6 speed to 8 speed

I have a road bike currently running old C record 6 speed with downtube friction shifters, but want to run some 8 speed wheels I have. Am I right that all I need to change is the rear mech?

Also considering old 6 speed indexed thumbies, as I will be running townie bars. But if I want to use the 8 speed wheels, would that work, just use 6 of the sprockets?

Your old mech will probably work with friction shifters on 8sp, providing that the number of teeth on the sprockets are within the derailleur's capacity.

I'm not sure about using 6-speed shifters on an 8sp cassette - the sprockets are closer together than what the shifters were designed for. You could just buy a 6 speed cassette and use it on the 8 speed freehub, though.

You also have to make sure that the shifter has the correct cable pull ratio for the derailleur to work properly.

Frame spacing?

Your old mech will probably work with friction shifters on 8sp, providing that the number of teeth on the sprockets are within the derailleur's capacity.

Yup.

I'm not sure about using 6-speed shifters on an 8sp cassette - the sprockets are closer together than what the shifters were designed for.

Yup.

You could just buy a 6 speed cassette and use it on the 8 speed freehub, though.

That won't work. 8 speed is wider (or at least 8 speed shimano cassettes are wider than 6 speed), you can use a spacer to fit a 6 speed cassette to an 8 speed freehub but the spacing will still be set for 6 speed and so, 8 speed shifters won't index properly.

You also have to make sure that the shifter has the correct cable pull ratio for the derailleur to work properly.

If it's all campag it should be fine.

Indexing at the time of C-Record was pretty hideous by today's standards (those Synchro shifters with the coloured plastic inserts). I'd just stick with friction shifting if you aren't already (it works very well with C-Record).

You should get a pretty decent range with both those shifters and an older Campag mech, but it's not guaranteed to cover all sprockets. Maximum sprocket size could be as little as say 24t or so on an older rear mech.

You'll very likely be going up to 130mm spacing on the rear for 8-speed.

That won't work. 8 speed is wider (or at least 8 speed shimano cassettes are wider than 6 speed), you can use a spacer to fit a 6 speed cassette to an 8 speed freehub but the spacing will still be set for 6 speed and so, 8 speed shifters won't index properly.

Yep, sorry, that's what I meant. 6sp will go on with spacers, and it'll only work with friction or 6sp indexed.

Thanks for all the advice/info. This is my first venture into road bikes. It's a Merckx Corsa Extra - soon to be townie/commuter. I reckon I probably will stick with the friction shifters, not least because they are a really nice shape. I may whack on the 8 speed wheels this weekend and see if they work as is. Take it from there.

Out of interest what hubs are you using? Shimano or Campag?

If the 8 speed wheels will fit between the dropouts you should be fine, but 6 speed may be 126 spacing and you will need 135.

8 speed campag has the same spacing between the cogs as 5, 6 and 7 speed.
A old Nuovo Record rear mech has enough travel for a 7 speed block, but I think for 8 speed you may need a later rear mech from the c-record/chorus era

The bike came with Gipiemme 6 speed (probably not the original wheels on the bike). I've got a set of Campy record laced to cxp30 with Campy cassette I'd like to use.

Looking at the mech I have compared to pictures of the 8 speed version, I've no idea how to tell the difference. I believe the intro of the Syncro changed it slightly?

Either way. I'll just give them a try and see what happens.

With friction shifters you can use any mech as long as it has enough travel.
It's only if you actually want to index that you need to get it all matching.