Why does my car do that?
How can I make it do what I want it to?
This is just a problem Porsches have, right?
Before pilot candidates begin to fly, they spend time learning a lot of the basic principles and properties of flight and airplanes in a classroom. In this column, I hope to cover topics that affect how vehicles respond in the same type of approach, since an understanding of some basic principles may translate to improving mastery of your car on the road and track.
The three questions above were frequent queries I heard during a recent car control clinic. In this first column, I would like to address some basics of vehicle dynamics, weight shifts and balance, and hopefully answer these questions. In future columns, I intend to explore oversteer/understeer/drifting, cornering techniques and how to adjust your line on the track. If you have questions or suggestions you'd like addressed, feel free to email me and I will try to answer them as well. For today, we need to cover some basic science to begin.
As with everything in the physical world, our cars are governed by the laws of physics. Newton's First Law (Inertia) states that objects in motion remain in motion and objects at rest remain at rest unless acted upon by an outside force (Miss E ? that explains why I can never get around to cleaning out the garage ? its against the Law!). Whether you drive a 997 GT3 RS, 356 or a Tucson City Bus, you directly control the outside forces of the throttle, the brakes, and the steering. Gravity, wind resistance and the friction between your tires and the road surface are also acting upon your vehicle. Obviously spoilers, body shape and damage, etc can affect wind resistance, but we'll assume it's largely beyond our control once we're behind the wheel. Gravity is also beyond our control (except for Walt Harrington ? I'm sure he had a gravity control device in his old 911!). It's the last item, the traction of our tires, which we can modulate to our advantage while driving.
Traction is a function of the road surface, the tire rubber compound and surface area, and the pressure between the tires and road. While we can't control the grip of the road surface, there are conditions while negatively effect it. Ice, snow, water, sand, gravel, debris, and cracks all deteriorate the quality of the road. For high performance driving, this means avoid the debris, sand, gravel, and "marbles" (chips of rubber from tires) to maximize the grip of the surface. We can address water and "the rain line" at a later time. As for our tires, a "slick" or similar tire with minimal voids between tread blocks has more rubber in contact with the track, which translates to more traction, all other things being equal. If you were driving in snow, sand or mud, increasing the tread void depth increases the surface area contact as these soft surfaces push into these voids, hence mud & snow tires for your SUV. The actual rubber compound itself is a crucial part of the tire's grip ? imagine an old dried out tire which is hard and smooth versus the soft gummy surface of a track once it has heated up. This is why racecars use slicks or "R compound" tires on the track, as they are a much softer, stickier rubber. In general, the lower the treadwear rating, the faster a tire will wear down, but the stickier it will be on the track, and it is not unusual for race tires to wear out and be replace several times during a race! Tire width is somewhat dictated by your car's design, but clearly as wide a tire as you can fit without rubbing will increase the amount of rubber you have on the road. Obviously, tire selection and its influence is dictated by car design & determined before you get behind the wheel, but after the road surface and the tire factors are managed, we're left with the last factor, the pressure between the tire and the road. This is where a good driver usesvehicle dynamics to his or her advantage.
Remember Newton and his First Law? The car's inertia will resist the forces you apply with brakes, throttle and steering. You experience this as a perceptible leaning of your car opposite the direction you are encouraging it to go. Push the throttle and the vehicle weight shifts to the rear, increasing the weight on the rear wheels. For a rear wheel drive vehicle, this is useful and reduces wheel spin as you accelerate. This is also why many of our cars have wider tires on the rear wheels to increase grip for acceleration. For a front wheel drive, however, this weight shift moves the weight off the drive wheels and may induce wheel spin or hop if done abruptly. In a similar way, when you apply the brakes, the weight of the car shifts forward (and you do too, pushing you against your seat belt) applying more weight to the front wheels. This is why most vehicles have larger brakes in the front than the rear, as the front wheels apply most of the stopping force. When steering, the vehicle's weight shifts towards the outside wheels of the turn, improving their grip, as it shifts off the inside wheels, reducing their traction.
In a perfect world, the reduced grip experienced by some wheels during these maneuvers would be exactly offset by the improvement experienced by the other. Alas, this is not the case, with the most traction obtained when the car is at constant speed in a straight line. Any change will upset this balance and degrade the total traction present. Your car's suspension is designed absorb bumps in the road while keeping all 4 tires in contact with the road surface, and yet keep the car as level as possible. The driver's job is to control the car in such a way as to optimize the functioning of the suspension. The more lean experienced, the bigger the loss of traction provided by the sum of all 4 tires, which is why high profile trucks won't corner as well as a go-cart. As you operate the controls, realize that the balance of the car will change with control inputs, and allow the weight shifts to occur, and the suspension to reach steady state. The smoother you apply steering/throttle/brakes, the less it will upset the suspension overall, and the better traction you will have to control the car. It also means that if the weight is shifted to the rear tires while you accelerate, there is less weight on the front tires for steering. This is why your car will drift toward the outside of the turn as you accelerate out of a corner, a phenomenon called "throttle steering". Let off gently on the throttle, more weight will transfer to the front tires, and the steering angle will increase. We'll explore this more when we examine oversteer and understeer.
One last facet of traction to consider before we move on is that there is a certain maximum traction available, and you can use that for accelerating, steering, or slowing, or a combination of steering with either of the other two, up to that maximum. The more you use for steering, the less is available for speed changes and vice versa. If you exceed that maximum, your tires will break loose and slide. This is why you'll hear Wendy Walker warning you against "braking and turning". While advanced drivers do this as "trail braking" you risk loss of traction and control if not done properly. It is sometimes suggested to imagine a sting from the bottom of the steering wheel to your right foot, so as you turn the wheel more it will lift your foot off the brake or throttle. Applying more "pedal" will similarly pull on the string and tend to return the wheel to a neutral position. I like this image because it reinforces that what your hands and feet do is interconnected and each affects the other.
Now that your head is spinning, go clear it with a spirited drive and start preparing for your next track session!
Keep the shiny side up and the greasy side down.
Greg
Joel demonstrates extreme weight transfer for his instructor.
By the end of the Car Control Clinic, the car was much more balanced,
Joel was a lot smoother and quicker as well.
Photo Credit: Rink Reinking