Polyurethane
Bushings and Friction
Polyurethane
bushings are a common replacement for rubber suspension
bushings. They reduce suspension
deformation under load, providing more precise cornering.
They also transmit road vibration, create ride harshness and
interfere with smooth transitions of the contact patch.
Ride
harshness is often blamed on the hard-compound of polyurethane
bushings. Though partly true, that is not the primary cause of
harshness.
In
fact, there is another process in play - friction.
Friction is the primary cause of ride harshness with
polyurethane bushings.
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| Rubber
Bushing
Rotates
via Deformation
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Rubber
bushings - how they work
For
all their faults,
rubber bushings allow suspension movement with very little
friction. Rubber bushings do not slide in their mounts,
they accommodate movement by deforming in a twisting motion.
There is no friction surface hence the friction is very, very
low. Rubber bushings resist movement due to the spring
rate of the rubber, not friction. Their behavior is
similar to a torsion spring though the spring rate is small.
Polyurethane
bushings - how they work
Aftermarket
polyurethane bushings are completely different than the rubber
bushings they replace. Instead of deforming, the
polyurethane forms a friction-surface that slides around the
steel suspension member or mounting point.
 |
| Polyurethane
Bushing
Rotates
on friction surface
|
Unfortunately
the polyurethane-on-steel friction coefficient is significant
causing them to "grab" the steel liners. With
the weight of a vehicle resting on the polyurethane the
friction becomes substantial. The problem is compounded under
high speed cornering loads or heavy braking. The problem
is further compounded if the polyurethane bushing fitment is
not precise or bushing alignment is poor. Grease will help
reduce the friction but doesn't last long, as demonstrated by
the many cars with squeaky polyurethane bushings.
Friction
and damping
Early
automobiles actually used friction-type dampers. Their
performance is horrible and use was quickly eliminated in
favor of hydraulic dampers.
The
key problem with friction dampers is static
friction and the resulting large force to start the
suspension moving. Once moving, the dampers begin
absorbing energy with relatively low kinetic
friction. Essentially the suspension is locked in
position until a large bump creates enough force to overcome
the static friction of the suspension. The result is a
very harsh ride that is insensitive to small bumps.
Contrast
this with a modern hydraulic damper that begins motion with
very low force. The damping action increases with the
speed of the damper. The suspension responds well to both
small and large bumps yielding improved ride quality, superior
tire-to-road contact, and road-holding.
Though
friction-type dampers are an extreme case, any friction in the
suspension causes similar ride harshness.
Stiction
Static
friction in suspensions is often called "stiction".
The word invokes an appropriate image of a sticking, jerky,
binding suspension that does not operate smoothly and only
responds to large inputs (bumps).
Unfortunately,
some level of stiction is present in all automotive
suspensions. Ball joints, shock seals, and bushings all
introduce some stiction. Stiction is the enemy of
road-holding performance and ride quality. Though it can't be
eliminated, all good handling cars take pains to minimize
stiction. True race cars use metal heim joints and
suspension bearings to minimize friction.
The
high-stiction characteristics of polyurethane bushings have
created their reputation for harshness.
Stiction
and performance
The
purpose of a performance suspension is to keep the tire
contact patch optimal at all times to maximize grip. For
a suspension to work it must move in response to bumps, road
contours and driver input. Stiction interferes with movement
attempting to lock the suspension in place.
Unfortunately, stiction is greatest under high corner and
braking loads - just when grip is most critical.
Stiction
also makes accurate corner
balancing of the vehicle nearly impossible. Stiction
creates corner weights that lack repeatability.
Measuring
stiction
With
the car parked on a level surface, lift one bumper corner by
hand extending the suspension as high as possible. Don't
simply release, but SLOWLY let the car return to normal height
under it's own weight. Do not push down. Measure
and record the bumper height.
Next
press down on same corner compressing the suspension. SLOWLY
allow the car to return to normal height. Measure and
record the bumper height.
The
difference between the two heights is a measure of the
cumulative stiction.
Several
factors influence the acceptable range of good values
including spring rate and vehicle type. But in general a
difference of 1/4 inch would indicate low stiction, a
difference of 1 inch or greater would indicate high stiction.
Reducing
stiction
Identify
and understand the operation of all friction points in your
suspension. This includes "A" arm bushings,
ball joints, steering tie rods, shock seals, shock mounts,
sway bars, linkages and anything that moves with the
suspension.
Ensure
that all these friction components are in top shape, replace
anything that is worn. Ensure that all items requiring
lubrication are properly lubricated.
Replace
high-friction elements like polyurethane bushings with low
friction alternatives. Low friction alternatives include
PolyBronze
bearings, rubber bushings, and heim joints / monoballs
as appropriate for 911,
914
and 944
series.
Ensure
that all bushing mounts are properly aligned. Misaligned
mounts result in pinching, binding and excessive stiction.
Click
for more information about suspension bearings for 911,
914
and 944
series.
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