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Suspension Compliance

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My track platform is a BMW 135i. The original front suspension arms are forged aluminum with a combination of spherical bearings and rubber bushings (one of them being fluid-filled). The original rear suspension arms are pressed steel with a combination of spherical bearings and metal-shelled rubber bushings at the joints. A simple upgrade path involves replacing most of the arms with their M3 counterparts, which are forged aluminum with stiffer rubber bushings and/or spherical bearings in place of the original rubber bushings. It seems clear that stiffer bushings or spherical bearings in forged arms mean less compliance should be the result, but testing reveals something more nuanced. The steel arms all incorporate features which provide for controlled buckling under overload compression conditions. The forged arms are all curved, which increases their compliance sufficiently that a compliance-based upgrade argument becomes a bit dubious. They are curved, by the way, to provide for a progressive failure under overload compression conditions. When progressing to aftermarket fully spherical straight arms, compliance will be significantly reduced, but overload failure modes will become more dramatic (something your insurance will be less than thrilled with if you car is street-driven).

This link provides some testing history that, while BMW-centric, may be of interest to others "upgrading" their suspension arms and bushings.


My car has now migrated to a fully spherical suspension as it is a dedicated track car, but that conversion was not without interesting detours.


Great insight and info as always, James

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