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Chainline and using spacers on a road BB

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Chainline and using spacers on a road BB

Old 05-13-15, 11:18 AM
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jbroadway
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Chainline and using spacers on a road BB

Hey all, building my 2003 Salsa Campeon last night I ran into an issue with chain rub on my Ultegra 6600 double cranks. In the small ring, the chain rubs the big ring when the rear is shifted in the 3 highest cogs of the cassette. I know that's considered cross chaining and is most likely the cause (frame out of alignment is another, and the worst). Just wondering if I can throw a 2 or 2.5 mm spacer behind the drive side BB cup to push out the chain line a bit to reduce the rubbing? Any ill effects of doing this?

Everyone is adamant about not needing spacers on a road BB, or not using those gear combinations etc. but maybe it's not the end of the world....What Would Sheldon Do?
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Old 05-13-15, 11:37 AM
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Interestingly, the triple version of that crank calls for a NDS spacer between the cup and arm -- presumably to make sure the arm is still spaced out far enough to clear the chainstay: http://www.shimano.com/media/techdoc...9830616000.pdf So there is a precedent for scootching the things around with this system. But that document also says not to use the small ring with the three smallest cogs, so...

I'm not as familiar with the HT style cranks, so the only question I have is whether the inner cover thing would still connect properly with both BB cups if the DS cup is now 2-2.5mm further away.

Other things to consider: whether you need a 52T/53T big ring. If you substituted a 48T or 50T, there would be less potential for the chain to rub when in the small/smaller cog combinations, because the diameter of the big ring would be smaller.

If it were a friction-shifted front, I'd consider adding thin spacers between the small ring and crank arm in order to scootch the two rings apart, which would increase the chain angle on the small/smallest gear combos, but would reduce/prevent the rub. That might mess up the front shifting though.
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Old 05-13-15, 11:39 AM
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That kind of rub when in the small ring and smaller few cogs is common particularly with compact 50/34 gearing but even with 53/39 cranks on a bike with short chainstays. Only sure cure is to avoid those gears. You can't space out a Hollowtech II crank like yours very much since the non-drive side spindle would bee too short for full spline engagement.
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Old 05-13-15, 11:40 AM
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Chainline is the center of the stack of cogs in the back .. on a double crank the other end of that parallel line is between those 2 chainrings .

Rubbing? The weird shapes of STI FD cages is to not rub until you are in a cross chain combination.

I know that's considered cross chaining and is most likely the cause
so stop doing it.
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Old 05-13-15, 11:45 AM
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Originally Posted by ThermionicScott View Post
Interestingly, the triple version of that crank calls for a NDS spacer between the cup and arm
That's because they're trying to use the same hardware for two difference specs.

A road double should be 43.5mm to the imaginary centerline between the two rings.

A road triple should be 45mm to the center of the center ring.

Hence the need for a 1.5mm spacer.

The OP shouldn't screw with anything if his chainline is 43.5mm which it likely is.
A standard install of a matching Shimano BB and crank puts it in the right place pretty much every time.
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Old 05-13-15, 11:59 AM
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Originally Posted by fietsbob View Post
Rubbing? The weird shapes of STI FD cages is to not rub until you are in a cross chain combination.
The OP doesn't have rubbing on the front derailleur cage. The chain is rubbing on the inner face of the large chainring and the fd has nothing to do with it.
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Old 05-13-15, 12:08 PM
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Originally Posted by andr0id View Post
That's because they're trying to use the same hardware for two difference specs.

A road double should be 43.5mm to the imaginary centerline between the two rings.

A road triple should be 45mm to the center of the center ring.

Hence the need for a 1.5mm spacer.

The OP shouldn't screw with anything if his chainline is 43.5mm which it likely is.
A standard install of a matching Shimano BB and crank puts it in the right place pretty much every time.
Yeah, but the spacer is on the opposite side from the rings. Perhaps the spindle is the same part for both cranks, and the DS triple crankarm itself attaches to it in a way that scoots it to the left by that 1.5mm, so the spacer is needed to take up the extra space. I dunno; I shouldn't speculate about it, and it's not really germane to this discussion.
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Old 05-13-15, 12:15 PM
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Originally Posted by fietsbob View Post
so stop doing it.
There it is! Haha yes, I will just have to avoid those gears. Coming from a DA 7900 with not a single noise to an Ultegra 6600 will just have to be an adjustment!

Thanks for the suggestions, guys!
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