Been asked about a bazillion times. 100% Nay won't work. The pull ratios are different.

 

8, 9, and 10 are the same cable pull ratio...except for 4700. It's 10 speed but the cable pull is the same as 6800/8000 and 9000/9100. You can use the 8 speed derailleur w/ any 8 or 9 speed shifter, and all 10 speed shifters except 4700 Tiagra. It won't work w/ ANY 11 speed shifter. You can use the 8 speed derailleur w/ any 8 or 9 speed mtb shifter as well, but no 10 speed (Dyna-Sys) shifter will work.

 

Exactly this. Shimano changed the cable pull ratio on 11-speed road and 10-speed mountain to pull more cable.

Unless this 8-speed rear derailleur is one you have lying around your bike parts pile, why would you want to buy an 8-speed one when 11-speed rear derailleurs are relatively cheap anyway?

 

This is a question that’s been on the back burner for a while now. Thought this would be the perfect time to lay it out here.
1. Rear derailleur works by the pulley moving across the width of the cassette. The cable pull is regulated by the amount of cable pull within the shifter mechanism. What’s stopping me from using a older model 8 speed derailleur with a 11 speed shifter? Or a 9 speed? The cassette hub is a constant fixed width. The only variable is the spacer width and the cog thickness. Theoretically, setting the high and low limits should take care of that. Let’s have the yay’s and nay’s on why it should or shouldn’t work.

As others have noted, as long as the cable pull is consistent then it will work. But when you say "older" derailleur, you might find that a newer 8s derailleur will work better than an older 8s derailleur just because there are improvements in derailleurs over time. Might not be an issue in your specific application.

 

So, how exactly are you going to get to 9,10, & 11 speed gears? ... when the shifters only go to ...8?
I think you're going to be missing some gears!

 

Reading comprehension issues or trolling? The OP said "8 speed derailleur with a 11 speed shifter"

 

OK, I got it backwards. I guess I got mixed up with another thread about/with a 11 spd hub/wheel.

 

Doesn't an 7 & 8 speed Dura Ace rear derailleur have a different geometry than everything else, that is 8,9, or 10?????

"1997 was a very big year for Dura-Ace. The system went to 9 speeds, and that was the most publicized change. In addition, however, the entire Dura-Ace system was redesigned and made to be inter-compatible with other Shimano components."

"The major difference between pre-1997 Dura-Ace and the rest of the Shimano lines is the cable travel of the rear derailer. Old Dura-Ace used a shorter amount of cable travel per shift. This has to do with the geometry of the cable attachment. Since the cable moved a shorter distance per shift, effects of cable friction or inaccurate cable adjustment were magnified.

With the introduction of the 6-speed 600EX S.I.S. group, the cable attachment on the rear derailer changed, so that the cable had to move farther per shift. This change reduced cable tension, reduced the effects of cable misadjustment and friction, and generally made for a more forgiving system. The same geometry was adapted for 9-speed Dura-Ace when it was introduced in 1997. All subsequent S.I.S. groups match the travel of the 600EX, except the 10-speed "Shadow" XTR derailers and shifters, which have an even longer cable travel.."

 

My shifter and derailleur set is in USPS limbo. Can’t even track the damn packet even with registered post and tracking. Last I know is that it’s here in country. DeJoy is taking all the fun out of the anticipation of the package. It’s been a month and a half. Everything else is here. Cassette, Crankset and brake. Beyond bummed.


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Shipping has been really weird lately. I ordered Campy brake springs from Nashbar and they took 4 weeks, but the 4 orders I got from overseas got here in 3 days (each one!). I think it's a USPS thing because the overseas orders all come by DHL.

 

Update: Finally got my bike switched out to the CRX group. Between keeping an eye on my kid who is virtually going to school, I haven’t had much time to work on the bike. Then my hydraulic brakes started acting up and finally had a friend of mine troubleshoot them today.
Will ride the bike tomorrow. And give my unscientific opinion about the group.


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My half assed review of the Sensah CRX 11 speed group that I bought on Ali Express to hopefully upgrade my brakes and bring my bike into the modern bike tech gestalt. I had a mixture of XT and XTR on my rigid carbon 29er that I bought online ten years ago in 2010. I had integrated 9 speed XT shifters with XTR front and rear derailleur with a 36/46 XT crank and no, that’s not a typo. This crankset was the downhill specific version that I used on my cross bike for a while. Just to make things interesting my shifters were the brake levers and rapid rise to boot. Brakes were hydraulic XT ice tech.
I preferred to ride as a rigid set up because of the trails in my backyard were dry, dusty singletrack in the foothills of the Sandia mountains.
One of the downside of the XT setup was that there was no adjustments to the levers. I have big paws and had to slide my hand to the edge to. Tap up or down on the levers to up or downshift. There were more than a few times when I accidentally downshifted and mumbled curses at the lever shifters design.
A few months ago, in the beginning of the pandemic, I came across the Chinese group set and I was skeptical but it stuck in my mind and the next month, I decided in the name of research and being stuck at home with a first grader which can explain poor decision making reasons on my side, I plunged into this and then waited and waited and waited because de joy took all the joy out of my plans by slow walking the USPS mail delivery. I finally got them and put them on my bike and finally ride after fixing additional glitches like my BB and brakes with help from a friend.

The shifters shift and the brakes stop . No revelations. Right now, this is a 1x running 11-36 in the back and the shifting is good. The brakes are a big improvement in feel and modulation at the levers. For some weird reasons, these levers work better than my XT levers although both are hydraulic.