Topics > Suspension Deformation
Topic - Suspension Deformation
happens to suspension components under load?
Race car suspensions feel precise and
responsive when compared to the vague, sloppy feel of production car
suspensions. Although many factors contribute to the handling differences
one key is the mechanism for
suspension attachment, the pickup points.
cars use rubber bushings between movable suspension components like
"A" arms and the chassis. Rubber bushings are used
for several reasons; they isolate vibration, have low friction, 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.
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.
with compressible rubber bushings
load causes loss of camber
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.
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.
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.
car pickup points are typically metal bearings (heim
joints, rose joints and monoballs), and hard-compound polyurethane
bearings deliver the best performance with very low friction, no
deformation and quiet operation. They are not widely used on
production cars because they cost more than rubber and absorb less
vibration. An exception is the ball joints used in most
production cars which are essentially monoballs.
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.
bushings squeak badly making them irritating for street
introduces substantial friction into the suspension creating a
harsh ride and erratic wheel
rates. Achieving proper fit can be difficult and normally requires custom
machining of the polyurethane.
polyurethane / bronze (PolyBronze™) bearings have emerged ( for 911,
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
with metal and metal/poly bearings
is maintained under corner loads
look at the same McPherson strut suspension with spherical bearings
and PolyBronze bearings (for 911,
installed in place of
the rubber bushings.
the suspension is able to sustain the cornering forces without
deformation. Camber is maintained such that the tire contact patch
remains optimal and tire grip maximized. The suspension moves
freely through its range of motion with very low friction, no
binding and no noise.
the rubber bushings is one of the most effective ways to improve
handling and performance.
Moreland - June 2003
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