CA343

On one gravel bike I am using a Shimano Ultegra RD-6800 GS with a Roadlink with a mtb 11-40 cassette and 50/34 crank and it works great. I can get big/big and small/small without any issues. I built up a new bike with Shimano Ultegra RX RD-RX800-GS (which has a clutch), used the DM Roadlink and 48/32 chainrings with the intention of using the same 11-40 cassette but cannot get the chain length to accommodate both big/big and small/small. It appears as if the clutch (both in on and off position) on the newer RD restricts the cage movement. I was wondering if anyone can explain this. I realize that the derailleurs are beyond their total capacity and the Roadlinks don't officially increase RD capacity but it is strange that it works on the older RD but not on the newer one.

Eggman84

Different length cage? Is the Ultegra have a larger rated capacity than the other? For the gravel RD, is the chain slack in small small and not able to go into large large? Assume you set up chain with same method for both drive trains. Be interesting to hear what the problem was (optimistically trust you'll figure it out).

Bill Kapaun

Might be as simple as a slight difference in RDER mounting geometry between bikes.
Swap the RDER's and see if the problem follows the RDER or the bike.

base2 

Some of the GRX atuff is factory crippled to only accept a 10 tooth jump in the front chainrings....the "RX" in the model number you specify makes me think you have a gravel minded derailleur. This is done by Shimano to force fit a niche or market space for the GRX product line to fit into

The road derailleur (rd8050, IIRC) & the XT/XTR derailleurs will accept a 16 tooth jump between rings & though the road Di2 derailleur says not to, we have an awful lot of people running 11-40 & 11-42 cassettes with road derailleurs. No roadlink & (apparently some) have the Di2 junction box programmed not to allow big/big. I'm not privvy to the details there.

I run Ultegra 6800 shifters with an 11-40 cassette, an XTR RD-M9000-SGS and a Jtek shiftmate8. Standard AbsoluteBlack 50-34 rings up front. Works fantastic & no need for roadlinks or running components out of spec. A standard XT rear derailleur would work just as well with the shiftmate & be a bit cheaper than the XTR.

FWIW: Next time around, if you'd like to save yourself some money, Sunrace (Sturmey-Archer) makes super decent cassettes. On the 11-42 between Sunrace and Shimano, the Sunrace has a 37 next to the 42 instead of a 36 which makes the bottom 3 hill climbing cogs more evenly spaced...& the cassette is 30% cheaper.

katsup

A road link is not needed with the Ultegra RX RD-RX800-GS and a 40T cassette. I've used this same derailleur with a 42T before, the front was 48/32.

Kapusta

Sounds like the new RD simply has less chain wrap capacity. Have you looked at what they are each rated as?

Iride01 

Clutch too tight? You aren't trying this out with the bike upside down are you?

CA343

No, I was not trying this with the bike upside down. However, after a couple of rides with a SRAM 11-36 cassette, I tried the 11-40 again and it now works much better. I think the clutch has worn in a little and loosened up enough that it shifts OK. It's not perfect in big/big or small/small but if I go there, no harm done.

CA343

Your setup is exactly what I was hoping would work. Why does yours work and mine does not.

Update: See my reply to Iride01.

katsup

Unknown, I had the setup on a Salsa Vaya and Soma Fog Cutter. I started with a 42T, then dropped to 40T for awhile before going to 36T. I never used the lower gears which is why I took them away.

The crankset at the time was a Praxis Alba 48/32 and the shifters were 105 R7000.

HTupolev

What are you referring to? Multi-ring GRX drivetrains generally use either a 16-tooth or 17-tooth front difference in the chainrings.

DorkDisk

He's probably thinking of CX, not GRX?

HTupolev

It's still unclear what that would be in reference to. Shimano does have a purportedly "CX-specific" front derailleur, the CX70, and it's rated for a front difference of 16T just like most of their 2x road front derailleurs.

base2 

True.

The RD-RX815 (for doubles) is limited to a max 34 cog, a max front differential of 17 & has a total capacity of 38.

The RD-RX817 (for 1x) has a max 42 cog, a max front differential of 0, & has a total capacity of 31.

