PERFORMANCE HANDLING TECHNIQUES
CONTENTS:
Handling
* Traction
* Balance
* Driving Experience
* Three Steps to Good Handling
Tires and Traction
* Increasing Tire Traction
* Tire Contact Patch Area
* Effect of Tire Pressure on Traction
Suspension and Steering Systems
* Lowering Your Car
* Springs
Antiroll Bars
* How Antiroll Bars Work
* Antiroll Bar Pros and Cons
Shock Absorbers
* Selecting Shock Absorbers
Aerodynamics and Handling
* Increasing Aerodynamic Downforce
* Automobile Rake
* Airflow Through the Car
* Aerodynamics on the Street
HANDLING
Traction
Traction is the ability of the tire contact patch to grip the road surface and is the most important of the handling parameters if higher cornering power is a consideration for a specific car or application. In any form of auto competition, increased traction is the most important consideration. Increases in traction may not be high on the priority list for street applications, however.
The most obvious way to increase traction is to increase the effective size of the tire contact patch. In addition to using a wider tire, the effective size of the tire contact patch and be increased by changing suspension settings to allow the entire tire contact patch to lie flat on the road surface during cornering.
Traction can also be increased by using a softer tire compound. The stickier tire provides more grip, but the tire will wear more quickly. A stickier tire may also provide less traction on slipper surfaces, such as wet roads.
Balance
For a car to exhibit good handling characteristics, the front-to-rear traction balance must be close to neutral. If the front tires lose traction first, the car pushes and the rear tires are just along for the ride, not really working to their maximum potential. In the opposite case, where the rear tires loose traction first, the car is loose. Neither condition is desirable in anything exceeding small quantities. The most important factors that dictate balance are the tires and the relative contact patch areas at the front versus the rear, the rates of the springs and the rates of the antiroll bars.
Achieving a good handling balance will improve the handling characteristics of the car. And in most instances, cornering power will improve also, with no detrimental effects on ride or responsiveness. Often, simple alignment settings can improve the balance of a car. On the other hand, changing springs or antiroll bars can have a major effect on the balance.
Driving Experience
Driving experience is especially important when a car is to be used for competition. Balance and responsiveness relate to driver experience. An experienced driver will likely want a different handling balance compared to an inexperienced driver. An experienced driver will also be able to utilize quicker vehicle response. Often, a newer driver in a responsive car cannot feel the feedback from the car quickly enough. It is possible to slow the response of the vehicle slightly so that a newer driver can keep up with the vehicle.
Three Steps to Good Handling
1. Requires the right pieces
2. Requires tuning the pieces to their maximum potential
3. Requires the use of good driving skills, not to extract maximum enjoyment form the improved handling of your vehicle, but also to live up to the potential of the vehicle from a safety point of view.
TIRES AND TRACTION
The most important aspect of handling is improved traction for the vehicle as a whole. While improved traction is the key, the goal is to increase cornering speed, reduce braking distance and enhance acceleration. The tire is the link to the road and, by itself, is the most important factor in the handling equation. Additionally, everything else you change in the suspension system will ultimately have an effect at the tire contact patch. Thus, understanding the basics of the tire and how it develops traction is paramount to making handling improvements.
Increasing Tire Traction
There are three factors that increase tire traction:
1. Increasing the friction between the tire and the road.
2. Increasing the number of rubber molecules at the tire contact patch.
3. Increasing the vertical load on the tire. These are rather general statements, requiring further explanation.
Increasing Friction Between Tire and Road
This can occur in two ways. First, the coefficient of friction of the road surface can increase. The coefficient of friction is an arbitrary measurement of grip created by a surface. The higher the coefficient, the greater the grip. Roads of varying materials - such as asphalt, concrete, dirt and so on - will have different coefficients of friction. This will let the tires grip the road more firmly. We have no control over this, short of choosing specific roads to drive on.
Second, we can increase the coefficient of friction of the tire. This is done by choosing a softer rubber compound tire. The softer rubber molecules will grip the road more firmly, but will also wear more quickly.
Increasing Amount of Rubber at Contact Patch
This can be accomplished in three ways. First, and most obvious, is a wider tire. Second, we can choose a tire with fewer grooves in the tread, thus increasing the area of the tire contact patch. This has obvious flaws on wet, slippery surfaces, however.
Increasing Vertical Load on Tire
The traction a given tire can generate increases with vertical load on the tire. Vertical load is the combination of vehicle weight resting on the tire contact patch, plus any aerodynamic downforce that may be present. The rubber molecules at the tire contact patch are further pushed into the road surface as the vertical load increases. This allows the rubber molecules to do more work.
