Playing with an 8BitDo Ultimate 2C Wireless controller
The majority of my playtime in ATS & ETS2 has been with an old wired Xbox 360 controller, which has honestly been fantastic except for the drifting/awkward sticks and combined trigger axis (can't control individual pedals at the same time.)
I've just gotten an 8BitDo Ultimate 2C Wireless controller (2.4GHz proprietary adapter or Bluetooth, PC and Android only.) Little did I know how much trouble it is to get it working well enough with these games.
Bluetooth
The first mode of operation I tried (after charging) was by pairing the Bluetooth mode to my PC's bluetooth radio. This was surprisingly easy and I was able to get in game with some level of controller detection. Setting it up quickly revealed some drawbacks. Right away, the Bluetooth mode presents the controller to the game as a generic x button, y axis gamepad. While the game doesn't officially recognize the distinct buttons, you do have a fair amount of control and bind-ability. IIRC, the trigger axes are actually independent. Controlling the game map is not good at all and I don't see a way of correcting that in the game its self. One nice thing about bluetooth mode is that the "L/R 5" button (small ones next to the bumpers) are independently bindable.
It was at this point that I tried Steam Input for the first time. While the game is running, I opened the Steam overlay and clicked on the Steam Input button in the main row of commands. I was given an option to enable it, which I clicked but didn't seem to do anything. I had to close the Steam Input window and reopen it to see that my click did take effect, enabling S.I support. This adds one or two additional input options in the Controls game settings menu, and I believe if you go through the setup wizard, it will actually select two of the three including the original Bluetooth controller. This caused some strange conflict between the game's bindings and Steam's input bindings. I'm not quite sure how Steam overrides the game's bindings, but it was causing chaos and confusion trying to do anything in game. This did improve how the game map handles, although the zoom in/out options (triggers) were also bound to the stick click commands, which were expanding the filter and re-centering on the player. This can go from mildly annoying to unusable. I believe this also had the side-effect of combining the trigger axes together into one.
I fiddled with the game's Controls settings menu, and specifically the Steam Input mapping mode option. It didn't seem to affect that much. Not satisfied with this, I overrode the Wizard from earlier and manually set the controllers to Keyboard+Steam Input Controller. This caused the trigger axes to not only be combined, but also digital in function (only outputs are -1 (full brake), 0, and 1 (full acceleration). Not great. I played with the settings in Steam Input for way too long and couldn't find a way to force this back to analog mode. I recall that this enables the button icons in game, which was nice. Still having trouble getting bindings to work properly though.
2.4GHz mode
It was at this point I sort of gave up and sacrificed a free USB-A slot for the 2.4GHz transceiver. This provides an XInput controller to the computer, although using that directly in game still presented some issues as before. I've kept Steam Input enabled, and set the Input Type in the game as Keyboard + Steam Input Controller (Xbox One/Series), which seems to be the most ideal situation now. Trigger axes are independent again. Game map controls work smoothly. I had to set the stick deadzones to their lowest possible in the main Steam Settings window, Controllers section and in the advanced menu for this specific controller. Hall effect sticks working nicely! Steering sensitivity is set to 1.50, non-linearity to 0.00, Steam input mapping mode to In-game mapping, and steering/pedal axes bound in Centred mode and deadzones all set to 0.
Hope this helps anyone out there who wants to use an 8BitDo controller with these games and is having difficulty. Curious if anyone uses a different configuration and how it works for them.