How Do Control Arm Bushings Affect Ride?

by Richard Rowe
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Conceptually, a vehicle suspension is a pretty simple thing. For thousands of years, humanity got by using little more than banded leather and rubber straps, which connected chariot and buggy axles to frames and allowed the axle to move over large bumps instead of forcing the frame to absorb the shock. These leather strips eventually became metal leaf springs, which turned into coil springs and torsion bars. Metal was an improvement in terms of durability and capacity, but it was a lot harder and heavier than leather and rubber, and so had trouble absorbing smaller bumps and vibrations. So suspension engineers went back to the drawing board -- which brought them right back to strips of rubber.

Control Arm Bushings

Control arms connect the wheel hub to the chassis, and pivot at both ends to allow the wheel hub to move up and down. On the rear of most vehicles, the pivot is a simple bolt that passes through a hole in the wheel hub carrier or a bracket on the axle, and a matching hole in the end of the control arm. On the other end of the arm, a second bolt passes through another bracket and hole and allows the am to pivot on the chassis. Up front, the wheel hub or "knuckle" has to turn, so the arm uses a ball joint on the hub side. A "bushing" is effectively a sleeve, commonly rubber, that slides in between the pivot bolt and the hole in the control arm and chassis or axle bracket. The sleeve is commonly a half-inch thick or more, and has a smaller, metal sleeve inside so the bolt shank doesn't directly rub on the rubber.

Bushing Purpose

Roads are highly textured things, with imperfections of all sizes. A vehicle's main springs handle very large bumps like potholes and speed bumps. The vehicle's air-filled tires act as a kind of secondary suspension, flexing to ride smoothly over smaller imperfections like pebbles, and small cracks and ridges in the road. But tire rubber is comparatively stiff and the tire itself is heavy; so if you want to absorb small bumps you need a light, soft spring. The smallest vibrations and imperfections in the road would travel right through the tire rubber and main springs, and into the vehicle chassis without a much smaller and softer bit of rubber to absorb them. Bushings deal with these smallest vibrations.

Affect on Ride

Because they primarily control the smallest of vibrations and tame small road imperfections, bushings contribute greatly to a reduction in what engineers call "NVH," or "noise and vehicle harshness." If you've ever ridden a hard-tail motorcycle or driven a go-cart, you've experienced a vehicle without bushings. Every tiny vibration in the chassis and the road sneaking up through the seat, through the steering wheel and floor, shimmying upward through your spine and rattling your fingers, bones, eyeballs and teeth. Over the short term, it's tolerable -- but over a long period of time, these tiny vibrations make covering any distance a physically punishing ordeal. Good bushings in an automobile all but eliminate these uncomfortable road vibrations, helping the vehicle to glide rather than thrum harshly over pavement at high speed. Luxury cars tend to have very thick or very soft bushings to make for a more serene driving experience.

Affect on Handling

The downside of using bushings is that they allow the suspension control arms to twist, wobble around and move in unintended directions. Rubber is squishy, so bushings introduce a degree of imprecision to the suspension movement. This reduces grip, and forces engineers to use stiffer main springs to maintain performance and suspension control. In racing and performance applications, the reduction in vibration and isolation from the road can itself be a problem; performance drivers need to "feel" the road, and require very precise real-time feedback to keep their tires on the limit of adhesion. For this reason, most race cars and many high-performance street cars use bushings of either solid metal, or near-solid polyurethane plastic. This makes the car feel much sharper and more precise, but it also makes it uncomfortable and harder to tolerate for long periods of time.

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