Both are incredibly limited in capability. Especially in comparison to either road (the 8050 for example which will happily accept a 40 or 42 cog, in spite of "ratings") or the XT/XTR that is rated at a 42 cog in 2x, 46 in 1x. This crippling of GRX is deliberate. Maybe thay figure flatland gravel of Kansas is where they want to create market space? I don't know. But, it's obvious they weren't thinking about the logging roads of the Pacific Northwest.

When I say deliberate, I'd like to recount an experience.

A friend of mine with a Salsa Warbird wanted lower gears.

Since she was on all Ultegra Di2 (& I didn't know the RD-8050 could be run so egregiously out of spec by 8 teeth,) I told her to buy the RD-RX815 (for doubles) thinking that as a gravel derailleur, it would share common engineering of the XT/XTR bretheren. What I had not counted on was the 34 tooth max cog spec to be hard/fast set, baked in as a means of design.

The new RD-RX815 derailleur bolted on, plugged in, & worked fantastic. That is, until I installed the 11-42 cassette. The first time I gently turned the crank on the test stand to run through the gears, the big/big combo stopped everything in its tracks mid-shift. The derailleur jammed against some sort of stop fashioned into the mechanism. "Maybe the I screwed up the chain length," I thought to myself. So, I installed a second quick-link & an inner chain link to effect a full link addition.

I ran through the gears on the stand again. Big/big now didn't stop the whole works, but small/small was super saggy & no amount of chain length, b-screw or other adjustment could satisfy both conditions. The GRX RD-815 could not accept the 11-42 cassette of it's GRX 1X brother & the 16 tooth differential of it's road RD-8050 cousin. The 38 max capacity had been factory forced as a matter of design. Forcing the additional 8 teeth of capacity to reach the 42 cog meant catastrophic permanent damage to the derailleur, the hanger, to "something" & I wasn't interested in finding out what.

The only way to have made the RD-RX815 work with an 11-40 or 11-42 out of spec cassette that would actually be a useful cassette for gravel, would have been a crankset with a much smaller differential (8 & 10 respectively.) Likely a crankset of the mountain variety, hence my assertion above.

In the end, for her Warbird, I put the RD-8050 back on, verified it to be the better, more capable choice, cautioned her against the big/big as being less than desireable & added the experience to my on-going grudge against Shimanos business practices.

Maybe the fault was mine for thinking GRX would represent the best capabilities of road & mountain. I don't think this is an unreasonable thought since "gravel" bikes tend to equally stradle both worlds. In this case, I was mistaken. GRX is a waste when all that is required to dominate the entire gravel component world be a unification of road/mountain pull ratios &/or allow road/mountain to cross communicate. That GRX exists at all is a false choice fallicy & represents artificial market segmentation that is an intentional disservice to the end user.

HTupolev

I find this surprising. I haven't used the RX800 or the Di2 GRX derailleurs, but my experience with the mechanical GRX parts is that they handle an absolutely enormous range very easily, despite the ratings. The cage on the multi-ring derailleurs is extremely long, and the b-screw is able to back them way off from the cogs. I have multiple friends using multi-ring GRX setups with 11-40 cassettes or larger and 16-tooth front differences. One of them is even running 48-32 and 11-50, although it's definitely sloppy toward small-small. But that drivetrain would need 55 (!!!) teeth of wrap to be perfect.


Sounds like you were dealing with wrap, not max cog.

Quote:

The derailleur jammed against some sort of stop fashioned into the mechanism.

Probably the b pivot. You can't really use a sprung b pivot when your cage pivot is clutched.

Cyclist0108

Turn the clutch off (or remove it) and see if the derailleur behaves correctly. It if doesn't shift smoothly and easily without the clutch, then the clutch isn't likely to make it any better.

The main point of a clutch is to keep the chain from coming off of a 1X front chainring. A useful side-effect is it reduces chain slap. If you have a 2X, you might be better off having the clutch off or removed.

The Shimano clutch in my opinion is poorly designed compared to Box and Microshift's designs.

Bill Kapaun

Update: See my post 3.