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Help with rear derailleur sizes
Hi all. Was hoping someone mechanically experienced might be able to help me with my move to a shorter cage rear derailleur and quieter drive train. I've got a 2008 Giant Reign 1 which i've modified to have just a single 32T chain ring up front and still the stock standard 11-34T cassette at rear. The stock standard long cage SRAM x9 rear mech is still there but I'm looking to buy a small or medium cage SRAM x9 for a tighter ride. Can i go to small? Thanks for any help. Happy trails.
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You should be able to run a short cage with that. The total capacity of the SRAM short cage is 30.
Total capacity is the size difference between the largest and smallest chainrings, and the size difference between the largest and smallest sprockets on the cassette added together. So if my calculations are correct then your setup comes to 23.
yeah cool...thanks buck! i think your calcs are correct. and that's all good news for me as i was hoping to go small. however it's also the answer that makes me nervous about buying...you see the 2010 giant reign x comes stock with a single ring up front and a 11-34T cassette just like me and it uses a med cage. why wouldn't they use smaller if possible? buck do you know where i can get the total capacity stats for sram's? can anybody else help shed more llight?
I found the specs for the SRAM deraileurs on their technical manual which comes in a big 12mb pdf file.
http://www.sram.com/en/service/sram/tech_manuals...
The 30 tooth figure I quoted was based on the 2009 manual.
According to the 2010 Giant Catalogue (http://www.giant-bicycles.com.au/emag.html) the ReignX (both 0 & 1) come specced with dual rings up front, hence the need for the Medium cage derailleur. They're both listed as 18 speed, but the change is that they come with a dual ring chain guide this year.
If you've only got a single ring, you'll be fine. My Glory8 came with dual rings (36,24), an 11-34 cassette and a Med Cage derailleur. I've since dropped the granny ring and installed a chain guide, but kept the 11-34 cassette (still in good nick). I broke my Med cage recently, so have changed to a short cage, and it works without a hitch (36t front - 11-34t back).
sorry sorry...need to make an edit to my last post...i meant to write...
the 2010 giant FAITH comes stock with a single ring up front and a 11-32T cassette and it uses a med cage. why wouldn't they use small if possible???
thanks again for help.
I don't honestly know why they'd use a Med cage on the Faith. Maybe they figure some people might change it to a dual-ring setup, and have a light version of the old Glory8 style bike? That way, with a Med cage, they only need to add a 2nd ring up front, and not change the derailleur. Especially given they've spec'd it with an 11-32 cassette...
All just speculation though...
Hi - just to add to the conversation with my experience, i run a 27/40 front and an 11-34 out back.......
with a SRAM X0 shortcage rear mech.
I can get all gears fine. It does mean you have to be very precise with the length of your chain (literally a link too few or too many either way and its compromised), but once its set you're good to go. it might be that SRAM dont encourage this sort of thing, but it can be done.
Its faster shifting, further from nasties and lighter than the longer caged mechs.
and less chain slap too right ants? (i sure hope there's that benefit because that's the reason i'm going shorter).
i'm even more confused now...the sram tech manuals are great (thanks buck) but they don't list a short cage option in the x9...yet they are selling such at the big online stores!
Works perfectly on my SX, I wouldn't even consider running a medium on mine.
very good article about rear derailleur cage sizing here:
http://forums.mtbr.com/showthread.php?t=284688
thanks for some brilliant advice everyone. only one last thing i still don't understand...
how can you get "better chain tension and less chain slap" from a shorter cage deraileur when, as sram tech manual and the article above states, "chain length will be the same no matter what derailleur you choose".
????
I think that statement is supposed to mean that the cage length doesn't determine the length of the chain, but only how far away the chain is extended from the main body of the derailleur - i.e. the spring tension of the der's main body should be such that no matter long cage or short cage, the overall chain length is the same.
A longer cage provides a greater leverage ratio at the outer pulley wheel relative to the cage's pivot at the der's main body. Forces affecting the chain at the outer pulley wheel compete with the spring at the cage's pivot to provide you with the spring tension required. Vice versa, forces causing rotational movement at the pivot are multiplied by the higher leverage ratio when it reaches the outer pulley wheel.
Thus, even given a constant chain length, chain slap would be minimised by a shorter cage because there will be less movement at the outer pulley wheel due to chain forces affecting the cage pivot/spring (usually from pedalling?), and vice versa forces affecting the chain at the outer pulley wheel (usually when backpedalling?) do not compete with the spring's forces leading to better spring tension.
okay i think i understand all that but if a shorter cage der extends the chain less far from the main body of the der then surely the cahin should be that much shorter.
or, to put it differently,
the distance between the two pulley wheels gets shorter when you go from a long cage to a short cage. shouldn't that same distance be taken out of the chain?
otherwise you end up with a chain that, yes is subject to less leverage from the action of the lower pully, but looser from the start.
my guess is: not necessarily. the lengths of the short vs long cage differ, but not by a huge difference. the difference can be effectively taken out by chain tension.
i.e. with a shorter cage you CAN run a slightly shorter chain, but i think sram's trying to tout the effectiveness of their der by saying "it's not necessary b/c der spring tension on our derailleurs evens out the slack" - that's true to a certain extent, but I believe there should be a noticeable difference in overall chain tension if you run a shorter cage w/o shortening the chain. this is amplified if you were not only changing your der cage but also reducing the overall number of max teeth on your bike (e.g. remove big ring, switch to a road casette). obviously also there is a maximum chain length beyond which spring tension will be NIL at your lowest cog-ring ratio so that won't save you - which is why long cage vs short cage was introduced in the first place.
/edit: sorry forgot - to a certain extent you can solve that last problem - by adjusting the tension screw that is set b/w the der and the dropout