Old Down Tube Shifters with no cassette
 

I found an old road bike in the trash that I've been fixing up. It has friction sun race clamp-on down tube shifters that I would like to keep on the bike. However, the bike does not have a rear wheel. Is there any way to tell what size cassette will work with these shifters or will any number of cogs work...or would I just be better off to buy new shifters?

Friction shifters should work with any number of rear cogs. The number of cogs in back is usually determined by the space between the rear dropouts - 120mm = 5 or ultra 6, 126mm = 6 or 7, 130mm = 8,9,10. Some bikes are other sizes (eg 128 to handle either 126 or 130 hubs). Steel frames are commonly re-sized by bending the rear stays if their owners want to switch the number of gears. Just don't do this to an aluminum bike since aluminum doesn't tolerate being bent like steel.

Welcome to BF and hope that trash bike works out for you - I sure like mine:D

i would keep those shifters, they are really good. You can unscrew them, without taking off the clamp, and clean them. I use them with an 8 speed cassette ...

I concur with the other respondents.

Yep, with friction shifing it doesn't matter how many cogs you have, as long as it fits your bike. I recently took off my 27" wheel with a 6 speed freewheel and replaced it with a 700c wheel with a 8 speed cassette. I had to change the limits on the derailleur, but all is fine.