Suspension
Binding
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.
 |
 |
|
Ball
joint and shock in normal
operating
range |
Lowered
car, ball joint / shock
range
exceeded and 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.
|
|
Bushing
with primary and
secondary
axis of rotation |
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
series} to provide incompressibility and freedom of motion on
multiple axis simultaneously.
|
| Axis of rotation
out of alignment with mounts |
Similarly,
the primary axis of rotation of a suspension member may be poorly
aligned with its bushings. 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.
 |
 |
| Linkage
in operating range |
Extreme
angle causes binding |
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.
Click
for more tech topics
|
