Ultimate Speed Secrets

by Ross Bentley

  If you brake too hard and lock up the front wheels, you will lose all steering control. If this happens, you will have to ease your foot off the brake pedal slightly to regain control, back to threshold braking. If you do this, you will most likely “flat-spot” the tires. This happens when the tires have skidded along the roadway and worn a patch of tire to the point where the tire is no longer perfectly round. You’ll know exactly when you’ve done this. You’ll feel a thumping or vibration in the car as the flat spot rotates.

  SLIP ANGLE

  Let’s take a closer look at slip angles. If you notice in the “Slip Angle vs. Traction” graph in Illustration 5-2, the peak traction limit, or lateral acceleration, is when the tires are in the 6 to 10 degrees of slip-angle range. Let’s look at four hypothetical drivers to see where on the graph it’s best to drive.

  ILLUSTRATION 5-2 The slip angle versus traction graph shows that a tire gains traction as it “slips,” up to a certain point, at which it begins to lose traction.

  ILLUSTRATION 5-3 Percent slip versus traction graph.

  Our first driver is probably inexperienced and definitely a little conservative. He consistently drives through the corners with the tires in the 2- to 5-degree slip-angle range. As you can see from the graph, the tires are not at their maximum traction limit. Driver 1 is not driving at the limit, and therefore will be slow.

  Driver 2 has a bit more experience and is known to be a little on the wild side. He consistently overdrives the car. But what does that mean? Well, he always drives through the corners with a slip angle above 10 degrees. In other words, he is sliding the car too much. It may look great, with the car in a big slide all the way through the corner, but the graph shows that in this range, the traction limit of the tires has begun to decrease from maximum. Plus, all this sliding about will increase the temperature of the tires to the point where they are overheated, further reducing the traction capabilities of the tires.

  Our final two drivers are consistently cornering in the 6- to 10-degree slip angle range. Both are very fast. Both are cornering at about the same speed. Both are driving the car with the tires at the limit. So, what’s the difference? Driver 3 is cornering in the upper end of the 6- to 10-degree range—about 9 or 10—while Driver 4 is around 6 or 7 degrees. Again, the cornering speed is the same, but Driver 3 is sliding a little more than Driver 4, causing more heat buildup in the tires.

  Both drivers will run at the front of the pack early in the race, but eventually Driver 3’s tires will overheat and he will fade. He’s the one complaining at the end of the race about his “tires going off.” Meanwhile, our winner—Driver 4—has gone on, consistently driving with the tires in the 6- or 7-degree slip-angle range and is praising the tire manufacturer for making a “great tire” and his crew for a “great handling car.”

  The goal, as this example demonstrates, is to consistently drive at the lowest possible slip angle that maintains maximum traction.

  Understand that the difference in speed between cornering with a slip angle of 2 degrees and 12 degrees may be 1 or 2 miles per hour, or even less. So you can imagine how much skill and practice it takes to be able to control the car well enough to stay between 6 and 7 degrees of slip angle!

  Now, I’m going to contradict myself. Sometimes you have to drive in the upper end of the ideal slip-angle range. If the tires are too hard a compound for your car (perhaps they were designed for another type of car), or the track temperature is low, you may have a difficult time getting the tires to their optimum temperature range. In this case, you may want to slide the car a little more, drive in the upper end of the optimum slip-angle range to generate more heat in the tires to achieve maximum traction. Consistent winners have learned to “feel” this and interpret their tire temperature readings, then adapt their driving style to suit.

  SPEED SECRET

  Drive at the lowest possible slip angle that maintains maximum traction.

  TIRE CONTACT PATCH

  I want you to really understand this, as this is the basis for much of what we’ll be talking about for a while and what will allow you to drive at the limit. There are only four small tire contact patches (the actual patch of tire or footprint that is in contact with the road at any one particular time) that are actually holding you and your car on the road. The larger the contact patch, the more grip or traction that tire has. Increasing the tires’ width obviously puts more tire footprint on the road. The result is more traction. Unfortunately, tire size on race cars is usually limited by the rules.

  ILLUSTRATION 5-4 The tire contact patch, or “footprint,” is the part of the tire that makes contact with the track surface as it rotates.

  VERTICAL LOAD

  A factor not limited by the rulebook, but one that has a great effect on the tire contact patch and the traction it offers, is vertical load, or pressure applied downward on the tire. By increasing this load on a tire, you increase the pressure applied on the contact patch. Thus (up to a certain point where the tire becomes overloaded), you increase the traction limit of the tire.

  ILLUSTRATION 5-5 Vertical load versus traction graph.

  Now, before you get any ideas of adding a 2,000-pound lead weight to your car, believing that all that extra load will put more pressure on the tires and give them more traction, think about this. Yes, the extra load increases the traction capabilities of the tire, but the work required by the tires to grip the road while carrying that extra load also increases. In fact, it increases even faster. It’s not a linear relationship, as noted in Illustration 5-5.

