Bicycle Mechanics - 10 spd shifter with 9 spd gears

this works, correct? thinking of getting a 10 spd Shimano 105 shifter with an existing Ultegra 9 spd der and cassette. just want to make sure it would work. can't think of any reason why not because the shifter just shifts the rear der one "notch" without regard to the distance shifted, correct? I just wont' have a 10th cog. So theoretically I could use a 8 spd shifter on a 9 spd system. Am I thinking about this correctly? Thanks!

No, you're not thinking about it correctly. The center to center cog spacing of a ten speed cassette is less than a nine speed cassette, so ten speed shifters pull less cable per click than nine speed shifters. But if you're really determined to do it, check this out:
http://www.sheldonbrown.com/gloss_aa-l.html#alternate

I've never tried the alternate cable routing like in the link, some say it works fine, some say it's not so great. But if you're determined to use ten speed shifters with a nine speed cassette, it would be worth trying. As for the "9 speed derailleur," that's a misnomer, the derailler doesn't care if you're running nine speed or ten speed, the "speeds" are in the shifters and the cassette-

Remember the shifters are by far the most expensive component so adding a 10-speed cassette and 10-speed chain will not increase your cost all that much and everything will work together properly.

this works, correct? thinking of getting a 10 spd Shimano 105 shifter with an existing Ultegra 9 spd der and cassette. just want to make sure it would work. can't think of any reason why not because the shifter just shifts the rear der one "notch" without regard to the distance shifted, correct? I just wont' have a 10th cog. So theoretically I could use a 8 spd shifter on a 9 spd system. Am I thinking about this correctly? Thanks!

I have the same setup as you. I use this (http://jtekengineering.com/shiftmate.htm)and it works great for me.

Remember the shifters are by far the most expensive component so adding a 10-speed cassette and 10-speed chain will not increase your cost all that much and everything will work together properly.


+1

10-speed shifters, 10-speed cassette, 10-speed chain, and you're good to go.

What "Wellbike" suggested is a free trick that work perfectly for an 8/9 speed system, and as I look at the proportions, it should work correctly for a 9/10 speed system.
If you don't like homemade solutions and want a commercial "proven" one, install a Shiftmate.

Basically, as Hillrider said, since the shifters are the most expensive part, it makes sense to buy the 10-speed when you need a new set of shifters. But for the rest, save your money, at least until your cassette is worn out.

Alternatively, look for a neighbourhood cycling club and get a set of used 9-speed shifters.

I found Sheldon Brown's alternate cable routing to yield mediocre shifting. The Jtek shiftmate was better, but even that left something to be desired. Bottom line is that 10sp shifter can work with a 9sp cassette, but "work" is the operative term. Both solutions work, but neither is as crisp as 10 to 10 or 9 to 9 shifting.