KasbeKZ

i'm planning to switch my old cannondale's shifters from the down tube to the brake levers, and i'm thinking that while i do so, i may as well switch from the odd-ball 6 speed to an 8 gear cassette. my question is, what are all the parts involved in this switch? i need the shifters, an 8 speed cassette, and an 8 speed derailleur? is that all? will i be able to find a cassette to fit my ancient hub? or will a new wheel be part of the project too....

Retro Grouch 

The first thing you need to do is to measure the distance between your rear dropouts in millimeters. That will drive your options. Modern road bikes are 130mm between the dropouts. Since your bike is 6-speed, it's probably going to be 126mm.

Assuming it's 126mm, all is not lost. Here's a possibility:
Acquire a 126mm over locknut rear wheel with a 7-speed freehub body. Buy a 105 or cheaper (important) 9-speed cassette and a new, narrower 9-speed chain. Throw away the biggest cog and install the remaining 8 cogs on your 7-speed freehub body - they'll fit. Install a set of 9-speed STI brake/shift levers on your handlebar. Presto! You now have an 8-speed drivetrain.

If your dropouts happen to have 130mm spacing you'll still need a new rear wheel with an 8/9/10 speed freehub body, cassette, chain and the STI levers. That's quite a bit of new stuff.

Don't ask me if the cost is worth it (especially the STI levers) that's for you to decide.

dabac

That part isn't too bad, you'll need to source the new brifters and some cable stops though. And the brifters needs to be compatible with whatever it is that you intend to run in the rear though.

To the best of my knowledge there is no body that can hold a 6-speed and an 8-speed cassette. 6-speeds have the smallest sprocket threaded and it's used to hold the other sprockets in place. 8-speeds OTOH have a lockring instead.

My recommendation would be to get the shifters and a "new" rear wheel, one that comes with an 8-speed compatible cassette from the beginning. Otherwise you're looking at a new hub, a relace and probably a new set of spokes as well.

RDs, particularly Shimanos, are surprisingly tolerant. The odds are quite good that your old one will still work, although by now it has probably earned its retirement too.

Going from 6 to 8-speed is likely to mean that your upgraded rear wheel will be wider over the locknuts. Depending on frame material and tolerances this might take some fiddling or be a non-issue entirely.

KasbeKZ

thanks for all the advise. to clarify though, i said 8 speed, but i really just want to go up to whatever is generally accepted today. what most the new road bikes have. i'm not die hard set on 8, but whatever is compatible with the most parts. i've just been told that i can't even get brifters (thanks for that word! i was wondering what to call them) meant for a 6 speed, even though 7 or 8 speeds will work moderately well.

z415

Most bikes nowadays have 9/10 speeds. Campagnolo just pulled off 11 speeds.

Bill Kapaun

Any drop out spacing issues will be the same between 8-9-10 speed.
9 speed has a noticeably wider range of gearing options, vs 8 or 10.

IF you are buying new shifters, I would go 9.
Chains are more expensive than 8 (8 speed uses 6-7-8 speed chain), but less than 10.
Cassettes aren't that much more than 8.
9 speed is a little more "robust" than 10.
It should also be slightly less "finicky" than 10, since the cogs are a bit wider spaced apart.
Just my $.02

KasbeKZ

that sounds like the plan. 9 sounds like that's where it's at. thanks again for all the advise!

now, one more question:

i have heard of using a 7 speed shifter on a 6 speed bike, but what about using a 9 speed shifter on a 6 speed bike? to spread out the expenses, i'll probably get just the shifter first, hope that it works with my current set up, then get the new wheel, cassette, and possibly derailleur later. will it work?

JiveTurkey

Unlikely. See: https://sheldonbrown.com/k7.html#spacing

The spacing between cogs is different between the two systems. The 9-speed shifter won't pull enough cable to get to the next cog. You'll get it to work on one--maybe two--cogs, but for the majority, the chain won't line up right.

KasbeKZ

ahh i see. thanks a lot.

San Rensho 

Theres lots of deals on Ebay for 9 spd since eveyone is upgrading to 10 spd. 105 is very robust and you can get the brifters used for very cheap. Go with a new cassette and chain however. Nashbar has a chain for about $15 and cassettes for about $25.