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Ultegra 6800
Has anyone tried it with an Ultegra 6800 RD ? I have tried a PowerHD 20Kg servo (6.6V), however it’s only able to pull one gear from the lowest gear. Horn radius is 15mm.
I haven't tried it on an Ultegra, no. I've used Shimano Alivio (with a 14mm diameter, so 7mm radius) and Tourney derailleurs (15mm diameter) with very good results.

If you're indeed using a radius of 15mm, so 30mm diameter, it's perfectly normal the servo can't pull that.
If the Ultegra has a short travel of cable, you should try smaller (12 or 13mm diameter) horns, and see if you can get enough travel to cover all sprockets.
On the other hand, there are a few things to check first:
- is the derailleur in good working order? Maybe it needs a lube
- is the cable flow good? I mean, is the bowden cable allowing a good fluid travel of the cable?
- is the servo getting good current flow? Try to measure it using a amp-meter. At the moment the servo isn't able to move the derailleur it should be drawing more than 1 amp. If amp is below that, either your battery isn't charged or there is something else limiting the current.

I suppose you're using the Shift4Me setup as described, including the 3D prints? If not, tell us more about your setup - it'll be easier to track down your problem.
Thanks for your fast feedback, I think the derailleur is Ok, very light using the normal shifter. The required pull length is 25mm, the servo does about 170deg so I’ll try a smaller diameter. The servo must draw high current as its getting quite hot. Batteries are LiFe type, almost no voltage drop even on high current and the servo is 6.6V LiFe type. 
Attached the STL file from the 32mm diameter.

can’t attach stl, screenshot instead

Attached Files Thumbnail(s)
ok, keep up posted when you've tried a smaller diameter. I would try a 22mm first and see if that covers it. If not, move up from there.
(10-04-2020, 05:11 PM)Zaphod Wrote: The required pull length is 25mm

Are you sure? That seems a lot. It is not the total derailleur travel that counts, it's the travel that's needed to cover all gears. I've measured it on a number of bikes if have laying around, no derailleur needs more than 20mm of cable travel.
If a typical derailleur requires 20mm cable pull, don't know myself, then I would design it so the servo gives around 25mm. Will make your life so much easier not working up against 2 hardware stops, the derailleurs and the servos. You will have more freedom to adjust the end stops in software.
On the other hand don't make the pull too big or you will loose accuracy and torque as the percentage of the servos travel used reduces.
I may have rounded up the cable pull somewhat, got a new 3D print ordered with less diameter (20 mm vs 32 mm). May take a few days as it is national holiday week in Shenzhen/China.
Will update results once received.
Ed, I agree. 
Zaphod, you may be interested in this thread I just posted about cable travel
Some maths for anyone who wants to calculate the size of the servo horn.

Diameter = cable pull x (360/(servo angle x π))

Explanation of the calculation in more detail.
The cable pull becomes a percentage of the servo horns circumference, which is (available servo angle / 360). We want the inverse of this so how many times greater than the cable pull does the circumference need to be. Therefore the circumference = cable pull x (360 / servo angle). Diameter = circumference / π.
So I got a smaller horn with 10mm radius. Not much luck with the 20kg servo, got another with 35kg & 270 deg travel. That one pulls the derailleur however the current during hold is quite high, I see like 820 mA on the largest sprocket, about 600mA on the 2nd smallest and like 9mA on the smallest (idle). That’s only the servo. I guess that can only come from the derailleur spring force and not any friction. Some pics attached.

Attached Files Thumbnail(s)

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