AERO LOADING
What your body kit is really doing

By Martin Donnon
(c) Copyright Express Publications. Reproduced under expressed permission. No copying permitted

While often fibreglass nose clips and rear wing sections are merely for decorative purposes, there was once a time when the aerodynamic function of a body addition was considered far more important than its cosmetic appeal. This time is returning. All the indicators are there that, once more, wings will be wings to enhance vehicle performance rather than gaudy fashion statements. It's a dead giveaway when we visit some of the Japanese tuning shops. In the past two years, their focus has gone from the traditional Veilside-style decorative body addition hardware to 'GT' products moulded directly from their premier JGTC Saloon Race Car Series. It's a little bit like the replica wings and noses in Australia, plugged off the V8 Supercar stuff for Fords and Holdens — except the Japanese product actually looks good. While this change has been undoubtedly good for the aerodynamics of the cars, it has also allowed current body styling to revert from overly fussy and complicated shapes to clean and functional lines. Anyone that's already labeled me as not a fan of the 'fibreglass for the sake of ugliness' fad has done so, correctly. What effect can a bodykit really have on the performance of your car? Plenty.

LOCKING Before we even open the Pandora's box of aerodynamic loading principles, there are many other effects that an ill-designed bodykit can have on the performance of your car. I've seen it many times, but very few get the clue that the nose design of a given vehicle has been specifically engineered to maximise the radiator airflow, allowing small lightweight radiators to do the job that previously required a much bigger core. All sorts of hot-weather testing is conducted with the cars idling in pre-heated ovens for hours, and then loaded to the hilt and held flat-out across deserts. The trick is to get the most effective duct in the nose of the car to allow the cheapest and smallest radiator to get the job done. That's all fine until you change the nose shape of the car, change the duct's efficiency, and then change the ability of the car to get air through the radiator.

 

While none of this is a hassle to a show car, a steadily climbing temperature needle after shoehorning a new front on is a dead giveaway that something's not right. The same thing can be said of intercoolers; the bigger the cooler, particularly if it's front-mounted, the greater the restriction they put on airflow, hence overheating potential increases, dramatically. Japanese aero specialist C-West was very quick to point out to us that any of its GT-spec front bars for semi-competition use not only had specially designed ducting to keep radiator flow levels up, they were also recommended for use with specific C-West aluminium radiators when big front-mounted intercoolers were installed.     LIFT AND DOWNFORCE Manufacturers very rarely release a bodykit design that actually promotes lift. When you consider that lift is the ability of the bodyshape to act like an aerofoil and create a high pressure under the car, that's a good thing. Anyone that saw the spectacular backflip performed by our own Mark Webber in the Le Mans Mercedes on the famous Mulsanne Straight will have seen a graphic example of a little too much aerodynamic lift unloading the chassis, then turning the car into a high-speed cartwheel.

Promoting lift isn't a good thing with a car that becomes unstable and 'wandery' at high speed. Why then don't manufacturers incorporate the opposite, full downforce, with the aerodynamic loading on the chassis actually increasing as speeds rise? The problem is that this costs them vital fuel efficiency and performance, as any effective downforce quite literally makes the car heavier, in some cases (high speeds), by several hundred kilograms. That's something else to remember as well. There is very little aerodynamic effect from any sort of bodykit below about 80km/h. You won't generate anything in the way of lift or downforce no matter what happens at less than that speed. However, once you pass the aero threshold and start reaching in excess of 120km/h, the effect of any aerodynamic influence on the car increases exponentially. For the trailer guys that only ever tootle their cars around on the street and enter them in shows, this all has very little relevance, but for those that actually get out there and drive their cars hard, there is some food for thought here. DRAG Aerodynamic drag is a nasty thing that automotive designers have been trying to avoid for years. The best way of describing drag is: the friction that is generated against the air when the car is punching a hole through it. Try running in knee-high water at the beach. That's an example of what your car's trying to do to the air at high speed.

 

All of the downforce in the world, although providing great stability, has its pay-off in increased drag, and drag literally robs power and costs speed. Examples of this can be seen in top-speed and 0-300km/h competitions where door mirrors are folded in flat and the gaps of panels et cetera are taped up. Anything that interrupts the airflow over and under) the car will create turbulence, and therefore drag. Eliminate any disturbance to airflow, eliminate any drag and you'll increase speed. It's not quite that simple, though. The trick is to balance the drag generated with a pay-off in downforce. For example, adjustable wing Formula 1 cars will run 'a lot of wing' (front and rear downforce) and sacrifice top speed at a track with a lot of medium/high-speed corners and very short straights. Move to somewhere like Hockenheim, in Germany, though, with a couple of stop/go hairpins and massive, long straights and top speed is the aim. There's very little downforce dialled in and hence very little drag. This was Mercedes' mistake at Le Mans. So much downforce and drag was removed from the car in search of top speed that they didn't realise the possibility of lift until it was too late.

AERO BALANCE While the rage seems to be the addition of a big wing to the back of a car, and only minor modifications to the front end, the reality is that this can cause its own problems. While a designer sits down and performs the calculations and testing required to match the rear downforce to the front downforce, very little of this is done in the aftermarket. There is the ability then to use a nose with less downforce generated than stock, combined with a massive adjustable rear wing that will make something like 10 times the downforce of the stock rear wing boot spoiler. As the speeds increase, the rear of the car becomes heavier and the front lighter, indicating that a car capable of slight oversteer under 80km/h would become neutral at around 110km/h and a dangerous plough-understeerer at anything more than 130km/h.

 

A proper approach is to keep the front and rear of the car balanced, with only slight trims to the wing angles required to actually tune the handling of the car. What some people don't realise is that with a current 'GT' wing set to full scale deflection' there may be in excess of 100kg pressing on your bootlid at high speed - hence the big strong brackets. CONCLUSION If you are into performance then be very careful what sort of body add-ons you specify for your ride, and then how you actually adjust them. If buying race-derived product from a company like C-West then expect something developed that will actually work as an aerodynamic accessory at speed. If, however, you are in it simply for the looks – ‘not that there's anything wrong with that' - beware that at high speed you may be pretty much on your own.

 

(c) Copyright Express Publications. Reproduced under expressed permission. No copying permitted