Weight is the enemy of motorsports. While the cars in nearly every series have weight minimums that they must meet, every bit that a team can get under that amount allows the team to distribute weight wherever it would like. AudiAudiGermany, 1909 > present83 models
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says that since 1999, it has reduced the weight of the monocoques used in its cars at Le Mans has been reduced by half. 

Audi has developed the R18 E-tron HybridAudi R18 e-tron quattroGermany, 2012 > present8 photos
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monocoque to support the front axle, body parts, hybrid system. The engine and transmission are bolted to the rear as stressed members of the chassis. It responsible for resisting the torsional and bending forces that the car faces while racing.

A major innovation has been new, lighter forms of carbon fiber. Carbon fiber has been used in racecars since the 80s, so it was natural for the 1999 R8RAudi R8RGermany, 1999 > present2 photos
would use the material. The major innovation for Audi's use of carbon fiber came in 2011 for the R18 TDIAudi R18Germany, 2011 > present7 photos
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. It was the first monocoque coupe to be made from a single form of carbon fiber instead of several elements bonded together. Anytime a team can reduce body pieces, it also reduces weight and increases rigidity. 

The current monocoque weighs half as much as the 1999 R8R and meets more stringent safety standards. Audi estimates that using carbon fiber reduces the weight by a quarter compared to using steel.

The engine has been specifically designed to be a stressed member of the chassis. The crankcase of the engine is formed into a ladder frame to further resist forces on the car. 

Audi started making the transmission case for the R18Audi R18 ultraGermany, 2012 > present11 photos
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out of carbon fiber in 2012. 

Smaller changes to lighten the car have included the introduction of a lithium-ion battery on the 2009 R15 TDIAudi R15 TDIGermany, 2009 > present4 photos
to save 7kg (15.4lb) and a carbon fiber gas pedal on the R10 TDIAudi R10 TDIGermany, 2006 > present4 photos
to save a few hundred grams.

“Audi’s LMP sports cars have continually become lighter, stiffer, safer in crashes and more efficient. There is hardly another motorsport discipline in which the creativity of the engineers is rewarded as highly as it is with the Le Mans prototypes," said Dr. Wolfgang Ullrich, head of Audi Motorsport.