  So traction increases with an increase in vertical load, but the work required of the tire increases faster. The result is an overall decrease in lateral acceleration, and therefore, cornering capabilities.

  However, there is a way of getting something for nothing here. Aerodynamics. Aerodynamic downforce increases the vertical load on the tires without increasing the work required of the tires. That is why an increase in aerodynamic downforce will always improve the cornering capabilities of a car.

  WEIGHT TRANSFER

  One of the keys to driving a car at the limit is controlling the balance of the car. In this case, “balance” describes when the car’s weight is equally distributed over all four tires (see Illustration 5-6). When the car is balanced, you are maximizing the tires’ traction. The more traction the car has, the more in control the car is and the faster you can drive around the track.

  ILLUSTRATION 5-6 The car, when balanced, has equal traction capacity on each tire.

  I’m sure you already know that as a car accelerates, the rear end tends to squat down. That’s because a percentage of the car’s weight has now transferred to the rear (see Illustration 5-7). When braking, the car nose-dives. The weight has transferred forward (see Illustration 5-8). In a corner, the weight transfers laterally to the outside, causing the car to lean, or “body roll” (see Illustration 5-9). The total weight of the car has not changed, just the distribution of it has changed.

  So, as a car accelerates and weight is transferred to the rear (the back-end squatting down) the pressure, or load, on the rear tires’ contact patch increases, resulting in the rear tire traction increasing. During braking, the exact opposite happens. The car nose-dives (weight is transferred to the front) and front tire traction is increased. While going around a corner, weight transfers to the outside tires, increasing their traction.

  ILLUSTRATION 5-7 Under acceleration, weight transfers to the rear, increasing rear tire traction.

  ILLUSTRATION 5-8 Under braking, weight transfers to the front, increasing front tire traction.

  However—and this is very important to understand—when the weight transfers onto a pair of tires, increasing their traction, weight is being taken off the other two, decreasing traction. Unfortunately, the overall effect to the car is a decrease in total vehicle traction.

  You can, and must, control this weight transfer to your advantage. Again, as the weight transfers onto a pair of wheels, pushing the
m into maximum contact with the road, we achieve better traction with those tires. Conversely, the tires that become unweighted lose traction.

  TRACTION UNIT NUMBER

  Let me explain it this way. If you were to quantify the amount of traction each tire has, and give it a corresponding number, that would be what I call the tire’s “traction unit number.”

  Let’s take a look at an example see illustration 5-10. With a car sitting at rest, or traveling at a constant speed, each tire has, let’s say, 10 units of traction for a total of 40 traction units gripping the car to the road. Now, when you corner, weight is transferred to the outside tires, increasing the vertical load on them, and therefore their traction, giving them 15 units of traction. But at the same time, weight is transferred away from the inside tires, reducing their vertical load and traction, resulting in only three units of traction each. The total traction for the car is now 15 + 15 + 3 + 3 = 36, which is less than you had before you caused the weight transfer by turning.

  As we have already seen in Illustration 5-5, vertical load versus traction is not a linear relationship. As load is increased on a tire, traction increases but not at the same rate as the weight increase. As load is decreased from the opposite tire, traction is reduced at a faster rate. The more the weight transfers, the less the total vehicle traction will be.

  ILLUSTRATION 5-9 Weight transfers laterally, to the outside of the turn when cornering, increasing the traction of the outside tires and decreasing the inside tires’ traction.

  BALANCE

  Obviously, it is impossible to drive a car without causing some weight transfer. Every single time you brake, corner, or accelerate, weight transfer takes place. However, the less weight transfer that occurs, the more overall traction the car has.

  ILLUSTRATION 5-10 The Traction Unit Number example demonstrates that as weight transfer occurs, the car’s overall traction limit is reduced. In other words, the better balanced you keep the car, the more traction it will have, and the faster you can drive through the turns.

  So your goal then is to drive in such a way as to keep the weight of the car as equally distributed over all four tires as possible. In other words, balance the car. How? By driving smoothly. Turn the steering wheel as slowly and as little as possible. If you jerk the steering wheel into a turn, the car leans, or transfers weight a lot. If you gently turn into a corner, the car does not lean as much. Squeeze on and ease off the brakes and gas pedal. Never make a sudden or jerky movement with the controls.

  Now you see why it is important to drive smoothly and how it can affect the balance and overall traction of your car. Again, the greater the weight transfer, the less traction the tires have. You play the major role in controlling weight transfer and maximizing traction.

  Weight transfer and balance also has an effect on your car’s handling characteristics, contributing to either “understeer,” “oversteer,” or “neutral steer.”

  UNDERSTEER

  Understeer is the term used to describe the handling characteristic when the front tires have less traction than the rears, and regardless of your steering corrections, the car continues “plowing” or “pushing” straight ahead to the outside of the turn. Think of it as the car not steering as much as you want, so it is “understeering.” Understeer, in effect, increases the radius of a turn.

