
    
Yes, @Kibbee. It's a Crosscheck. Not nearly as big as the Pugsley but still wide. Probably the narrowest of their FFF's. I have 35mm's on it now and that's as big as it'll go (with fenders) I think. –  IAmNaN Feb 12 '13 at 22:04
    
I won't really have issues with this since these cranks are wider than the old ones. It's good to know to look out for this. I was even wondering if I would have to worry about tire rub on the chain since I"m putting 2.55" tires on the back, but I don't think this is going to be a problem. –  Benzo Feb 13 '13 at 12:44
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I need spacers, as noted by AlexCuse.

It looks like the spec I was looking for was 'chainline' in the case of my crank, which is measured from the centerline of the frame to the middle ring in my case (this varies by crank type). I can probably estimate at the number of spacers by taking the bb shell size divided by 2, add the width of one BB cup that sits outside the shell and add the spacer size to get the resulting chain line.

That would be (68mm "bb shell" / 2) + 10mm "bb cups" + X "spacer size for one side" = 49 "quoted chainline for my crankset".

So, I'd say it was 34 + 10 + x = 49, solving gives x = 5. So, I would assume I'd need 1x 5mm spacer on each side, give or take. (I'm estimating the bb cup size, everything else is quoted from the manufacturers specs).

See sheldon brown's notes on chainline: http://sheldonbrown.com/chainline.html

Park tool chainline guide: http://www.parktool.com/blog/repair-help/chainline-concepts

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You most likely only need one 5mm spacer on one side or the other. Modern BB shells, and consequently, cups pretty much only come in two sizes: 68mm and 73mm. Spindles are made to match one or both of these specs and having too many spacers in there can cause problems too. It's also super easy to measure the shell. Just take the cranks off and it's a direct measurement with a ruler. –  jimirings Feb 13 '13 at 13:43
    
Actually, my calculations are off a bit since I've got a triple and chainline for mountain triple is calculated to the middle ring, not the small ring. I need to compensate for that in the calculation by subtracting the distance from the inner edge of the small ring (or maybe just where the crank interfaces with the bottom bracket) to the inner edge of the middle ring. –  Benzo Feb 13 '13 at 13:44
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