Topics > Polyurethane Bushings and Friction
Topic - Polyurethane Bushings and Friction
root cause of road noise and vibration
Polyurethane, delrin and other plastic
compound bushings are a common
replacement for rubber suspension bushings. They reduce suspension
deformation under load, providing more precise cornering.
They also transmit road vibration and create ride
harshness is often blamed on the hard-compound of polyurethane bushings. Though partly true, that is not the primary cause of
fact, there is another process in play - friction. Friction is
the primary cause of ride harshness with polyurethane bushings.
Rubber bushings - how they work
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.
- how they work
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.
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.
automobiles actually used friction-type dampers. Their performance
is horrible and use was
quickly eliminated in favor of hydraulic dampers.
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
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.
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
friction-type dampers are an extreme case, any friction in the
suspension causes similar ride harshness.
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
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
high-stiction characteristics of polyurethane bushings have created
their reputation for harshness.
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
also makes accurate corner
balancing of the vehicle nearly impossible. Stiction creates
corner weights that lack repeatability.
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
press down on same corner compressing the suspension. SLOWLY allow
the car to return to normal height. Measure and record the
difference between the two heights is a measure of the cumulative
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.
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.
that all these friction components are in top shape, replace
anything that is worn. Ensure
that all items requiring lubrication are properly lubricated.
high-friction elements like polyurethane bushings with low friction
alternatives. Low friction alternatives include PolyBronze Bearings for 911,
monoballs for 911,
that all bushing mounts are properly aligned. Misaligned
mounts result in pinching, binding and excessive stiction.
Moreland - January 2004
to our newsletter and receive the latest tech topics when
they are released.