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.