MORE CURVES
The challenge comes with multiple curves. Remember, high speed driving is active, not reactive. Here is another place where looking ahead is important. If you are looking straight in front of your car you can't make a single curve straighter. If you are looking through one curve, you may not be ready for the next curve. In other words, the perfect exit for curve 1 may not be the perfect entrance for curve 2. It is quite probable that at some point through the curves your exit from a curve will place you in such a poor position for the next curve that you won't make it through. This is why it is so important to look as far ahead a possible at all times and not fixate on any particular location. Constantly scan the road from as far as you can see to a few car lengths ahead of you (unless there is a car in front of you). You should be planning your line through everything you see and should be constantly revising that line based on the new road coming into view.
If you are looking into the distance, you'll notice that the road takes on a new shape. Many times it loses its curvature. There are several roads in the vicinity of my house which have three bends in a row, but I take them in a straight line without crossing a road marker line. However, remember that if you can't see the road ahead and aren't familiar with it, the apex you choose based on what you see could be too early, and an early apex means that you could run out of road on the exit. A late apex is a safe apex even though it isn't the fastest apex. Be conservative on unfamiliar roads. Still drive a line, but plan late apexes to be safe. Your late apex may turn out to be the end of the turn, but at least you're still on the road.
By the way, there is no singularly perfect apex for a given curve. The proper apex is car and weather dependent. When I say car dependent, I mean down to the last suspension, tire, and weight distribution set up. A 100lb driver driving TVR 2500M VIN 3225TM will probably want a different apex than a 200lb driver in the same car. For that matter, the addition of a passenger in 3225TM will make the difference as will the amount of gas in the tank. Next time I will discuss how to throw your weight around, and you'll see how weight distribution affects the turn. Before the discussion on throwing your weight around, I want to explain the dynamics of what you're trying to accomplish by throwing your weight around.
FORCE OVALS
I am going to use something I call a force oval to illustrate the effects of weight on traction. A force oval
is a graphical representation of the maximum frictional forces which can be exerted by a given tire contact patch.
For this discussion forget that the tire contact patch changes shape and size as the car flies down the road. For
now just consider a snapshot of the tire contact patch. The force oval will
change with differing tire contact patches, but the principles remain the
same. The illustration to the right is a force oval. The first thing to note is
that frictional forces occur in the direction opposite to motion of the tire
surface. (Remember that as your car goes forward the surface of the tire
touching the road goes towards the back of the car.) The for and aft
maximum frictional forces are not necessarily the same, particularly in the
case of tires with directional treads. The left and right maximum frictional
forces may also be different as is the case with Pirelli P77, if I understand the
principle behind them correctly. The tread on the outside of the car is
supposed to be softer for better cornering while the tread on the inside of the
car is supposed to be harder for better wear during highway driving. The
vertical axis will always be longer than the horizontal axis, since the tire is designed for more strength going
forward than sideways. The end of the straight up axis represents the maximum friction during the car's forward
acceleration. The end of the straight down axis represents the maximum friction during deceleration. The ends of
the straight left and straight right axes represent the maximum friction during perfectly lateral acceleration. (1) Any
point defining a combination of forward, braking, and lateral motion of the tire tread which is within the force oval
represents a condition where the tire is adhering to the road. Any point outside of the force oval represents a
condition where the tire has lost adhesion to the road (there are still frictional forces between the tire and the road,
but they are kinetic friction forces). Therefore, for the sake of survival and your bodywork, you want to keep your
combinations of forward, braking, and lateral motions within the force oval. This still leaves a lot of combinations.
Remember how the end of the straight up, down, left and right axes represents maximum frictional forces in pure straight line or pure lateral motion. Well, the perimeter of the oval represents the maximum frictional forces of combinations of longitudinal and lateral motion. Therefore, to get the most out of your tires, you want to use combinations of forward motion, braking and turning which keep the forces on your tire on the edge of the force oval. Any combination of lateral and longitudinal motions which puts you inside the perimeter is a waste of the friction you have available.
Now that I've thrown all this theory at you I will give you an
example of what it means in the real world so that you'll know how simple
what I just explained really is. Say that you are going straight and using the
maximum friction available to you. This puts you at point A on the force
oval. If you make a sudden 30 degree turn to the right without changing your
speed you will put yourself at point B which is outside of the force oval and
beyond the limits of the tire. What this shows is that you have to back
down on the use of forward friction in order to use lateral friction. For the 30
degree turn you want to get to point C. The simple way to accomplish this
is to jam on the brakes, slow down to point D, and then turn your 30 degrees
(Path S). Or you can slow down smoothly while concurrently turning
smoothly so that your adhesion capability follows path T. Path S takes you
inside of the force oval which means that you aren't taking advantage of all
of the friction available. Path T lets the car use the maximum available friction at all times. Now you know why
SMOOTH analog methods are better than jumpy digital methods.
Next time I will show you what you're trying to accomplish by throwing your weight around using force ovals. Until next time happy flying.
1 Physics Lesson: Velocity is a vector representing speed and direction. Acceleration is a vector representing a change in velocity (either or both speed and direction). Force is required to change the velocity of an object, or in other terms, accelerate an object (Newton's Second Law: F=ma). Lateral acceleration occurs when an object travels a uniform circle at a constant speed. Since the object is at constant speed it is not going faster or slower, hence acceleration and deceleration are not occurring in the straight forward direction. However, the object is changing direction and, therefore, velocity. The acceleration and, therefore, force are in the lateral direction only (to change the direction) and not in the longitudinal direction (no change in speed). This condition is perfectly lateral acceleration.
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