To answer the OP's question, I'm going with "Yes". You're ss derailer will work finewith your ?x28 freewheel, when coupled with your 52/42 crankset.
To bring the discussion to a more interesting area: what DOES determine the maximum size cog an RD can handle? The cage's length has more to do with chainwrap capacity, of course. And, most of shimano's current road derailers list a 28t max cog, whether we're looking at ss or sgs versions. (Which backs up Hillrider's comment). However, I have a short-cage 105 rd from ca.1990 that was doing duty with a 14x34 6speed freewheel when i got it, so...? Also, many recent short-cage mtb rear derailers from shimano and sram can rock 34t cogs, but can't handle excess wrap...
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Originally Posted by surreal
So, what does determine the max cog size?
The position of the upper (jockey) pulley is the determiner. The derailleur's geometry and the length of the frame's derailleur hanger control how much room there is between the jockey pulley and the cogs and the further away it is, the larger the cog that will clear the pulley. The "B" screw can adjust the clearance within a range but at it's maximum there is no more room.
MTB derailleurs have their jockey pulleys positioned lower than road derailleurs so they will clear a larger cog.
BTW, the max cog rating of a derailleur makes some assumptions about the derailleur hanger too. There are some road frames that will not allow the manufacturer's largest specified cog to be used with a given derailleur and others that will let you far exceed the manufacturer's spec. I've heard of frames that won't clear a 27T cog with any of Shimano's road derailleurs and others that will let you use a 32T cog with the same derailleur.
interesting, and quite helpful, information. I just bought an old BStone 300. The original owner put a 105 ss rear derailer on there back in 91, running with a 32t on the freewheel. It had surprised me. The 90s 105 rd looks really weird to my eye, but the geometry of the thing seems standard enough. Got my head to scratchin'...
I made a "custom" 11-28 cassette for my road bike. I ran it on two bikes, a Specialized with 53/39 and an RX-100 derailer and a Trek with 52/42 and a 600 Ultegra derailer. It works fine on both bikes. I'd like to try something larger (~30) but all I have right now is a 32 and I don't want to jump that high.