Bicycle Mechanics - road STI and MTB derailers—Compatable?

I am helping a friend adding STI 8 speed to his old bike. I can get 8 speed RSX STI and wondered what derailers I can mate it to. I looked at the prices of rear and front derailers and noticed mtb. equipment prices are much lower. Could I mate the shifters to a deore 8/9 speed rear derailer sold on Nashbar http://www.nashbar.com/profile.cfm?c...t%3A%20Rear%20
What about the front? Anyone have an idea. He is not looking for the cutting edge or super high performance. I have a chain casette and rear wheel for 8 speed he can use.

Rear derailleurs will work fine but the front MTB derailleur does not operate at the same cable pull ratio as the Road STI shifter. Try picking up a SORA front 8 speed derailleur, they are inexpensive and will do the job.

I am helping a friend adding STI 8 speed to his old bike. I can get 8 speed RSX STI and wondered what derailers I can mate it to. I looked at the prices of rear and front derailers and noticed mtb. equipment prices are much lower. Could I mate the shifters to a deore 8/9 speed rear derailer sold on Nashbar http://www.nashbar.com/profile.cfm?c...t%3A%20Rear%20
What about the front? Anyone have an idea. He is not looking for the cutting edge or super high performance. I have a chain casette and rear wheel for 8 speed he can use.

No problem in the rear, or the front if you're using a double chainwheel.

If you have a triple in front, that turns into a can of worms.

See: http://sheldonbrown.com/front-derailers

Sheldon "Doubles Are Easy" Brown

Thanks for the replies.
Seldon, I love your website, lots of great info.

absolutely

bump of sorts. I'm also putting RSX STI's onto my trek 3x7 speed. though i have Deore cranks and deraillers.. will that work or will i also have to get the Sora front derailler to shift right?

bump of sorts. I'm also putting RSX STI's onto my trek 3x7 speed. though i have Deore cranks and deraillers.. will that work or will i also have to get the Sora front derailler to shift right?

The answers above also apply to you. Try it with what you have but you may have problems with the front derailleur, and road front derailleurs don't always work so well with a mountain crankset. A touring crankset would work better with a road derailleur and road shifter. You will probably need to do some experimentation, forfunately derailleurs are inexpensive.

And the shifters must match the number of cassette cogs.

Al

An MTB crank will work ok with a road fd even though the cage curvature isn't ideal. In the past Trek equipped some of their road bikes with 7-speed RSX which came with 46/36 double or 46/36/26 triple cranks and they also used RX-100 road fd's intended for 53/39 or 52/42/30 chainrings. They shifted just fine.

maybe mine is a mish mash of things someone put together.. it's 50/44/28 hopefully things will work.. or i just make it work mostly for the middle and big gear and call it good enough for now. got the bike cheap and am not really looking to put much $ into it.. so we'll see how it goes