This article first appeared in Racetech magazine
Anthony Davidson’s terrifying accident on the Mulsanne straight at this year’s Le Mans 24 Hours once again thrust the issue of cars flipping in to the spotlight. Throughout sports car history it is a fact that cars have caught air underneath their floors and flipped.
The problem stems from the fact that the car is designed to produce downforce when travelling straight, not when sliding at high yaw angles. When a car begins to slide much of the downforce is lost, if a car then hits a kerb or digs into a gravel trap it allows air to get underneath the car and it will flip. As LMP cars have a large plane floor you get a situation where there is a high pressure underneath the car causing it to leave the track, usually upwards.
The list of incidents throughout sportscar history is long; here are just a few examples and instances of cars having got airborne, often meaning devastating accidents.
John Surtees, Lola T70 at Mosport 1965
Brian Redman, Lola Can-Am at St Jovite 1977
Stefan Bellof, Porsche Group C at Nurburgring 1984
John Nielsen, Sauber Group C at Le Mans 1985
Masahiro Hasemi, Nissan Group C at Fuji 1991
Takao Wada, Nissan Group C at Fuji 1991
Yannick Dalmas, Porsche GT1 at Road Atlanta 1998
Mark Webber, Mercedes LMP900 at Le Mans 1999
Peter Dumbreck, Mercedes LMP900 at Le Mans 1999
Bill Auberlen, BMW LMP900 at Road Atlanta 2000
Michele Alboreto, Audi LMP900 at Lausitzring 2001
Marc Gene, Peugeot LMP1 at Le Mans 2007
Hideki Noda, Lola LMP2 at Le Mans 2008
Ben Devlin Lola LMP2 at Sebring 2009
Nicolas Minassian Peugeot LMP1 at Paul Ricard 2011
This list (which are just selected incidents) illustrate that these kind of accidents are nothing new and apart from a gap when sports prototypes were not prolifically manufactured between 1993-1996, there have always been ‘flying’ accidents. There have been many notions, theories and studies as to why they occur but the answers are relatively simple. Sports cars are large surface area cars and when their floors are exposed to wind then they lift.
Former Lola designer Julian Sole has been one of the key figures in LMP design since the LMP675 that he created in 2001. He has seen different iterations of rules and fully understands the reason why there are reasonably frequent incidents like we saw Davidson suffer back in June.
“It’s very difficult once a car has its underbody angled to the wind for a car not to take off,” says Sole. “Imagine just putting your hand out of a car window at 200mph! If you angle your hand its ok but when you start putting it square to the wind then it’s going to get pushed up and back. It’s exactly the same with a car, especially one with a large surface area like a sports car.
The ultimate way to stop these kind of incidents would be to reduce the surface area but then this standpoint is heading towards creating an open wheeled style car like a formula car which goes against whole philosophy of two-seat sports cars.
“The problem has been studied in great detail since the ACO were forming the present regulations in 2003/4,” explains Sole. “In particular when the new LMP regs came in to affect in 2004 there was great emphasis on addressing the issue. Looking at the profile of the floor and the side view of the car it is clear that they are now designed to try and keep them as much on the ground as is possible. So the base shape of the cars is very different from the start of the last decade and there have been smaller changes that have a large visual impact. The three changes made for the 2012 season were:
1. Shaped plank- designed to create low pressure under the car at high yaw angles.
2. Fin- creates high pressure on the top surface
3. Wheel arch holes- reduce low pressure on wheel arches hence reducing the low pressure on the top surface so reducing the overturning moment.
These three measures have certainly helped reduce the overturning moment but the problem is with a large flat floor as soon as you get air underneath the car there you get a big overturning moment.
Comparing the present LMP cars with Group C cars from 25 years ago you can see that the flat bottomed Group C cars were very susceptible to taking off such as when John Nielsen flipped at Le Mans with the Sauber C8 in 1985 and Stefan Bellof did the same at the Nurburgring 1000kms in the works Porsche 962 a year earlier. There were also a couple of Japanese incidents in the 1991 season at the end of the main straight at Fuji after punctures. The one involving Masahiro Hasemi in the Nissan R91CP can be viewed on Youtube and a very scary accident it was too, not dissimilar to Davidson’s although this time a puncture is to blame.
“Now though with the diffusors and stepped underfloors and angled side panels the cars have much more aerodynamic grip,” says Sole. So what is the solution? “Well there isn’t a clearly defined one,” continues Sole. “If you start taking some of the bodywork area off the cars it can make them more unstable in a straight line and much more difficult for the driver to keep control of the car with that level of reduced downforce.”
“The early studies that were done in to reducing the risk of cars taking off were done by the FIA, mostly in windtunnels,” says Sole. “This was really a reaction to the Mercedes accidents in 1999 when the CLK’s produced more lift than downforce, especially when cresting rises or in the wake of other cars. The FIA used an independent study which was conducted by John Piper and his Piper Design business. More recently the studies have primarily been conducted using CFD. Most of the constructors have contributed to the stability tests and most of the specific ideas were generated in the Technical Working Group (TWG) meetings that all the constructors attend. Most of the constructors tested their own ideas before feeding them back in to the TWG meetings and the ACO/FIA on what the best way forward was. The changes that were applied last year; plank, front bodywork holes and taller fin were examples of these studies that were shared to the governing bodies.”
So the conclusion appears to be that preventing these accidents is pretty much impossible without drastically changing the rulebook, look of LMP cars and in turn the whole ethos of endurance racing. If the dimensions of the car were changed to essentially a single seater look with perhaps fairings or silhouette bodywork then the lineage of two seater sports cars to road cars is lost forever.
The governing bodies can mitigate somewhat the seriousness of the impacts with greater emphasis being put on cockpit safety and the eradication of gravel traps perhaps. These factors will help but contact, punctures or just driver error at 150mph+ will almost always run the risk of cars getting air and resulting in the accidents that we have seen throughout the history of sports car racing.