Double-wishbone suspension has been around for awhile, but the later versions have seen the most improvements. Double-wishbone suspension incorporates two wishbone-shaped arms, also called A-arms, placed in parallel on each side of the axle for each wheel. One end of the wishbone arm connects two joints to the chassis, while the other single joint connects at the steering knuckle, or wheel pivot point. A coil spring and shock mounts to the wishbone arms, to control vertical movement. Double-wishbone suspension allows independent wheel travel in several smaller axis, and depending upon the design, a vehicle owner can discover how this unique suspension works and how it is applied to modern vehicles.
Coil Spring Type 1 Wishbone
Coil spring type 1 wishbone suspension uses two A-arms stacked over each other. The arms are sometimes designated as upper and lower control arms. The base of the A-arms have two connection joints each, which mount to the vehicle chassis. The single end joints of the A-arms connect to the top and bottom of the wheel spindle, allowing a vertical hinging movement. The bottom A-arm functions as the main load carrier. A coil-over spring connects the bottom A-arm to the top shock tower. When the A-arms arc through their vertical movement, it allows a slight side-to-side motion, known as "scrub." During normal A-arm travel relative to the chassis position, a toe or steering angle and camber angle motion is allowed.
Coil Spring Type 2 Wishbone
The coil spring type 2 wishbone suspension has a double A-arm configuration similar to the type 1, although the bottom control arm can have a single support rod (arm) that connects to the chassis at one end and the spindle at the other, which takes away the "A" design. Type 2 wishbone suspension is not as popularly used as the type 1, since it requires more vertical room to accommodate the top-mounted shock and spring.
Multi-Link Wishbone Suspension
Multi-link suspension represents the latest innovation in double-wishbone suspension design and operation, commonly used in the Audi A4 and A8 cars. The double A-arm design feature is retained, but each arm of the upper and lower wishbones are separate constructions instead of one-piece, solid units. They join at the top and bottom of the spindle, similar to type 1 and 2, but have extra pivot joints in their length, which comprises four suspension arms. The extra pivot joints allow the suspension geometry to change when the wheel is turned, applying torque to all four suspension arms. This unique design provides improved road-holding qualities and allows for multiple suspension compensation and adjustments. In certain designs, shocks mount independently from the coil spring.
Double-Wishbone Suspension Advantages
Double-wishbone suspension lends itself well to calibration of each moving joint in the system, since the load is more evenly spread between parts. With the variations of double wishbone systems, design loads are more simplified and can be tuned for different load, speed and cornering applications. Part design weights can be reduced and customized since load factors for each part are more readily known. Double-wishbone suspension allows for a more complete negative camber gain through maximum "jounce" -- rocking or bouncing -- travel, unlike the McPherson design, which only allows negative camber gain at the beginning of jounce travel.
Double-Wishbone Suspension Disadvantages
Double-wishbone suspension contains more moving parts than the simplified McPherson strut design. Replacement and service costs are higher for the added double-wishbone parts, and they often require multiple adjustments, which adds to labor time. Due the the complexity and added parts, the double-wishbone design adds extra weight to the vehicle chassis.
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