Senior Member
Join Date: Nov 2005
Location: NW Ohio
Posts: 3,419
Bikes: 1984 Miyata 310, 1986 Schwinn Sierra, 2011 Jamis Quest, 1980 Peugeot TH8 Tandem, 1992 Performance Parabola, 1987 Ross Mt. Hood, 1988 Schwinn LeTour, 1988 Trek 400T, 1981 Fuji S12-S LTD, 197? FW Evans
Mentioned: 24 Post(s)
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There may not be such a thing as a "double or triple rear derailer", but the cage still has to take up the slack when you shift from the large ring to the smallest. Derailer specs normally list the max cog size, followed by the max number of teeth. You subtract the smallest cog from the largest, and do the same for the chainwheels, then add those two numbers together to determine how many teeth you need to take up the slack chain if you ran the small/small combo. Some people will accept some slack in order to have a granny gear that is only used on rare occasions. So on my bike with a 48-38-28 and a 14-32 freewheel, I need (48-28)=20 plus (32-14)=18 or 38 teeth. My road bike with 52-42 and 14-28 freewheel only needs 24 teeth.
All that being said, I agree with you that they are giving you a line of bull. If the bike shifted okay before and not now, they messed something up. It sounds to me like the chain is too short, maybe just a couple links.