The Story of Knee Action (GM)

What Was GM Knee Action?

Posted on  by MCG

General Motors didn’t invent independent front suspension, of course. But the automaker was among the first to bring the feature to the American mass market in 1934 with the colorful name Knee Action.

This is not widely understood today, but the development of independent front suspension was driven not by a desire to improve handling, as one might expect, but by a serious need to improve the ride. With their traditional beam front axles, which required stiff front spring rates to fight shimmy and other issues, cars of the ’20s and early ’30s suffered relatively poor ride quality. In fact, the ride was usually better in the front seat than in the rear, thanks to the disparity in front vs. rear ride frequencies, which produced unpleasant pitch motions that tossed the rear seat passengers up and down.

General Motors engineers, led by Maurice Olley, carefully studied these issues with the K² rig, a modified Cadillac sedan with an extreme range of weight distribution adjustments. Their work determined that isolating the front wheels from each other would permit front spring rates that were equivalent to the rear, producing the superior “flat ride,” as they called it, they had been searching for.

In March of 1933, a demonstration for the heads of all five GM car brands was arranged with a pair of Cadillacs using two different independent front suspension systems, and they were all so impressed with the ride improvement that they were sold on the spot. As CEO Alfred P. Sloan remembered it, GM technology guru Charles F. “Boss” Kettering proclaimed, “We can’t afford not to do it.” The feature was adopted across the board by Chevrolet, Pontiac, Olds, Buick and Cadillac for the 1934 model year and marketed under the colorful name Knee Action.

The IFS selected by Chevrolet president William S. Knudsen for the Chevy and Pontiac divisions was the Dubonnet system, named after its originator, French auto racer and aperitif heir André Dubonnet (1897-1980). The springs and damper were enclosed in a single housing (above) that pivoted on the kingpin, with the wheel supported by a trailing arm and radius rod. Knudsen reportedly chose the Dubonnet type for its potential advantages in high volume production, a natural concern for Chevrolet.

The cutaway illustration above reveals the insides of the Delco-manufactured Dubonnet-type unit, employing two concentric coil springs and a double-acting hydraulic shock absorber, with the top cylinder managing bump motion and the bottom cylinder handling the rebound. For the 1934 introduction, the system was offered as standard on Pontiacs and on the Chevrolet Master line. (However, the price-leader Chevy Standard continued to use a conventional beam axle and parallel leaf springs.) With its large trailing arm, the Dubonnet-type IFS does indeed resemble the human anatomy, and we can see how the term Knee Action came naturally to the GM marketing people.

Meanwhile: Buick, Oldsmobile, and Cadillac opted to go with the second of the two IFS systems presented, an SLA (short-long arm) arrangement (above) that was not so different from the double-wishbone suspensions of today. (One noteworthy difference: a lever-action shock absorber doubled as upper control arm.) To our eye, this style of IFS does not resemble the human form very much, except perhaps an extremely bow-legged one. Still, all the GM car divisions adopted the term Knee Action in the sales pitches for their IFS systems. In 1937 Pontiac switched from Dubonnet to conventional IFS, and Chevrolet followed in 1939, but the term Knee Action continued.

For a while, it appeared that Knee Action might achieve the Kleenex effect and become the generic, industry-wide term for IFS. But the other Detroit automakers were very soon offering their own IFS systems (minus Ford, notably) and the phrase gradually faded from public use. It was never embraced by the GM Engineering staff, it would appear, and the term finally disappeared from GM sales materials after 1954. Still, we can remember when old-time GM dealer mechanics referred to independent front suspension as “Knee Action,” drawing quizzical looks from their younger co-workers.