Aerodynamic efficiency shows significant improvement - wind tunnel provides new Opel Astra V8 Coupi with crucial fine-tuning. When Opel go to the starting grid of the third DTM race in the United Kingdom this coming weekend, the first victory of...
Aerodynamic efficiency shows significant improvement - wind tunnel provides new Opel Astra V8 Coupi with crucial fine-tuning.
When Opel go to the starting grid of the third DTM race in the United Kingdom this coming weekend, the first victory of the Calibra V6 at Donington Park nine years ago will be 'old hat.' More than ever, a perfect set-up will be crucial on this beautiful, but highly demanding circuit. The high-speed, selective up and downhill bends in particular demand a car with effective aerodynamics and a well-balanced set-up. Conditions under which the Astra V8 Coupi can take full advantage of its aerodynamic efficiency, which has been clearly improved compared to its predecessor.
As in the Formula 1, with the high-tech touring cars contesting in the DTM series, aerodynamics are playing an increasingly important role. "Even minute changes are noticeable, translating into tenths or hundredths of a second - and in the DTM, where things get extremely tight, this is of crucial importance," said Timo Scheider (Opel Team Holzer), who finished in fourth and fifth place at the most recent race in Zolder, scoring points as did teammate Michael Bartels, who finished third in the qualifying race. "The impact of aerodynamics has risen tremendously. The closer you get to the car in front of you, the higher the tendency to understeer, because there is an interference of air flow from that vehicle. Overtaking has thus become more difficult," added Joachim Winkelhock (Opel Team Phoenix).
Up to 250 kph in the wind tunnel
One of the major development objectives for the 2002 DTM Astra was a notable increase in aerodynamic efficiency, calculated as a quotient of downforce and drag. The coefficient of drag which, in the production Astra Coupi reaches an excellent Cd-value of 0.28, has clearly been raised for the racing Coupi, owing, for example, to enormous wheel arch extensions, but also to air ducts for cooling water, oil, brakes and gearbox.
"The impact of these air ducts on drag and downforce, resulting from such factors as the location and design of the air intakes at the front apron must be kept at an absolute minimum," said Martin Gerspacher, aerodynamic expert with Opel Performance Center (OPC).
Extensive simulations helped create a 40-percent scale model of the racing Astra used for investigating air flow and downforce behaviour in the wind tunnel of Fondmetal Technologies in Italy. Because DTM regulations only allow one particular aerodynamic configuration per racing season, these investigations were soon followed by wind tunnel tests of the original vehicle at the University of Stuttgart and, of course, on the track itself. "At the end of the day, what counts is the stop-watch and lap times," OPC project manager Dr. Ulrich Pfisterer summed up the aerodynamic efforts of the team.
The wind tunnel in Stuttgart is currently rated as one of the world's most advanced facilities, enabling road simulation tests of vehicles up to a speed of 250 kph. This is accomplished by five air-cushioned steel running-belts, one between the wheel track and four for driving the wheels. Velocity plays a decisive role, because the aerodynamic forces rise at a square relative to the increase of speed. Consequently, doubling the speed from 100 to 200 kph quadruples the aerodynamic forces that come into play.
In pursuit of aerodynamic balance
Aerodynamic forces lead to higher wheel loads, which means that higher forces can be transmitted via the tyres. This results in higher cornering speeds, better braking deceleration and higher traction when accelerating from corners. The rear aerofoil is the most conspicuous aerodynamic component. The restrictive technical regulations of the DTM prescribe both the spoiler profiles and their position. A considerably higher developmental effort is invested in the air flow underneath the vehicle. Whilst a flat underbody is mandated between the axles, the front splitters and the diffuser provide for the so-called ground effect.
Based on a carefully crafted shape worked out in wind tunnel tests, the air underneath the front spoiler is accelerated, with the higher air flow velocity creating a suction or vacuum-like effect on the vehicle.
"The difficulty lies in the fact that the flow must not be interrupted under any conditions, thus ensuring a constantly sufficient level of downforce," explains Martin Gerspacher. Front flaps located on the left- and right-hand sides of the vehicle front, which may be removed for performing the balanced vehicle set-up, increase downforce. The diffuser, which sucks out the air from underneath the vehicle, thus creating a suction or vacuum-like effect at the vehicle's rear, works in a similar way as the front splitter. The wedge shape resulting from a change in the vehicle's position through front and rear ground clearances is an another major factor in this.
"The objective is to achieve an optimum of aerodynamic balance, resulting in driving behaviour that is as neutral as possible, without compromising optimal wheel load values," Gerspacher added. The definition of aerodynamic balance is derived from the distribution of the downforce coefficient to the front and rear axles.
Individual driver needs
The differing characteristics of the DTM race tracks and the individual needs of the drivers require utmost exploitation of aerodynamic potentials. As such, the Norisring requires the least amount of downforce, whilst the Sachsenring, for example, requires a very high-level one. On the other hand, a high degree of downforce frequently leads to understeering, because the 'aero-balance' has been shifted excessively towards the vehicle rear. "This often results in conflicting interests, which means that optimal balance is always a compromise," according to Michael Bartels.
As a general rule, the driver, in conjunction with the vehicle engineer, at first develops the set-up of vehicle parameters like track, camber, dampers, springs, etc. in order to achieve optimum mechanical grip. Normally, aerodynamic balance is only developed in the second step, which also considers personal driving styles. Timo Scheider prefers a car that understeers, enhancing his ability to 'head' for bends, whilst Manuel Reuter, the winner of the 1993 race at Donington, has great difficulty handling a race car that understeers. At the end of the day, the only thing that counts are the lap times and the chequered flag.