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Geeky read: Why did this famous crash happen?

23w ago

28K

Racecar crashes can be horrifying, but none get close to the craziness of those witnessed at the 1999 Le Mans 24 hour race that led to many questions being asked about car design, engineering practice and the rules that the teams adhered to. 

During the LMPGT era, five incidents occurred that involved cars ‘somersaulting’ through the air.

Here are three of them:

Porsche 911 GT1

BMW V12 LMR

And finally, the most famous of them all, the Mercedes-Benz CLR

It is the latter of these three motoring giants that I'll focus on. This is because – during the halo race of the series (the Le Mans 24 hours) – Mercedes cars flipped three times throughout the weekend. The first incident occurred during practice, the second during qualifying and the final and most-documented incident during the race itself.

Now YouTube-famous, the crash of Peter Dumbreck will go down in motorsport history as one of the most spectacular accidents ever and has influenced Le Mans car design to this day.

The incident

Early in the race, Peter Dumbreck in the Mercedes-Benz CLR was slipstreaming a Toyota GT-One in the run up to Indianapolis corner. After gathering pace through the seriously rapid section of the track, Dumbreck moved out of the slipstream of the Toyota just as both cars were travelling over a crest in the run-up to Indianapolis corner.

The car then seemingly took-off from the track, flipping in the air multiple times before crash landing over the track barriers into an area of deserted woodland.

Ouchtown: population Peter Dumbreck

Due to this sequence being filmed by the world’s media, many conclusions have been made surrounding how the accident happened and who was to blame, with the general consensus being that the crash was a combination of the car’s poor aerodynamic design, coupled with the sudden movement out of the leading car’s slipstream.

Although many motorsport fans take this as gospel, there could potentially be multiple other factors that caused such a dramatic crash.

Let's talk about the car, the Mercedes-Benz CLR

Isn't she a beauty

To understand the engineering behind the car (and therefore the crash), the first investigation should be into the Mercedes-Benz CLR's design. The CLR was built within the regulations for LMPGT (Le Mans Prototype Grand Touring) which meant that it only had to vaguely resemble a road car but had bespoke motorsport underpinnings. A large-capacity V8 was nestled within a carbon fibre and tubular spaceframe monocoque, keeping a cockpit that resembled a road-going Mercedes-Benz of the time.

The engineers maximised the car’s dimensions but – unlike the other manufacturers – they took advantage of some new diffuser rules to the max. By using a relatively short wheelbase compared to its competitors, the CLR instead opted to use the length of the car left over for large overhangs (the areas past the limits of the wheelbase) to incorporate large front and rear diffusers.

The overhangs were pushed to the max

Although this method could potentially lead to increases in downforce through the implementation of the diffusers, the overhangs (260mm longer than some competitors) may have led to an inherent instability by producing large surface areas (known as the ‘throat area’) for incoming air to try and act upon in an upward manner, creating lift. If air were to get under the car, these large overhangs would act as ‘paddles’ which would create a large torque underneath the car, potentially pivoting it about an axle.

Pitch angles could be killer

Another potential engineering failure within the car’s design was the general setup of the cars for the specific Le Mans race. The track used for the 24-hour race is notorious for its long straights, meaning that top speed becomes one of the main aims. This factor brings pitch angle into the equation.

The pitch angle of a car is the angle of its nose relative to the tarmac below it – a positive pitch angle means the car is slightly facing upwards to the sky and a negative angle pitches the car slightly downwards into the road, maximising the frontal area of a car and increasing downforce.

A heavily pixelated picture of negative pitch angle

Most LMPGT cars generally used a slight negative pitch angle (-1.5 to -2.5 degrees) to utilise large rear wings to maximise downforce on the driven wheels. This downforce brings with it a large drag force on the car meaning that top speed is sacrificed for acceleration and cornering speed.

The Mercedes engineers knew that they had a particularly capable engine at their disposal and decided to alter the pitch angle of the vehicle to a more neutral setting to make the car as fast as possible down the famous Mulsanne Straight.

With the car now riding at around -0.7 to 0 degrees of pitch, a higher top speed could be achieved, but this left the car susceptible to undercurrents of air that would in the past have been dealt with via a more extreme pitch angle. Robert Dominy (a researcher at the University of Northumbria) investigated the influence that pitch angle had upon a scale model of the Mercedes-Benz CLR.

By directly simulating the conditions and setup of the car (pitch angle -0.7 degrees, speed 320kph), an almost-linear relationship was found between downforce and pitch angle, with an angle of +0.4 resulting in front axle downforce of just 2000N. At these positive pitch angles, the centre of pressure was found to move quickly forward which corresponds with the nose lifting in an unwanted manner.

Now, one way to suddenly make a car’s pitch angle positive in a bad way is to drive quickly over the brow of a hill. Research has shown that downforce on the Mercedes’ front axle only decreased by a small amount in the run-up to 0 degrees of pitch. But once past +2.0 degrees of pitch, downforce quickly reduced to the point where lift force was being generated at the front – not what you want in a racing car.

The point of no return was +2.4 degrees, with 400kg of downforce over the rear axle being cancelled out by the same amount of lift at the front axle. From this point on the front of the vehicle lifted clear of the tarmac, increasing the pitch angle rapidly and sending the car quickly up into the heavens.

Could slipstreaming have caused the crash?

There was plenty of potential slipstreaming action during Mark Webber's crash during practice

An outside factor that has been attributed to these crashes is the air disturbances caused by slipstreaming. By tucking in behind a car in front, a driver can place himself within a more-laminar flow of air, reducing drag and achieving a higher speed to overtake.

