cccorlew

Currently I have an 11-32 in back. I'd like to go 11-36, but my Ultegra derailleur will only handle 32.

So, RX 800? RX810? RX812? They all have a 34 tooth max, but I'm hoping I could stretch it to 36. Th 812 says it's only for 1x bikes, but how would it know? Why would it care?

I have a gravel bike with a 46-30 crankset. I'm planning on a week long off road tour with way too many climbs. I ride 7000 miles a year, but on the road. I wouldn't call myself strong.

Please help me pick a new derailleur that will work for me. I have Shimano 11 speed brake/shifters, so it has to be one of these I think...

Thanks

Bill Kapaun

Chain wrap capacity.
A 1X only has to wrap the difference between the smallest & largest cogs/
Add a ring and it also has to wrap that additional difference.

HTupolev

They don't all have a 34T max; the RD-RX812 has a 42T max

The RD-RX812 is a 1x derailleur in two ways.

First, it's only designed to wrap enough chain to handle a 42-11 cassette, which is 42-11 = 31 links.
Whereas an RD-RX810 is designed to handle up to an 11-34 cassette along with a 48-31 crank, which is 34-11+48-31 = 40 links. To handle this, the RD-RX800 and RD-RX810 use a longer tension cage than the RD-RX812.

Second, on the RD-RX812, there's a lot of distance between the tension cage pivot and the jockey wheel. This causes the position of the jockey wheel to be heavily dependent on how much chain is currently being wrapped. When the drivetrain shifts to a bigger cog, the tension cage swings counter-clockwise (when viewed from the drive side), lowering the jockey wheel. This means that the derailleur will actually adapt itself to the cassette somewhat; if you switch to a smaller cassette, the jockey wheel will stay close to the cassette when you're in the now-less-big big cogs, so shifting won't degrade much when you switch between different cassettes.
HOWEVER
Front shifting ruins this scheme. When you shift to a bigger chainring up front, the rear derailleur swings counter-clockwise and pulls the jockey wheel away from the cogs, affecting the rear shifting. This obviously isn't desirable. So, the RD-RX800 and RD-RX810 keep the jockey wheel close to the cage pivot, so that front shifting has a minimal effect on rear shifting. Instead, they rely mostly on the geometry of the parallelogram to manage the distance between the jockey wheel and the cogs.

cccorlew

Thanks all! This is info that is very useful. I feel educated.

Bill Kapaun

Me too-

sch

Ditto. However there have been a number of posters on this and other forums that have used Ultegra 11spd
(both Di2 and cable pull)
on 11-36 cassettes, some with and a few without the $10 (on Amazon) RD mount extenders popularized by
Wolftooth (who ask $20 for theirs). FWIW I recently rebuilt my 2008 tandem with the Shimano RD7800
long cage left on but extended with a Wolftooth and it functions fine with a 10 spd 11-36 cassette.
Might be worth saving $100 and try an extender instead. (based on $110 prices for the RD RX810)