A bit of googling suggests that it’ll work as the cassette spacing is the same between 11 and 10 speed, just need to set the limit screws appropriately on the 11 speed mech. Anyone running this setup?

It’ll be 11 speed di2 with a 10 speed cassette, probably all ultegra.

Given the price of an 11spd road cassette, and the fact you’re running Di2… Why???

Could half understand it if you’d scored a 2nd hand 105 groupset for peanuts, but honestly, from someone that works on other peoples bikes fixing their own bodged jobs half the time, just buy a bloody 11spd cassette! If you don’t get everything quite right, and write a Di2 rear mech off as a result, it will be 5x more expensive than just buying the cassette in the first place!

Op might want to do it as he doesn’t have a suitable 11spd rear wheel at the moment.

I have used an 11 speed cassette (with the 11 tooth cog removed) on a 10 speed freehub with 11 speed shifters and it worked just fine with the limit screw set correctly. The only slight issue I had was the 12 tooth cog was not an “end” cog so it didn’t have the grippy teeth bits in it for the lock ring. That was on a mechanical setup though.

If a 10 speed cassette is the same spacing then I can’t see why it wouldn’t work.

Thanks, sounds good.

Yes, mostly an issue with wheels, freehubs, turbos, etc. I don’t need the extra range of 11 speed for this build.

On the turbo I use an old 10sp wheel with 105 10sp cassette in an otherwise 11sp Uletgra set up. Without any twiddling it misses a couple of gears – but fine in the main for what I want on a turbo.

Thanks, what do you mean by “misses a couple of gears”?

Hmm wonder if I could get an 11-23 and a 12-25 cassette, both 11 speed, and swap the 11 and 12 from the 11-23 for the 12 of the 12-25 and then use the locking properly? Would that work?

Sorry, I’m an idiot. I’ll just use a 10 speed cassette!