calculating correct length axle (fitting new chainset to old mtb)

[John.w]

I have a n.o.s deore chainset (2002 compact drive)to replace worn set
on 93 stumpjumper, have reamed and faced bb shell, fitted "new style"
cartrige bb, and wish to know if it is a case of trial and error (ie:
see if it jumps or not) or if there is a calculation/specific method
to know if i need to fit a new/different length axle, or if my
original will do.

old kit was 93 dx chainset with 170mm sq taper cranks and bio-pace
full size rings, ill still be using a 7sp hg cassette (a new one tho)

A friend "thinks" if when the chain is on middle front ring and 4th
cassette ring, it should "line up straight" when looking along length
of bike??????

 

[Sheldon Brown]

John.w wrote:

> I have a n.o.s deore chainset (2002 compact drive)to replace worn set
> on 93 stumpjumper, have reamed and faced bb shell, fitted "new style"
> cartrige bb, and wish to know if it is a case of trial and error (ie:
> see if it jumps or not) or if there is a calculation/specific method
> to know if i need to fit a new/different length axle, or if my
> original will do.
>
> old kit was 93 dx chainset with 170mm sq taper cranks and bio-pace
> full size rings, ill still be using a 7sp hg cassette (a new one tho)
>
> A friend "thinks" if when the chain is on middle front ring and 4th
> cassette ring, it should "line up straight" when looking along length
> of bike??????

That's not wrong.

See also: http://sheldonbrown.com/chainline

and: http://sheldonbrown.com/bbsize

Sheldon "Numbers" Brown
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Phone 617-244-9772 FAX 617-244-1041
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[David Damerell]

John.w <[email protected]> wrote:
>A friend "thinks" if when the chain is on middle front ring and 4th
>cassette ring, it should "line up straight" when looking along length
>of bike??????

There's no formal method for this; you want to have the chain as straight
as possible in the gears you expect to use.

For instance, I have a 52/34, but only use the 34 rarely on steep
inclines. Therefore I try to arrange that the 52 lines up with the third
smallest sprocket, my usual flat cruising gear, which also neatly lines up
the 34 with the largest sprockets, those being the ones I use it with.
--
David Damerell <[email protected]> Distortion Field!