Most of these terms are heard frequently around the track — some in our novice meetings held before each Driver Education event — while others are used by instructors in the process of teaching.
Every corner has three parts to it; the entry, apex and exit. The apex represents the transition point between the entrance and exit of a corner. Physically, it is the point during the turn where your inside front wheel is closest to the inside edge of the corner. You can early apex, late apex, or correctly apex a turn. The correct apex to a turn is where the car is in its most stable condition. It is at this point where you will be able to get the maximum acceleration out of a corner.
The line is a physical line of movement that a car takes through any given turn. A line can be good, bad, right or wrong. The right line around a track is generally the one which gets you around the safest and fastest. In driver’s education however, it is not always the fastest line which is taught. Because of the emphasis on safety, in many instances the line taught involves late apexing corners which provides greater safety and allows a greater margin for error.
Slip angle is the angle between the wheel rim and the tire tread or more generally, the angle between where the car is headed vs. where the wheels are pointed. A slip angle occurs during cornering when tires are subjected to side forces. The greater the slip angle, the more there is a tendency for the tires to slide.
In general, used to describe the situation where the rear end of a car gets loose (in other words the rear tires lose grip) and the rear end wants to come around. More specifically, it occurs when the rear tires have a greater slip angle than the front tires. The result of oversteer is that the nose of the car is pointed toward the inside of a turn which can have the result of decreasing the radius of the turn. For this reason some (advanced) drivers induce oversteer.
Understeer occurs when you turn the car into a corner and it still has a tendency to go straight. Also referred to as pushing or plowing. In this situation the front tires have a greater slip angle than the rear which means that the rear tires have better grip or traction than the front. Understeering will increase the radius of a turn. Most street cars are set up with a bias toward understeering.
Heel and Toe
With modern pedals, this is often misleading to new students. Also called heel and toe downshifting or double clutching, this technique is used when approaching a corner, because it allows the driver to brake and downshift at the same time which is the smoothest way to accomplish the brake-and-downshift process.
The technique involves using the ball of your right foot which is completing the braking process while rolling over the side of your foot (or in a few cases, using your heel) to blip the throttle (which raises the engine RPMs) to match the speed of the wheels in the lower gear. While your right foot is doing the above, your left foot (at an appropriate point in the braking zone) is pushing in the clutch, your right hand has taken the car out of gear, paused in neutral while your blipping the throttle and when the engine RPMs have reached the appropriate level you move into the lower gear and release the clutch.
Some drivers let the clutch out after moving into neutral and then push it in again to move into the lower gear, hence the term “double-clutch.” There is no reference here to “appropriate level”; while each gear certainly does have its own appropriate RPM range, depending on which lower gear you’re changing to, most drivers are not watching their tachometers at this time but are relying on sound and feel.
That is why this technique is employed by more advanced drivers; it takes a certain amount of seat time in order to develop the sense of when to release the clutch just by the sound of the engine and feel of the car. This is a technique that is better practiced in a large parking lot or quiet back road until it feels natural before attempting it on the track.
How many times have you heard an instructor talk about being smooth? Frequently! Smoothness is attained by keeping the car well-balanced at all times. What does that mean? Optimally, it means having the car’s weight equally distributed on all four wheels. If the car’s weight is evenly distributed on all four wheels, it means you will be able to maximize the traction (or adhesion) of all four tires. And, the more rubber you have in contact with the road, the better you are able to control the car, not to mention the faster you will be able to go. One of the first upgrades new DE students make is to exchange their street tires for softer compound tires. These tires have less tread, softer compound rubber and, therefore, more tire is in contact with the road surface. This enables greater adhesion and hence more safety at speed.
Unless you are traveling in a straight line, and very rarely even then, is the car’s weight evenly distributed on all four tires. The car’s weight is constantly being shifted from side to side, frontwards and backwards and even up and down. Your goal as the driver is to manage these weight shifts or transfers which occur while braking, shifting, cornering and accelerating in order to take advantage of every bit of traction that is available. By the way, there is a limit as to the amount of traction your car has available. This is commonly defined through the conceptual use of the Friction Circle.
