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derailleur cage length: short, medium, or long?

qgwpqgwp Posts: 53
edited October 2010 in MTB buying advice
Short, Medium, or Long cage lengths for 3x9 MTB derailleur?
Is there a simple formula to determine optimum cage length?

I have Sram X.9 medium cage derailleur.
Sram 22/32/44T chainset.
Sram PG990 11-34T cassette (changed from PG950 11-32T).
Set as per standard chain length guide (big-big diameter cogs, + 2 links).

I suspect, maybe ideally a long cage would be recommended?

Current X.9 medium cage/chain length allows enough slack to inadvertently change to big-big cogs without jamming; also fine in small-small cogs, if continuous tension is kept up (if not kept up, the chain is very loose, & slapping frame).
Both extremes I obviously don’t use with any intention as a rule anyway.

I thought I had no problem with this setup, as I shouldn’t, & don’t as a rule use near either of those extremes of gear ratios.
Most importantly, I thought it best having enough free play chain capacity to occasionally inadvertently go big-big without jamming (& most probably ripping/bending components), rather than worrying about occasional inadvertent opposite of chain in small-small cogs clattering & marking frame (I’m not fussed about more marks on frame anyway).

My local Halfords upon servicing this week, told me to shorten chain. I explained that I realized chain was slack in small-small cogs since I’d changed cassette from 11-32 to 11-34, but if chain was shortened, would jam in big-big cog, & probably cause damage. They were still insistent to shorten chain, so I came away with feeling that these supposed pro’s seem to have less sense than I on servicing my bike…


  • AnonymousAnonymous Posts: 79,678
    quick rule of thumb:
    1-chainring up front - short cage rear mech
    2-chainrings up front - medium cage rear mech
    3 chainrings up front - long cage rear mech.

    This is in the FAQ. Try searching before posting.
  • qgwpqgwp Posts: 53
    Hmmm, very droll “yeehaamcgee“…

    I tried searching beforehand, but advice is apparently conflicting on multitude of forums.
    Also, your advice seems to conflict much of other conflicting advice… So I’m still none the wiser…

    Is there a real simple proper formula to determine optimum cage length?
    Or maybe I should re-phrase that for “yeehaamcgee“, just in case misunderstood, a recognised bona-fide formula (not just some smart asses sarcastic opinion…)?
  • chedabobchedabob Posts: 1,133
    Your setup seems fine, just stop using big-big and small-small. Take advice from Halfords employees with a pinch of salt. There's some great guys there, but there are some proper numpties.
  • supersonicsupersonic Posts: 82,708 Lives Here
    Halfords are talking utter bollocks. Always size the chain to go round big/big, unless you a 100% sure you will not hit the combo. Ever.

    Then size your mech for the capacity you require. If you don't mind some slack in other odd combos, then try a smaller mech.
  • thel33terthel33ter Posts: 2,684
    The length makes such a tiny difference anyway you are best going for long if you aren't sure, but Yeeha's method is pretty sound.

    I normally go for whichever is cheapest, which always seems to be the long cage ones, hence my long cage Dura-Ace on my roadie, picked up for £20 new :D
    And now you know, and knowing is half the battle
    05 Spesh Enduro Expert
    05 Trek 1000 Custom build
    Speedily Singular Thingy
  • AnonymousAnonymous Posts: 79,678
    Ok dokey, Where do I begin?
    qgwp wrote:
    I realized chain was slack in small-small cogs since I’d changed cassette from 11-32 to 11-34
    Since your smallest cog would be the 11 tooth one, and you changed one 11-tooth cassette for another 11-tooth casette, there is no difference in small-chainring/small-sprocket tooth counts is there?

    Apart from that, you don't appear to understand how to use your gears.
    Big/big is by and large, a no-no. Sure you can have your chain long enough not snap the mech off, but there are other issues, such as the angle of the chain, relative to the sprockets and chainrings. It will unduly stress your chain, and cause premature failure.

    On top of that, my advice is sound.
    If you want to know specifics of how much range a specific rear mech has, check it out on Shimano, or SRAM's websites.

    But you know what? Since it's so near crimbo, I'm in a good mood, and you're obviously "special needs", here's some pertinent info for you...

    how to choose rear mech length

    How to use your gears
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