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The Opel DTM Race Car

The technology of the 460hp DTM racing Opel Astra V8 Coupe.

Courtesy of Opel

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The awesome DTM Opel Astra V8 Coupe runs a 460hp 4-litre V8 - and enormous ventilated carbon brakes. Nowhere are those brakes more in evidence than at the Norisring racetrack.

The Norisring has a peculiar characteristic: A driver will only use the brakes three times during each of the 44 laps of a race on this circuit. Before entering the 'Grundig-Kehre', his foot will be on the centre pedal for five seconds, decelerating from 260 to under 50 km/h, before the 'Dutzendteich-Kurve', the braking event will last about four seconds to slow the car down from 225 to just under 60 km/h, and before the 'Schoeller-S', a speed of just below 200 will be reduced to around 90 km/h in three seconds. With a lap time of just over 50 seconds, this adds up to a total of twelve seconds during which drivers will have their foot on the brake. The remainder is nothing but sheer acceleration, except for a bit of throttling down in the chicane.

Brakes

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Ventilated 380-millimetre carbon discs and six-piston aluminium calipers at the front, ventilated 340-millimetre carbon discs and four-piston aluminium calipers at the rear - this is the standard braking system for DTM race cars, supplied by UK specialist AP Racing. Anti-lock braking systems (ABS) are prohibited.

While production vehicles achieve a maximum deceleration of 0.8 to 1.1g, DTM cars nearly reach 2.0g. And while the production vehicle needs about 40 metres to brake down from a speed of 100 km/h to zero, the V8 Coupé needs only 25. This high level of braking power is the result of the ratio between vehicle weight, the roadholding capability of the standard Dunlop tyres and the energy transformed by the brakes. To illustrate this point, accelerating the Opel Astra V8 Coupé with a mass of 1,150 kilograms - including driver and fuel - to a speed of 200 km/h requires a distance of 200 metres and takes 6.4 seconds, thanks to about 350kW of engine power. Decelerating the same vehicle, however, only requires a distance of 63 metres, which equates to the braking system absorbing about 1,100kW of power in the process. This in turn means that 1.1 megawatts of power are transformed into heat within a mere 3.6 seconds!

It comes as no surprise then that brakes tend to overheat. The critical point is when the heat passes from the discs, which develop temperatures of up to 550 degrees Celsius, via the brake pads to the calipers, because the brake fluid reaches its boiling point at about 230 degrees Celsius. This results in vapour locks which may lead to a "soft pedal".

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"Pit stops are particularly critical, because you keep your foot on the brake during the tyre change, with heat being continually transmitted, and this results in a higher risk of developing vapour locks," says Michael Bartels (Opel Team Holzer).

To prevent this, the brake calipers are cooled down by water. When approaching the pit lane, the driver will push a button on the steering wheel that activates a device which sprays water on the calipers, reducing the temperature to below 150 degrees. DTM regulations allow a five-litre supply of water for brake cooling.

One way to define the differences in driving styles is the way drivers use the brakes, with some slamming the pedal harder and others using a more gentle "touch." While one driver may prefer more braking power at the front, another may prefer stronger braking force to be applied to the rear wheels. This distribution of braking force is adjusted via a brake balance bar which absorbs the pedal force, causing pressure to be built up within the two master brake cylinders via two coupling rods. Also, there are drivers who choose to use their left instead of their right foot for braking, like Timo Scheider and Manuel Reuter.

"I've been braking with my left foot for a year and a half," says Scheider, "because that keeps me from having to switch from the gas to the brake pedal as well as enabling me to stabilise the car with the brakes, while continuing to accelerate. It wasn't really that difficult to change my ways, after all, I used to brake with my left foot in karting as well."

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The Norisring circuit takes the brakes to the ultimate test, but the drivers as well.

"The brake pressure you need is hard and long, at other tracks you normally spend considerably less time with your foot on the brake," says Alain Menu (Opel Euroteam) who won his first podium position at the Sachsenring. And Timo Scheider adds: "This puts excruciating stress on any driver. I once suffered a cramp because of this."

The centre pedal is being slammed with ultimate force particularly at the Norisring. "It may well be that you find yourself with aching muscles on the Monday after the race," says Winkelhock. "That's why sitting properly is important for optimally applying this force."

Aerodynamics

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One of the major development objectives for the DTM Astra was a notable increase in aerodynamic efficiency, calculated in terms of downforce and drag. The coefficient of drag which, in the production Astra Coupé reaches an excellent Cd-value of 0.28, has clearly been raised for the racing Coupé, 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.

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"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 km/h. 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 km/h quadruples the aerodynamic forces that come into play.

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. While a flat underbody is mandated between the axles, the front splitters and the diffuser provide for ground effect.

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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 to the front splitter. The wedge shape resulting from a change in the vehicle's position through front and rear ground clearances is 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.

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, while 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.

Safety

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When the technical regulations for the DTM, which was relaunched in 2000, were first established, safety was stressed as a major integral component and has been continually expanded since then.

