EDIT: After some searching, it appears this type of derailleur is currently available in the form of folding bike derailleurs, such as the Shimano RD-FT35. It is a SIS 6/7 speed compact derailleur with chain capacity of 17 teeth, max cog size of 28t. As a bonus, it's very cheap. It seems to be targeted as an OEM item, and details are a bit hard to find, but appears to be perfect for this application. I ordered one to try.
Was there ever such a thing as a rear derailleur for a single chainring, but a 6 or 7 speed freewheel?
What I want is a derailleur optimized specifically for 14-28 or 14-34 freewheel, but a single chainring, and indexing compatibility doesn't matter.
The reason I ask is I fix up lots of cheap bikes that are being thrown out and make them rideable again. It's become almost a vocation at this point. In the case I get a good frame and wheels, I even try to make them nice.
I usually throw away the front derailleur and go with a single chainring, for cost and simplicity. That is, when I don't outright convert to single speed, I only keep the rear gears. I ALWAYS change to friction shifting, so compatibility is not an issue.
The common freewheels still available seem to be Shimano 6 speed 14-28 or 7 speed 14-34 "mega range" freewheels, or generic equivalent freewheels.
Since I don't need the range of chain length from multiple chainrings, this means even the 6-speed derailleurs, which are designed for 3x6/18spd gearing, are overkill in terms of chain pull, and result in a chain longer than it needs to be. At the same time, I need a derailleur that can shift onto the 28 or 35 tooth sprocket of the freewheel.
I would really like a derailleur with a shorter cage just for 3-speed to 7-speed freewheels only, with a single chainring. Are there any rear derailleurs, new or vintage, that are like this? If not, what would be the closest?