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Needle Bearing Suspension

Right idea, wrong solution

PolyBronze™ Bearings are a low-friction replacement for factory rubber suspension bushings. PolyBronze delivers excellent ride quality and extreme road-holding performance (for 911, 914 and 944 series) . The product is vastly superior to traditional polyurethane/plastic bushing alternatives.

We are sometimes asked why PolyBronze incorporates bronze bearings instead of needle or roller bearings. One might conclude that if a bronze bearing is good, a needle must be better. But a quick look at the facts shows that for automotive suspensions needle bearings are a poor choice.

Both types of bearings deliver very low friction when new and properly fitted. But wear issues and misalignment cause serious problems with needle bearings. For these reasons bronze bearings prove superior in automotive suspension applications.

Races wear at needle bearing contact
points only, small angular rotations
cause uneven hill-and-valley wear

Angular rotation and wear

Needle bearing races wear only at the contact points of the rollers. For large angular rotations (shafts that spin round-and-round), the races wear evenly.

false brinelling

But suspension components oscillate back and forth, rotating just a few degrees. Small angular rotations displace lubricant from the needle contact points, depositing lubricant between contact points. The result is dry bearing contact points and accelerated race wear in a hill-and-valley pattern. This process is known as false brinelling or fretting.

Once false brinelling develops, friction of the roller bearing goes up.

In contrast, bronze bearings maintain full contact with the race and deliver even wear for small and large angular rotations. Friction is consistently low over the bearing life.

Hill-and-valley race wear pattern detail

Misalignment and bearing compliance

Control arms and spring plates are each supported by two bearings that pivot on a common axis. Any misalignment of the bearings can cause binding. Broad manufacturing tolerance of factory suspension mounts yield mount alignment that is less-than-precise even in static unloaded conditions. This is no problem with soft rubber OEM bushings, but is a problem with incompressible bearings. This misalignment is compounded as the chassis flexes under load.

The relatively small profile of bronze bearings allows the patented fitment of a polyurethane jacket in PolyBronze bearings. This elastomer layer compresses slightly as needed, keeping mounts in alignment (even as the chassis flexes) without pinching and binding.

In contrast, the physical size of needle bearings makes the inclusion of an effective elastomer layer difficult/impossible within factory mounts that were not designed for their large size. With no elastomer layer, any misalignment pinches and binds the metal needles. The condition gets worse when the chassis flexes under load in turns, braking, bumps and acceleration - just when suspension performance is most critical.

Bronze bearing wears evenly,
full contact on race

Summary

In closing our PolyBronze Bearings (for 911, 914 and 944 series) use bronze bearings because they show superior wear characteristics for small angular rotations seen in automotive suspensions.

Their small profile facilitates packaging with the polyurethane jacket that is so critical to fitment and maintaining proper alignment.

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