For all you engineers out there — trivia questions:
- In what year was the concept of the Friction Circle publicly introduced?
- Who introduced it?
Think of it in these terms: a tire cannot exceed 100% of any one function or combination of functions at any given time. If, for example, you use all the traction available (100%) to brake in a straight line, you will not have any percentage left over for turning. If you need to turn while braking, you have to reduce the percentage used for braking in order to have some left over for turning. 100% adhesion is not available in two directions at once!
Seeing an example of tire contact patches is a great way to show the effects of weight transfer on your car. In reality, a tire’s contact patch is about the size of a standard postcard; this is the amount of rubber in contact with the road surface. These tire contact patches expand (when weight is over that wheel) or contract (when weight has been shifted away from that wheel).
When accelerating (in a straight line) weight transfers to the rear wheels, expanding the rear wheels tire contact patch but decreasing the front wheels tire contact patch. The reverse happens under braking. Add a corner and lateral weight transfer takes place. Turn to the right, and the weight shifts to the left; turn to the left, and the weight shifts to the right.
Assuming you have just finished braking or lifting, during the entrance to a right hand turn, the largest tire contact patch is at the left front tire with the left rear tire patch being almost as large.
Imagine a string with one end attached to the bottom center of your steering wheel when it is in a neutral (straight) position and the other secured to your accelerator in the depressed position. When you turn your wheel, it forces the accelerator to ease up. As the wheel straightens out, you are able to fully depress the accelerator. There is a balance between accelerating and turning. that adds up to the 100% discussed above under weight transfer.
Similarly, imagine the string attached to the brake pedal instead. The same principle applies. In short, you cannot turn the wheel without giving up some acceleration or braking power.
This is the practice of braking at the limit of tire adhesion while still keeping control of the car and not locking up the wheels. Start by very gradually braking later and later as you approach a corner. This allows you to learn what your particular tires’ adhesion limits are without adding the effects of lateral weight transfer. As you gain more experience you will want to try trail-braking.
This is the practice of braking and cornering at the same time. As a beginning student, however, you are taught to brake in a straight line. Trail-braking allows you to maintain your straightaway speed longer before having to brake for a corner because you can corner and brake simultaneously. You still begin your major braking on the approach to a corner which means that you have transferred weight to the two front tires and enlarging your front contact patches.
To maintain that extra adhesion in order to give yourself added steering capability, you continue to brake, but not as hard as you did at the beginning. This frees up some of that 100% of your tire, allowing you to apply the remainder of the 100% to your steering. Just as you apex, you can “unwind” the steering wheel for the exit, thus straightening out your tires and lessening the need to steer. You have also freed up some of that 100% that you can now apply to the throttle. The straighter the wheel, the more throttle you may apply which transfers more weight to the rear wheels. It is this greater adhesion in the rear that is needed for accelerating out of the corner.
Trailing Throttle Oversteer
We have explained oversteer as a situation where the rear wheels lose traction in a corner. The rear wheels can lose traction in a corner if you lift off the accelerator, transferring weight to the front tires and off of the rear ones. Therefore, the rear of the car becomes light and because of the cornering forces the rear end swings to the outside of the corner.
Experienced drivers in certain situations will actually induce oversteer, maybe not by lifting entirely off the accelerator but by trailing or by slightly coming off the gas enough to swing the rear of the car around the corner.
Depending on the type of corner in which this technique is used, it may be the fastest way through the corner, as long as you remember smoothness. The weight transfer that is constantly taking place during the transition between braking and accelerating increases the need for smoothness. Abrupt changes in weight transfer will upset the balance of the car and you may end up without traction when you need it the most! That is why smoothness is emphasized so strongly at Driver Education Events.