Road Cycling - 8 speed derailleur with 7 speed shifters?

Could I run a Shimano 600 Tri-color rear derailleur with 7 speed 105 downtube shifters, and have it indexed? I know I would need to get a 7 speed cassette. But could I index the 8 speed derailleur to 7 speeds?

yes.

Yep. The derailleur really doesn't care much about what it's shifting, as long as the cassette and shifters agree with each other.

Yep. The derailleur really doesn't care much about what it's shifting, as long as the cassette and shifters agree with each other.

..sounds like a movie I saw once.

Yes. A derailleur itself doesn't have its own indexing. The indexing comes from the shifter.

As long as you're using a Shimano indexed compatible derailleur it'll pretty much shift OK. There are some small considerations in terms of cage width and blablabla but in general it'll do fine.

Older Dura-Ace deraileurs have a different cable pull than new DA or 600/Ultegra/105/etc and won't play nice with them.

Older Dura-Ace deraileurs have a different cable pull than new DA or 600/Ultegra/105/etc and won't play nice with them.

Correct, but conversely you can get 7 and 8 speed DA shifters to play nice with non DA derailleurs by using an alternate cable routing. But back to OP, yes 7 speed shifters work with 8 speed derailleurs. I'm running 7 speed DA downtube shifters to an 8 speed DA rd right now.

I have seen this said over and over and even been told the same by two LBSs and some very knowledgeable folk, however the truth of the matter is that the spacing between the sprokets in a 7-speed casettte is *NOT* the same as that of an 8 speed (or 9 for that matter). Sheldon Brown (who else?) has a very good chart showing the spacing for various casettes. All indexed shifters are designed to move the rear derailleur horizontally by the exact distance between the sprockets for the casette it is supposed to match. If I recall a 7-speed has a spacing of 5mm while an 8 speed has a spacing of 4.8mm. So while there are those of you who have run 7-speed shifters with 8-speed casettes, as I have, they will *NOT* operate optimally. Think about it, if you have the derailleur adjusted correctly for the smallest cog then by the time you get to the next one you are 0.2mm off, a tiny amount to be sure so no problems, now shift to the next. you are now 0.4mm off and so on. By the time you have shifted 5 gears you are 1mm off, some may say thats not much but you will definitely get some noizse, chain wear and slower shifting. Cogs will also wear out faster.
You could of course adjust the lateral position of the derailleur to be correct for one of the middle gears of an 8-speed casette, which means that at the outer reachable gears with a 7-speed shifter you are at most 0.6mm off. Still enough to tell the difference for those who care about ultra smooth shifting.

I have seen this said over and over and even been told the same by two LBSs and some very knowledgeable folk, however the truth of the matter is that the spacing between the sprokets in a 7-speed casettte is *NOT* the same as that of an 8 speed (or 9 for that matter). Sheldon Brown (who else?) has a very good chart showing the spacing for various casettes. All indexed shifters are designed to move the rear derailleur horizontally by the exact distance between the sprockets for the casette it is supposed to match. If I recall a 7-speed has a spacing of 5mm while an 8 speed has a spacing of 4.8mm. So while there are those of you who have run 7-speed shifters with 8-speed casettes, as I have, they will *NOT* operate optimally. Think about it, if you have the derailleur adjusted correctly for the smallest cog then by the time you get to the next one you are 0.2mm off, a tiny amount to be sure so no problems, now shift to the next. you are now 0.4mm off and so on. By the time you have shifted 5 gears you are 1mm off, some may say thats not much but you will definitely get some noizse, chain wear and slower shifting. Cogs will also wear out faster.
You could of course adjust the lateral position of the derailleur to be correct for one of the middle gears of an 8-speed casette, which means that at the outer reachable gears with a 7-speed shifter you are at most 0.6mm off. Still enough to tell the difference for those who care about ultra smooth shifting.

That's not what the OP is asking.

That's not what the OP is asking.

You are right, I misread the question. To be clear yes the derailleur will not matter, its the shifter and the cassette that should match for optimal performace.

Okay, thanks guys.