Ask Joe Mechanic: Steering and Suspensions Part 4

Continuing our discussion from last week, the next factor for suspension design and tuning is called the roll couple percentage. This is a determination of handling balance, which is the wheel rate of each axle in roll as a ratio of the total roll rate. This lateral roll transfer is controlled and adjusted by using an anti-roll bar.

            The next factor is weight transfer. Weight transfer takes place during any change of motion, whether cornering, acceleration or braking. It is calculated at each wheel and is a comparison of the highest load to the static or standing weight on each wheel. There are four factors that control weight transfer, the distance between wheel centers (wheelbase in the case of acceleration and braking, and track width in the case of cornering). Also, the height of the center of gravity, the weight of the vehicle, and the rate of acceleration of deceleration are determining factors.

            Unsprung weight transfer is dependent on weight transfer but includes more factors. This includes all the weight not controlled by the springs. That would include the wheels and tires, hubs and spindles, brakes and rotors or drums, and half the weight of the control arms and axles. For calculation, they are put through the same forces as for weight transfer.

            Sprung weight transfer is the weight transfer of the weight resting on the vehicles’  

springs. To calculate this, you need to know the sprung weight, the roll center heights front and rear, and the sprung center of gravity, which will be higher than the normal center of gravity. Also needed is the roll couple percentage. 

            Jacking force is the total vertical force exerted on the suspension links. This is determined with use of the roll center, with the higher the roll center meaning a higher jacking force.

            Travel is the total distance that a suspension can move from the top of its stroke to the bottom. If a wheel can be forced upward against its stop, this is called bottoming. Bottoming is an extremely dangerous situation as it can cause a catastrophic loss of vehicle control. This can be cause by quite a number of things including; the suspension hitting its limit stop, a broken or worn spring, strut or shock, tires coming into hard contact with a body part, part of the car hitting the pavement, etc. Lifting is the opposite situation where the tire actually looses contact with the road because the suspension is fully extended. This can also cause a dangerous situation, especially if it takes place in a curve. Off road vehicles require limit straps or stays so that with the extreme suspension travel that they experience, that the coil springs do not come out of their perches or cause damage to the suspension bushings and links. The opposite effect is accomplished by use of a bump stop, usually made of rubber, which prevents full compression of a suspension.

            Damping is the control of motion by the use of the valving of shock absorbers. This is also a compromise between comfort and control. Damping controls the resistance and the speed that a suspension moves up and down. If properly controlled and adjusted, the vehicle will return to its normal ride position in a minimum amount of time with a minimum amount of discomfort.

            The next factor is camber control. Camber will change due to wheel travel, body roll and suspension movement. In general, a vehicle’s optimum control and tire wear occurs with one to two degrees of negative camber off vertical. Some racing applications may run as high as seven degrees negative. Many older rear-wheel drive cars and trucks actually ran positive camber. Mounting placement and suspension geometry controls camber.

            Roll center height is a product of suspension instant center heights and is a critical determining factor in analyzing weight transfer, body roll, and front to rear roll stiffness. This particularly is critical to controlling jacking forces. Instant center is an imaginary arc through the wheel and suspension intersecting points when viewed from the front. This helps to determine how weight transfer affects the deflection of the suspension.

            Anti-dive and anti-squat are percentages that refer to the dive that occurs when braking or the rear of the vehicle squatting during acceleration. If a vehicle is rear drive with inboard brakes and half shafts such as a Jaguar uses, this is not a factor, but for most vehicles it must be controlled. Forward anti-dive and squat are much more critical to control due to the necessity to maintain vehicle control. These factors are used to help determine the percentage of braking front to rear, better known as brake bias.

            Flexibility and vibration in suspension is determined by the size and composition of suspension bushings. There can also be detrimental vibrations caused by the flexing of structural parts such as during accelerating in a hard turn. Another factor is how to insolate high frequency shock and vibrations. For this, consideration must be made to the design of the suspension components. Tires, springs and shocks will tune out most vertical vibration, but lateral noise and vibration must be filtered by the suspension bushings and components.

Unsprung weight is an important factor. Unsprung weight is those components such as wheels, tires hubs, spindles and brakes that are not controlled by the suspension. The lower the unsprung weight, the better it is. This is the reason for the popularity of alloy wheels and also now seeing the usage of aluminum in suspension components. 

            Space occupied is critical in front wheel drive vehicles. McPherson struts require much less space than most other designs. This is also a reason why most vehicles do not use inboard brakes even though it reduces unsprung weight, although cost is another reason.  Force distribution is the matching of the suspension mountings too the frame design in regards to strength, geometry, rigidity and materials.

            Air resistance or drag is another consideration, especially with today’s high importance on fuel efficiency. Some vehicles actually use a height adjustable suspension to lower drag at higher speeds. Also, you are now finding suspension components that are made from oval as opposed to round tubing to cut drag. Also, in many higher performance cars, you will see the spring/shock assemblies moved inboard out of the airstream and being controlled by rocker arms or pull rods. 

            The final factor is something that enters into almost every facet of our lives, that being cost. Even though it is not the most efficient, most rear wheel drive vehicles, especially trucks, still utilize the solid, unsprung rear axle as it is still the most cost effective rear wheel drive system.

            We have now covered the factors that are required to be considered when designing a vehicle suspension system. Starting next week, we will explore the history of and current types of suspension systems in use.

Portions of this post were sourced from www.wikipedia.org.