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elliptic spring

Corvette leaf spring

Since 1963, transverse leaf springs have been an integral part of the suspension of GM's Chevrolet Corvette. This article concerns the practical differences between leafs and coils, popular misconceptions, and the design considerations that led to their inclusion in the Corvette.

Traditional use of leaf springs

A leaf spring is a long, flat, thin, and flexible piece of spring steel or composite material that resists bending. The basic principles of leaf spring design and assembly are relatively simple, and leafs have been used in various capacities since medieval times. Most heavy duty vehicles today use two sets of leaf springs per solid axle, mounted perpendicularly to support the weight of the vehicle. This system requires that each leaf set act as both a spring and a horizontally stable link. Because leaf sets lack rigidity, such a dual-role is only suited for applications where load-bearing capability is more important than precision in suspension response.

Leaf springs on the Corvette

All six generations of the Corvette have used leaf springs in some capacity. The basic arrangement for each generation is listed as follows:

  • C1 (1953-1962):

Front: Independent unequal-length double wishbones with coil springs
Rear: Rigid axle supported by leaf springs and longitudinal control links

  • C2 (1963-1967), C3 (1968-1982):

Front: Independent unequal-length double wishbones with coil springs
Rear: Independent suspension with trailing and lateral links supported by a centrally mounted leaf spring

  • C4 (1984-1996):

Front: Independent unequal-length double wishbones with transverse fiberglass mono-leaf spring mounted to allow for anti-roll effect.
Rear: Independent suspension with trailing and lateral links supported by a centrally mounted fiberglass mono-leaf spring

  • C5 (1997-2004), C6 (2005-):

Front: Independent unequal-length double wishbones with transverse fiberglass mono-leaf spring mounted to allow for anti-roll effect.
Rear: Independent unequal length double wishbones with transverse fiberglass mono-leaf spring mounted to allow for anti-roll effect.

In the C2 and subsequent generations, a leaf spring is mounted transversely in the chassis and used in conjunction with several independent suspension designs. Common to these post-C1 Corvettes, the leaf acts only as a spring, and not a suspension arm or a link. Because it is not required to stabilize the wheels, the leaf functions in much the same manner as a coil spring. This configuration obviates the drawbacks and imprecision associated with traditional Hotchkiss (non-transverse leaf spring) suspensions.

Although commonly referred to as a "leaf spring suspension" it is more accurately calls an independent double A-arm suspension.

Motion of a transverse leaf spring

The following images show the movements of an independent suspension using a transverse leaf spring. For all images:

  • The suspension arms are green
  • The chassis is blue
  • The uprights are gray
  • Leaf springs are dark gray
  • Pivot links connecting the ends of the springs to the suspension arms are red

Illustrations #1 and #2 show independent left and right leaf springs mounted rigidly to a chassis. In the first illustration, the suspension is at rest. As a left wheel moves up in the second illustration, the left spring flexes upward, but the right spring remains unaffected. Because the two springs are not connected, the movement of one wheel has no effect on the spring rate of the opposite wheel. While the C2, C3, and C4 Corvettes used a continuous spring instead of the split spring of the illustration, left and right spring rates remained independent because the spring was rigidly mounted at its center to the chassis.

Illustrations #3 and #4 show an independent suspension with a single transverse leaf spring, an arrangement similar to that used on the C5 and C6 Corvettes, and the front of the C4 Corvette. While at rest in illustration #3, the leaf forms a symmetric arc between the left and right sides of the suspension. Under the compression of both wheels in illustration #4, the widely-spaced chassis mounts allow the spring to pivot; the ends of the spring flex upward and the center moves down.

The leaf spring as an anti-roll bar

The extent to which a leaf spring acts as an anti-roll bar bar is determined by the way it is mounted. The fundamentals of this explanation appear in the Michael Lamm link at the end of this article, as well as in several patents. US Patent #6189904 is of particular relevance.

A single, loose center mount would cause the spring to pivot about the center axis, and push one wheel down as the other was compressed upward. This is exactly opposite an anti-roll bar, and has not been used on any generation of the Corvette.

A single, perfectly rigid center mount that held a small center section of the spring flat against the frame would isolate one side of the spring from the other. No roll or anti-roll effect would appear. The rear spring of the C2, C3, and C4 has this type of mount, which effectively divides the spring in two. It becomes a quarter-elliptic spring.

The C2 and C3 Corvettes from 1963 until 1983 used a rear transverse leaf spring with a central rigid mount. The spring was constructed of multiple steel leafs with plastic anti-friction liners and closely-spaced mounts. These traits prevented it from acting as an anti-roll bar.

Since the C4, the Corvette has had widely-spaced double mounts on the front. The rear spring has had double mounts since the C5. The spring is allowed to pivot about these two points. When only one wheel is compressed as in illustration #5, the portion of the spring between the mounts assumes an "S" shape, bending in two directions. As a result, the spring force applied to the right suspension arm is reduced as the left side compresses, like an anti-roll bar. The caster, camber, toe-in, and general orientation of the left wheel remain unchanged.

This anti-roll effect is the result of the compound bend, the "S-bend", that the spring must make when the wheels are not level. A compound bend requires the spring to assume a tighter bend radius. The tighter bend radius requires more force than a larger one thus greater force must be applied to the spring.

When both wheels are level the force applied by the spring to the suspension is even between the sides.

When only one side of the suspension is compressed, the leaf spring is forced into an "S-bend" shape. This results in a compound bend in the spring, as opposed to the single bend in the case when the wheels are even. The compound bend has the effect of increasing the stiffness of the spring on the side which is being compressed. At the same time, the bend reduces the force on the side which is not compressed.

