Ultimate Speed Secrets

by Ross Bentley

  DYNAMIC BALANCE

  Getting back to balancing the car, there is also what I call “dynamic balancing.” Few cars have a perfect 50/50 weight distribution to begin with. Most purpose-built race cars are midengine with a weight distribution around 40 percent front and 60 percent rear, as this is close to ideal for a race car. Production-based front-wheel-drive cars are usually closer to 65 percent front and 35 percent rear. Only production-looking tube-frame race cars (Grand-Am GT, NASCAR, etc.) are close to 50/50 weight distribution.

  Realizing this, a driver must compensate by controlling the weight transfer to balance the car into a neutral handling state (no understeer or oversteer). To do this, the driver may have to effect the weight transfer so that statically there would be more weight on the front or rear, but dynamically the car is perfectly balanced.

  Look at it this way: Let’s assume your race car’s static or at-rest weight distribution is 40 percent front, 60 percent rear, and it is set up to oversteer at the limit (either on purpose or because you haven’t been able to find the right setup). While driving through a 100-mile-per-hour corner, you know you could go quicker if the car oversteered less—if it was neutral handling. To make the car oversteer less, you will have to cause some weight to transfer rearward by squeezing on the throttle. This would change the weight distribution to approximately 35 percent front and 65 percent rear. At speed through a corner, dynamically, this is balanced.

  BRAKE BIAS

  Keeping this weight transfer in mind, an important factor in braking is how the brake bias is set or adjusted. Braking forces are not equally shared by all four wheels. Due to the forward weight transfer under braking, and therefore more front tire traction, most of the braking is handled by the front brakes. So the brake forces will be biased toward the front. This is why all vehicles have larger brakes on the front wheels than on the rear.

  Actually, you want to adjust the brake bias so that the front wheels will lock up just slightly before the rears. This is a more stable condition, as it gives you more warning of a skid. You will feel it in the steering immediately if the front tires begin to skid. Plus, if the rear tires lock up first, the car will tend to skid sideways.

  Different conditions will require a different ratio, or bias, of front-versus-rear braking forces. In the rain, because there is less forward weight transfer to the front (because traction limits are lower, heavy braking is not possible without locking up), you will have to adjust the brake bias more to the rear. Some cars also change dramatically as the fuel load lessens during a race. This is where a driver-actuated brake-bias adjuster is beneficial.

  Practically all purpose-built race cars have an adjusting mechanism for changing the bias. Learn how to “read” and then adjust your race car’s brake bias. With production-based race cars, you will pretty much have to live with the bias that the factory built into the car.

  AERODYNAMICS

  Aerodynamics really only come into full effect at relatively high speeds. Only very sensitive, experienced drivers will feel the effects of aerodynamics at anything under 60 miles per hour. Beyond that, aerodynamics play a big role in the handling of a race car, and therefore, you must learn as much as possible about how to adjust and feel the effects.

  In the simplest terms, a race driver is only concerned with two aspects of aerodynamics: drag and lift (both negative and positive). Drag is the wind resistance or friction against the body of the vehicle that effectively slows down the car. Lift is the effect the air has on the weighting of the vehicle: Positive lift is the lifting up of the body (which is what airplanes like), and negative lift is the downforce on the body (which is what cars like, or should I say, what drivers like). This keeps the car in contact with the road.

  Aerodynamics can influence the balance of a car and cause it to either understeer or oversteer. This is referred to as the car’s “aerodynamic balance.” Sometimes, a car that understeers at relatively low speeds will begin to oversteer at higher speeds. The low-speed understeer is a result of suspension design. But as the speed increases, bodywork design (including wings if present) begin to affect the situation. A vehicle with more downforce on the front-end than on the rear (possibly due to spoilers, wing adjustments, etc.) will have more traction on the front tires as the speed increases, resulting in high-speed oversteer. It is important for you to understand the difference between suspension-induced and aerodynamic-induced handling characteristics.

