conventional wisdom on MTB derailleurs?

ant said:

A friend was putting a new chain on his mountain bike, and was wondering aloud about how long to
make it. I read, a long long time ago, Sheldon's big-big-and-one,

Although I recomend it, I can't take credit for it.

Quoted message said:

and have been using it without a single exception for the last 80 chains i've installed. no
exaggeratin'. and as i always test gears after installing the chain, which puts me through both of
the crossed combinations, id come to believe that the method was infallible.

so, i told my friend to step aside and let me show him my 'infallible' method. as it turns out, it
left the chain so slack in the small-small that there was a sizeable droop in the chain even after
the der had folded itself all the way back.

shocked, i had to come to grips with an explanation to protect my ego 😉

does he need a longer derailleur cage, or is this standard fare for mtbs? his explanation: i never
use the big ring with the bigger rings in the back. But that wouldnt do it for me on my bike.

This "droop" doesn't do any actual harm, however riding in the small-small combination with a modern
MTB drivetrain is EXTREMELY abusive. The itty-bitty granny ring can put an enormous amount of pull
on the chain, and the itty-bitty 11 tooth rear sprocket is only sharing the load amoung 5 or at best
6 teeth/chain rollers.

Even leaving the bad chain angle out of consideration, using this combination cause greatly
increased chain and sprocket wear. The small-small combination should _never_ be used on a triple
crank setup, especially a modern "compact" type.

Even the second smallest rear cog should be avoided when you're using the granny chainring.

See also: sheldonbrown.comgloss ca m.htmlOpen ↗

Sheldon "Not A Problem If You Use Your Gears Properly" Brown
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617-244-9772 FAX 617-244-1041 harriscyclery.comharriscyclery.comOpen ↗ Hard-to-find parts shipped Worldwide
captainbike.comcaptainbike.comOpen ↗ sheldonbrown.comsheldonbrown.comOpen ↗

ant said:

A friend was putting a new chain on his mountain bike, and was wondering aloud about how long to
make it. I read, a long long time ago, Sheldon's big-big-and-one, and have been using it without a
single exception for the last 80 chains i've installed. no exaggeratin'. and as i always test
gears after installing the chain, which puts me through both of the crossed combinations, id come
to believe that the method was infallible.

so, i told my friend to step aside and let me show him my 'infallible' method. as it turns out, it
left the chain so slack in the small-small that there was a sizeable droop in the chain even after
the der had folded itself all the way back.

shocked, i had to come to grips with an explanation to protect my ego 😉

does he need a longer derailleur cage, or is this standard fare for mtbs? his explanation: i never
use the big ring with the bigger rings in the back. But that wouldnt do it for me on my bike.

I have the same problem with my Cannondale Jekyll -- and I'm using a long cage XTR DR. But if I make
the chain shorter, I'll probably break or bend something if I accidentally hit the big-big
combination (night riding). Also remember that the small-small combination is to be avoided because
of the angle the chain makes with the gears.

David