This form of racing reduces the interaction of air with the diffusers situated under the car, and it can be seen from the footage that the crashes seen throughout the LMPGT era were during overtaking manoeuvres after slipstreaming, meaning that this factor could have been a defining element of the crashes. The general consensus and conclusions taken from the footage within the motoring world is that the cars suddenly entered an area of turbulent air displaced by the car in front.

Although this may seem like a self-explanatory solution to the argument, it is hard to believe that slipstreaming was the only factor that lead to the cars flipping, or even that it was the leading factor. After watching the footage and investigating the cars at hand, it seems more likely that the flipping of the cars was due to a combination of factors, with slipstreaming being an ancillary component to more prominent causes.

This thought process is backed up by researchers at the University of Northumbria who investigated the effects of slipstreaming, concluding that very little of the lift force characteristics of the cars were affected by moving out of the slipstream of a car in front.

Things haven't gone well for this Porsche

What about the track?

In the aftermath of the race weekend, Mercedes-Benz blamed the track design for the incidents that occurred. The team claimed during press conferences that the radius of the crest in the run-up to Indianapolis corner was too severe for the flat-bottomed nature of the cars and therefore should have been altered in-tandem with the new car design rules.

Another complaint from Mercedes was the severity of the kerb heights which influenced Mark Webber’s crash during qualifying. The team’s complaint was that the sudden change in pitch angle by clipping such extreme kerbs resulted in air acting in such a way that produced a flipping moment underneath the car.

The aftermath

The aftermath of each crash provided some insight into the shortcomings in the safety processes and regulations governing unsafe car designs. For example, even after the initial crashes in practice and qualifying, the CLRs were able to race after the ACO (the governing body behind the organisation of Le Mans) allowed a mechanical engineer to come up with a quick solution to increase downforce at the front axle.

Christian Horner (of Red Bull F1 fame) advised the Mercedes-Benz team to place front canards (dive planes) on the car, using small carbon composite wings on each corner to push the front wheels into the ground. This ‘quick fix’ obviously had very little impact on the aerodynamic properties of the vehicle, considering the most violent of the crashes (Dumbreck's) happened after this modification was made.

Here you can see the canards placed on either side, over the Bridgestone advertising

It is speculated that Mercedes retrospectively performed tests at its own test track to find the cause of the accidents, but the tests apparently produced ‘no conclusion’. The FIA investigation did however involve hauling up the organisers for allowing the cars to race, leading to the ACO providing a press release in reply to both Mercedes and its governing body. It predictably stated that the blame should solely be put on the Mercedes team for its poor car design.

The ACO also emphasised that the circuit had passed all scrutineering by the FIA and any blame of the circuit setup was simply trying to pass over responsibility, cleverly dodging the wrath of the FIA as well as planting the blame firmly on Mercedes.

How this crash changed motorsport forever

The results of the crash show just how important these crashes were to the engineers and governing bodies at hand, as multiple changes were made to rules and tracks that have held fast ever since.

The most apparent change has been in the rules governing LMP car design nowadays, with the main alterations being in the overhang limits, wheel arches and aerodynamic equipment. The front and rear overhangs are now limited to 1000mm and 750mm respectively; substantial decreases over the older cars and further emphasising car design as a leading factor in the crashes.

The overhangs of the Porsche 919 Hybrid are definitely stumpier than those on the elongated CLR

The latest Le Mans Prototype racers also now feature a large cut-out at the top of the wheel arch that allows incoming air from underneath the car to exhaust upwards and away from creating high pressure concentrations that could lead to a lifting force.

Large ‘shark fins’ have also been added to streamline the air travelling down the side of the car. This enhances the interaction between the air flow over the car and the rear wing, pushing the car down into the tarmac for higher downforce and cornering speeds.

It also seems that the FIA considered the complaints of the teams and modified the Le Mans circuit as well as other circuits in the racing calendar, reducing the radius of crests and flattening kerbs to decrease the chances of a sudden change in pitch angle.

The incoming air that sneaks under the front splitter of the car is also somewhat exhausted upwards, cleverly making its way to the shark fin and rear spoiler

The impact of these crashes was emphasised further by the collapse of the LMPGT category for the LMP900 category that radicalised the designs of the cars, specifically for aerodynamic purposes. The most prominent change came with teams opting for an open cockpit format instead of the closed cockpits of the Mercedes and Porsche cars of the time.

It was found that the closed cockpit (used to reduce drag and make the cars look more ‘road-like’) created a significant proportion of lift compared to the open cockpits seen in the likes of Formula 1 and IndyCar racing. Although this initial shift was undertaken, closed cockpits have made a comeback in recent years due to advancements in technology and driver safety.

The Audi R8 was all-dominant in the LMP900 category, pushing on from where the LMPGTs left off.

Conclusions...

The series of crashes that occurred in the late '90s obviously shook the foundations of endurance racing and were embarrassing for the teams and regulators involved. The flips that occurred have never been seen to the same extent at Le Mans since, showing that the actions undertaken tackled the problems and thus found the causes of the flying cars.

In terms of deciding upon the defining factor, it can be seen through research and the study of general engineering principles behind the cars that the crashes were caused by a combination of track design, car design and a lack of engineering analysis of the cars at hand. Although slipstreaming seems to be a common factor in the crashes and is jumped to as the popular conclusion, I feel it was of minor influence when compared to the change in pitch angle produced by the car’s design and setup, along with the severity of some of the track geometry.

The LMPGTs were so god damn pretty, but WOW could they be unstable. The CLR may be one of the best-looking racing cars to ever take to Le Mans and if it wasn't for the freak accidents that occurred, one of them may have gone on to take the 1999 title.

Sadly, it'll be remembered for one thing and one thing only – taking to the skies.

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