The weight of the car doesn't have any relation to its aerodynamic performance, I am curious to know the method you use for your calculation?
Basically the problem encountered during cornering is producing the force to sustain the centripetal acceleration force:
mv^2/r where m = mass, v = velocity and r = radius of turn. For a lighter car, this force is proportionally lower.
Friction = CoF x Normal reactive force (Fn)
Hence CoF x Fn = mv^2/r [1]
Where there is no banking Fn = mg with zero downforce.
Where this is downforce Fn = mg + downforce (Df). Or (m + downforce)g if downforce is somewhat incorrectly given in kg (mass unit not force).
If mg is relatively small, it can be seen that for a given kgf of downforce the percentage increase in Fn is larger, hence the percentage increase in v in [1] is larger. Hence why 600kg F1 cars major in high speed cornering.
Substituting and re-arranging [1] gives:
CoF x [mg + Df] = mv^2/r = CoF.mg + CoF.Df
--> CoF.Df = mg[v^2/gr - CoF]
so [CoF.Df.r/m + CoF.r.g]^0.5 = v (max. cornering speed) = {CoF.r [Df/m + g]}^0.5
So we see cornering speed is proportional to the sqrt of bend radius and CoF and also the sum of the downforce to mass ratio and gravity (a constant on Earth).
If we state Df in kg then.
v (max. cornering speed) = {CoF.r [Dfg/m + g]}^0.5 = {CoF.r [g(Df+m)/m]}^0.5
From [(Df+m)/m]^0.5 we see that downforce equal to 44% of m gives a 20% increase in v for a given corner on a given suspension and tyre setup relative to zero downforce. This is roughly the case with a P1 at 160mph. Stated another way, at maximum downforce setting a 133mph corner (at zero downforce) becomes a 160mph corner.
Using Df = 1/2.Cd.A.density.v^2 we see that a 111mph corner becomes a 124mph corner too, for both the P1 and LaFerrari.
