On Fri, 04 Jun 2004 22:20:27 GMT, "John Rees" <
[email protected]> wrote:
>
>"Sheldon Brown" <[email protected]> wrote in message
>news:[email protected]...
>| B wrote:
>|
>| > I have an XTR mid range rear derailer on my bike with a double crank.
>| > Can I change cranks to a 48/36/24, with a 12-25 rear cassette?
>| > Do I need a XTR long cage, or will my mid cage work?
>|
>| Yes, it will work just fine.
>|
>| Sheldon "Couldn't Be Better" Brown
>
>Teach us to fish. Can mere mortals figure out this sort of thing? Or are
>the necessary mathematical calculations only understandable to those who
>design wheels that are lighter than air, or real man saddles ;-)
>
Technically this setup exceeds Shimano's specs for the XTR GS
derailleur-
http://bike.shimano.com/mtb/XTR/componenttemplate.asp?partnumber=RD-M952-GS
33 teeth capacity.
48-24 = 24
25-12 + 13
Added, that makes 37 teeth total.
BUT Shimano is assuming that you will want to wrap a 48/25 combo as
well as a 24/12 combo. Both of which are very unlikely and not good
gearing practice.
If you set up the chain length so you can wrap the largest combo-
48/25- then you will not damage anything if you go to 22/12. What
*will* happen is that the cage will not be able to wrap all of the
extra chain without having to pivot backwards so far that on the
bottom of the rear derailleur the chain will be riding across *both*
pulleys, and most lilkely will be slack anyway. The noise is bad and
the slack chain rattles and shakes, but you don't break anything, and
if you have any sense at all you get out of that combo as quick as
possible,
I am more concerned by the XTR's stated maximum cog of 34 teeth. I
wonder if you can get clean shifting across a 12-25. When I used a
Deore SGS on a 12-25, I couldn't adjust the B-screw or whatever they
call it to get the pulleys close enough to the rear cogs at both the
smallest and largest cogs. I could feel some sloppiness in the
shifting which was annoying.
Then again, other people have done similar setups with excellent
results.
