DIY rear derailleur / Chain length Help

[Arrowz]

in all my brilliance I decided to buy a 9 speed shimano cassette, mtb, I think it is an XT. Then I
bought a long cage sram 9.0sl with the matching shifter. The whole idea was to be able to run the
long cage "downhill" type derailleur so help with the chain slap, I'm tall, so that crankset is
waaaayyy out there.

Now of course, the derailleur didn't come with any hint as to the range of motion. So I checked my
mountain bike and winged it. What I ended up doing was putting the chain on granny gear in the front
and the 9th sprocket in back (theoretically shortest chain need). At this point, I set the chain
length so that the derailleur idler gears are straight up and down.

The problem is, even in the middle gear in the front, the derailleur is maxed/straight horizontal in
3rd. If I downshift to 2nd, it overlaps the cassette and everything gets stuck. Now it seems like
the obvious solution is to make the chain longer, since I'll probably never use the granny gear over
4th or so anyway, it seems pretty silly that I did that.

Now, before I go chain breaking and trial-and-erroring, do you guys have a rule of thumb you use for
this sort of thing? Should I instead measure from the top end, i.e. biggest chainring, highest
sprocket? Which way would the derailleur idler gears point at the longest possible chain need?

I'm pretty good at remembering not to shift below 3, but still, I want it to work!

alan

 

[Rorschandt]

arrowz <[email protected]> wrote in news:[email protected]:

>
>Should I instead measure from the top end, i.e. biggest chainring, highest sprocket? Which way
>would the derailleur idler gears point at the longest possible chain need?

largest sprocket to largest chainring. The derailleur arm should be less than parallel with the
ground. The more the better, provided that when you shift two the smallest diameter gears (front and
back) there is some chain tensioning done by the derailleur.

or look at http://www.sheldonbrown.com/derailer_adj.html

>
> I'm pretty good at remembering not to shift below 3, but still, I want it to work!
>
> alan

 

[Andrew Douglas]

Alan;

There should be no reason you can't use every possible combination of chainring/cog on a bent,
provided you're not "cheating" by using chainrings that exceed the max. capacity of the derailleur
(I have my Wishbone set up this way).

ANYTHING that doesn't cause the derailleur cage to collapse completely
(i.e. allow the chain to actually rub against itself) when in the granny ring/small cog will work.

Yes, you need to add some links.

To further explain "cheating:" You can come up with chainring/cassette combinations that result in
unusable gears, but the flip side is that you get a wider gear range. On my Wishbone the crankset is
(IIRC) 56-36-26, or something like that. The granny might be 28. The cassette is 11-30. I have a
gear range of (IIRC) 24 - 125 gear inches.

BUT I can only use the two largest cogs when in the granny. If I try to upshift the rear, the rear
derailleur wraps up fully. So I have to first upshift the front.

Hope this makes sense....

Andy D.