"Elements like space frames, carbon fibre monocoques and crash boxes comprise a unit with exactly defined maximum load and stress values that must be confirmed by tests," said Opel motorsport boss Volker Strycek. "The DTM cars are currently representing the ultimate in safety standards."

The DTM driver has exchanged his racing seat, which used to be bolted to the chassis, for a safety cell that basically equates to a Formula 1 monocoque. This carbon fibre cell contains the six-point safety belts, the in-moulded seat bucket, the pedals and, with Opel, the fire extinguisher reservoir as well.

"Dimensions, load and stress values are precisely specified," said Dr Ulrich Pfisterer, DTM project manager of Opel Performance Center (OPC). The cell, which must weigh a minimum of 25 kilograms, must be confirmed to hold up to a load of just under 8 tonnes in test stand trials - and over a period of 30 seconds at that.

"At first, sitting in such a box was an unfamiliar experience, but now, I wouldn't want to do without this safety element any more," said Michael Bartels (Opel Team Holzer).

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A head restraint eclipsing the driver's head on the sides is an integral component of the safety cell and seat shell unit. Opel had a major part in initiating this development. Due to the lateral extension of the head restraint, the seat came to be nicknamed as a "wing chair." The maximum distance prescribed between the two lateral supports is 40 centimetres, and even the head restraint must be able to resist a load of nearly one and a half tonnes. Both the carbon fibre cell and the space frame must be cushioned with crash-absorbing materials in the vicinity of the driver. Regulations specifically require the use of Neopolene foam by BASF for this purpose.

The "seat box" is firmly bolted to the tubular frame. More than 85 metres of steel tubing - specifically, standard 115 CrMo 14 aviation steel - were used for the frame structure, following in-depth engineering calculations and clear specifications established by the regulations. After all, the tubular frame not only ensures the requisite stiffness of the body, which influences the driving properties of the V8 Coupé, but is also an integral component of the overall safety concept. The impressive tubular construction, which has nothing in common with the roll-over bars of the past, must be able to resist exactly specified stress parameters as well. As such, the tubular frame in the vicinity of the B-pillar has to be able to withstand a vertical force of up to nine (metric) tons, with only a maximum 25-mm deformation of the roll-over section permitted.

Whilst static load and stress tests are prescribed for the preponderance of the parts, all of which are monitored by Deutscher Motor-Sport-Bund (DMSB), the crash boxes installed at the front and rear must be subjected to both static and dynamic testing. Similar to the crumple zones in production vehicles, the impact structures made of carbon fibre are mounted to a sled, simulating a vehicle crashing against a concrete wall at a speed of 46 km/h. The permissible deceleration values of 40g on average roughly equate to those of a production vehicle. The testing buck weighs a total of 1,200 kilograms, approximately the same as a DTM vehicle with a full fuel tank.

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The HANS system, which made its way into the series last year already, has since become an integral element of the safety package. HANS stands for "head and neck support." HANS is a carbon and aramide fibre supporting device resting on the driver's shoulders and pressed against the body by the shoulder belts. Fastened to the driver's helmet by two flexible connections, HANS, an U.S. development, provides better support to the cervical vertebrae, while reducing the biomechanical forces exerted on the head and neck in case of an impact.

"HANS is an excellent invention that has already proven its worth time and again," said Manuel Reuter (Opel Team Phoenix), while Yves Olivier (Opel Euroteam) added: "HANS offers major benefits, although the system reduces mobility as well as limiting the driver's vision to the right and left."

Despite fireproof underwear, headgear and racing overalls, despite fire extinguisher systems and fire protection walls between the engine compartment and the cockpit as well as the safety tank, the driver's ability to leave the cockpit at lightning speed can be crucial in case of a crash. To ensure that this can be done, DTM regulations impose specific requirements: Wearing full gear and belted into the seat, with the steering wheel in place and the doors closed, the driver must be able to leave the cockpit in seven seconds through the door on his side, which must be capable of being opened - and in nine seconds through the door on the co-driver's side. At the DTM season opener at Hockenheim, all drivers had to pass this test, which Opel's Michael Bartels, Alain Menu, Yves Olivier, Manuel Reuter, Timo Scheider and Joachim Winkelhock mastered superbly. The quickest, by the way, was Timo Scheider - the 23-year-old climbed out of the cockpit in a mere 3.9 seconds.

Suspension

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From the heavy bumps at the Norisring or a completely flat track at the EuroSpeedway Lausitz - the suspension springs and, first and foremost, the shock absorbers play a crucial role in ensuring that the 460-hp touring cars never lose optimal touch with the ground.

"A spring is still a spring, but shock absorbers, which are now adjustable in virtually every respect, have seen incredibly fast technological advancement," says Opel works driver Joachim Winkelhock, emphasising the importance of the dampers.

"Shock absorbers convert the vibrational energy generated by the suspension springs as they cushion the vehicle body against the road into thermal energy," explains Marco Gehlen, engineer and damper specialist with Opel Team Phoenix.

Working on springs and shock absorber settings, the Opel drivers in conjunction with their vehicle engineers fine-tune the specific set-up to be used for the respective race track. Of course, this includes other set-up parameters like toe, camber, castor angle, anti-roll bars, aerofoils, etc.

