I am pretty new to cycling, and bike mechanics, but i have been building and racing cars and motorcycles for years, so i have a pretty good technical understanding of most basic mechanisms.
can someone explain what the difference is between a "good" rear derailleur and a cheap one?
i have read a lot of reviews and i understand that the better RD's shift more smoothly, more precise, etc etc.
it appears to me that they are all made relatively the same. So, weight aside (i could care less about a few grams here and there for what i'm building) what is the difference between a Sora and a Dura Ace RD (Mechanically speaking; spare me all the "it feels better, shifts smoother fluff please)?
I'll be building up a cyclocross bike soon, and i want to know where to put my money.
In short the better ones are overall stiffer. The separate pieces are more rigid, and the joints have less play. On a bike with a cheap/worn RD the RD will often lean towards the wheel while a good RD will pont straight down. For a car reference think of it as one car with all polyurethane bushings in the suspension and one car with worn and sloppy rubber bushings.
Quality of materials and tolerances are the big differences. This means things won't wear out as quickly (there are a lot of pivots on a rear derailleur), and therefore will over time be more precise as things start to wear. Weight is a part of it, too.
Don't forget that shifters are the brains of the operation, though! Cheap derailleurs will shift great (until they wear out) if you have nice shifters. My XTR shifter/Deore RD shifts much better than my XT shifter/XT RD. I know those are mountain parts, but my experience with road (Ultegra, 105, Dura-Ace, Tiagra) parts is comparable.
I'd say the curve of quality is asymptotic - as you approach DA level, your gains are minimal, but from crappy to 105 / Ultegra, there are real gains.
Pretty much like car stuff - Light, cheap, strong - pick any two!
I personally wouldn't drop below 105 road stuff if I was buying new. Probably the best value for the money.
The main thing giving good shifting is mechaism adjustment, which applies to Dura-ace as much as Sora. Sora has cheaper materials at the pivots which will wear quicker. If you want a wide gear range you will need a long cage rear derailler.
Bike mechanic and car mechanic use the same side of the brain. The good stuff are made out of closer tolerances as mentioned. I've heard from bike mechanics about the cheap stuff being "soft" as in break easy, not meant to be repeately disassemble/re-assembled. Good stuff can take punishment and last. More expensive (duh!).