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Changing BB - without front derailleur BB Bracket
#1
I'm replacing most of the drive gear on my Avanti MTB due to wear and a bit of damage here and there. Up till now it has run Shimano Acera gear and now I'm replacing with Shimano Alivio gear (Front / Rear Derailluer, Gear Selectors, Crankset and Bottom Bracket).

The issue I have at the moment is the old Acera front derailleur was Bottom Bracket mounted and the new derailleur is seatpost tube clamped.

Now, having said that I've pulled out the old Bottom Bracket and identified it as a Shimano BB-LP27-E which is 68mm x 110mm(ish- only using a ruler).

Because I am replacing the derailleur, which was Bottom Bracket mounted, with another which is Tube mounted, there will no longer be the 3mm derailleur bracket on the Bottom Bracket.

The way I understand it, is by not having the 3mm bracket attached to the Bottom Bracket, the Chain Line will be 3mm closer to the centreline of the frame when I install both new Alvio Front Derailleur and Crankset.

I would just install the new crankset etc and measure the chain line however I have not yet purchased the Crankset / BB yet, so I can only calculate at the moment.

The old Bottom Bracket feels a little rough so I'll be replacing this as well. Does the above imply that I need to install a Bottom Bracket with a different length Spindle, ie 113mm (as opposed to the existing 110mm), or is a spacer utilised to make up the thickness of the derailleur bracket.

Steve
[color=#0000CD]Percussive Maintenance.........

The fine art of whacking the crap out of something to get it to work again[/color]
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#2
Hi Steve,

If the old set up had the correct chainline with a 3mm thick bottom bracket mount, I would go with your idea of getting the next bottom bracket with a 3mm longer spindle. The extra length is on the drive side, so that should give you similar chain line, but your left crank will be 3mm further left.

Here's lots of bottom bracket info here, including some information on chainline: http://www.sheldonbrown.com/bbsize.html
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#3
If you get a BB with extra length then the length MAY be divided on both sides.
I would try the new crankset first and measure the chainline.
Consult Shimano tech doc's for the specified chainline dimension.
Such as:-
http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Alivio-Acera/FrontDriveSystem/SI-F920A-En_v1_m56577569830612827.pdf

Chainline is not relative to the chain but to the dimension required for the front derailleur to work properly.

I would look at the tech' doc' related to your front derailleur which will tell you the chainline required for it. Then see the tech' doc' for the Alivio chainset which will advise the BB required.
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#4
Cheers guys, I have consulted the tech docs and have realised that the BB for the old Acera crankset isn't necessarily the same for the new Alvio crankset.

Will be ordering the BB that is specified to be used for the combination of Alvio derailleur and Crankset, think it was 118mm from memory.

Steve
[color=#0000CD]Percussive Maintenance.........

The fine art of whacking the crap out of something to get it to work again[/color]
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