Vertical load can be increased by adding weight to the vehicle, so that the tire contact patch sees more vertical load. While this will increase traction, the work that each rubber molecule at the tire contact patch must do also increases. Cornering speed, baking distances and acceleration will not improve. In face, the performance in there categories will actually deteriorate.
Tire Contact Patch Area
How large should the area of each tire contact patch be? This is an important question. The initial obvious answer is: as large as possible. But that is not true. Yes, we want a large tire contact patch, but if it is too large for a given car, rolling resistance increases, and may negate any performance improvements added by traction gains. Additionally, as a tire increases in tread width, it becomes more difficult to keep tire contact patch flat on the ground when it is most needed.
A basic rule of thumb is that as engine horsepower increases, the need for larger contact patches also increases. A corollary to this is: If the tires fit in the stock fender wells without rubbing during suspension travel or body roll, them the tire is probably not too large.
The area of the tire contact patch is one factor that determines the handling horsepower of a car. The area of the contact patch should be proportional to the load on the tire. If the load at the front is equal to the load at the rear, then the contact patch areas should also be equal to each other. If they are not, then the end of the car with larger tire contact patches will have more traction capability than the other end, and handling balance will be affected.
Effect of Tire Pressure on Traction
The pressure in the tire has a big effect on tire traction. The pressure doesn't really affect the grip of each rubber molecule, but it certainly can affect how many of the molecules at the tire contact patch are in contact with the ground. A specific tire on a given car with a given load will have only one correct tire pressure. In practice it is a narrow range of pressure, within about 3 pounds per square in. (psi). If the tire pressure is outside this range, the tire contact patch is deformed and not fully contacting the road surface. In other words, fewer than the possible number of rubber molecules are in contact with the surface of the road.
If the tire is overinflated, the edges of the tire will lose contact with the surface, and traction is reduced. Fewer rubber molecules are carrying the same load, so the tire will operate at a higher slip angle for a given cornering force, and the maximum cornering force will be lower. If the tire is underinflated, the center of the tire contact patch will not maintain optimum contact with the road surface, and again, fewer rubber molecules than possible will be doing the same work, resulting in higher slip angles for the same load and reduced cornering power.
SUSPENSION AND STEERING SYSTEMS
What function does a suspension system serve? Quite simply, its job is to keep the tire contact patch flat on the road surface, thus allowing the maximum number of rubber molecules to work as much as possible.
Lowering Your Car
Once of the most important ways to improve handling is to lower the center of gravity. This increases traction by reducing weight transfer and reduces body roll for a given cornering force. The easiest way to lower a car is with the springs. Shorter springs lower the ride height easily and quickly. But there are many pitfalls awaiting those who lower their cars. Some are obvious, others are not.
On the obvious side is reduced ground clearance. When a vehicle is lowered, it rests closer to the ground; in motion, during bump travel, especially over speed bumps and driveways, the bottom of the vehicle is more likely to contact solid ground, with possible damage to a variety of components. On the street, this is crucial, because the real world is much bumpier than the racetrack or autocross course.
There are a number of less obvious potential hazards when a car is lowered. The most important is suspension bottoming in bump travel. When a car is lowered by the springs (as opposed to lower-profile tires), less bump travel is available. When the suspension bottoms on the bump stops, the problems is minimal because the bump stops are designed to slowly stop suspension travel. In many cases, the lowering will take all of the bump travel, leaving the car resting on the bump stops. This increases the suspension rate drastically, and can cause serious problems and dangerous handling characteristics.
Even worse is the case where the bump stops are removed. The bump stop is designed to progressively limit bump travel and to keep the shock absorber from bottoming out. When the suspension bottoms out, the spring instantly rises to infinity and a sudden loss of traction can occur at the end of the vehicle if the car is cornering near the limit of adhesion. If the shock absorber bottoms first, the valving will be blown out, and little shock damping will remain. Wither situation can have, at minimum, expensive consequences and, at worst, tragic results.
How to Lower Your Car
It is common practice to cut springs to lower a vehicle. From a materials standpoint, cutting springs is not a bad thing to do, assuming the proper methods are employed. But cutting springs can cause bottoming problems. The shorter spring will have a slightly stiffer spring rate, but usually not enough to limit bump travel adequately to keep the suspension from bottoming out over bo9mps. This often leads to the problems described above. And in many instances, vehicles with chopped springs ride horribly. The dollars savings often prove to be false.
