Dual Motor Z axis works 
Printing my hotend mount at max 120mm/sec I have some underextrusion after retraction, and there is still Z-wobble … I wonder why every printer I built has more or less z wobble. How do You guys build your moving bed axis?
So many times, Z wobble has to do with over constrained Z screws, so any curve or warp in the screw distorts the frame. Sometimes z artifacts are misunderstood though. Screw-induced z wobble will show a periodic shift that matches the pitch of the screws. Not sure what is going on in your case.
Maybe off-topic a bit: the hotest fins are not cooled in your hotend mount.
@Jeff_DeMaagd I found some loose screws on my Z axis, after fastening them it’s much better now 
To my defense, they were beneath the print bed, I just checked them after I printed different bed holder parts .
@Florian_Ford Thats right, maybe I could move the fan opening down a bit, but I have no problems at all with that print head. But usually I only print ABS and sometimes PETG.
If you don’t cool the first fins you’ll have a longer segment where the filament is viscous and not solid and the longer this transition segment the less predictable is for the firmware to work well with the parameters. Basically retracts, etc can’t be counted on the same way you do when this transition segment is kept to a minimum. You might not be seeing that in your current prints but then you can only imagine your prints looking better .
If you want some example take a look at my 8mm XCarriage compared to the 10mm XCArriage on thingiverse for the corexy design I am working on: http://www.thingiverse.com/mbthor/designs
This is the angled fan mount I ended up with (the above link is maybe difficult to follow for what I was suggesting): https://is.gd/fmTcxD
@Florian_Ford I already saw your design some time ago, my first impression was : looks complicated I designed my hotend mount to be as small as possible, and it’s a one-piece-to-print design. For that it works really well. I had some problems with PLA (leaking) wich may have been caused by the lower fins not beeing cooled enough, leading to heat creep, maybe I’ll test that with my reserve printer with the fan as low as possible. I was a bit surprised that it worked so well, it’s even only cooled by a tiny 25mm fan.
Ans so far I havent seen any big retraction problems, as long as I have the print temp right. Only when printing too fast (above 100mm/sec) theres that underextrusion after retraction, but I think it’s the hotend not keeping the temp when too much filament is going through. It’s getting less when I increase the temp when printing fast.
yeah, it has to be a bit complicated because I keep the nozzle right below the XRods which makes clamping the hotend quite tricky. It has to be clamped at the groove level while having the XCarriage below that to accommodate the LM10UUs …well
The other one I really like that is the same style is the Tank Extruder Mount built for RigidBot (http://www.thingiverse.com/thing:589515)
I see you have the vertical XRods, I have a quite tiny design for that with even adjustable belt tensioning but I’ve steered away from vertical rods arrangement due to the reasons mentioned above.
Anyway, you can find a way to put the fan lower for sure. Good luck.