Do I have this right? Drivetrain ?

[Tony O]

again this is for Nashbar Cyclocross build. Going with 11-34 cassette in back, Sugino Compact and Shimano 9spd STI up front (still to be purchased - I would prefer Campy but people have said that thumb shifter is awkward with Midge bar.) My question has to do with derailleurs. If I have it right, I can pretty much use any derailleur in the back that is compatible with shifters - mtb or road, keeping in mind chain length. I want a short cage for the back, but do I NEED to use a Road front derailleur? (to go with STI).

learning...but slowly.
Tony O

[isotopesope]

rear derailleurs have a cassette limit. you won't be able to use a 11-34 cassette with a short cage derailleur. short cages typically can't handle anything bigger than a 27t. if you're going to run a compact crank, why not go with something like a 12-25?

as far as the front derailleur, they also have a limit of teeth difference. most road front derailleurs can handle a difference of 15 teeth. triples typically are 22t. fsa, ird, and campy all make compact specific ones, which have 16t ranges. a compact needs the 16t of difference to work with the standard 50-34t gearing. so, depending on your chainrings, you may need to run a compact specific one. however, if you're going with a more cross specific 46 or 48 big chainring and the 34t smaller, you could just use a standard front der.

[Tony O]

thanks for the info/reply. I guess I should have stated that my crank has 48/36 for chainrings (wish it was 48/34). The reason I'd prefer an 11-34 is that on my current JTS, I have a 53/39 and 11/34 in the back (that's the factory setup). The 39/34 combo is barely sufficient to get me up some of the hills I have been climbing. I usually go mtn biking with people who have proper mtn bikes and granny gears galore and though on our usual 9 mile climb, I have no problem keeping up with them, there are times when I'd like to be able to "rest" by using lower combo (or is higher combo) and then there are times when I straight up cannot climb a steep incline (though often neither can they) because of going to slow, etc.

re: rear derailleur, so not short cage. ok. I guess I need medium cage? would seem like to me, long cage with 11/34 would take up too much chain in the 48/34 combo or am I off again?

cheers
Tony O

[isotopesope]

perhaps if you're needing more spin and you're doing more mountainish riding, you should be running a triple? :eek: or perhaps try to get the smallest chainring possible for that compact crank's bcd? i don't know what to tell you. my mountain bike is a single speed 29"er and i can climb most stuff with 30:18.

[vanwaCX]

rear derailleurs have a cassette limit. you won't be able to use a 11-34 cassette with a short cage derailleur. short cages typically can't handle anything bigger than a 27t. if you're going to run a compact crank, why not go with something like a 12-25?

In practice, I've seen guys running Ultegra short cage dérailleurs with 11-34 cassettes. Shimano publishes a limit but with the narrow spaced front rings found in cross, a short cage is more than up for the challenge of big cassettes in the back.

However, it you're racing, a 12-27 coupled with 36/46, 36/44, 38/46, or 38/44 is the bees knees.

Edit: I see you're trail riding with this bike. I'd say the 11-34 with a compact crank with a 36/46 would be pretty sweet.

[2wheel-lee]

It's not an issue of the short cage with an 11/34. I use a short cage Shimano XT derailleur on my DH mtb that uses an 11/34, but only one chainring up front. However, if you're using an 11/34 cassette, you have to should an mtb derailleur. Because of the angles of the linkage, the mtb derailleur will swing down enough to clear the larger diameter 34. A standard Shimano road derailleur is only designed to swing down enough to clear a 27t cassette. A road derailleur will not work properly with a 34t cassette.

The difference between cage length is to be able to take up so much chain slack, such as running a triple chainring up front, vs. a double.

[Tony O]

thanks again guys. So there are no issues with indexing on STI shifters with mtb derailleurs?
If not, then I will probably go with mtn bike derailleur for the reasons you state. One thing I should mention, and it's not to argue wrong or right, but I have a '03 JTS and the factory setup is in fact, a road Shimano 105 derailleur with a shimano hg 750 11-34 cassette in back. It shifts fine on the road, but of late, it's been slipping in the 39/34 position. I've been doing a lot more mtn biking of late, in fact I'm thinking that I <gasp> may have to purchase mtn bike instead, which I am really resisting doing but these downhills on hard, rutted fire roads and single tracks while gripping the brakes makes my arms and wrists hurt like a SOB.

cheers
Tony O

[Vibes]

I'm using a rear XT short cage with an 11-32 cassette and a Dura Ace bar end shifter and it shifts great. The rear shifting is the same for road and MTB. The rear short cage XT has a larger range than a typical road derailleurs. On the same bike I'm also using a front XT with a road 50-34 compact and it works great but the Dura Ace front derailleur bar end shifter is friction mode only so it doesn't matter about the small difference in front shifting between road and MTB chainring spacing.

You may have shifting problems if your chain is not the right length and blame it on using the wrong derailleur. Make sure you size your chain by putting the chain in the smallest cog on the back and largest chainring in the front. The chain should be long enough so the two idler cogs line up one directly centered above the other. If you have to choose between a slightly shorter chain and a slightly longer chain go with the longer. Once you have spliced the chain together shift it (cross chained) to the largest cog in back and keep it in the largest chainring in front. The chain should be stable running in this mode although it will probably sound stressed. If it will not shift all the way up to the largest cog in back then you will have to add two links.

I recently changed my 11-32 cassette to a 12-26 because I was spinning too much and it was slowing me down. I didn't have to modify the chain for this and the shifting is still great. The XT allows me to run whatever cassette I want on the back.

JT