Originally Posted by
valve_timing
also my 89 came with a hyperglide 7 speed cassette on it. and shimano 52/42 biopace crank i know the smallest cog is 13t gotta go home and check to see what the largest is . but i wanted to know what the largest rear cog the RD will handle. Just want something for climbing .
The rear derailleur should handle up to a 28T big cog on a 7-speed freewheel or cassette. I was skeptical about this myself a couple of years ago, but made it work just fine with Suntour GPX and comparable Shimano rear derailleurs.
Main issue: Be sure the chain is long enough to shift into the 52T big chainring and 28T rear cog. Even if you plan to avoid ever using that combination, some day, some way, when you're egg-beatering up a climb, legs thrashing like a Yorkie dog paddling, thighs on fire, all form and grace lost 30 seconds ago, desperately trying to avoid slowing down to a walking pace and toppling over or actually bailing out and walking... you'll shift into the dreaded big/big combo.
And don't count on drivetrain noise as a guide to avoiding crosschaining. When ambient noise is high from wind, traffic, or if we listen to music or podcasts while riding, we won't even hear crosschaining. And if the drivetrain is optimized, including super-slick lubes like wax, Rock 'n' Roll Absolute Dry with PTFE, or your favorite lube of choice, it's harder to hear or feel any crosschaining.
If I'm installing a new chain and don't have the old chain as a guide for the correct length, I'll err on the side of cutting the chain too long, then remove one link at a time until it'll safely handle the big/big combo without being too long and flopping around smacking the seat stay.
Some older rear derailleurs have the "B" screw for adjusting the distance between the RD pulleys and freewheel/cassette. It'll help when stuffing a 28T big cog into a setup intended only for 21 or 24 tooth big cogs. Takes a bit of fiddling and testing on a work stand or indoor trainer to get it right. But once done it's usually set and forget, won't need to be adjusted again.
With Biopace rings, which are eccentric -- not quite oval but not round -- there will be slight differences in chain "length" throughout the cycle. So check the shifting thoroughly on a work stand or indoor trainer (or have a helper hold up the bike with the rear wheel off the ground/floor) while you shift through the entire gear range several times. Before riding I always check on the indoor trainer with the resistance set normally, which emulates road riding conditions pretty well -- although it's not unusual for a glitch to become apparent only on the road.