Think of it this way. Imagine you are in a gigantic parking lot, driving in a circle. Now imagine that you lose all traction in the front. Since your steering wheels no longer have any grip, they cannot turn the front of the car. Thus, the car will travel in a straight line, and you will experience oversteer. If, instead of your front wheels losing traction, your rear wheels lose traction, then the front of your car will continue to turn and follow the circle, but your rear will continue in a straight line. This is Oversteer, and will put you into a spin if not corrected since the rear of the car does not turn with the front of the car.

It is important to note why Oversteer occurs, as this will help you understand why Understeer does not become Oversteer. Oversteer requires traction in the steering tires to introduce rotation to the car. With no rotation, there can be no Oversteer. Try locking up all 4 wheels sometime and you will see that the car just travels in a straight line.

Weight distribution comes into play in inducing Over or Understeer, and in how easy it is to correct. I’m going to write an article at some point about the finer points of this, but can give you a quick run down now. Basically, the tires at the heavier end have to work harder. A front-heavy car works the front tires harder, and reaches and exceeds their traction limit first. Thus, front-heavy cars tend to understeer more (all other factors being 100% equal, which they rarely are).

More important is what happens when you begin to Oversteer. With a rear-heavy car, there is now a lot of weight swinging out to the side. More weight has more inertia, and requires more grip to correct. Thus, rear-heavy cars can be more difficult to control in Oversteer.

And while most cars are either front or mid-engined, Porsche 911s are all rear-engined. So rear-heavy cars are a bit more common than you would think.