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9 speed 105 STI shifters with 8 speed cassette
I am waiting for a call from a guy that is selling some used 105 (9 speed) STI shifters at a good price. I currently have an 8 speed cassette. I have looked around this site and others and cannot seem to find a good answer. The bike (Trek 2120) originally came with 105s, but that was back in '97 when 8 speed was still fairly standard. The previous owners replaced them with Soras and while they are functional, I really hate the thumb shifter on them as I have to take my hand off the drops whenever I want to go faster. For a reference, here is the bike here and it's the same other than the shifters: http://www.bikepedia.com/QuickBike/BikeSpecs.aspx?Year=1997&Brand=Trek&Model=2120&Type=bike

The basic questions are these: Can I run a 9 speed shifter on the 8 speed cassette? Or do I need to upgrade to a 9 speed cassette? I do know that upgrading to a 9 speed cassette will require a new chain and I may as well get a new one with a new cassette.

Will I have any problems with the front derailleur? I have a triple and the shifter is a triple.

Will freehub with an 8 speed take a 9 speed?

Thanks for any help. I will probably buy the shifters regardless as I think I can easily sell for the same price I buy them.
No, you cannot combine 8spd cassette and 9spd shifters. You will need a new cassette and chain. Rest should work.
Thanks Joe,

When I went to buy them and talked to the guy, he said he was running them on an 8-speed and adjusted the rear derailleur and made sure he had it in the lowest sprocket setting when he did the adjustments. I do some volunteer work at a community bike shop and think I will talk to one of the mechanics there tomorrow night. I will probably spend the money there if they tell me it won't work without the upgrade. I know SRAM sells inexpensive replacement cassettes and the chain should be that expensive.
As Joe says 8 speed and 9 don't match properly.
You may get some semblance of indexing but it won't be perfect.
7 clicks (1 to 8 sprockets) on a 9 speed shifter and you won't get the full movement across the cassette distance.
Ride hard or ride home alone!
Okay, thanks for the previous help. Now I have a new situation.

I decided the cassette and chain would need to be replaced eventually, so did not fuss with seeing if the 9 speed shifters would work on the 8 speed cassette. I replaced the old cassette with at SRAM PG950 (11-28) cassette and 9 speed Shimano chain. I worked out the adjustments and seemed to work fine. But, lately I have been getting this skipping on the middle cog on the cassette. It does not jump shift gears, but seems to try. This only happens on that gear as far as I know. If it does on others, the one next to it. it never happens on any of the other 6. 42-17 seems to be a good gear for an urban environment.

Any thoughts as I looked at the teeth and they look fine. I would understand how to adjust it if it was the high or low.

maybe you need to give the barrel adjuster a turn. Other than that: can you pinpoint the exact conditions?

Oh: It might be a stiff link (though that would show up quite bad on the small gears)
I have looked at the chain and don't see a stiff link. And, as you said, it would show up on the other cogs as well.

As far as exact conditions, it seems to happen the only when I am on the 5th gear and it seems to happen right after I have shifted. It does seem to smooth out a bit once I have ridden on it a while. It is just a weird thing as it is not happening in the other gears. I have fiddled with it a bit and will try the barrel adjuster one more turn. I have looked on here and on bikeforums to see if others have had this problem, too and can't really find a fix.

Hm. how old is the rear dérailleur (spring worn?)? Is the hanger (and the dérailleur) straight?
Okay, I think I have it fixed. I had the problem with changing three tubes during the process, but did adjust the barrel adjuster. I took a close look and it was just a little off and going towards the higher gears. I have taken it on a few 20 mile rides and it has seemed to work fine.

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