Beating Around the Bushings: Suspension & Steering Joints

May 5, 2015 | Reply More
BMW pioneered the all-aluminum vehicle suspension. The resulting weight reductions and strength gains helped pave the way for rapid adoption of multi-link suspension and four wheel independent systems. When diagnosing and repairing these complex suspensions, technicians must go beyond traditional bushing replacement. They must also consider the other components that may be affected by the bushing installation process, or, based on the age of the replaced bushings, are likely to also need replacement soon.
[caption id="attachment_1963" align="aligncenter" width="615"]The new BMW 3 Series CoupÈ and Convertible - steering wheel (01/2010) In BMW’s Electric Power Steering (EPS) system, a torque sensor measures the force the driver applies to the steering wheel, and adjusts the vehicle’s steering characteristics. BMW calls the system “low maintenance” – translation: You can replace rubber boots and adjust alignment, but everything else is factory-sealed.[/caption]

Unsprung: The Revolutionary BMW E39

The BMW E39 1995-2003 5-Series was the first production vehicle sold with almost all-aluminum front and rear suspension. The axle carrier, control arms, outer strut tubes, steering knuckles, -- everything except the wheel bearings, were aluminum alloy. The result was a significant savings in unsprung weight, which benefitted handling, ride quality, and fuel economy. Aluminum alloy control arms were lighter while simultaneously stronger and smaller. The increased space and power these smaller components provided allowed BMW engineers to pack even more chassis control technology onto the vehicles, even while decreasing overall weight. Larger brake systems, double wishbone and double-pivot front and multi-link rear suspension, and higher precision suspension angles were just a few of the benefits of aluminum alloy use. The BMW E39 quickly earned a reputation as a superior handling vehicle.

Absorb and Isolate

On the BMW E34, E28 and E32, the upper control arms are attached to the front subframe through bushings that handle a lot of the load on the front end when braking. These bushings are typically fluid-filled rubber cylinders surrounded by a metal sleeve. Bushings are typically used in BMW control arms and also in thrust arms. On all but the sportiest platforms, BMW uses either soft rubber or hydraulic bushings for driving comfort. For racing applications, harder rubber and urethane compounds provide more durable bushings. These tougher bushings contribute to a stiffer suspension than is desired by most luxury vehicle owners. The bushing end of a control arm helps isolate the vehicle body from vibration and noise due to minor road surface roughness. Bushings also help maintain wheel positioning, preventing undesirable camber and toe changes as the vehicle moves over uneven road surfaces, or through hard turn or braking maneuvers. The resulting torsional, or twisting, load on the bushings over time causes the rubber to weaken and the fluid to leak out. The driver will eventually notice reduced suspension performance and poor handling. Dry cracks, leaks and rattles are signs of excessive bushing wear. Bushings can wear so much that, in extreme cases, the wheel can lean enough to touch suspension components. But don’t automatically assume that the bushings are the only parts that need replacement.

The Joint is Jumping!

The other end of

 

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Category: Steering + Suspension, the bimmer pub

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