Handling: If you don’t have a car that handles right and predictable, you have very little chances of winning a race. Handling is an abstract term. Perhaps the best way to define it is the driver’s feeling and impression of a car’s reponse to cornering, braking and accelerating. The driver very definitely has to enter into the situation, because every driver’s feedback of a particular car response will be slightly varying. Because of this, we ask you to accept many theories and principles without our presenting a sound mathematical proof.

Understeer & Oversteer

Understeer - is one of the two basic reactions of a vehicle when steering forces direct a car off its straight heading. Understeer is a resistance to any steering wheel angle being added to stay on the radiused path of the turn. Understeer is pushing or plowing of the front end of vehicle.

Oversteer - is the other basic reaction of a vehicle to steering. This means that the rear end of the vehicle is loose, requiring less and less steering angle to keep the vehicle on the radiused path of the turn.

Understeer and oversteer can be transient or momentary condition, and both may occur in a single turn. Basic oversteering handling may still understeer while entering a turn because the front tires need some time to develop side bite. Cars with extremely powerful engines, can cause the rear wheels to spin at most any time is called power oversteer. This happens when you stand on the throttle when exiting a turn, the car suddenly oversteers to a certain degree.

The two most important factors that influence understeer and oversteer are weight distribution of the vehhicle and roll resistance (accomplished through selective spring rates). Having a basic design influence on these factors are front and rear roll center locations and center of gravity height location.

All factors affecting the vehicles handling act through the vehicle’s tires to the road surface. The most important tire charactoristic is its development of a slip angle when lateral accelerations are applied. No matter what type construction, the pneumatic tire is deformed while cornering. Therefore it will follow a path between the steered angle and straight ahead.

Slip Angle - is the difference between the line the tires are steered on and the line they actually follow. Slip angle is actually a drift angle. The angle of tire slip can depend on vehicle speed, lateral acceleration, vertical load on tire, coefficient of friction of the rubber, and inflation of the tire.

We can then say that if the outside rear tire of a vehicle has a larger slip angle than the front outside tire, it will oversteer. If the outside front tire has larger slip angle than the rear tire, the vehicle will understeer.

1: A front heavy vehicle will oversteer because the outside tire on the front heavy vehicle, will require a larger slip angle to handle the heaveir weight loading.

2: Increasing the roll stiffness (with stiffer spring rates) on one end of a vehicle will vreate a large slip angle on that end of the vehicle.

3: When the roll center is raised on one end of a vehicle, it will increase the load transfer and thus the slip angle at that end of the car.

4: Varying tire pressure will vary the slip angle. Lowering the inflation will lower the load carrying capacity of a tire, so slip angle will be increased.

Cornering - The vehicle is taken from straightaway acceleration. The car is decelerated using brakes and engine drag, and iat the same time is steered into the approaching turn. Chassis weight is pitched forward during deceleration, and centrifugal force pulls and rolls weight to the outside of the vehicle during cornering.

There are three modes of cornering.

1: Entering the corner the outside front wheel will be the most heavily loaded, the outside rear the next, with the inside front third and inside rear the least heavily loaded. Our object is to keep all four tires contact patches working equally for best cornering ability, you can see why a car heavily biased to the rear in a static condition is a help to keep more wieght on the rear tires.

2: Now the deceleration pitch starts to dimish and centrifigul force is the greatest as the apex of the trun approaches and is passed. In this mode the two outside wheels are carrying almost an equal amount of weight and the two inside wheels are equally light.

3: The accerlated exit. The drive tries to get back on the throttle as quickly as possible to accelerate out of the turn. The vehicle piches weight to the rear of the vehicle, and increases centrifigul force somewhat. This causes the outside rear tire to be the heaviest loaded, followed by the outside front, the inside rear and then inside front.

Roll Couple Distribution - is a controlling factor of understeer and oversteer. Increasing roll stiffness at one end of a car will produce a greater tire slip angle of the outside tire.

To control a car that is oversteering you would increase the roll stiffness at the front end of the vehicle. Install a stiffer outside front spring or stiffer anti-roll bar. The reverse is also true for an understeering car.

Roll couple is adjusted with spring and anti-roll bar rates. The stiffer end of a vehicle will lose traction first. So if a car’s front suspension is stiffer that the rear, the roll couple distribution will produce understeer because the front end is handling more weight transfer.

Center Of Gravity Height - is the geometric center of weight location of the vehicle. The CGH is the point above the ground at which this exists. All forces of acceleration, lateral acceleration and deceleration act on the entire car though this CGH. The lower the CGH, the better the cornering ability of the vehicle. Production vehicles have a CGH from 18 to 25 inches above the ground. When componets are rearranged the total vehicle lowered, the stock cars racer’s CGH can be 14.5 to 17 inches.

Roll Centers - Every vehicle has a front roll center and rear roll center. They are independant from each other. The roll center is a theorectical point determined by the pivot point location and angles of the suspension linkages, about which the particulae end of the vehicle rolls. Front and rear roll centers are joined bya straight line running through the center of the call call the roll axis. During conrnering, the car will roll about the roll axis. The relationship between the CGH and roll axis determines body roll taking place during conrnering.. The greater distance between the CGH and the roll axis, the greater the body roll angle with a given lateral acceleration. The guideline for building a race car is generally the front roll center will lie 1-3 inches above ground and rear roll center will lie 8-12 inches above ground.

Spring Rate - is the stiffness of a spring as expressed in pounds per inch. To obtain this rating, force or wieght is applied straight down on the spring to compress it one inch. If it requires 275 pounds of force or mass, then its rate is 275 pounds per inch. The terms soft and stiff define the springs rate in relationship to another. A soft spring does not require as much force to compress it one inch.

Ride Rate - is how the vertical dynamic loads which the wheels encounter are carried to the passenger of the vehicle. Soft springs as used to control deflections which are fed into the frame and structure to assure a soft ride.

Wheel Rate - is the efeective spring rate of the spring at the center of the wheel. The spring rate is always softer when considered at the wheel because the sping always is located at some distance from the wheel. The further the spring from the wheel, the softer the wheel rate.

It is difficult to designate with great accuracy which spring rates should be used on each corner of the car. There are many varibles which can change spring rate requirements. The basic selection for a chassis with a car weighing approximately 3000-3500 pounds running on a ¼ mile or ½ mile fairly flat track. Right front 1000 pound per inch Left front 800 pounds per inch Right and left rear 300 pounds per inch the front anti-roll bar should be one-inch diameter with 12 inch arms.

Wedge - Coil springs have jack screws built on top of them in the frame while leaf springs are mounted on adjustable spring shackles. Changing vehicle corner height with either method produces a weight transfer. When the right front weight jacking screw is screwed down, the chassis is moved up on the spring and weight is transferred tot eh left rear corner. This is called assing wedge to the chassis. Because the total weight must remain the same, the left front and rightrear corners of the chassis get light as more wedge is jacked in.

Checking the Wedge - Place a 9/16 inch socket on a hydraulic jack and jack up under the center of the rear end housing. This give the housing a pivot point as it is raised. If the car is on level surface, the right rear tire will lift off the ground first before the left rear tire if wedge has been jacked into the chassis. If the right rear lifts up a half inch before the left lets, then it is said that there is a half inch wedge in the chassis. For fine tuning, no more that ½ to ½ inch of wedge is used.

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