Silverstem

Before spending too much money, an Ultegra r8000 (11spd) r-derr will accurately shift an r6800 10spd cassette? Will I need to adjust the derr so that it shifts looser between the cogs,.. (limiting screws, or can can it be done with cable length)? I understand a 10 speed cassette has slightly more distance between the cogs than an 11 speed... Thks.

CliffordK

NO (well, mostly).

Find a 10-speed rear derailleur & shifters, or upgrade everything to 11-speed.

https://en.wikibooks.org/wiki/Bicycl...ing_Dimensions

10 speed and less has a 1.7 shift ratio.
11 speed has a 1.4 shift ratio.

With the exception being 10-speed Tiagra 4700 which uses the 1.4 shift ratio for 10-speed, and thus will work with Shimano 11-speed derailleurs.

dsaul

I would be interested in knowing what shifter you plan to use with that derailleur, since the Tiagra 4700 and GRX 400 would work. Otherwise, the answer is "No", that derailleur will not correctly shift a 10 speed cassette with any other shifter.

shelbyfv

AFAIK 6800 is 11 speed. If you do have a 10 speed group (other than 4700) you will need an older 10 speed rear derailleur such as 6600 or 6700. The 105 and DA counterparts would work as well.

Silverstem

I planned to upgrade drivetrain to cabled r8000. But I don't think I want to drastically tighten the spokes on the rim to get that extra width. I've already adjusted the spokes to accommodate a 10 speed cassette. I have to ask first cause it's a lot of money to spend if it won't work after installation. ):

Basically, I don't believe all the cogs on a cassette are necessary. I understand there are 12 and 11 speeds going around, and that's too much, I think. It'll make for a lot smoother riding, but it'll also keep your hands, (fingers), very busy. I think 10 is a good number of speeds to shift. The real speed comes from the chainrings.

shelbyfv

8000 isn't for you if you want 10 speed. Also, you don't have a good grasp of 10 vs 11 cassette and wheel compatibility. Do some more research.

Crankycrank

Agree with the above posters as well as the fact that a 6800 cassette is 11sp not 10sp. There are possible ways around some setups to achieve what you want but it gets complicated and we need more info on your current hub, cassette tooth range you want to use and shifters you plan on using. Highly unlikely you can do this for cheap though. If all you need is a derailleur for 10sp shifters and cassette then you can substitute various models to replace whatever you have but no Shimano 11 sp will work.

Iride01 

Can't you find a 10 speed DR? Usually 10 speed stuff is cheaper than 11 speed stuff. Tiagra 4700 should be great for 10 speed rears. What shifters do you have? That might make a difference for what DR you can put on. Maybe not, I'm not to up of what series components were affected by cable pull ratios.

If you are buying the Ultegra 6800 DR for better shifting, then realize it gets is better shifting from working with all the other components of that 6800 group.

Anything else and it's your experiment and your risk.

CliffordK

11 speed freehubs are wider than 10 speed freehubs.

It may depend on the brand. Shimano 11 speed freehubs are not compatible with Shimano 10 speed hubs.

One exception is the Shimano 11-34T cassette will work on 10 speed freehubs. Some vendors have modified other slightly smaller cassettes to fit too.

One option would be to only install 10 sprockets of an 11 speed cassette, and appropriate spacers. Some of the sprockets will be on a common carrier, and with shift gates, it may not be perfectly straightforward to choose which one to drop.

I believe most (or all) Shimano 12T sprockets will work in the first position (with the appropriate locknut). So, if you had a 11-12-13-etc cassette, you may be able to simply discard the 11T, and add adequate spacers behind the cassette.