I had the privilege of attending the full two-day Winter Driving School at Steamboat Springs, Colorado, featured in the December '91 Roundel. This school is held on a snow packed track on the side of a hill. The surface of the track is very representative of adverse road conditions which one might experience in winter driving. The front straight allows speeds of about 60 mph. Elsewhere on the course are an agonizingly slow series of tight turns and a 35 mph circular skid pad on the side of the hill. We students drove the Jeep vehicles owned by the school. These included rear and 4 wheel drive Cherokees, front and 4 wheel drive Summits and front and AWD Talons, including the 190 hp TSi which I drove at every opportunity. We were encouraged to push the vehicles to the limits of traction and not to worry about sliding into the soft snowbanks. With perfect weather (light snow) and 1-2 inches of powder on the course, we headed into a great weekend. I knew that the BMW 325iX which I drove in the winter was extremely competent and forgiving on the snow and that some of the techniques would not be applicable to iX driving, but I was excited to learn all I could about vehicle control with a variety of vehicles. And occasionally, I must drive an ordinary two wheel drive vehicle.
Saturday, we concentrated on familiar driving school themes such as being smooth, braking in a straight line, and taking the proper line through turns. (If you haven’t participated in you local chapter’s driving school, you really are missing an opportunity to have a really fun time and improve your driving skills.) We also covered three rules of winter driving which are counter to instinctive reaction:
- releasing (pumping) the brakes in an emergency,
- applying light throttle when the rear end starts to break loose in an oversteer situation, and
- slightly unwinding the steering wheel when the car refuses to turn in an understeer situation.
These and other correct reactions make sense, given a few basic principles which apply in all driving conditions, but which are particularly applicable on slippery surfaces:
- a sliding locked tire has no preference for direction and no ability to affect vehicle direction,
- an accelerating vehicle transfers weight to the rear tires, giving them more traction and reduces traction and effectiveness of the front tires, and
- lifting off the throttle or using the brakes transfers weight to the front tires making them more effective in steering the vehicle and reduces traction at the rear.
We practiced several maneuvers which demonstrated these principles. First was emergency braking in vehicles without ABS. We were taught to moderately apply and release the brakes about once a second. (Threshold braking as we teach in our chapter driving schools is not very effective or easy because the driver cannot hear the tires start to lock up on the snow as one can on dry pavement.) By releasing the brakes enough to allow the wheels to resume rotation, directional stability and steering can be maintained between lockups. This technique is the manual equivalent to the functioning of ABS.
We then practiced recovery from oversteer situations in which the rear end of the vehicle tries to come around due to lack of traction. You have experienced this situation and know to steer in the direction of the skid. If this situation was induced due to excessive throttle, the driver must lift off the throttle and expect the rear end to come around even more because of the transfer of weight to the front tires and off the rear tires. If this situation arises when the throttle is only lightly applied, the driver can probably save the situation by adding slightly more throttle in addition to steering into the skid. Both of these reactions are not instinctive and require practice.
In an understeer situation, the vehicle refuses to turn and plows to the outside of the turn. This is a typical characteristic of front and four wheel vehicles, including the iX. The instinctive reaction is to lift off the throttle and turn the steering wheel even more. Lifting one’s foot off the throttle is the correct reaction because this transfers weight to the front tires and makes them more effective in turning the vehicle. However, the other proper reaction is to unwind the steering wheel slightly to straighten the front wheels. This allows them to start rotating again and affect the vehicle direction rather than simply allowing the vehicle to plow straight ahead. (Installing a stiff rear sway bar reduces this trait.)
One exercise was particularly useful in demonstrating this technique. First we insured that we slowed in a straight line and then turned the steering wheel a little more than you might otherwise expect. By turning in a little extra at the start of the turn, the wheel can be unwound slightly through the turn. This avoided an understeer situation in which the instinctive reaction is to crank in more and more steering.
We also practiced handbrake turns in which the rear wheels are locked up to cause the rear end to slide around through a tight turn. This is particularly useful on front wheel drive vehicles at higher speeds when the driver stays on the throttle and causes the vehicle to rotate by slowing the rear wheels with the handbrake and causing them to slide. This technique is only effective in a limited way on four wheel drive vehicles because the use of the handbrake tends to slow all four wheels.
On Sunday, we learned and practiced left foot braking techniques in which the right foot applies throttle and the left foot applies the brake lightly to accomplish different things depending on the type of vehicle. One of the few drawbacks of ABS is that these techniques (which require locked wheels or wheels which are turning at different rates due to braking) are not very effective. On a front wheel drive, left foot braking can be used to induce oversteer in the same way that the handbrake is used. Entering a sweeping turn, the driver stays on the throttle and gradually applies the brake with the left foot. Vehicle speed is maintained and there is no abrupt transfer of weight to the front tires caused by lifting off the throttle. The rear wheels tend to slow their rotation and cause the rear end to slide sideways. The amour of resulting oversteer can be easily controlled by the amour of pressure on the brake and throttle. Once the driver is sure of making the turn, the brake can be released and the car will accelerate out of the turn.
On a rear wheel drive, left foot braking can be used for just the opposite effect: to induce understeer. (Unfortunately on late model BMWs this technique is not effective due to the ABS. On the rear drive Cherokee and on several Volvos I drove this year on the lake at Georgetown, however, it was incredibly useful.) Consider the following situation on a rear drive car. The rear end has broken loose and is sliding in an oversteer situation. To recover quickly, the driver stays on the throttle to keep the rear wheels turning but applies the brakes to slow or lock the front wheels. The rear wheels which continue to turn straighten the vehicle very quickly as the front wheels slide. I know this sounds strange,but you’ll be amazed at how effective this is when you practice this with a non-ABS rear drive vehicle.
On the four wheel drive Cherokee we used left foot braking to effectively and undramatically slow the vehicle in a turn and to tighten the radius of the turn. When the vehicle on the skid pad was just starting to slide outward due to lack of traction, a light application of the left foot to the brake while staying on the throttle slowed the vehicle without the abrupt and unsettling weight transfer which would have occured if the right foot were lifted from the throttle and used for braking. The front of the vehicle undramatically tucked in slightly to decrease understeer and restore balance and control.
I’ve had some success with these technique on the iX, and I am still learning. I encourage you to do as I do -- take every opportunity to experiment with your vehicle so that in an emergency situation you know how your vehicle will respond and how you should respond. As we quickly learn in our driving schools and racing experiences, the performance of our vehicles is more likely limited by the driver than the vehicle itself. Practice can be very rewarding and a lot of fun. Enjoy!