  The balancing of the suspension and aerodynamics is what setting up the car is all about. Many hours are spent developing the handling in the slow corners with suspension adjustments, then changing aerodynamics for the ultimate balance between downforce (front or rear) and drag. Unfortunately, the increase in downforce (resulting in higher cornering speeds) means more drag, resulting in less straight-line speed. The lift (downforce) to drag ratio is quite a compromise.

  Another important factor for a driver is how a car leading another will affect the following car’s speed and handling. When a leading car blocks the air, reducing the wind resistance for the second car, it is called “drafting” or “slipstreaming.” This allows the second car to travel faster, to perhaps pass the leading car or even back off the throttle slightly to conserve fuel.

  Another, often forgotten, factor comes into play here as well. Particularly with winged or ground-effect cars, the car relies on a certain airflow for downforce. When that airflow is blocked by a leading car, the second car’s cornering ability will be reduced. That is why you will see a car catch up to another quite quickly and then struggle to get past. When it’s running by itself, it is quicker; but when its airflow is reduced, it is no faster than the leading car. As a driver, you must recognize this and not overdrive while following closely behind another car. Perhaps, the best strategy is to “take a run at the leading car.” In other words, hang back a little until you get enough momentum from the draft to quickly pull out and pass on the straightaway.

  The first time I drove an Indy car on an oval track, I couldn’t believe the effect other cars around me had on the handling of my car. If there was a car in front of me, it took away a lot of the air flowing over my car, causing it to understeer. If there was a car close to my tail, it seemed to make the airflow over the rear wing less effective, causing my car to oversteer. I learned quickly to make note of the other cars’ positions and to predict what they would do to my car. By the way, this doesn’t just happen with Indy cars. Any car that relies on aerodynamic downforce for grip will be affected to some extent.

  Any car that depends on ground effects for downforce has another little trick for you to consider. The faster the car, the more downforce, and therefore traction, it has. This can make for an uncomfortable situation when you begin to drive a ground-effects car. If you reach a point when it feels like the car is at its limit, you may have to drive faster to get more downforce. Once you go faster, the car has more grip, and it feels like you’re nowhere near the limit, and you’re probably not!

  SMOOTHNESS

  As I mentioned earlier (a number of times!), balancing the car is one of the most important and probably most difficult aspects of driving. But, again, it is the key. Whenever one or two tires become unweighted due to weight transfer from braking, cornering, or acceleration, they lose traction. So, obviously, you want to cause as little weight transfer as possible. How? By driving smoothly! The less abruptly you apply the brakes, turn the steering wheel, or use the gas pedal, the smoother you will drive, and the more overall traction the car will have. In other words, don’t abuse the traction the tires give you.

  We’ve seen how important controlling the weight transfer in the car is and how to do this with your controls, but you also have to accomplish this with extreme smoothness. If you jerk the steering into a turn, it immediately transfers excessive weight to the outside of the car, reducing traction. Now you have to wait until the car’s weight settles down and is balanced again—taken a set—before being able to corner at the limit and accelerate out
of the corner. This wastes time.

  SPEED SECRET

  Smooth is fast.

  The car should be driven absolutely as smoothly as possible all the time. Practice this in your everyday driving. Don’t pounce on the gas pedal; squeeze it on and ease off gently. Don’t slam on the brakes; squeeze them smoothly and progressively to the threshold braking limit. Don’t yank or jerk the steering wheel; smoothly and gently feed in the required steering input that your eyes looking well down the road tell you. Don’t bang the shifter into gear; simply place it in gear—with finesse.

  Keep in mind that each tire has a specific, limited amount of traction. If you exceed that traction limit, the car will begin to skid or slide. The smoother you drive, the easier it will be to stay within those traction limits. A tire achieves a higher traction limit if it is gradually built up to that limit. In other words, if you enter a corner and quickly jerk the steering wheel into the turn, or jab at the brake pedal when trying to panic stop, you haven’t given the tires a chance to gradually build up their traction forces. They will not be able to hold on, and a skid or slide will result.

