Sram GXP chain line adjustment?

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Sram GXP chain line adjustment?

Postby 2dads » Fri Feb 08, 2013 12:20 pm

Hey guys,
tech question. recently got hold of an unused omnium crankset to go on an old clemont frame i'm building up as an introduction to fixed gear for myself. id done the maths and was convinced the chainring would clear the chain stays but on installation realised that id failed to account (due to a lack of supplied info and not over =sight on my part, surely) for the massively chunky 144 spider.

now to my Q.......

my bb shell is about 67.2 mm if i was to hone off a mm or two from the non drive side and pack the drive side cup with a 2.5 - 3 mm spacer we would be in business.

can anyone comment on the bb shell max width tolerance for the road gxp bb?
what potential problems i'd have by doing this?
best material for the spacer? test case i just used was 3mm perspex cut with a 35mm and 44 mm (ID ~41) carpenters hole cutters and seems sweet but is it worth getting a perfect fit brass one machined up?

thanks guys.

allan

btw im new to this forum from newy.
"Every time I see an adult on a bicycle, I no longer despair for the future of the human race" - H.G. Wells
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by BNA » Fri Feb 08, 2013 8:07 pm

BNA
 

Re: Sram GXP chain line adjustment?

Postby Nobody » Fri Feb 08, 2013 8:07 pm

2dads wrote:my bb shell is about 67.2 mm if i was to hone off a mm or two from the non drive side and pack the drive side cup with a 2.5 - 3 mm spacer we would be in business.
...
what potential problems i'd have by doing this?
I suppose it comes down to how heavy you are and what gearing you're running as to how long the BB will last, if it fails at all.

As you probably already know, one reason why square taper is still reasonably popular is the ability to tune the chainline with different length axles.
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Re: Sram GXP chain line adjustment?

Postby 2dads » Sun Feb 10, 2013 9:10 am

thanks nobody,
after a good inspection of the length of the internal sleeve between the cups (it'll be well long enough as I'm pretty sure they just make the one for both 68road and 78mtb bb's) and how much of the non drive side machined (bearing interface) axle surface will contact the non drive bearing (it should almost cover the whole bearing width as its made wider than the bearing).

so i think ill be in luck without cutting into the bb shell at all. will torque it all up first with a 2.5 mm spacer from a shimano bb, it'll be really close and may still hit if the setup flexes a bit.if it hits i think ill get a 3mm one made up.
"Every time I see an adult on a bicycle, I no longer despair for the future of the human race" - H.G. Wells
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Re: Sram GXP chain line adjustment?

Postby HLC » Sun Feb 10, 2013 9:30 pm

2dads wrote:thanks nobody,
after a good inspection of the length of the internal sleeve between the cups (it'll be well long enough as I'm pretty sure they just make the one for both 68road and 78mtb bb's) and how much of the non drive side machined (bearing interface) axle surface will contact the non drive bearing (it should almost cover the whole bearing width as its made wider than the bearing).

so i think ill be in luck without cutting into the bb shell at all. will torque it all up first with a 2.5 mm spacer from a shimano bb, it'll be really close and may still hit if the setup flexes a bit.if it hits i think ill get a 3mm one made up.


Why not just massage your chainstay with a ball hammer to give the clearance??? obviously this rides on the quality of the frame. Clamont's / Geoff Scott frames are typically pretty nice so wouldn't be my first option.

Sounds like your BB shell has already had a little bit honed off it. IIRC, Sram GXP BB's are for a 68mm bottom bracket shell..
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Re: Sram GXP chain line adjustment?

Postby 2dads » Tue Feb 12, 2013 6:41 pm

Thanks HLC,
but yeah its a pretty sweet lugged geoff scott frame from reynolds tubing that was bought, stored and never used by the person i bought it from so ill run the risk of destroying a bb before taking a ball pein to the frame. pretty sure itll work out, ill let you know how it goes.

allan.
"Every time I see an adult on a bicycle, I no longer despair for the future of the human race" - H.G. Wells
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Posts: 5
Joined: Fri Feb 08, 2013 9:46 am
Location: Hamilton


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