i have a 1988 cannondale SM500 with suntour X1 rear derailleur and suntour 5-speed freewheel.

i started getting chain skipping on the smallest cog. the derailleur shifted fine and smoothly with indexed shifting and everything lined up fine and was silent when pedalling, so i didn't think i had an indexing or chain alignment problem. i measured the chain and sure enough it had over 1/8" of stretch over 12 links. upon cleaning and inspection of the freewheel, i so no visible wear on any of the cogs. so, i bought a new chain (KMC), removed, degreased and relubed the derailleur, installed the new chain and re-adjusted the derailleur for indexing.

now, it skips even worse! but still only on the smallest cog. there is no slippage on any other cogs and the indexed shifting works perfectly. again, i looked very carefully and can't detect any smoothing or wear around the teeth and the chain seems to seat perfectly around the cog with no visible gaps. and again, the derailleur pulleys align perfectly with the cog. when determining the length of the new chain, i wrapped the chain around the largest cog and chainring combo, bypassing the derailleur, then added two links.

i'm wondering if the skipping can be caused be inadequate derailleur tension and/or the distance between the cog and the first derailleur pulley? i'm no expert on derailleurs and i've never played with either of these adjustments. i don't want to mess with these until someone chimes in.

any other thoughts?

thanks in advance!

The rear cogs on which it skips are shot.

2nd for the bad cog. i had the same exact problem you mentioned, got a new chain- etc. and it was a single worn cog that was at issue. you need a new cassette.

wouldn't i be able to see the wear on the one cog? the teeth look as good as on all the other ones.

No.
You will have your proof the cog is worn when you replace it and the problem disappears.

Take a look at the pictures here on worn gears. It's not a question of what the teetch look like but their shape.

https://sheldonbrown.com/chains.html

On small cogs, the wear and pitch mis-match with a new chain can be significant even if the teeth look normal. The chain is more perceptive than you are. It knows for sure what it doesn't like.

aw, shoot. so it looks like i need a new freewheel. bummer!

Considering you got 20 years out of the last one, $15 shouldn't be too big an investment.

true, but now i have to buy the suntour-specific freewheel tool plus a chain whip, right?

now for part deux: what freewheel should i upgrade to? or should i stick with a NOS suntour off of ebay? i'd like to keep the quality on par with (or better) than what was stock. i think the index shifter is good for 5- or 6-speed freewheels only. again, i am pretty inexperienced with derailleurs/freewheels and am learning my way through bike repair with this bike.

I would also get a chain-stretch gauge. Chains need to be checked often and replaced before they prematurely wear the cogs and rings.

Al

Some suggest chain and cassette/freewheel replacements to happen at the same time.

As do I.

I might be barking up the wrong tree here, but did you cut the chain down to the right length? i.e. the same as the old?

yeah, a chain gauge would be nice. i used sheldon's method of holding a ruler against the chain with some tension on it. i measured the distance past 12" that link #13 starts. it was easily 1/8" too long for every 12 links.

yes and no. i did not cut it the same as the old since the old had stretched. but i did cut it to the same number of links .

Yeah, that's what I meant, sorry.

Oh, no! He doesn't want to do that!
When I bought a chain-stretch gauge, I promptly had to buy two chains and two cassettes.

A chain stretch guage is really

1) a minsnomer
2) a 12" ruler

Go with the gauge. Far simpler to use which means you'll do it more often. Speaking of misnomers: a ruler is for drawing straight lines. A 12" scale would be far better than a ruler.

Al

Your bike has two cassetts and two chains?!

Al

"Your bike"???
Who has only one?

I use a steel ruler and a Park CC2. I get consistent results between the two. With the CC2 it is important to not use too much pressure, just take out the slack. If you bend the pins you can throw away the tool.

Al

Very true.

Al