It appears that this core might have parts that are hard coded to using an XBOX 360 controller.
I have seen this with the WinVice cores as well, it's not completely abstracted to the Retropad interface.
Having said that, I do have it working perfectly now and below are some of the gotchas and work arounds I needed.
1) I was using the HTML5 Gamepad Tester here:
https://html5gamepad.com/ to find the correct axis for my Logitech MOMO steering wheel.
The problem is it did not match how RetroArch saw them, so make sure to bind keys in Retroarch -> Input -> User 1 Binds to find out the correct ones to use.
When using combined pedals, that HTML5 tester showed Axis 0, within Retroarch it showed Axis 6! Some differences when using seperate pedals as well.
2) Works using combined or seperate pedals as long as correct axis is set.
3) In my autoconfig file for my Logitech MOMO, I
MUST map the pedals to L2 and R2. (Which are XBOX triggers)
If it set them to anything else, including other axises, I will only get digital values. (Not pressed or fully pressed pedals, no variable.)
Another issue was I would get pedals values that once they got to 50% would also change the value of the other pedal even though they were on seperate axises.
My guess is that being full axis pedals, forcing them to use XBOX triggers which are half axis fixed that issue.
4) Do
NOT set any of the controls within Retroarch to ACCEL or BRAKE. Yes you read that right and it seems odd, but for the pedals to work in full axis mode, this was needed.
All the other controls still need to be set within controls in Retroarch: steering directions, horn, low gear, high gear, etc.
If I set ACCEL and BRAKE within Retroarch controls, to L2 and R2, I get the problem with pressing the pedals more than 50% affects the other pedal.
Here is a link to video of it working in action:
https://www.dropbox.com/s/jar20tv8oesdquj/Video%202019-12-18%2C%2011%2007%2026%20AM.mov?dl=0
You can clearly see I don't map ACCEL or BRAKE within Retroarch controls.
Hopefully this helps someone else, but what a frustrating journey tracking this down and getting it work. So happy because I get much better performance using flycast over Demul on my messly i3 in my cabinet.