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Tech Topic - Suspension Deformation

What 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.

Production 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.

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. 

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 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.

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. 

Unfortunately, polyurethane bushings squeak badly making them irritating for street cars. Polyurethane 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.

Hybrid polyurethane / bronze (PolyBronze™) bearings have emerged ( for 911, 914 and 944) 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.

McPherson strut with metal and metal/poly bearings  camber is maintained under corner loads

Let's look at the same McPherson strut suspension with spherical bearings (for 911, 914, and 944) and PolyBronze bearings (for 911, 914 and 944) 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 tire grip 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. 

Chuck Moreland - June 2003

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