Suspension Mods, Binding, & Geometry
Vehicle suspensions are designed for smooth movement of the wheels through an operating range. This range is bounded by the use of bump stops; rubber bumpers that prevent breakage and physical contact at the limits of motion. Within this operating range the suspension should move freely, controlled by springs and dampers (shocks).
The term “binding” describes a suspension that does not move smoothly through the operating range. Binding causes dramatic changes in wheel rate as the suspension compresses. Understeer / oversteer tendencies shift rapidly, making handling unpredictable. Ride quality and road grip suffer.
Needless to say, a properly designed suspension should never bind.
Performance suspension modifications such as lowered ride height, geometry, combinations of 3rd party parts and custom parts may cause problems if done incorrectly. No surprise, binding conditions happen all too often in modified suspensions.
If you are going to modify your suspension, understand common causes of binding. Select properly designed performance parts and combine them in ways that do not bind. Check operation over the full range of motion. Following are common conditions that cause binding.
Exceeding Range Of Travel
Ball joints, shocks, tie rods, bushings and other components have limits for angular or linear motion. If individual limits are exceeded these components will bottom out resulting in binding and potential breakage.
Bump stops in stock suspensions prevent individual components from reaching limits. Components are thus protected.
Aggressively lowered cars often have bump stops cut down to regain lost suspension travel. The increased suspension travel may allow individual components to reach range of motion limits causing binding.
Even if bump stops are not altered, altered suspension pickup points can change angle of operation or extension/compression of individual components. As an example consider devices that extend the range of camber adjustment. These camber plates and compensators may cause ball joints to operate at extreme angles, potentially exceeding their limits.
Physical Interference
Swapping components of different size than stock may cause physical interference. Larger springs, shocks and wheels/tires may physically contact bodywork or suspension components and effectively bind the suspension.
Add-on components can also cause interference. Examples are added sway bars or coil-over shocks to cars not originally so equipped. The added components may physically contact other components as they move through their range.
Axis Of Rotation
Through deformation, rubber bushings allow a large range of angular motion along a primary axis of rotation. Some bushings pivot only along the primary axis, others along two or more axis through compression.
Unfortunately polyurethane bushing replacements sometimes find their way into bushings that require multiple axis of rotation. Nearly incompressible, polyurethane binds along any secondary axis.
Polyurethane is an inappropriate choice for such applications. Correct performance replacements for rubber bushings would incorporate spherical bearings ( for 911, 914, and 944) to provide incompressibility and freedom of motion on multiple axis simultaneously.
Similarly, the primary axis of rotation of a suspension member may be poorly aligned with its bushings, due to accident damage or poor design/fabrication. This typically happens when the axis passes through two bushings that are not collinear. Alternately the bushing position may be changeable by alignment adjusters. If the axis can not be similar adjusted, binding will result. Properly designed bushing mounts will ensure proper alignment.
Linkage Operating Angles
Lever arms and pushrods connect sway bars, steering assemblies, remotely mounted shocks and other suspension components. Operating angles and leverage change as they move through their range. Near the center of the operating range the leverage is relatively stable. Exceed the operating range and operating angles become severe, leverage changes rapidly and can bind.