Suspension
Deformation
Race car suspensions feel
precise and responsive when compared to the vague, sloppy feel of
street car suspensions. Although many factors contribute to the handling differences
one key is the mechanism for
suspension attachment, the pickup points.
Street
cars use rubber bushings between movable suspension components like
"A" arms and the chassis. Rubber bushings are used
for several reasons; they isolate
vibration, accommodate flexing and misalignment, they are
inexpensive, quiet, maintenance free and long lasting.
It's the ability of rubber bushings to compress and deform that
accommodates the movement.
Unfortunately
that same compression and deformation prevents rubber bushings from precisely
positioning suspension components. Cornering, bump and braking forces
compress bushings causing loss of alignment as
components shift about unpredictably. The result is a vague and
imprecise suspension that hurts performance.
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McPherson strut
with compressible rubber bushings
corner
load causes loss of camber
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An
example is the popular McPherson strut front suspension used on
many cars including Porsche 911. The production 911
uses rubber bushings to attach the front "A" arms and
at the top of the strut.
As
shown in the illustration, cornering forces at the tire are
transferred to the rubber bushings, compressing them and causing loss of camber.
Bumps and heavy braking also cause deformation. Similar
deformation happens at the rear suspension.
Varying
combinations
of cornering, bump and braking forces create wandering alignment
settings front and rear making the car difficult to control. For
this reason, rubber bushings are not typically used in race cars.
Race
car pickup points are typically metal bearings (heim
joints, rose joints and monoballs), and hard- compound polyurethane
bushings. Metal
bearings deliver the best performance with very low friction, no
deformation and quiet operation. They are not typically used on
production road cars because they cost more than rubber and transmit more road
vibration.
Polyurethane
bushings have been frequently used to replace rubber bushings in an effort
improve performance. Nearly incompressible
under suspension-level forces, polyurethane is able to maintain alignment
settings under load. Polyurethane absorbs more road noise and
vibration than metal bearings, however not as much as
rubber.
Unfortunately,
polyurethane
bushings squeak badly making them irritating for street
cars. Achieving proper fit can be difficult and normally requires custom
machining of the polyurethane. Failure to achieve precise
fitment results in friction, binding or slop.
Hybrid
polyurethane / bronze (PolyBronze™) bearings have
emerged that are superior to old-style polyurethane bushings.
PolyBronze bearings combine the low friction, silent operation
and precise fit of metal bearings with the vibration isolation
characteristics of polyurethane bushings. PolyBronze does
not have the squeaks and installation difficulty of
polyurethane bushings.
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| McPherson
strut with metal and metal/poly bearings
camber
is maintained under corner loads
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Let's
look at the same McPherson strut suspension with spherical
bearings and PolyBronze
bearings installed in place of the rubber bushings.
Now
the suspension is able to sustain the cornering forces without
deformation. Camber is maintained such that the tire contact
patch remains optimal and traction maximized. The
suspension moves freely through its range of motion with very
low friction, no binding and no noise.
Upgrading
the rubber bushings is one of the most effective ways to
improve handling and performance.
Click
for more information about suspension bearings for 911,
914
and 944
series.
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catalogjun03.pdf