"Fine-tuning the shocks is performed by such minute 'notches' that the results can only be verified through data analysis, and rarely by the driver's perception," says 'Jockel' Winkelhock (Opel Team Phoenix). "A smooth set-up can be deceiving," adds Timo Scheider (Opel Team Phoenix). "You'll be enjoying a comfortable ride but the disadvantages will show in your lap times."

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As a general rule, the efficacy of the shock absorbers is investigated during in-depth rig testing prior to any DTM race. This includes the development of different parameters, depending on the characteristics of the circuit as well as the individual needs of the drivers.

Racing shocks can be adjusted externally, enabling their performance and efficacy to be modified even on short notice. Generally speaking, shock absorbers have a bi-directional effect, i.e. the bump and rebound stages. For both directions of travel there are two speed ranges: bump low speed and bump high speed, rebound low speed and rebound high speed. The force is generated at the moving main piston and, additionally, at the compression stage valve.

"Damper parameters are extremely complex. Any type of progression can be adjusted. The initial and follow-on travel of the wheel suspension can even be adapted to the special characteristics of corners," says Timo Scheider.

Whether the vehicle will have a softer or stiffer set-up primarily depends on the race track.

"The Lausitz-Ring, for example, is very flat and has no high kerbs. That's why the shock absorbers in particular can be set for higher stiffness because, on the EuroSpeedway, they don't have to respond to sudden bumps," says Alain Menu (Opel Euroteam).

The hairpin bend at the Lausitz-Ring, which leads back to the start and finish line via the banked corner, will be a crucial point for the vehicle set-up. "The question is how far you can take stiffness with your set-up and at what point you will lose too much time in the hairpin, because a softer set-up would actually improve traction."

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The shock absorbers are adapted to the respective suspension spring. The optimal selection of the spring rate - in racing, spring rates of up to 200 newtons per millimetre (N/mm) are being used, while production vehicles have spring rates between 20 and 60 N/mm - requires a compromise between aerodynamic and mechanical grip.

"Aerodynamics require stiff springs in order to keep the ride height as constant as possible to achieve steady downforce," says Dr Ulrich Pfisterer, DTM project manager with Opel Performance Center (OPC).

Mechanical grip is increased by a soft set-up, enabling the wheel to optimally follow any bumps on the ground. Production vehicles have no downforce in this sense of the word. Instead, ride comfort, which is decisively influenced by the shock absorber configuration as well, is of critical importance.

Racing shocks are high-tech components manufactured in extremely small volumes. In addition to performance and adjustment options, light-weight design and optimised friction resulting from the use of special seals are important considerations. It thus comes as no surprise that the price of a shock absorber designed for racing is about 50 times higher than that of a high-volume production shock used in road cars. DTM regulations, by the way, prescribe so-called coil overs with the spring concentrically located around the damper. For that reason, the damper has been designed to enable the spring to be quickly exchanged during tests and practice sessions.

"Finding the right compromise and optimal vehicle balance is a difficult task," says Manuel Reuter (Opel Team Phoenix). "After all, springs and dampers are only two of numerous parameters involved in the vehicle set-up."

Opel Astra V8 Coupé (DTM) Specifications
Engine Type V8-engine, front, longitudinal, cylinder bank angle 90 degrees, cylinder clearance 102 mm, four chain-driven overhead camshafts, four valves per cylinder
Displacement 3998 cm
Bore 94 mm
Stroke 72 mm
Compression 13.0:1
   
Output app. 340 kW/462 hp at 6750 rpm
Max. torque app. 510 Nm at 5500 rpm
Engine management Bosch MS 2.9.2
  Emission control closed-loop three-way catalytic converter
  Lubrication dry sump
   
  Transmission longitudinal transaxle racing transmission (standard DTM component, manufacturer: optionally X-trac or Hewland), 6-speed, straight-cut gears, non-synchromesh, sequential actuation, fixed gear ratios, variable transmission ratio; carbon fibre clutch; mechanical limited-slip differential; rear-wheel drive
   
  Body tube frame with integrated CFC-driver safety cell, integrated safety concept with side- front- and rear-mounted crash boxes, double rear wings with DTM standard profile
   
Fuel tank 70 litres
Length 4470 mm
Width 1850 mm
Height 1250 mm
Wheelbase 2670 mm
Track 1600 mm front, 1550 mm rear
   
Suspension double wishbone suspension, front and rear;
  central wheel fastening; adjustable anti-roll bars, front and rear; compression- and rebound-adjustable dampers, power-steering
   
Wheels front 9x18", rear 11x18"
Tyres front 240/650 R 18, rear 280/650 R 18
(DTM standard tyres, manufacturer: Dunlop)  
   
Brakes ventilated carbon discs, front (diameter 380 mm),
  ventilated carbon discs, rear (diameter 340 mm), six-piston-aluminium-calipers, front and rear
(DTM standard components, manufacturer: AP)
   
Minimum weight 1,080 kg, including driver

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