lhill 

My vintage bike with Biopace 42/52 chainring and 12/28 cassette often “skips” when shifting from 3rd into 4th. Most of the time my crank arm will skip only a few degrees but occasionally it will skip 15 or 20 degrees. Before I start messing with any adjustments, I would love to hear from an expert on their opinion about why the skip only between cogs 21t to 18t.
Skipping to me means that as I pedal and shift from 3 to 4, the chain jumps a few teeth before it is fully engaged with the 18t cog.
Thanks for any help!

shelbyfv

I'm no expert, but if you are hesitant to try adjusting your shifting, I'd suggest taking it to a shop. Have them check your chain for wear. Most likely you need a new chain and cassette.

KevinF

So it sounds like the chain moves over the cassette without really engaging it? That sounds like the chain and the cassette are worn differently; i.e,. chains "stretch", and once they exceed a certain tolerance, they won't reliably engage the cogs anymore. It's common for certain cogs (i.e., the frequently used ones) to get worn out faster than other cogs.

There's an excellent write up here: https://www.sheldonbrown.com/chain-wear.html

lhill 

Thanks guys, but chain and cassette are brand new, less than 200 miles on each.

billridesbikes

Probably the cable tension is a little too loose or too tight on the rear derailleur.

With the bike on a stand, start with the smallest cog and start shifting from the smallest to the next largest cog as you shift. Pay attention to how easily the chain shifts to the next cog. If it is struggling to get to the next cog (i.e. 3>4) it means the cable it too loose. Turn the barrel adjuster counter-clock wise about 1/4 or 1/2 a turn and repeat until the shifting is smooth.

Now go backward and start from the largest cog to the next smallest if the shift is delayed or stuck in the large cog there is too much tension in the cable. Turn the barrel adjuster clockwise until the downshift is smooth.

Repeat until all the gears shift smoothly up and down.

90% of shifting problems can be fixed this way, and If the shifting was fine previously there should be no need to adjust any of the screws on the derailleur unless you somehow bent the hanger.

shelbyfv

Even more reason to take it back to the shop