Shimano RX800 Ultrega derailleur, will it work with 11-42?

On pretty much my dream gravel bike I am running an XT derailleur, 11-42 XT cassette, and 105 hydro shifters.

I’m running a Shiftmate to help the ratios, but the shifting still isn’t spot on. The LBS (who did the original build) have suggested a new RX800 derailleur, will this solve the problems?

From the madison website the RX800 says the maximum cassette size is 34 t, but other information on the web suggest it will work with larger cassettes.

Hoping to sort it in time for the dirty reviver!

An 11-40 will work: https://road.cc/content/feature/246424-how-get-ultra-low-gearing-gravel-bike-adventures

11-40 works with my r7000 105 mech without mods so i’d reckon the ultegra will. chain lasted just over a 1000 miles so a bit of elevated wear. no clutch on the 105 either and it’s never dropped a chain and i use the checkpoint on MTB trails

Rocketdog, does that work OK in the big/big combo? In know that you’re not supposed to ride that combo, but I know that if were riding I’d inevitably forget and shift into it so would rather have a setup that was safe in any sprocket/chainring combo. A few years ago for an Alps trip I fitted a 32 cassette to my 105 short cage setup with a supposed max of 28t capacity intending never to move into the big/big combo and it ran sweet as anything in all other gear combo’s…until the inevitable happened and I inadvertantly shifted into the big big combo….a horrible crushing noise later something was ver amis with the drivetrain and the chain had become jammed in the cage, twisted the cage, shattered the lower jockey wheel and very nearly spun the mech around and through the seat stay. Luckily I stopped pedalling before it did. A 40 minute road site heath Robinson hack later I managed to complete the rest of the ride and get home…lucky. So I’ve learned my lesson and realise you need to have a

I had tried installing an Absolute Black sub-compact, but it moves the chainring line inboard by 2.5mm and I can’t adjust my from mech to accommodate and with it being a press fit BB I can’t use spacers to make back that 2.5mm, so have to look for a sprocket solution to get some lower gears. 11-40 would be fine (11-36 would be better but they don’t do that in 11 speed).

My other option is to go with a Deore XT rear mech and a Wolftooth TanPan, but not sure how well they work or not.

Thanks.

Rocketdog, does that work OK in the big/big combo?

running 1 x 11 here, no 2 x road groups is designed to run big big, it really doesn’t like that

I’ve been using an XTR M9000 11sp rear mech with an 11-42 XX1 cassette on R685 shifters through a Lindarets Tanpan. My shifting is perfect.

Thanks rocket dog, appreciate your not supposed to but it’s at lease safe to do so, so I would’nt want to run a non safe configuration.

Thanks daffy, I think that might be my best option.

cross chaining is at best elevated chain wear, at worst the rear mech lunching itself into the chainstays

I’m using an 11-40 SLX cassette with an RX mech – looks like you could squeeze 11-42 in there but I didn’t try it..

This is 2×11, with chainrings mixed from CX and road on the front – currently using 46-34 (after trying out 50-34 and 46-36 !)

On the road with 25mm tyres I almost exclusively stay in the 46, cross chaining to use 46-40 with no problems (little bit noisy) but when I put on 40mm G-ones and ride up and down things I probably shouldn’t, having the 36 and even the 36-40 is very worthwhile for me. I am not blessed with ridiculous leg power.

I’ve just fitted an RX800 with an 11-40 using a roadlink type extender and it works well, there is easily enough extra adjustment to fit in a 42 I reckon. The only issue I’ve found is the shift from the 40 to the next sprocket is a little slow, if I switch off the clutch then it’s normal so I might try and fiddle with the tension and see if it call be improved without reducing the point of the clutch

the new 105 works with 11-40 slx cassette. Was just a case of adjusting the b limit. Still a bit more to go in that so I reckon 11-42 would work fine.