The best way to lower a vehicle is with springs designed to lower a specific amount, with increased spring rates that will minimize the possibility of bottoming during bump travel. Even in this instance, most aftermarket spring kits include new bump stops, designed to work in the specific application. The reliable and competent suspension and spring manufacturers like Eibach, Suspension Techniques and others have engineered a package that addresses these problems and eliminates them.
Cars with MacPherson strut suspension systems cause another set of problems. Most strut cars can be lowered about 1 in. with no bottoming problems, as long as the spring rates increase accordingly. If the car is to be lowered more than 1 in., then, in most cases, the strut tubes must be shortened or relocated to allow adequate bump travel. If this is not done, the same bottoming problems will occur as previously described. A competent fabricator can accomplish such work. When struts are shortened, often a shorter strut cartridge (shock absorber) must be used to insure adequate bump travel. Good suspension shops know the scoop and find the right parts for the job. But beware! Not all vehicles have the right strut cartridges available. Check before you begin work.
Overall, the nest way to lower a car for the street is with one of the aftermarket kits available for this purpose. The knowledgeable manufacturers have created packages that improve the look, lower the center of gravity and improve handling performance, while eliminating most of the hassles and possible problems.
Review on Springs
Springs have two important jobs: to keep the suspension from bottoming or the chassis from bottoming on the road surface; and to keep the tire contact patch on the road surface over bumps, dips and so on.
These two criteria require opposite spring rate needs. Since maintaining tire contact with the road is crucial for good handling, we use the stiffest spring possible that allows good tire contact with the road. The bumpier the road, the softer the spring.
We use the suspension frequency as a design tool for specific types of road surface conditions. The suspension frequency is proportional to the wheel rate of the spring and inversely proportional to the sprung weight resting on the spring. The wheel rate of a spring is equal to the spring rat in pounds per inch times the motion rations squared.
Progressive rate springs offer substantial advantages over standard springs, allowing lower suspension frequencies and lower ride heights without bottoming. The progressive rate spring comes closer to meeting the opposing needs of soft spring rates for improved tire contact over bumps and stiff spring rates to allow minimum rid heights without bottoming.
ANTIROLL BARS
The antiroll bar which is also called a sway bar or stabilizer bar, controls the amount of body roll while cornering. The roll resistance provided by the antiroll bar is added to the roll resistance provided by the springs. The total roll resistance determines the total amount of body roll for a given situation. The total roll resistance at the front of the vehicle, compared to the total roll resistance for the entire vehicle, tells us the roll couple distribution; in other words, the front versus rear roll resistance. This determines the handling balance of the vehicle. If we have too much rear roll resistance the car will oversteer. If we have too much front roll resistance, the car will understeer.
For handling improvements, we use the antiroll bar for two purposes: controlling the amount of body roll, and controlling the roll couple distribution, which determines where weight is transferred.
How Antiroll Bars Work
The antiroll bar resists body roll while cornering by twisting. When the body begins to roll, the arms on the antiroll bar will twist the main section of the bar. This resists additional body roll. The arms are attached to a suspension arm (usually the lower control arm) on each side of the car. The other end of the arm (the arms are usually part of the main bar) is attached to the main bar.
On independent suspension systems, the bars link the left and right sides, causing the suspension to no longer be completely independent. If the wheels encounter the same bump or dip, then the antiroll bar does not work. But when only one wheel hits a bump or dip, the antiroll bar adds to the spring rate by resisting, adding to the suspension frequency. An excessively stiff antiroll bar can cause tire contact problems over single wheel disturbances.
The stiffness of an antiroll bar is determined by the stiffness of the material the antiroll bar is made of, the diameter of the main bar, the effective working length of the main bar and the effective length of the arms.
Almost all antiroll bars are made from materials of similar stiffness. The diameter of the bar affects its stiffness to the fourth power: if you double the diameter of the bar, the stiffness will be sixteen times greater. Small changes in diameter can have a major effect on roll resistance. The effective length of the main bar is inversely proportional. To the stiffness, as is the arm length. For most applications, it is difficult to change the effective length of the bar, but the diameter of the bar can be altered, and the arm length can be easily adjusted.
Antiroll Bar Pros and Cons
It may seem that the antiroll bar is the answer for tuning the handling of a vehicle. It is an important factor in handling, nut it is not the answer. It can be used to fine-tune the handling balance, and to limit body roll for improved tire contact with the road. Antiroll bars allow the springs to do their job, but there are limits. First, we can have too much roll resistance overall; second, the bars can provide too large a percentage of the total roll resistance.