  Accelerating too hard or not smoothly enough through a corner transfers excessive weight to the rear, decreasing traction at the front and causing understeer.

  Most drivers’ first reaction to understeer is to turn the steering wheel even more. Don’t! This increases the problem because the tires were never designed to attack the road at an extreme angle. The tires were meant to face the road with their full profile, not with the sidewall. So the tires’ traction limit has now been further decreased.

  To control understeer, decrease the steering input slightly and ease off the throttle gently to transfer weight back to the front. This increases the traction limit of the front tires and reduces speed. Once you have regained front tire traction and controlled the understeer, you can begin squeezing back on the throttle. Obviously, this easing off and getting back on the throttle will destroy your speed on the following straightaway and upset the balance of the car. So make sure you accelerate smoothly the first time.

  ILLUSTRATION 5-11 An understeering car does not steer, or turn, as much as you want along the intended path. This is also called “pushing” or “tight.”

  ILLUSTRATION 5-12 An oversteering car steers, or turns, more than you want along the intended path. This is also called “loose.”

  OVERSTEER

  Oversteer is the handling characteristic in which the rear tires have less traction than the fronts, the back end begins to slide, and the nose of the car is pointed at the inside of the turn. The car has turned more than you wanted it to, so it has “oversteered.” This is also called “being loose,” “fishtailing,” or “hanging the tail out.” Its effect is to decrease the radius of a turn.

  Turning into a corner with the brakes applied, or lifting off the throttle in a corner (“trailing throttle oversteer”) causes the weight to transfer forward, making the rear end lighter, thus reducing rear wheel traction. The result: oversteer.

  Also, if you accelerate too hard in a rear-wheel-drive car, it will produce “power oversteer.” What you have done is used up all of the rear tires’ traction for acceleration and not left any for cornering. To control excessive power oversteer, simply ease off the throttle slightly.

  To control excessive oversteer, just look and steer where you want to go. This forces you to turn into the slide, or to “opposite lock,” thereby increasing the radius of the turn. At the same time, gently and smoothly ease on slightly more throttle to transfer weight to the rear, and thus, increase traction. Whatever you do, avoid any rapid deceleration. This will most likely produce a spin as you decrease the rear-wheel traction even more.

  NEUTRAL STEER

  Neutral steer is the term used to describe when both the front and rear tires lose traction at the same speed or cornering limit and all four tires are at the same slip angle. Sometimes described as “being in a four-wheel-drift,” this is ideally what a driver is striving for when adjusting the handling of the car and trying to balance it.

  I love the feeling when I’m controlling the balance of the car with the throttle, driving through a fast, sweeping turn at the limit. If the car begins to oversteer a little, I squeeze on more throttle to transfer a little weight to the rear; if it starts to understeer, I ease off slightly, giving the front a little more grip. When it’s done just right, all four tires are slipping the same amount (the car perfectly balanced, neither oversteering nor understeering) in a perfect neutral steer attitude through the turn.

  In terms of how the car is set up, however, most drivers prefer a little understeer in fast corners, as it’s a more predictable, safer characteristic, and oversteer in slow corners to assist in pivoting the car around the tight turn.

  TAKING A SET

  “Taking a set in a turn” describes when the car has finished all of its weight transfer. It is the point in a turn where all the weight transfer that you are going to cause has occurred. The car is most stable when it has taken a set and can be more easily driven to its limit then.

  How quickly the car takes a set in the turn is largely a matter of how the shock absorbers are adjusted and how you drive. As you turn into a corner, the quicker the weight transfers, the quicker the car takes a set. The sooner the car takes a set, the sooner you can drive the car at its limit, and the faster you will be.

  Why? Remember the Traction Unit Number example. As weight transfers, the tire traction available is reduced. Once all the weight transfer that is going to occur has occurred, and the car has taken a set, you can then work with the traction available and drive at the limit. If you don’t make the weight transfer happen quickly enough, you spend most of the corner waiting for the car to take a set. Therefore, you wait a long time before
you really know what traction limit you’re working with. If you don’t drive smoothly—causing a little weight transfer, then a lot, then less, then more again, all through the same corner—the car will never take a set. It’s difficult to drive at the limit when that limit is constantly changing.

  Before you get any ideas about making the weight transfer occur too quickly, however, think about the Traction Unit Number example again. If you quickly transfer weight by jerking the steering into a corner, the effect will be more overall weight transferred and therefore less overall traction.

  ILLUSTRATION 5-13 A car whose handling is neutral has equal slip angles front and rear; an understeering car has larger front slip angles that the rear; and an oversteering car has larger rear slip angles than the front.

  So your goal is to make the car take a set in the turn (get to its maximum weight transfer and stay there) as quickly as possible without causing any more weight transfer than necessary. That means use smooth, precise, and deliberate actions with the controls.