after struggling all day to get my mech working which has got a severe case of the problem described in this thread
http://www.singletrackworld.com/forum/topic/help-needed-my-x-9-rear-mech-sticks-in-1st
i had the bright idea to commandeer my road bike's mech and tried it on my mtb. It is a sram rival short cage. Now you might think that short cage would cause it not to work, but i've only got a 14-25 (yes 25) cassette on there and the only limitations of the sram rival road mech is that the max sprocket is 27. It's exact actuation, so it should work.
But here's what happens: in 9 to 3 (9 being smallest) it's perfect. Then a further click pushes it all the way over 2 and into 1. If I move the shifter to position 1, it feels like it's struggling to pull enough cable through to index, then when i click the shifter back from 1, the cable 'snatches' at it and the shifter goes from 1 to 3, straight over 2.
so I sacked off that idea and put it back on my road bike.
But i just wondered, if it's exact actuation,i.e. 1:1 cable pull to mech travel, then why doesn't it work?
I don't think it is the same cable pull as the mtb mechs.
Road bike and MTB have different pull ratios as do Shimano and Sram, so if you're using sram mech and Shimano shifters its not going to work in a month of Sundays.
Maybe it's because it's a ratio rather than a figure, so your two sides of the ratio don't match anymore if you swap bits? So if 1:1 means that if you move the shifter x amount the mech also moves x amount, and 'x' is different for road and MTB, then you've no longer got 1:1.
Not sure that's not utter rubbish now...
Sram do make a double tap rapid fire unit for flat bar road bikes. That would give you the right actuation I guess.
Oh and you can do things like get campag to work with Shimano, So I recon its not impossible to make sram and shimano work together!
I thought 1:1 meant that if the shifter pulls through X mm of cable, then the mech moves that same X mm.
But i suppose you've then got the question of 'yeah but [i]which bit[/i] of the mech moves Xmm'...
it was almost like it was too tight to begin with - i had to actually slacken off the cable in 9th for the shifter to be able to pull as much cable as 9 shifts pulls.
Guess i was just slightly confused to find sram road shifters actually pull different amounts of cable to sram mtb shifters.