The MacPherson strut is a type of automotive suspension system that uses the top of a telescopic damper as the upper steering pivot. It is widely used in the front suspension of modern vehicles and is named for American automotive engineer Earle S. MacPherson, who invented and developed the design.
Earle S. MacPherson was appointed the chief engineer of Chevrolet's Light Car project in 1945, to develop new smaller cars for the immediate post-war market. This gave rise to the Chevrolet Cadet. By 1946 three prototypes of the Cadet design had been produced. These incorporated the first MacPherson strut independent suspension both in front and rear. The Cadet project was cancelled in 1947 and the disgruntled MacPherson was enticed to join Ford. Patents were filed in 1947 (U.S. Patent 2,624,592 for GM) and in 1949 (U.S. Patent 2,660,449 for Ford), the 1949 patent citing designs by Guido Fornaca of FIAT in the mid-1920s. The strut suspension of the pre-war Stout Scarab could have been an influence and long-travel struts in aircraft landing gear were well-known by this time. French Cottin-Desgouttes utilized a similar design, albeit with less sophisticated leaf springs, however the Cottin-Desgouttes front suspension was in turn inspired by American engineer J. Walter Christie's 1904 design.
MacPherson designed the strut for all four wheels, but it is normally used for the front suspension only, where it provides a steering pivot as well as a suspension mounting for the wheel.
The first production car to use MacPherson struts is often listed incorrectly as the French 1949 Ford Vedette, but it was developed before MacPherson with an independent front suspension based on wishbones and an upper coil spring. Only in 1954, after the Vedette factory had been purchased by Simca, did the revised Simca Vedette switch to using front struts.
A MacPherson strut uses a wishbone, or a substantial compression link stabilized by a secondary link, which provides a mounting point for the hub carrier or axle of the wheel. This lower arm system provides both lateral and longitudinal location of the wheel. The upper part of the hub carrier is rigidly fixed to the bottom of the outer part of the strut proper; this slides up and down the inner part of it, which extends upwards directly to a mounting in the body shell of the vehicle. The line from the strut's top mount to the bottom ball joint on the control arm gives the steering axis inclination. The strut's axis may be angled inwards from the steering axis at the bottom, to clear the tyre; this makes the bottom follow an arc when steering.
To be really successful, the MacPherson strut required the introduction of unitary construction, because it needs a substantial vertical space and a strong top mount, which unibodies can provide, while benefiting them by distributing stresses. The strut will usually carry both the coil spring on which the body is suspended and the shock absorber, which is usually in the form of a cartridge mounted within the strut (see coilover). The strut can also have the steering arm built into the lower outer portion. The whole assembly is very simple and can be preassembled into a unit; also by eliminating the upper control arm, it allows for more width in the engine compartment, which is useful for smaller cars, particularly with transverse-mounted engines such as most front wheel drive vehicles have. It can be further simplified, if needed, by substituting an anti-roll bar (torsion bar) for the radius arm. For those reasons, it has become almost ubiquitous with low cost manufacturers. Furthermore, it offers an easy method to set suspension geometry.
Many modern implementations replace the lower control arm by a wishbone. An anti-roll bar is optional and if present is attached by a ball-jointed rod to the spring-damper or by a ball or elastomerically jointed rod to the wishbone.
Advantages and disadvantagesEdit
Because MacPherson struts are packaged with a significant structure in the front crash structure of the car, it is easier to engineer cars that pass more stringent small overlap crashes with struts over a double wishbone suspension. Notable examples include the Honda Accord and Civic as well as the Mercedes E-Class which both adopted struts to improve crash performance. The overall simplicity of the design also means there are fewer joints in the suspension to wear over time, affecting handling and steering feel. Inverted monotube struts can also provide extra rigidity in the front suspension as seen in the Porsche 911 GT3 and Cayman GT4 as well as the Subaru Impreza WRX STI. Finally, struts can package more efficiently than other types of front suspension, which allows for significant cargo space in rear/mid engined cars like the Porsche 911 and Boxster.
Geometric analysis shows it cannot allow vertical movement of the wheel without some degree of either camber angle change, sideways movement, or both. It is not generally considered to give as good handling as a double wishbone or multi-link suspension, because it allows the engineers less freedom to choose camber change and roll center. Cars that have cockpit adjustable ride height generally cannot have MacPherson struts because of the camber changes that are an unavoidable part of the design. Ride suffers because the shock absorber has almost the same vertical motion as the wheel, so there is relatively little leverage to break the stiction in the seals. A standard single pivot MacPherson strut also tends to have positive scrub where the center of the steering axis is offset from the center of the front tires, which results in torque steer.
Despite these drawbacks, the MacPherson strut setup is still used on some high performance cars, because they tend to have relatively small suspension travel, and so do not see the kinematic problems.
Porsche 911 up until the 1989 model year (964) used a similar strut design that does not have coil springs, using torsion bar suspension instead, after that point all Porsche 911s have had front MacPherson struts.
In recent years, General Motors and Ford have introduced a modified strut setup, "Hi-Per Strut" and "Revoknuckle" respectively, that split the strut into two components that handle the up-and-down flexibility and steering dynamics separately. The benefits of this design are greater surface contact and reduction in torque steer. The drawback are the additional weight and cost but less expensive than either a double wishbone and multi-link setup. Honda also introduced another variation strut setup called "dual-axis" applied in the suspension setup of the Civic Type-R. Another variant of the MacPherson strut is the double pivot front suspension which splits the lower wishbone into two while retaining the standard upright design of the MacPherson strut. This allows for better control of steering geometry and scrub radius while allowing for a larger brake assembly.
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