  Think of the tire’s traction limit as the force it takes to snap a piece of string. If you gradually and gently pull two ends of the string, it requires a lot of force to break it. However, if you quickly jerk the string apart, it snaps with much less force, just like a tire’s traction limit.

  So, everything you do behind the wheel must be done smoothly. When turning into a corner, turn the steering wheel as gently and slowly as possible. This will make the turn smooth! When braking, squeeze the brake pedal, don’t jab at it. Believe me, if you squeeze the brakes, you will stop faster and with more control than if you quickly jabbed at the pedal. So, always think squeeze when applying the brakes or the throttle. Progressively squeezing the gas pedal down will give more controlled acceleration, even when trying to accelerate in a hurry.

  It is better to be smooth than to be fast. Speed will come with practice, practicing smooth driving. Trying to drive fast before learning to be smooth is a mistake. You will never be as fast as if you learn to be smooth first and let your speed pick up naturally.

  Once again, the slower and smoother you move behind the controls, the more in control you will be, and the faster your car will be.

  TRAIL BRAKING

  “Trail braking” is a term used to describe the technique of continuing braking while turning into a corner. In other words, you brake and turn at the same time. There is a specific reason for doing this, as explained by understanding the Traction Circle.

  TRACTION CIRCLE

  The traction circle is a simple graphic way of showing the performance of any driver in any car. Basically, it is an X-Y axis graph produced by a computer data acquisition system of the g-forces during braking, cornering, and acceleration that the car experiences while being driven around a track. See Illustration 5-14.

  First of all, 1 g-force is equal to the force of 1 times the weight of the vehicle; i.e., if a 2,000-pound car is cornering at 1.0 g, there is a centrifugal force of 2,000 pounds pushing outward on the car.

  Consider that a tire has relatively equal traction limits in each direction—braking, cornering, or acceleration—say 1.1 g, for example. In other words, the car and tire combination is capable of braking at 1.1 g, cornering at 1.1 g, and accelerating at 1.1g before the tires begin to break away and start to slide. If you exceed the tires’ traction limit, they will begin to slide, slowing you down and, if not controlled, resulting in a spin. On the other hand, if you do not use all the tires’ traction available, you will be slow.

  ILLUSTRATION 5-14 The Traction Circle is a simple X-Y graph that represents the g-forces that a car experiences while being driven around the track. In this graph the circle is shown at a theoretical limit of 1.5 g’s.

  These g-forces can be measured and graphed as you drive through the corner. If you use the proper driving technique, the graphed line will somewhat follow a circle—the traction circle—telling you that you are using the tire’s full potential.

  In the transition from one directional force to another, say from braking to cornering, there are two ways to get from one limit of traction to the other. You may, upon reaching the end of the braking zone (where you braked at 1.1 g), suddenly lift off the brakes, and then turn the steering wheel into the corner (building up to 1.1 g of cornering force). The second option is to gradually ease off the brakes while progressively applying more and more steering angle and overlapping some of the braking and cornering. This is called trail braking.

  In the first scenario, the car goes through a short period (perhaps only a fraction of a second) where the tires are doing little work. They are not being used to their full potential. This wastes time, no matter how short, because the car cannot instantly change from straight-line braking to a curved path. The second scenario, which keeps the tire and car on the outside edge of the traction circle graph, is a much faster way of driving a race car. It is also the smoothest way of “building” traction forces, which as we know, generates higher cornering speeds.

