The 1999 24 Hours of Le Mans witnessed a series of shocking and unforgettable moments: the Mercedes-Benz CLR race cars dramatically flipping and becoming airborne. These incidents remain a significant point of discussion for motorsport enthusiasts and engineers alike. Many have asked, what exactly caused these spectacular and concerning crashes involving the Clr Mercedes? Was it merely bad luck, or were there deeper design and aerodynamic factors at play? As an automotive repair expert familiar with the intricacies of high-performance vehicles, let’s delve into the technical aspects that contributed to the Mercedes CLR’s airborne episodes at Le Mans.
The root causes of the CLR Mercedes issues can be traced back to a combination of its dimensional architecture, a relative lack of overall downforce compared to modern standards, and a degree of unfortunate circumstances coming together at the wrong time.
Dimensional Architecture and Aerodynamic Sensitivity
The Mercedes CLR was designed to adhere to the maximum length regulations of the time, measuring 4890 mm overall. Its wheelbase was relatively short for a Le Mans prototype at 2670 mm, coupled with a substantial 1080 mm front overhang and an even more extended 1140 mm rear overhang.
In 1997, regulations permitted LMP and LMGTP cars with flat bottoms to incorporate a rear diffuser extending from the rear wheel centerline to the bodywork’s trailing edge. Mercedes maximized this allowance on the CLR, resulting in a rear overhang significantly longer than its competitors. To put this into perspective, the Toyota GT-One had a 990 mm rear overhang, the Audi R8C 940 mm, and the Nissan R391 just 880 mm, compared to the CLR Mercedes’s 1140 mm. The front overhang at 1080 mm was comparable to rivals, perhaps slightly on the longer side. However, the front diffuser underneath was notably small and less aggressive than those found on competitors like the Toyota GT-One or BMW LMR. Adding to this unique geometry was the CLR’s wheelbase, the shortest in the LMP category, contrasting with the Toyota GT-One (2850 mm), Audi R8C (2700 mm), and BMW LMR (2790 mm).
1999 Mercedes Benz CLR
This dimensional combination created a highly sensitive aerodynamic platform. A shorter wheelbase paired with long overhangs means that even minor changes in the car’s pitch – such as during braking or acceleration – could lead to significant variations in ride height at the extreme front and rear ends of the car. Reports of the CLR Mercedes porpoising on various sections of the Le Mans track throughout the race weekend strongly suggest an underlying instability in its aerodynamic platform.
The Role of Coupe Bodywork and Downforce Deficit
The closed-cockpit coupe body style of the CLR Mercedes also played a role. The shape of a closed cockpit can inherently generate lift, which the car’s downforce must counteract. While most race cars of this era generated considerable downforce to overcome this, the CLR appears to have been operating with a smaller margin for error. Designers often accept the lift potential of a coupe for the aerodynamic drag reduction benefits it offers, but in the CLR’s case, it became a critical factor.
Anecdotal evidence suggests the CLR Mercedes team used softer rear springs. While not definitively confirmed, if true, this would further compound the issues. Softer rear springs, sometimes employed at high-speed circuits to enhance straight-line speed, can, under certain conditions, contribute to aerodynamic instability. At high speeds, rear downforce would compress the softer springs, lowering the rear and potentially reducing drag for better top speed. However, this could also make the car more susceptible to pitch changes and aerodynamic imbalances.
Adrian Newey Consultation and Downforce Enhancement Attempts
Between practice sessions and the race, facing mounting concerns, the Mercedes team reportedly consulted with renowned Formula 1 aerodynamicist Adrian Newey in a bid to find a solution. One outcome of these discussions was the addition of front nose dive planes to increase front downforce. Both CLR Mercedes cars started the race with these dive planes.
Cars of this era, in general, were relatively low on downforce compared to contemporary racing machines, especially in Le Mans trim. Data from the open-top Nissan R391 LMP900, a competitor, indicates downforce levels between 2000-2500 lbs at 200 mph.
Mercedes-Benz, in a press release following a warm-up crash, stated that the dive planes added up to 25% more front downforce, aiming to reassure everyone of the car’s race viability. Assuming a 45/55 front/rear downforce split and a total of 2000 lbs, a 25% increase on the front (900 lbs) would provide an additional 225 lbs, a plausible figure. Rebalancing the split to 45/55 would then increase the total downforce by around 500 lbs. However, the fundamental issue remained: the relatively low overall downforce of cars in that era.
The “Moment” of Takeoff
Bringing these elements together helps explain the sequence of events leading to the CLR Mercedes becoming airborne. In most instances, the CLR was following closely behind another car. This “dirty air” from a leading car significantly reduces downforce on the car following, particularly at the front end. Simultaneously, the CLR encountered changes in track elevation, such as cresting an undulation or running over a curb. These factors induced a change in the car’s pitch.
Reduced front downforce due to turbulent air, combined with a pitch change from track variations, acted upon the already aerodynamically sensitive CLR Mercedes. The long overhangs and short wheelbase amplified the effects of even slight pitch changes, leading to a more substantial than expected loss of front downforce. As the low pressure zone under the front of the car diminished, approaching zero, the inherent lift generated by the cockpit and upper bodywork began to assert itself, further lifting the nose.
Meanwhile, the rear wing continued to function effectively, maintaining downforce at the rear and creating a pivot point around the rear wheel centerline. As the nose lifted, the extended rear diffuser, projecting significantly beyond the rear wheels, moved closer to the track surface and began to generate more downforce, paradoxically exacerbating the lifting effect at the front. Ultimately, the underside of the car became exposed to the airflow, and the lift from the cockpit and the now exposed underfloor surface overwhelmed the remaining downforce, resulting in the dramatic airborne incidents.
Despite the alarming second incident during the warm-up session, Mercedes-Benz controversially chose not to withdraw from the race. Rumors persist of a possible earlier incident during testing, though these remain unconfirmed. In the years since, Mercedes has largely distanced itself from this chapter of its Le Mans history, and a return to the legendary race seems unlikely. The CLR Mercedes flips serve as a stark reminder of the complex interplay of aerodynamics, design choices, and unforeseen circumstances in motorsport, highlighting how even slight miscalculations can lead to dramatic and unexpected outcomes.
