Looking for what criteria set the following Shimano rear derailleurs apart from one another.

Models: TZ50, TX35, TX55, TX75 -- They range in price from $10 to $20

They all have a max 34T capacity, 43T wrap, 6/7 speed compatibility, same listed weight, light action design, and direct mount or dropout mount options. The TX series all have "Smart Cage, Mega Range" compatibility.

Thanks, Paul

poking at Shimano's documentation, the 35 appears to have the cable adjuster on the side pointing towards the rear, while the 55 adn 75 have it on a pivot where it points more up the seat stay and a roller for the cable to wrap around. past that, the differences are probably largely cosmetic. its all really low end junk.

The modern Tourney performs and endures surprisingly well for low end junk.

it does at that, but I'd still rather have a low end (or older) deore, thats mostly forged alloy instead of stamped steel and plastic.

The TZ50 is the only one specifically designed for 6 speeds. All the others are designed for 6/7 speeds.
Aside from that there are differences in attachment points. The TX55, TX35, TX75 are all conventional mount rear derailleurs that screw into a derailleur hanger. The TZ50 and FT30 are claw type that mount to the rear axle.

All of them are designed for city use ie city hybrids, mtb styled city bikes, and ebikes and the derailleurs are very reliable when used there. If you want more gears I'd suggest looking at another derailleur like the Alivio, Altus or Acera which are also city bike derailleurs. If you want to go offroad I'd suggest looking at the Deore which is designed for treckking. If you're starting with a Tourney - there's really no point looking at a RD higher than Deore because at that point the frame probably will be the weakest component in the equation anyway.

+1

It shifts a 9 speed cassette just fine on my 86 Rockhopper "upgrade".

+1

Ugly, large, and heavy but remarkable in terms of performance and durability. If I was going to buy a new RD though, I would spend the extra $5 or whatever to get an Altus/Acera/Alivio RD.

Its about the cost of producing it .. I expect the whole process is automated , assembled by machines.