We all know what suspension bushings do. But beyond replacing stock rubber bushings with polyurethane models, what is the science behind these parts? What goes into determining what works best for a given application? To gain a better understanding of how these parts work, we contacted the people at Whiteline U.S.
First, there is a scale of suspension bushings that relates to their use. It ranges from rubber bushings for road cars, then polyurethane, which comes in varying stiffness, brass bushings and then spherical joints at the other end of the scale.
For street-driven cars, rubber is the material of choice because it is excellent at absorbing energy, minimizing energy transfer and improving ride quality, and reducing noise vibration harmonics.
According to Whiteline literature, “Original rubber suspension bushings are compressed or chemically bonded to the suspension components in which they are installed. In the case of a steel-cased bushing, the rubber is compressed at the time of manufacture and chemically bonded to the metal center sleeve and outer steel shell. The rubber bushing is normally press-fitted into the suspension component such as a control arm, then bolted to the chassis. In this way, the capacity for suspension movement or rotation is restricted. The rubber suspension bush design requires delicate compromise, as it must be soft enough to permit sufficient rotational movement while maintaining alignment settings.”
Whiteline originally designed its polyurethane components to replace brass bushings in vehicles competing in hill-climb events. Brass was great for competition use, but it was too harsh for normal street use, as are spherical joints, which are reserved for high-performance applications.
The ideal compromise is polyurethane, and even it ranges in hardness depending on formulation. However, today the durometer value is just one element considered when engineering a bushing for specific application and outcome. The chemical makeup, additives, and ratios play a huge factor in a poly compound, and net positive performance results on both ends of the spectrum.
For example, Whiteline bushings come in one of three grades of hardness in each application, with No. 70 durometer being its softer material, No. 85 the intermediate and No. 93 durometer the hardest. Because of advances in chemical composition, today’s materials have higher resistance to tear, abrasion and compression while being softer overall than original formulations.
Whiteline also debuted innovative grease-free sway bar bushings, at the Performance Racing Industry show in December. Whiteline calls it the PTFE lining, which is bonded to the inner bore during the casting process. It reduces heat buildup and squeaking, and eliminates the need to periodically lubricate the bushings.
Whiteline published a fascinating white paper on the subject of bushing deflection in sway bar end links. The accompanying chart shows the difference between a noncompliant link such as a spherical joint (K) and a compliant link that uses polyurethane bushings (k). Compliance refers to the ability of the bushing to absorb or dissipate loads/energy within the chassis and joint. Noncompliant joints are designed to minimize bushing deflection, which increases energy transfer throughout the chassis.
Line (K) indicates that the stress-strain curve of the noncompliant link is constant in its force vs. deflection curve. The second dual-linear line is for a compliant link with polyurethane bushings, which indicates the initial bushing deflection to the point of max deflection, then reverts to the same trajectory as the top line (K). Although the graph is simplified for the sake of understanding, it shows a polyurethane bushing will deflect to its limit, then assume the same deflection curve as a noncompliant link. Whiteline calculates the curves shown in the chart as follows:
Noncompliant Links
K (noncompliant) = A x E/L
K = non-compliant
A = cross sectional area
E = material modulus
L = length
Compliant Links
K (compliant) = k x K/(2 x K + k)
K = stiffness as above in noncompliant case
k = stiffness of the individual bushings
As you can see, you must determine the value of the noncompliant (K) before you can find the value of compliant (k), because there are in effect three springs in series in an end link set that uses polyurethane bushings. The two softer springs at either end represent the compliant bushings and the third “spring” is the material used for the link itself.
For more information and other white papers, visit: www.whiteline.com.au/suspension_parts_resources.php?category=HowTo