  So what you must do (what the traction circle is telling you to do) is to continue the braking into the corner-entry phase (trail braking) so that, while the tires are in the process of building up cornering force, they are still contributing braking force. Or brake at 100 percent of the traction limit (1.1 g) along the straightaway up to the corner, and begin to ease off the brakes as you turn in, trading off some of the braking force for cornering force (90 percent braking, 10 percent cornering; then 75 percent braking, 25 percent cornering; then 50 percent, 50 percent; and so on), until you are cornering at the limit (using 100 percent of the traction for cornering, at 1.1 g). Then, you start to straighten the line through the corner, “unwinding the car” out of the turn early, so that the tires have traction capacity for the acceleration phase (90 percent cornering, 10 percent acceleration; 75 percent cornering, 25 percent acceleration; 50 percent cornering, 50 percent acceleration; and so on).

  ILLUSTRATION 5-15 These two Traction Circle graphs represent two ways of driving the same corner. In the graph on top, a lot of the tires’ potential is not being used, and time is being wasted. The graph on the bottom shows the correct way to take the corner; the tires’ full traction potential is being used.

  The real key to the traction circle is the smooth progressive overlap of braking, cornering, and acceleration. If you follow the old advice “Do all of the braking in a straight line, go through the corner at maximum cornering force, then accelerate in a straight line,” you are going to waste a lot of the car’s potential and a lot of lap time. You must “drive the limit” by balancing and overlapping the braking, cornering, and acceleration forces to keep the tires at their traction limit at the edge of the traction circle. This will lead to the fastest possible lap and to another type of circle, the winner’s circle.

  As I said before, tires do have a limit to their traction. If you are using 100 percent of that traction for cornering, you can’t use even 1 percent for acceleration. The traction circle demonstrates how a tire’s traction limit can be used and shared. It shows that if you are using all of the tire’s traction for braking, you can’t expect to use any for cornering without easing off the brakes. If you are using all the traction for cornering, you can’t use any for acceleration until you begin to “unwind” or “release” the steering (straighten the wheel). If you are using all the traction for acceleration, you can’t still be cornering near the limit.

  Think of the throttle and brake pedal as being connected to the steering wheel. More steering angle means less brake or throttle pedal pressure. More pedal pressure means less steering angle. Too much steering angle combined with too much pedal pressure puts the tires beyond their traction limit.

  ILLUSTRATION 5-16 This illustration shows the relationship between what the driver is doing in the corner and the Traction Circle graph.

  Too much steering angle for the amount of braking or acceleration (or vice versa)
will cause the car to exceed the traction limit, usually at one end of the car before the other (understeer or oversteer). This can sometimes “trick” you into believing there is a handling problem with the car, when it most likely is your technique, asking either the front or rear tires to do more than they are capable of doing.

  When I attended my first racing school, I was taught to do all my braking in a straight line on the approach to a corner, then turn into the corner. Over the next couple of years I gradually learned by trial and error to trail brake. But when I started to race a Trans-Am car a few years later, I had to improve my trail braking. It was the only way to go fast in one of those cars. So over the next couple of weeks, at night in my street car, I would practice trail braking well into the corners of a deserted industrial park. I didn’t have to go fast. I just practiced the technique of trailing off the brakes while turning into the corner, then squeezing back on the throttle while unwinding the steering out of the corner. It really was an effective way of improving my technique.

  The traction circle really demonstrates the key to driving fast is balancing the pedal application with the steering angle. Learn how to overlap the braking, cornering, and acceleration, and you will drive the limit.

  As we’ve seen in the last chapter, to be a winner, you have to use the tires’ traction limit. Once you have built up the tires’ braking, cornering, or acceleration forces, keep them there. Drive the limit.

  I know it sounds easy to say, but that’s what it takes. Entering the corner, brake at the traction limit. That’s threshold braking. As you reach the point where you begin turning into the corner, ease off the brakes as you turn the steering wheel. The more you turn the wheel, the more you ease off the brakes (trail braking), until you are completely off the brakes. At this point your vehicle should be at the tire’s maximum cornering traction limit. As you start to unwind the steering coming out of the corner, you start to increase the acceleration until you are at full throttle onto the straight. (See Illustration 6-1.)