What’s the weight difference between a brand new Shimano 105 RD-5800-GS, and an all-steel SunRace M2T? Take a guess.
Did you guess 11g? If so, congratulations.
Of course, weight isn’t everything. The 105 is Shimanos newest offering, packed with decades of technological advances. The Sunrace is a Shimano Skylark derived design, which later became the Shimano Eagle equipped on many Schwinns, and eventually one of the many Tourney designs, which the Sunrace, being index compatible is probably patterned off of.
Shimano has a dropped parallelogram. That means the knuckle pivots aren’t inline with the linkages, and required a big heavy B-knuckle. The advantage is it allows the jockey pulley to be pulled in closer to the hanger bolt, allowing a reduction in the jockey to cassette gap for better shifting. Before the slant parallelogram, it also allowed the linkages to be closer to tangential to the axle, rather than radial. With inline radial designs, the A-knuckle actually moves farther from the axle as it moves over smaller cogs, the opposite of what you want to happen.
Then it has a slant parallelogram, copied from Suntour. The linkage pivots aren’t perpendicular to the A-knuckle (pulley cage) pivot and the B-knuckle (hanger bolt) pivot. Although this doesn’t really add much weight, it allows for much better shifting accuracy by moving the A-knuckle in a diagonal path that better follows the cassette.
The 105 unit also has a B-adjust screw. Again, this is a feature that allows you to get the derailer as close to the cassette. There’s no adjustment on the Sunrace unit, which relies on the preset spring tension. But at least it has a spring which helps it stick closer to the cassette and adapt to cassettes of various sizes. Many old derailers had no adjustment or movement at the B-knuckle, and simply relied on overshift and trim to get you to the next gear.
Amusingly, the Sunrace jockey pulleys are 12g vs the Shimano’s 19g, probably the only thing lighter about it, thanks to the fact that it has no dust covers for the bushings, and the pulleys only have 10t to the 105’s 11t, which also, as you guessed it, moves it closer to the cassette among other things. Both of the lower pulleys are simply the plastic pulley running on a metal sleeve, although the Sunrace spins more sloppily and more easily, and the unidirectional Shimano jockey is beveled on one side of the teeth to better accept chain from the chainring even at extreme chain angles without skipping. Unlike the Sunrace, Shimano uses a different pulley on top. It features a 2-piece centeron bushing, allowing for precisely controlled lateral play that makes it less touchy to misadjustment, and taller teeth, so it can hold onto the chain better when it is trying to get the chain to come off of cassette cogs.
There’s some nice touches like the fact like it’s overall better made, the spring is covered up on the 105, and not exposed to the elements to muck up with dirt and grime on the Sunrace. It’s got allen head screws for the most part instead of the hex nut and screws found on the Sunrace making it easier to service. The fact that the head of the top jockey is on the back is also nice. It has a nosed ferrule in the adjuster to reduce friction, although the primary reason is almost certainly to reduce the fraying that would overwise happen with their coated cables. It’s also mostly made from aluminum, and is considerably less likely to rust into a solid immovable object. Both are index compatible, but one is indexed for 5-7 (and in theory, 8, 9 and old 10) and the other is indexed for 11 speed.
There’s a dozen reasons to spring for a new modern derailer. If it were a Tiagra, it would be actually heavier. But upgrading because of “heavy” steel components, I’ll let you decide if 11g is an actual reason to upgrade from bargain basement to 105.
Fun fact: While Dura Ace RD-9000 went down a few grams from RD-7900, Ultegra RD-6800, 105 RD-5800, and Tiagra RD-4700, which all use the new 11-speed (new 10-speed) actuation ratio, compared with the old 10-speed versions. This was done to compensate for the additional friction found on first generation Shimano under-the-tape shifter cables (Dura Ace 7900, Ultegra 6700, 105 5700), needing a redesign of the derailer geometry. You can tell if it’s a new actuation ratio derailer by the mounting point of the cable. If it’s between the linkage pivots, it’s old. If it’s almost all the way over to the A-knuckle, it’s new.
And if you were curious, an Ultegra triple braze-on with a Shimano 34.9mm adapter compared to a LXIANG clamp-on meant for a 42t max chainring. No, it’s not a fair comparison.