Finished my undertray test some hours ago. I am not happy, but not sad either.
The positive first: You definitly feel the difference. The balance changed adding grip to the rear. So the diffuser did his work. I am not sure with the rest. The car definitely felt like it was pulled down on the straight and entered the fast corner more controlled. I also didn't need to brake as strong into this corner.
Now the negative:
The fastest laptime was very close to my todays best without undertray, but the consistency was very bad. Very few laps were within one tenth in succession. Most differed around two tenth. Without undertray I was within one tenth for 5-10 laps before a bad lap came and these laps were within the same tenth of my todays fastest. So the tight infield makes the car difficult to drive because it behaves unexpected.
Then the undertray needs a lot more ride height to prevent or at least reduce scrub. 2mm isn't enough, so the cars cog is rised significantly.
Indoor on high grip ETS-carpet I don't think a 1/10 tc chassis will benefit.
Lets end with a good thing: The undertray was tough enough to withstand a (small) crash and all abuse I gave it on track.
