A lot, I can't remember the numbers off the top of my head, but I was testing to see how the car would react to different settings, testing extremes in geometry, wings, and suspension.
I decided to test out how rake affected the handling, and was pleasantly surprised how planted the car felt with added rake. It didn't just give it a higher tendency to oversteer, as you'd expect in a low downforce car or a road car, it gave the rear a lot more high speed stability, making me think rake was also modeled aerodynamically in cars like the FA with floors and diffusers designed to produce downforce.
I wanted to see just how much grip I could get out of the floor with extra rake, and kept adding more and testing. I have only done this with the FA, which has 2011 style F1 EBD to aid the diffuser. In case you don't know the basic principles of this system, they placed the exhaust outlets on either side of the diffuser, at the throat end, to use the exhaust plume as a "fluid skirt" to seal the sides, preventing the turbulent air from the spinning rear tyres "tyre squirt" from entering the diffuser and messing with it's flows.
Unlike early EBD concepts, which had exhaust gasses inserted directly into the diffuser itself, in order to energise the flow, this newer design was a way of replicating the old ground effect era skirts, which not only prevented tyre squirt getting into the diffuser, but also prevented the high pressure air above the floor from spilling over into the low pressure side at the diffuser end. All of this allowed much higher rake to be run before the diffuser would stall.
Running higher rake has two main benefits aerodynamically.
1) It gives the entire floor an angle, similar to the old GE cars venturi tunnels, with the leading edge very close to the ground, and the diffuser throat and trailling edge much higher. This gives a significantly larger expansion ratio which in turn increases rear downforce, or more accurately, increases the downforce produced at the diffuser throat, which is forward of the rear wing, so kind of rear-mid downforce.
2) It enables you to get the front wing closer to the ground, which will allow it a better interaction with the ground. In essense, the front wing runs in ground effect too, so getting it closer to the ground will drive it harder.
This of course is highly dependent on the diffuser being capable of being driven at such high angles without stalling. Turbulent air entering the diffuser will reduce the amount of laminar air flow inside it, leading to flow separation, or stalling. To prevent this, the front wing and barge boards are constructed so as to create and direct vortices to the edge of the floor, to help seal edges of the floor. The exhaust plume is much more powerful at doing this, but is more useful at the edge of the diffuser throat, where it can keep tyre squirt out, and seal the diffuser, where the floor edge will be at it's highest.
Of course, there will always be a point where the angle will be too steep for the laminar flow inside the diffuser to stay connected at speed, which will stall it too, even with perfectly sealed sides.
Sorry about the massive explanation if you already know all of this, but I'm sure it will help some people to understand how and why it works.
Back to my testing. I kept increasing the rake angle to see how much grip I could get out of it, iirc adding 5mm increments at the rear after the front was as low as I was willing to put it (to avoid bottoming out the front wing). I reached a point where one 5mm increase in rear ride height turned the car from feeling like it was on rails, to having massive oversteer at high speed. I lost a lot of rear downforce, which made the car horrible to drive. It was still ok at low speeds, but once I was moving at a decent speed, the rear end would break away as though I'd turned the rear wing all the way down, or enabled DRS before turning in.
Due to the EBD on the FA, I reckon the rake angle it can run will be significantly higher than the FB, as that doesn't have the same amount of sealing of the diffuser. I haven't tested it with the FB though, so I'm not sure how much it can handle.
Edit: There are also a number of other factors that need testing to find the optimum rake angle for the FA. As the likelihood of flow separation increases with air flow speed, the optimum rake at low to mid speed will be higher than at high speed, so that brings spring stiffness into the equation as something that could allow a higher rake at low speed, and less at high speed as the downforce forces the rear ride down. Fuel load will also change the rake angle if the springs aren't stiff enough, which would mean your quali rake angle won't be the same as the race start. Obviously the flows won't be as accurate as real life, so it'll be interesting to find out how complex this effect is. I remember Doug from SMS talking about ride height affecting drag due to the diffuser's interaction with the rear wing.
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