THE RACING LINE

Introduction

What is the shock unit as discussed in the IndyCar II and NASCAR manuals?

Suspension systems are comprised of three major components: the springs, the shock absorbers and the anti-roll bars. Papyrus has combined the spring and shock absorbers into a single component, referred to as the shock unit.

The purpose of the springs in a racing car is to maintain the wheel in contact with the road surface at all times. No contact, no grip. By using independent suspension systems each wheel can move indendently from the car body and the other wheels. The springs also restrain the movement of the car body.

How do springs work?

The strength of a spring is defined in the units of force, or weight, per distance, i.e. lbs per inch or kg/mm. As the spring is extended or compressed from its resting or static position, it resists this movement by exerting an oppossing force that is proportional to the amount of movement, ie if a spring is rated at 500 lb/in, then a 1 inch compression will result in the spring resisting with a force of 500 lbs and with a 2 inch compression, the spring will produce 1000lbs.

The spring will produce these forces in compression or extension and the force will attempt to return the spring to its static position.

Why do we need shock absorbers?

When the front left wheel hits a bump in the road, the spring at this wheel will be compressed and it will force the wheel down. The spring will tend to force the wheel past its static position and the spring will be extended. The resulting oscillating motion will continue for a long time until friction eventually brings the spring back to its static position. This cyclic motion is very hard on the car and the driver. To quickly reduce the amount of time for the spring to stop bouncing up and down, a shock absorber is attached inside the spring between the wheel and the car body for the NASCAR race cars. In IndyCar and Formula One cars, the springs and shock absorbers are placed inside the car body, but the theory is the same.

The resistive force of the shock absorber is proportional to the velocity or speed of the spring. The faster the spring motion the greater the resistive force, referred to as damping, of the shock absorber. The force is generated by forcing a liquid in the shock absorber through a series of holes. Actually the name of shock absorber is incorrect as the spring absorbs the shock and the shock absorber dampens the motion of the spring.

In effect, the spring resists the movement of the wheel and the shock absorber resists the velocity of the wheel. You can adjust the stiffness of the shock unit from soft (0 %) to stiff (100%) for each wheel.

Anti-Roll Bars

The anti-roll bar is a bar that connects the two front or rear wheels and transfers weight from the outer tire to the inner tire during a turn, as opposed to the springs which allow the wheels to operate independently. The purpose of the roll bar is to reduce the amount of roll and to increase the amount of lateral load transfer from the outer to inner wheel during cornering. The advantage of the anti-roll bars is that it allows the driver to adjust the stiffness of the front and rear wheels during the race and therefore modifying the understeer/oversteer characterstics of the car.

How the suspension affects the oversteer and understeer balance of the car?

Cornering traction is the ability of a tire to resist the cornering or sideways force generated when a car is going through a corner. When the cornering force required of a tire is greater than the cornering traction limit of the tire, the tire loses it grip.

What is understeer and oversteer?

Understeer occurs when the driver is attempting to turn the car into a corner and the car continues in a straight line. The front wheels reach the cornering traction limit before the rear wheels. The rear wheels push the car into the wall instead of following the front wheels through the corner.

Oversteer causes the car to go into a spin. In this case, the rear wheels reach their cornering traction limit before the front, so as the front tires are going around the corner, the rear tires prefer to go straight and the back end overtakes the front of the car.

To simplify the explanation of the understeer/oversteer characteristicsof a race car, the cornering force of the front and rear tires will shown as a single vector at each end of the car.

Keeping Forces Balanced

If the racing car below is properly balanced, ie in nuetral steer without oversteer or understeer, the centrifugal force generated by the car turning will be balanced by the rear cornering force and the front cornering force. As shown in Lesson 1, the centrifugal force acts through the center of gravity, and the rear and front cornering force act throught the center of their respective tires.

Two rules to remember when going through this discussion:

• The cornering traction of two tires with the same amount of loading is greater than the cornering power of one tire with twice the load. This is shown by tire testing rather than basic physics.
  
• The set of wheels with the greatest amount of load transferred to the outside wheels has less cornering power.

For the car to be in balance, two equations apply:

1. Centrifugal force = Front Cornering Force + Rear Cornering Force
   
   The cornering force of a tire is based on the vertical load on each tire and the corresponding cornering traction or grip. The grip of each tire is then added together to determince the total cornering force at each end of the car. The relationship between the vertical load of a tire and its grip is found during testing, such as Damon Hill doing test laps at Estoril or Jed pushing a tire across his garage with a bathroon scale.
   
2. Torque produced by the Front Tires = Torque Produced by the Rear Tires
   
   Torque, or moment, is a force multiplied by distance, and normally has units of foot-pounds (lbs * ft) or kg * mm. For example, the torque produced by the front tires is equal to the total front cornering force multiplied by the distance from the front tires to the center of gravity.

For neutral steering:

Rear Cornering Force * Distance from the centerline of the rear tires to the Center of Gravity
=
Front Cornering Force * Distance from the centerline of the front tires to the Center of Gravity.

In the figure above, the car is attempting to turn to the right. Lr is Left rear, Rf is Right front, etc.

This force diagram shows that if the front cornering force is greater than the rearing cornering force, then the car will go into oversteer. The reverse occurs when the rear cornering force is greater than the front cornering force, and the car understeers.

Why doesn't an understeering car spin like the oversteering car?

Remember from Lesson 1, that the centrifugal force that is being resisted by tires is inversely proportional to the radius of the turn. The greater the radius of the turn the smaller the centrifugal force produced, of course the smaller the radius, the greater the centrifugal force produced. The understeering car is in effect increasing the radius of the turn and is therefore returning to more stable postition, hopefully before hitting the wall.

The oversteering car decreases the radius of the turn and becomes more unstable, resulting in a spectacular spin into the kitty litter.

Adjusting the balance

As discussed earlier, the roll bar is used to control the amount of load transfer between sets of wheels. In this example, if too much load is transfered off of the inner rear tire it will lose grip, and rear cornering force will be reduced, resulting in a spin. Reducing the stiffness of the anti-roll bar can reduce the amount of load transfer at the rear. An alternative to solution might be to increase the stiffness of the front anti-roll bar to help balance the front and rear grip.

Or to decrease the stiffness of the suspension to improve the grip.

Or improve your driving skills by spending more time on the track, or reduce the amount of wheelspin coming out of a corner thereby increasing the rear wheel grip.

Or by reducing the amount of turn in during cornering of the front tires to increase their grip.

Or......

The point being that adjusting the anti-roll bar is only one aspect of getting your IndyCar or NASCAR under control on the Papyrus tracks. The main advantage of the anti-roll bar is that it can be adjusted during the race by the driver.

"If it was easy everyone would be doing it" Martin Brundle.

This article was based on endless laps on the Vancouver track with IndyCar trying to get my time below 60s and the following books, which are highly recommended for anyone interested in Racing Engineering:

• Tune to Win, Carroll Smith, Aero Publishers Inc, USA
• Race Car Engineering and Mechanics, Paul Van Valkenburgh, HP Books, USA