Originally Posted by
Steve_sr
Wonder why Shimano doesn't mention the max cog size of 40-42 in the spec for the RX-810? Their "consumer" literature only mentions the 11-34/36 cassette with the 2X group.
Shimano generally only specs derailleurs to handle what they intend to sell them alongside, which in this case is an 11-34 (RD-RX810).
They're also conservative in only spec'ing things that they test and guarantee to work dead-on first-try if you follow the setup procedure. When I say that friends have gotten "pretty reasonable results", I don't mean that they just bolted the parts to their bicycles and it instantly shifted as crisp as a 53-39 11-25 Dura Ace racing drivetrain. There's often some messing around with b-screw adjustment and/or hanger extenders, and sometimes the shifting ends up slightly balky in the high gears. When exceeding wrap, you also need to pay close attention to chain sizing and be careful about drivetrain behavior in the cross-chained gear combos: using Shimano's big-big-plus-1" sizing method, this means that the drivetrain may get very slack in the small-small region, the high gears on the small chainring.
Originally Posted by
DorkDisk
812 takes a path more suitable to larger cogs.
Sort of. Like most "1x" derailleurs, it uses a large offset between the jockey wheel and the cage pivot, which causes the jockey wheel to move downward and away from the cogs as the derailleur wraps more chain. This causes it to take a path more suitable to larger cogs when a larger cassette is used, and more suitable to smaller cogs when a smaller cassette is used. This allows a single derailleur design to behave well over a wide variety of cassettes, and can help a 1x drivetrain perform better if the user is occasionally swapping in a smaller cassette than it was originally set up with.
The challenge is, the offset between jockey wheel and cage pivot means that the position of the jockey wheel is
also affected by wrap from front shifting. This is why the RD-RX810 puts its jockey wheel concentric with the cage pivot: the position of the jockey wheel doesn't get affected by front shifting at all, instead the path taken by the jockey wheel is determined
entirely by the slanted parallelogram.
The drawback here is that the RD-RX810's parallelogram isn't slanted quite as much as you'd want for super-wide cassettes. So if you set the b-screw (and/or use hanger extenders) so that it clears a very large cassette, the jockey wheel will end up pretty far from the cassette when you're in the small cogs, which can result in balkier shifting in the high gears.
Both derailleurs have their drawbacks when used in a drivetrain with both a super-wide cassette and wide front shifting.