DELL WD15 DOCK with Macbook Pro 2018 - max resolution issue

@Copainbig  Are you actually able to get 60 Hz when running that display through the WD15?  The reason I ask is that the WD15's documentation indicates that it supports single displays only up to 1440p 60 Hz or 4K 30 Hz.  3840x1600 is between those two, so I'm not sure what's possible there.  But the reason the HDMI adapter works differently is that when you connect an HDMI adapter directly to the system, all 4 high speed lanes in the USB-C connector are allocated to video.  With the WD15, the dock sets up the USB-C link to carry both video and USB 3.0 data simultaneously.  Carrying USB 3.0 requires two high speed lanes, which means your available video bandwidth is cut in half.

In terms of why the two systems behave differently, I'm not sure since I don't spend much time on Macs.  You could try using SwitchResX to see if you can force additional modes.  That's a popular utility on Macs for people who want more control over their display setup.  But unless it just happens to be possible to run 3840x1600 at 60 Hz through the half-bandwidth USB-C video link available through the WD15, you might really want to consider getting a Thunderbolt dock like the WD19TB, which when paired with those systems would be be able to tap into at least 4x more display bandwidth than the WD15.

Hi!

Thank you for your answer.
I tried multiple tools like switchResX but they do not help me in setting the right 16:9 resolution.
I could try the Thunderbolt option... But modern macs only have USB-C interfaces... So not sure it would help.

@Copainbig  Modern Macs all have USB-C ports that support Thunderbolt 3.  The only Mac I'm aware of that has a non-Thunderbolt port USB-C port was the 12" regular MacBook, which didn't last long.

Hi again and thanks for the help.

Indeed, TB3 seems to be the USB-C shaped TB. So I ordered a Dell WD19TB and a TB2 to TB3 adapter... Fingers crossed!

Thanks again for the help guys! 

Ok... I guess that me being dumb is a part of the problem here.

Tbh, I just picked the DELL because we use that one at work for macs, and I am not really into hardware.
I did not know CatDigit, but I just noticed that it is sold on the Apple website, so I guess that it is, indeed a better choice on a compatibility point of view.

Thank you so much for your help, it is already ordered. 
I will finally get that ultimate home-office setup!

@Copainbig  Good choice.  And fyi it just occurred to me that since you mentioned you were using HDMI, the HDMI output on the WD15 may have been a limitation as well.  The newest HDMI standard available at the time of the WD15 was HDMI 1.4b, which I'm pretty sure would not support 3840x1600 at 60 Hz.  The HDMI adapter you're using likely supports HDMI 2.0, which does.  But even if you had been using the WD15's Mini-DisplayPort output using something like a Mini-DisplayPort to DisplayPort cable to connect to the U3818DW, I suspect you'd still have had a bandwidth constraint on the link between the system and WD15 itself.  And on that subject, I'm not sure which CalDigit dock you ended up switching to, but since neither the TS3+ nor USB-C Pro Dock has an HDMI output, but both have a DisplayPort output, I would recommend getting a regular DisplayPort cable to connect it to your U3818DW.

@Copainbig  You're welcome, but...in addition to there being only one guy in here who's been helping, what are you planning to do with that TB2 to TB3 adapter?  Thunderbolt 3 when it's available uses a USB-C port as its physical connector.  That's why your MacBook Pros that support Thunderbolt 3 offer it through their USB-C ports.  The WD19TB is a Thunderbolt 3 dock, and as such it uses a USB-C connector.  You'll be able to plug the WD19TB directly into your MacBooks, no adapter needed.  You wouldn't be able to use the WD19TB with an older Thunderbolt 1 or 2 system, and if the WD19TB itself were a TB1 or TB2 dock, then it wouldn't be able to provide power to charge your system, since that wasn't possible through the Mini-DisplayPort connector used by those older standards.

I'm also not entirely sure why you're using Dell docks with MacBooks in the first place, unless you also have Dell systems.  If not (or honestly even if so, in certain cases), the CalDigit TS3+ or USB-C Pro Dock, the latter of which supports and will use TB3 if available on the attached system, would probably be better choices.  The WD19TB just got a firmware update to improve support for Macs, but you might need access to a fully supported Dell system in order to install it unless the unit you ordered happens to have it pre-installed.  That's how the WD15 worked, at any rate.  The firmware update occurred during boot time, and as such could only be installed Dell systems that supported updating the firmware of that dock, which wasn't even all Dell systems that had suitable USB-C ports.

You might want to look a bit more carefully into the technology you're trying to use.