Let's examine how an antiroll bar works. When a turn is initiated, the outside suspension moves into bump and the inside into rebound. The antiroll bar begins to twist, with the outside end of the bar lowering and the inside end of the bar rising. The bar pushes down on the outside suspension, while it tries to lift the inside suspension. At the inside wheel, this is the opposite of the way a spring reacts. The spring pushes the inside wheel down, and the bar lifts the inside wheel.
If the bar is too stiff, the inside wheel is unloaded too much and, if it is the drive wheel, may cause wheel spin as power is applied at the exit of a turn. This is a serious problem if the vehicle is not equipped with a limited-slip differential, and has high horsepower. It will be worse in slow turns. The problem is unlikely in low horsepower circumstances, and less likely with a good limited slip. This is the reason we have found it nest to have a higher suspension frequency at the end of the car with the drive wheels. The optimum percentage of roll resistance provided by the bars seems to be between twenty-five and fifty percent of the total roll resistance. Springs provide the balance of the roll resistance.
Choosing the correct bar rates is a complex process, requiring a significant amount of data and plotting. To truly calculate the proper bar rates, the center of gravity height must be known, as well as the roll center locations and camber change curves. The work involved is extensive, and is best left to an experienced designer or antiroll bar manufacturer.
Additionally, when complex bends are required in a bar design, the true rate cannot effectively be calculated. The actual rate of the bar must be measured on heavy-duty test equipment. The easiest approach is to purchase antiroll bars from an experienced manufacturer or to consult with a suspension design expert.
Overview
The antiroll bar is used to limit body roll, allow the springs to keep the tire contact patches on the ground over disturbances, and to adjust roll couple distribution.
Antiroll bar rates are easily adjusted to fine-tune roll couple distribution. The major rate changes are best accomplished with the bar diameter, and fine-tuning adjustments with the arm length.
The antiroll bar works by loading the outside tire, but lifting the inside. Excessive antiroll bar stiffness can hurt handling and traction, especially at the drive wheels during the exit phase of a turn. Excessive roll resistance will cause the inside wheel to lift off the ground in a turn.
Finding the best compromise can be time consuming, but usually rewards substantial dividends to those who make the effort.
SHOCK ABSORBERS
If springs actually absorb shock, then just what do the shock absorbers do? The primary purpose of the shock absorber is to dampen vibrations or oscillations. In other countries, shock absorbers are called dampeners. The goal is to keep the spring from bouncing beyond one full cycle. If you have driven a car with bad shocks, you know how uncomfortable the ride is when the car bounces over every bump, dip and rut. The properly rated shock absorber stops this.
The shock absorber has two jobs. First, it must control oscillations of the unsprung mass, that is, the wheels, tires, hubs, and so forth. Second, the shock absorber must control the sprung mass of the car. The spring does most of the work on bump travel, while the shock controls the return motion with rebound travel. Over dips, in rebound travel, the shock slows the downward movement of the body, allowing a more level ride. For this reason, the bump resistance is about one-third the rebound resistance for street use.
The shock absorber must be compatible with the spring rates to work effectively. It controls the rate of weight transfer, and therefore the transient handling response of a vehicle.
A stiffer shock absorber means quicker weight transfer and faster vehicle response. It is best when the rate of weight transfer at the front is the same as the rear. If the rate of weight transfer is different from to rear, handling balance will likely be affected. The end with stuffer shock absorber settings will reach peak lateral acceleration first.
It can be difficult to distinguish if a transient handling problem is caused by the shock absorber rates, or by driver inputs. Trying different combinations of shock absorber settings is the key to fine-tuning transient handling response.
Selecting Shock Absorbers
One of the most difficult tuning jobs to improve handling is selection the shock absorber. Since the shock has a major affect on ride and handling, it can be tricky business walking the fine line between good handling and good ride. In some cases, specifically competition, the nod goes to handling; ride is not a consideration. But on the road, while the pleasure of blasting a few quick corners is enhanced with firm shock valving, the price is a harsh ride for many commuter miles, often with less that sympathetic passengers.
There are two important questions to ask when selecting the right shock for your car. First, do you need adjustable shocks for the kind of driving you do? Second, will you take the time to actually adjust them? Honest answers to these questions will save you time and money, and not take away from the driving pleasure you demand from your car. In most cases, especially where the vehicle is only driven on the street, nonadjustable shocks or shocks where the adjustment is easily made at the top of the shaft are the only practical solutions.
