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Wings and Things PDF Print E-mail

I was following a BMW 3-series recently and got to thinking about the so-called spoiler on the boot lid.  Purely cosmetic:  there is absolutely no way it is going to have any aerodynamic effect on the handling of the car, that’s for sure.  Why?  Because by the time the air flow reaches the boot lid it is so disturbed it will not flow evenly over the surface of the wing.  In other words, turbulent air is of no use in creating significant downforce.

And in fact, the drag the spoiler inevitably creates may actually reduce the performance of the car rather than enhance it.

Let’s look at other cars with these strange and often unsightly appendages.  The Mazda MX-5 springs to mind.  It has a neat little boot spoiler but with the hood either up or down there is absolutely no chance clean air is going to reach the wing.  Completely unnecessary and simply a clever marketing ploy by Mazda to relieve the customer of a few more dollars.

Then there are the small saloons from Honda, Toyota and Mitsubishi.  All are available with an optional rear spoiler, but it is not going to do anything to improve the car’s handling or road-holding.

So does any wing work on a saloon car?  Well, many years ago, in touring car racing in Europe, BMW introduced what was fondly named the “Batmobile”.  This fearsome machine had a roof-mounted spoiler designed solely to smooth the air flow onto the high-mounted rear wing.  It worked but it was rather unsightly.

ImageCloser to home, the Nissan Skyline GTR has an adjustable rear wing as well as an underbody diffuser which “sucks” the car down onto the road.  This mighty monster chucks out a healthy 280 bhp but a change of chip could easily boost this to over 400 bhp.  That’s when you need significant downforce.

The Subaru Impreza WRX has a handy rear spoiler, but the recently launched Prodrive rally version has an even bigger wing with a series of right-angle fences across its plane to prevent loss of downforce.  Prodrive’s technical director David Lapworth admits it does not so much contribute to the effective handling of the rally car but it does produce stability, which in turn gives the driver greater confidence.

Audi’s highly successful TT Coupé was originally introduced with smooth flowing lines, but a series of high-speed accidents, mostly in Germany where speed limits are somewhat more relaxed, resulted in an expensive recall to equip the cars with a tiny rear spoiler as well as ESP (electronic stability program).  In this case it really was a spoiler, ruining the beautiful asymmetric shape of the TT.

Porsche has a small spoiler located in the rear engine cover of its 911 series, which rises automatically at higher speeds to improve stability.  There is also a cockpit switch to raise the spoiler if the driver feels the need to pose around town.

One of the best examples of wingless flight is probably the Ferrari 360 Modena.  Not a spoiler in sight, yet this car is capable of speeds approaching 300 km/h.  And it does so with the utmost stability thanks to a large pair of rear diffusers.  Ferrari spent more than 5,000 hours in the wind tunnel to perfect the 360 Modena and ensure negative lift no matter what the ground clearance.  Ferrari achieved a negative lift figure that actually progressively increases as the car picks up speed.  A figure of 180 kg (downforce) is claimed as the Modena reaches 290 km/h with slightly more weight on the front axle.

The wings that do not work are those that are stuck on the boot lid, and appear to have little in the way of a profile.  Remember, the aircraft principle of lift, where the air over the wing has to flow faster than that beneath the wing’s profile.  A spoiler is essentially an upside-down wing, designed to exert downforce.  No profile, no aerodynamic effect.

And one thing that should be remembered, should your insurance company discovers that you have added a spoiler – a modification which strictly speaking should be notified in writing – you may find there is a loading of the annual premium which far outweighs any appearance advantage you might feel the wing adds.

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© 2019 Jeff Heselwood. All rights reserved.
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