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  • Undercar aerodynamics

    On my MC (Z3), I've previously made various aerodynamic parts for the underside, by gutting a pair of rear silencers and then putting flat bottoms on them and incorporating them into a rear-diffuser design and it seems to give the car high-speed stability. So, I thought I'd try something similar on my Z4MC.

    Normally, most production cars develop aerodynamic lift, so what I've attempted to do is use the known information about the way that air flows over and around a moving car in a way which should allow the underside of the car develop lower air pressure, thus preventing aerodynamic lift from occurring.

    The first thing that I've done is ensure that I've got some positive rake (i.e. belly-pan lower at the front than the rear), which I've been able to do by adjusting the height with my AC Schnitzer Racing suspension. I've lowered the front as much as I can, without the possibility of grounding, which keeps the car very "street able".

    Although the OEM Z4MC doesn't have a full under tray, it's pretty tidy and relatively uncluttered underneath, but I thought that various bits and pieces could be fitted which should help to prevent aerodynamic lift. All the parts that I've made either attach to the car using existing fittings or attach to existing holes in OEM parts, or in the case of the rear diffuser, I've welded brackets onto a spare pair of silencers I've previously modified, where I've removed the OEM silencers and stored them. I like all mods to be easily removable and that do no permanent damage to the car.


    The front of the car already has "fangs" which are small splitters on the bumper, and there is also a small rubber "air dam" which fits between the fangs, so the amount of air entering under the car is somewhat restricted, which is good.

    Behind the bumper, there is an OEM plastic under tray, which should assist with creating a zone of low pressure under the bonnet, but thereafter, the air passing under the car is very much left to its own devices. So it's from here to the rear of the car where I've examined how I might be able to improve things.

    The photo below shows that I've made 2 longitudinal strakes which attach to existing holes in the metal undertay which forms part of the front suspension/subframe assembly. The idea of these is that it keeps the air flowing under the central part of the car, flowing straight towards the rear, where fast flowing air is at the lowest pressure.

    To the side of the car, just behind the front wheels I've fitted 2 deflectors at 45 degrees, made from aluminium and rubber. The idea of these is that the air passing from the front wheel, leaving as vortices behind the wheels for the arches, will be directed outwards and away from the underside of the car: this should act as a "curtain" along the length of the cills, and therefore act like side-skirts, thus preventing air coming under the sides of the car in front of the rear wheels.





    The photo below shows a deflector placed inside and in front of the rear wheel. The idea of this is that if air does come in from the sides, then it will be directed longitudinally and should therefore interfere less with the air travelling longitudinally along the central part of the undeside of the car.





    The next photo shows one of the 2 small strakes that I've attached to the OEM diagonal brace at the rear of the car. The purpose of this is to ensure that the air in the central section of the car continues to travel longitudinally, thus travelling quickest and maintaining low pressure.






    At the rear of the car, I've made and fitted 4 vertical strakes, which attach to the rear silencers to form a diffuser arrangement.











    The two photos below have been taken to show the parts as fitted, and I've deliberately positioned the car and the camera to illustrate the parts on the car. However, when normally looking at the car, you wouldn't ordinarily be aware of the parts unless you knew that they had been put on the car. All the heights of the deflectors and strakes were calculated prior to fitting to allow the car to get over speed humps etc. and after a 1,500 miles round trip to The Ring and back, on and off the ferry, and having done over 150mph, the parts (and car) have remained perfectly intact.









    Obviously, without testing in a wind tunnel, I don't really know what the real effects of fitting these parts might be. However, I have attempted to observe what I hope happens by using a pressure washer and filming the results, as below.


    These two photos show that air travelling under the car besides the inside of the front wheel gets expelled sideways from underneath the car, which should reduce pressure under the car. Hopefully, this expelled air "curtain" prevents air entering in front of the rear wheels, especially at higher speeds.









    These two photos show that if air does enter under the cill, in front of the rear wheel, then the deflector besides the rear wheel and the strakes attached to the silencers, prevents air from entering the longitudinal central section beneath the car, thus maintaining the smoothest and fastest flow beneath the car for maintenance of low pressure along the underside of the car.







    I'd already fitted AC Schnitzer Racing suspension, uprated H&R ARBs and Rear Trailing Arm Limiter kit as well as set the geometry to my taste, but with these aeroparts, the car now seems to handle with less effort and feels planted and very stable at all speeds, with an improved fuel economy, suggesting a reduction in drag.

    My next project is to attempt to produce easily fitted and removable front splitter and rear wing.
    Last edited by exdos; 14-07-2014, 03:59 PM. Reason: more information.
    /// Exdos ///
    "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

  • #2
    Interesting stuff John look forward to seeing a rear wing fitted

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    • #3
      nice work. With the new M4 being in Austin Yellow I can see a resurgence in Phoenix.

      I like!


      My BSL. Breadvan Sport Lightweight.

      S54 - 18" BBS RS GT/KW V3/H&R/Rogue TM/AC Schnitzer Flippers/Brembo Calipers/CSL Discs/PFC Pads/20mm Spacers/Carbon Fibre Roof/Fibreglass Bonnet/Fibreglass Boot/Lexan Window/Custom Cages Roll Cage/Lithium Battery/Recaro Pole Positions/Schroth Harness/Randy Forbes Braced Floor/Airbags Delete/Lightweight Door Cards/UUC EVO3 Short Shift Kit/E34 M5 Clutch/UUC Single Mass Flywheel/Vibra Technics Competition Engine Mounts/3.46 Diff/Prospeed Exhaust/200 Cell Cats/Strong Strut Brace/ESS+ASA Supercharger Kit/Alpha N/497bhp - 1315KG

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      • #4
        Professor Pat-Pending strikes again

        will you be at Silverstone John ?
        [GROWING OLD IS COMPULSARY ---- GROWING UP IS OPTIONAL

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        • #5
          Originally posted by ragerover View Post
          will you be at Silverstone John ?
          I'm afraid that I don't have a ticket. It's a shame that you just can't pay at the gate on the day.
          /// Exdos ///
          "Men who try the impossible and fail spectacularly are infinitely superior to those who reach for nothing and succeed" --Napoleon Bonapart

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          • #6
            Originally posted by exdos View Post
            I'm afraid that I don't have a ticket. It's a shame that you just can't pay at the gate on the day.
            Totally agree I can rarely plan anything as far in advance as buying a display pass requires (though totally understand they have a lot of planning to to with regards to that)
            Not actually in the least bit fat

            2000 S50 Titanium Silver / Black
            ACS Suspension, Exhaust & Type III Wheels
            Simota carbon air intake | Strong Strut Front Brace

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            • #7
              you could always come down for the day and park with the door dingers outside..

              I wouldn't do that either
              [GROWING OLD IS COMPULSARY ---- GROWING UP IS OPTIONAL

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