Thread: Ideal Rake?
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      01-02-2013, 11:01 AM   #32
exdos
Second Lieutenant
England
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Drives: Z3 M Coupe(S54) and Z4 M Coupe
Join Date: Apr 2008
Location: UK

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Quote:
Originally Posted by tikamak View Post
Having this cleared away, I can join the discussion again

Can you please send me your alignment, front and rear height in mm taken from the bottom end of the wheels to the gap of the fender.

To make it easier for both us please fill it in here so that i compare it to mine:

Front:
Height =
Caster =
Toe (total toe) =
Camber =

Rear:
Height =
Caster =
Toe (total toe) =
Camber =
Not a problem.

Here's the information you want.
Front:
Height = 592mm (OEM = 607mm ± 10mm)
Caster = OEM
Toe (total toe) = 0 degs
Camber = -1.6 degs (pins removed and OEM top mounts at maximum negative camber)

Rear:
Height = 605mm (OEM = 594mm ± 10mm)
Toe (total toe) = 0 degs
Camber = -1.15 degs
Caster doesn't apply at the rear.

Additionally, I've measured the height of the cill on a level floor immediately behind the front wheel (=138mm) and immediately in front of the rear wheel (= 158mm). Although the Z4MC doesn't have a flat floor or undertray, the bottom edge of the cill, being straight, gives an indication of a rake angle, and with my figures, this produces a positive rake angle of 0.66 degrees (OEM = +0.55 degrees). Whilst my rake angle has only been increased slightly, in degrees, the real effect of this change is that the gap between the bumper and the road has been reduced by about 25mm, which reduces the volume of air passing under the car, which according to the laws of Physics (Bernoulli), creates a zone of low pressure beneath the front of the car, which is also encouraged by the aerofoil shape in the OEM plastic undertray beneath the engine. In turn there is a zone of high pressure on top of the hood, with the net effect of reducing the tendency to lift.

By ensuring that there is a positive rake angle beneath the car, this additionally helps to increase the speed of air passing under the front of the car, which contributes to the increase in low pressure beneath the front of the car.

As I've said in an earlier post, the OEM springs at the front are made of 11mm wire, which permits excessive lift under acceleration, which opens the gap at the front of the car between the bumper and the road, probably by as much as 40mm or more, whereas with stiffer springs of aftermarket suspension, there is considerably less lift. Therefore, with an OEM set up, under acceleration there will be a dynamic reduction in rake angle, which might open the gap between the bumper and the road by a further 50mm, and by a total of some 75mm more than might occur with my car with an increased static rake angle and with stiffer aftermarket front springs which prevent lifting under acceleration. I can only see this as an advantage and not irrelevant as suggested by those claiming that attention to rake angle is unimportant.
Appreciate 0