The major shock absorber manufacturers do a good job of creating a valving for each application that is a good compromise between performance handing and ride. And most of the companies make a specific racing-application shock for competition purposes. It is not truly possible to have the best of both in a single set of shocks.
The shock absorber must be compatible with the spring rates to work effectively. It controls the rate of weight transfer, and therefore the transient handling response of a vehicle.
A stiffer shock absorber means quicker weight transfer and faster vehicle response. It is best when the rate of weight transfer at the front is the same as the rear. If the rate of weight transfer is different front to rear, handling balance will likely be affected. The end with stiffer shock absorber settings will reach peak lateral acceleration first.
It can be difficult to distinguish if a transient handling problem is caused by the shock absorber rates, or by driver inputs. Trying different combinations of shock absorber settings is the key to fine-tuning transient handling response.
AERODYNAMICS AND HANDLING
Increasing Aerodynamic Downforce
The reason to increase aerodynamic downforce is simple: the load created by aerodynamic downforce increases the load on the tires. As we have seen, increasing the load on the tires increases traction, but the aerodynamic load does not add weight to the car, so the engine, brakes and tires do not have to work harder to accelerate, brake or corner the vehicle. This increase in cornering force is nearly free. The price for the increased downforce is more aerodynamic drag.
How do we increase downforce on a production vehicle? The basic method is to change the airflow characteristics over and under the car. This is the principle behind the wing. A wing creates lift by altering airflow over its surface. Each side of the wing has a different surface shape, and the air must travel a greater distance across one surface. The air flowing across the longer surface may travel faster than the air flowing across the shorter surface. The faster-flowing airstream creates less pressure, while the slower-flowing air is at a higher pressure. This pressure differential creates lift, or in the case of the car, with inverted wings, downforce. The wing also causes aerodynamic drag determines the efficiency of the wing shape. For a given wing profile, the larger the wing aspect ratio, the more efficient the wing will be. The aspect ratio is the comparison of the chord (the width at the widest section) of the wing to its length. The longer a wing of given width, the better the downforce-drag ratio becomes. Gliders, with their long, slender wings, have the highest efficiency of any aircraft wing.
A greater wing angle of attack will create more downforce than a smaller angle of attack, but will also create more aerodynamic drag. If the angle of attack is too great, the wing will stall and drag will increase while downforce decreases dramatically.
Automobile Rake
Another way to increase downforce on a production car is to set the vehicle on a slight rake, so that the front is lower than the rear. This increases surface area and improves flow over the body, but it also increases frontal area, which causes aerodynamic drag to increase as well. Rake is usually measured by the difference between front and rear ride height. The normal amount of rake, and a good starting point for most cars, is 0.5 in., with the front lower. Rake over 1 in. is excessive. Like setting wings and spoilers, rake adjustment requires testing and trial and error to find the best compromise between downforce and drag.
Rake is the tile of the chassis from level in side view.
Most cars use some positive rake to increase downforce and minimize airflow underneath the vehicle. Positive rake can be overdone, resulting in more drag due to an increase in effective frontal area.
In all cases, negative rake causes front-end lift and drag increases. Negative rake should always be avoided.
Ride height has an effect on aerodynamic drag. All cars generate the least drag when the ride height is as low as possible. This reduces the effective frontal area to the minimum, and reduces turbulence underneath the vehicle. Unless dictated by rules, the ride height should be as low as possible, taking suspension travel into account. If the car bottoms, either stiffer springs or higher ride height is needed.
Airflow Through the Car
Airflow around the car is only half the picture. One of the most substantial sources of aerodynamic drag is airflow thru the car. Any place air enters the car is a source of aerodynamic drag and so on can reduce drag. The exiting of the air from the car is also worthy of attention. The worst place to exit air s under the car, but that is exactly where most air exits. We have already seen that air flowing under the car causes increased aerodynamic drag. The goal should always be to exit airflow through the sides or top of the body structure. Fenderwells are often accessible and easy places to vent airflow. Forcing airflow under the car will increase drag and reduce any downforce.Aerodynamics on the Street
Possibly the best reason to add an aero kit to your vehicle is looks. Some of the kits available not only improve performance, but enhance the look of the car as well. If you are in the market for an aero kit, make sure the design will actually enhance high-speed handling and stability. Also check the fit and finish of the parts, Installation can be difficult, so check into the availability of factory or dealer installations.