Ok, I have spent the better part of an afternoon trying to correct a skipping chain problem on an old peugeot that I have set up with a first gen shimano freehub with a brand new cassete. the chain doesnt skip or anything at all in any other gear. I tried a new chain and that didn't fix the problem. I threw on a different RD than the crappy simplex (shimano 600...old pre indexed)..that didnt help either. In fact given its lack of b screw adjustment and that I installed it with a random SIS hanger it is tensioning the chain worse than the simplex on the little cog. (and yes, the chain is as short as it can be to work on the big cog. )
This takes us to the casette itsself. it is a seven speed model on a 6 speed body so I installed it without the 6th cog, but with spacers with these casettes, the 7th cog is the lockring. the spacing is the same as between the other cogs The drivetrain functions fine with the wheel from my bianchi, which has an even farther outboard little cog. (yeah, maybe thats telling me somthing)
My theory: as this is a total frankensystem, with nothing exactly right for any other part, I belive the chain is catching momentairily on the 5th cog at the point in my pedalstroke where I am applying maximum force through the drivetrain. this problem only happens when the drivetrain is under load, I cant replicate it on my bike stand.
My plan: add a .5mm spacer I saved from the previous cassette. requires building a 2nd chainwhip or taking it to the LBS: niether of which is happening tonight.
My question: If this doesn't work, are there any other likely explanations for why this is happening? I figured I'd ask now so that in the unfortunate event that a spacer doesnt solve all my problems I might have some other ideas on hand, because this is my last one. It should have been my first.
EDIT I forgot to mention this is a 10 speed to five conversion, with the outer chainwheel ground off, so It is kind of important for the forbidden cross gear combo to be useful.