cut 'n' paste from Discount Tire:
Q. What can happen if I only use two winter tires?
A. Running only two winter tires can cause you to lose control of any vehicle. Here's why:
Front Wheel Drive Vehicles Even though the front wheels are responsible for steering, acceleration and most of the braking, don't forget about the braking done by the rear wheels. If the rear wheels are not equipped with winter tires too, you are essentially disabling the rear brakes due to lack of traction. The following is an example of what can happen using only winter tires on the front.
Traveling along in your neighborhood at 25 mph in just light snow, you begin to slow down to make a right turn. As you apply the brakes, your winter tires are doing their job, giving you all the traction you need to slow down. At the same time, the all-season tires on the rear are giving you much less traction causing the rear of your vehicle to slide around. Perhaps at slow speeds no harm is done, but what if this had occurred at highway speeds?
Rear Wheel Drive Vehicles Many people think that winter tires on the rear will solve the acceleration problem in ice and snow. However, getting your vehicle to accelerate is only half the battle, because you still need to stop. The majority of braking is done with the front brakes, and failing to put tires designed for cold weather on the front of the vehicle can have disastrous consequences. Also, keep in mind that steering is the sole function of the front tires. With insufficient traction on the front tires, it's like not being able to firmly grasp the steering wheel.
Now that you know this, how confident would you feel if your vehicle was equipped with only rear winter tires? What if you were driving on snow, ice, or even cold pavement at 45 mph and suddenly had to brake and swerve to avoid an accident? Chances are you would be unable to stop and turn in time.
