Complex sentence: The whaletail was originally designed for Porsche 930 and Porsche 935 race cars in 1973 to reduce rear-end lift and so keep the car from oversteering at high speeds, the rubber-edges of the whale tail spoilers were thought to be "pedestrian friendly".
Split into simpler sentences: The whaletail was originally designed for Porsche 930 and Porsche 935 race cars in 1973. Designed to reduce rear-end lift and so keep the car from oversteering at high speeds, the rubber-edges of the whale tail spoilers were thought to be "pedestrian friendly".