Simanaitis Says

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DOING THE CHAPMAN STRUT, AND OTHER MOVES

THE FIRST automotive front suspensions were beam axles with steering spindles at each end, only one step past a kid wagon’s pivoted assembly. Sturdy, simple and inexpensive, it communicated bumps on one side directly to the other. However, beam front axles (and variations such as split-beams with central pivots) had their adherents for years.

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Beam front suspension. Bugatti Type 35. Image by Curt Johnson from http://goo.gl/0Bftgb.

One of the most artful was the Bugatti design, a beam suspended on a pair of semi-elliptic leaf springs arranged longitudinally. The springs passed through square openings of the hollow tubular beam; simple, lightweight, elegant—and likely the devil to fabricate (especially in contrast to the traditional standard technique of a couple of U-bolts).

Independence of the front suspension separates left/right action, but it also opens a path to wheel shimmy. Various means have been used to control IFS shimmy.

My favorite traditional IFS is on the Morgan (and, later, Lancia): the sliding pillar. The term “traditional” is appropriate because H.F.S. Morgan used sliding pillars on his first car in 1909 (http://wp.me/p2ETap-14b). Apparently independent of this, Vincenzo Lancia came up with them on his Lambda model in 1922 (http://wp.me/p2ETap-1CM).

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Sliding pillar independent front suspension. Early Morgan trike.

The idea is to have the steering spindles ride on vertical pillars of the chassis, with coil springs providing the suspending medium. In theory, this means the wheels are perpendicular to the road (a Good Thing for handling)—but only so long as the pillars remain perpendicular to it too.

In a limber chassis like the Morgan’s, this second perpendicularity is up for grabs, but extra-firm springs solve the problem deftly, in effect by making the entire willowy chassis part of the suspension.

To quell an IFS’s shimmy tendencies, Morgan devised steel damper blades that work fine when their friction is just right. (The Dreaded St. Malvern’s Dance can occur otherwise.)

In the 1930s, GM’s Maurice Olley brought science into independent front suspension designs. By contrast, Ford eschewed IFS until the end of the 1940s.

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Double-wishbone IFS. Image by RB30DE.

For a long time, double wishbones were the standard IFS design around the world. As the term implies, upper and lower arms control each front wheel’s vertical motion, with a coil spring the typical suspension medium.

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Coil-over IFS. MGB upgrade. Image from Classic Conversions, http://goo.gl/zJwLtm.

This double-A-arm design is still common, ranging from coil-over units in racing cars to applications in heavy-duty vehicles.

In 1945, Earle S. MacPherson was appointed chief engineer of Chevrolet’s Light Car project. He and his team devised what has become even more ubiquitous in auto IFS design: the MacPherson strut.

Alas, GM canceled the project; MacPherson left for Ford—and the first production cars to use MacPherson struts were English Fords. (My 1958 Consul Convertible had them.)

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MacPherson strut IFS. 2012 Honda Civic.

With a MacPherson strut, the top of its telescopic damper unit serves as the upper steering pivot, with a coil spring typically wound around the strut. A lateral link provides its lower location. The design is light and inexpensive to manufacture. It offers less freedom in choosing handling dynamics than a double-A-arm design, but not so much that plenty of high-performance cars, Porsche 911s among them, use MacPherson struts.

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Chapman strut, as fitted to a Lotus Twelve. Image by John Chapman (Pyrope).

By the way, if used in an independent rear suspension, these are often called Chapman struts, honoring Lotus founder Colin Chapman’s idea of using them back there. ds

© Dennis Simanaitis, SimanitisSays.com, 2014

4 comments on “DOING THE CHAPMAN STRUT, AND OTHER MOVES

  1. Bill Urban
    December 3, 2014

    Years ago a Tom Wilson word picture stuck with me, when he answered a question about ladder chassis vs. monocoque – suspension characteristics can only be optimized when they have a rigid structure to act against (or words to that effect).

    • simanaitissays
      December 3, 2014

      Or, in Morgan’s case, just make the springs extra firm and let the entire limber chassis act as the suspending medium.
      (I’ll “suspend disbelief” in this case…)

  2. Mike B
    December 6, 2014

    My 1976 Capri had classic McPherson struts in front. The only official adjustment was for toe-in; all else was set if the body was straight. Of course, that wasn’t true: caster was a bit off resulting in unfixable pull (though one shop did a little work with a sledge that helped), and movement of the anti-roll bar in its rubber bushings made things a little flexible too. It worked, but I wouldn’t want that suspension on a heavier car (though the Capri was no lightweight for the time) or one where you want steering precision. But golly it was cheap to build (though not, at the time, to replace shocks in)!

  3. Bill Rabel
    December 8, 2014

    I remember an old dirt track racing maxim, “Any suspension will work if you don’t let it”.

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