This transfer of force from one side to the other is exactly the same way an traditional anti-roll bar functions. In the case of the Corvette the effects of the anti-roll bar and leaf spring are additive. The operation of the leaf spring and anti-roll bar do not affect each other but their forces add together at the wheel. This additive property allows the Corvette engineers to use a smaller, lighter anti-roll bar as compared to using a separate left and right spring (coil or leaf).

When this system was initially in design for the C4 Corvette, the engineers had hoped that the resulting anti-roll rate would be sufficient to eliminate the need for a conventional front anti-roll bar. A supplemental anti-roll bar was ultimately implemented because sufficient roll rates could not be obtained with the anti-roll forces of the leaf spring alone. [See Michael Lamm reference]

Transverse leaf springs within independent suspensions

Advantages

  • Less unsprung weight. Coil springs contribute to unsprung weight; the less there is, the more quickly the wheel can respond at a given spring rate.
  • Less weight. The C4 Corvette's composite front leaf weighed 1/3 as much as the pair of conventional coil springs it would replace. Volvo reported that the single composite leaf spring used in the rear suspension of the 960 Wagon had the same mass as just one of the two springs it replaced.
  • Weight is positioned lower. Coil springs and the associated chassis hard mounts raise the center of gravity of the car.
  • Superior wear characteristics. The Corvette's composite leaf springs last longer than coils, though in a car as light as the Corvette, the difference is not especially significant. No composite Corvette leaf has ever been replaced due to fatigue failure, though steel leafs from 1963 to 1983 have been. As of 1980, the composite spring was an option on the C3.
  • As used on the Corvette, ride height can be adjusted by changing the length of the end links connecting the leaf to the suspension arms. This allows small changes in ride height with minimal effects on the spring rate.
  • Also as used on the C4 front suspension, C5, and C6 Corvettes, the leaf spring acts as an anti-roll bar, allowing for smaller and lighter bars than if the car were equipped with coil springs. As implemented on the C3 and C4 rear suspensions with a rigid central mount, the anti-roll effect does not occur.

Disadvantages

  • Packaging can be problematic; the leaf must span from one side of the car to the other. This can limit applications where the drivetrain, or another part, is in the way.
  • Materials expense. Steel coils are commodity items; a single composite leaf spring costs more than two of them.
  • Design complexity. Composite monoleafs allow for considerable variety in shape, thickness, and materials. They are inherently more expensive to design, particularly in performance applications.
  • Cost of modification. Due to the specialized design and packaging, changing spring rates would require a custom unit. Coil springs in various sizes and rates are available very inexpensively.
  • Susceptibility to damage. Engine fluids and exhaust modifications like cat-back removal might weaken or destroy composite springs over time. The leaf spring is more susceptible to heat related damage than conventional steel springs.
  • Perception. Like pushrod engines, the leaf spring has a stigma that overshadows its advantages.

Racing concerns

  • Running stiffer springs left-to-right would require either asymmetrical spring mounts or an asymmetric spring. However, a few companies such as VBP offer kits that allow independent adjustment of spring rate and ride height at all four corners of the car.
  • Regulations often prohibit the use of leaf springs; NASCAR does not allow them.
  • The more compact shape of a coil spring can allow for variation in more suspension design and spring placement. Because a transverse leaf spring must span the width of the car, open-wheel cars are too low to use them. The leaf spring would have to pass through the gearbox or the driver's legs.
  • Coil springs are not car-specific. A Porsche, an LMP, and a Ferrari can all use a spring custom wound on the same generic equipment. Custom composite leaf springs require expensive retooling and cannot be used across car models.
  • The characteristics of coil springs in a performance environment are known, and racers will use what they know. Most race teams do not have adequate experience with leaf springs to use them in this capacity.

Carroll Smith is quoted in his book, Engineer to Win

If I were involved in the design of a new passenger vehicle, however, I would give serious consideration to the use of a transverse composite single leaf spring of unidirectional glass or carbon filament in an epoxy matrix. This would be the lightest practical spring configuration and, although space constraints would seem to limit its use in racing, it should be perfectly feasible on road-going vehicles, from large trucks to small commuter cars. (Since I wrote this paragraph the new-generation Corvette has come out with just such a spring to control its independent suspension systems-at both end of the car.)

Transverse leaf springs in other vehicles

In addition to the Corvette, a composite transverse leaf spring has been used on other GM and non-GM vehicles.

  • Volvo 960 (Wagon only)
  • Volvo S90
  • Mercedes Sprinter vans (transverse in front only)
  • VW 1-Litre-Car prototype car
  • GM W-platform cars- (Lumina, Grand Prix, Regal).
  • GM E-platform cars- (Eldorado, Toronado, Riviera, Reatta).
  • Mercedes Smart ForTwo (used with MacPherson Struts)
  • Indigo, a Swedish made, low volume roadster. Due to the anti-roll properties of the transverse leaf spring setup the car does not use a separate front anti-roll bar.

Many small European cars such as the Fiat 128, the Yugo, and the Triumph Motor Company small chassis cars (Herald, Vitesse, Spitfire, GT6) used transverse steel springs in similar fashion. The Yugo's steel spring used twin attachment points and did provide anti-roll capability.

Recent research on the design concept has been performed. In 2006 Ford Global Technologies was granted patent #7029017 for Wheel suspension for a motor vehicle with a transverse leaf spring

References

  • http://www.carbibles.com/suspension_bible.html
  • http://www.autozine.org/technical_school/suspension/tech_suspension1.htm
  • http://www.circletrack.com/techarticles/ctrp_0602_leaf_springs/
  • http://forums.corvetteforum.com/showthread.php?t=1918713

GM Patent #5425829 desribing a method for creating variable rate composite leaf springs. Includes background information on benefits to transverse leaf springs used with conventional suspension designs

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