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.

Ask Joe Mechanic: Steering and Suspensions Part 3

The suspension of a vehicle serves a number of different purposes. The first and probably

most important is that it locates each wheel in relation to the others and provides a stable platform for steering and braking control of the vehicle. Additionally, it enables overall driving enjoyment and safety. The second purpose of a vehicle suspension is to isolate the passengers and cargo from bumps and irregularities in the road surface. Generally, these two requirements are at opposite ends of the suspension-tuning spectrum, so it requires a compromise that optimizes control without sacrificing comfort.

            The most important thing with any suspension is to keep the tire in constant contact with the road. Considering that each tire only has a few square inches in contact with the road surface at any given time, it is of utmost importance to maximize that contact. This is accomplished by using springs, shocks and linkages to properly locate each wheel. In many vehicles, there are different systems used at each end of the vehicle.

            If we had roads that were perfectly smooth, we would not need suspension systems. But as we all can attest to, our roads are far from smooth. If we did not have a suspension to absorb the bumps and jolts, the tires would never be able to stay in contact with the road surface. Newton’s first law states that for every action there is an equal and opposite reaction. This means that for every upward bump, this would launch the wheel upward with an equal force. For every pothole, the vehicle would first be forced downward and then upward as we exit the hole. 

            As stated earlier, the two most important factors are the ride and handling. These two things can be broken down into three determining factors, which must work together. The first factor is road isolation, which means finding a way to absorb the shock of road irregularities without causing undo motion or upsetting the stability of the vehicle. The second factor is road holding. This means a number of different things. It is regulating the transfer of motion from front to rear during acceleration or from rear to front during braking. It is also the transfer of weight from side-to-side to minimize weight transfer in order that the tires maintain a constant contact and friction patch with the road. The third factor is cornering. This means the ability to maintain directional stability and control weight transfer and road contact during changes in direction. This also includes controlling body roll. 

            To develop a working suspension system, which will control all these things and give us a ride quality that is acceptable for the type vehicle we have requires consideration and computation of quite a few factors. I have compiled a list of twenty-two factors, which must be considered in any suspension design for it to work properly. There could even be more factors to consider, but these are all definitely part of any design study. I plan to list each, along with a brief description of what each is. I won’t attempt to make you understand how each is determined, but give basic information to illustrate how complex this system can be to design, despite the finished product looking quite simple. 

            The first factor is spring rate. This factor determines the ride height of the vehicle and also the amount of force required to compress the suspension a set distance. When a spring is compressed or is extended, there is a force exerted that is proportional to the change.  The spring rate is determined by the change in the force it exerts divided by the change in defection of the spring. This is why a compact car has much smaller springs than a pickup truck. 

            The second factor is wheel rate. Basically, on an independent suspension vehicle, this is very similar to the spring rate. But on a vehicle with a solid axle such as a pickup truck, there are other factors that weigh in. When a solid-axled vehicle is cornering, there are different effects depending if it is accelerating or braking. For this reason, the springs are located as closely to the wheels as possible to limit the twisting motion.

            The third factor is extremely difficult to determine because it is affected by so many of the other considerations. This factor is known as the roll rate. Roll rate is influenced by the vehicle ride height, the center of gravity, the vehicle’s sprung weight, the track width, the spring and shock rates, the roll center heights front and rear, the stiffness of the anti-rollbars, and tire pressure and construction. The roll rate will often differ from front to rear to allow for turning ability and steady state handling. The roll rate does not change the amount of weight transfer in a vehicle; it actually controls the speed at which those changes occur. 

            We will continue this suspension system factors discussion in coming posts. Some information for this post obtained from www.wikipedia.org.

Ask Joe Mechanic: Steering and Suspensions Part 2

This week we will continue the discussion of steering systems and then get into the basics

of suspensions. Last week we discussed the common types of steering, recirculating ball and rack and pinion. A system that has been tried by a number of manufacturers in recent years is four-wheel steering. There are two types, active and passive.

            In active four-wheel steering, there is a set of steering linkages connected to the rear wheels similar to what is used in the front. Most rear steering is electronically controlled with a system of sensors and actuators. In most types, the rear wheels will turn differently dependant on speed. At low speed, such as for parking, the rear wheels will steer opposite what the front wheels do to reduce the turning radius required. Meanwhile, at higher speeds, such as during highway driving, the electronic controls will turn the wheels in the same direction as the front wheels, which will increase directional stability. General Motors offered a system of this type on Chevy and GMC trucks and Tahoes. However, the response was limited, with only about 16,500 vehicles being sold in the three years it was offered so they discontinued it in 2005. It is offered now mostly by higher end brands like BMW, Infinity, Porsche, Lexus and some Mazdas.

            Passive four-wheel steering is a more commonly seen system, and on many cars, you are not even aware that it is there. Basically, through the design of linkages and bushings, the lateral forces generated during turning at higher speeds will turn the rear wheels slightly inward to increase the directional stability of the vehicle.

            One other very important feature of the steering system is the collapsible steering column. This feature was originally introduced by Mercedes in 1959 and was adopted by the American automakers during the 1960s after extensive lobbying by Ralph Nader. This was much more important with the recirculating ball steering systems where the steering box was mounted forward on the frame.  However, even with the firewall mounted rack and pinion systems of most cars, it is still an important safety feature that most people do not even think about.

            In the next post we will start to explore suspensions and the different types of systems and how they work. We will also examine some of the calculations that go into determining how a system is designed.

Elements of this post have been sourced from www.wikipedia.org.