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| 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, 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 tight 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 |
A single, perfectly tight 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. | ||
| Since the C4, the Corvette has had widely-spaced double mounts on the front. The rear spring has |
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 the suspension compresses and the end of the leaf is pulled up, the center of the leaf spring between the two mounts moves down. This in turn reduces the spring force on the wheel on the opposite side of the car. In this way, the leaf acts like an anti-roll bar. | ||
| When both sides of the suspension compress, the center length between mounts forms a U, with each wheel contributing. When one wheel moves up while the other moves down, the leaf is forced to make an “S” shaped bend. One wheel resists of the motion of the other, again like an anti-roll bar. The C4 engineers had hoped that the resulting anti-roll rate would be sufficient to eliminate the need for conventional anti-roll bars. It was not, but those required were smaller and lighter. | When both sides of the suspension compress, the center length between mounts forms a U, with each wheel contributing. When one wheel moves up while the other moves down, the leaf is forced to make an “S” shaped bend. One wheel resists of the motion of the other, again like an anti-roll bar. The C4 engineers had hoped that the resulting anti-roll rate would be sufficient to eliminate the need for conventional anti-roll bars. It was not, but those required were smaller and lighter. | ||
| The C2 and C3 Corvettes from 1963 until 1983 also used a transverse leaf spring with two mounts, but it was constructed of multiple steal leafs with plastic anti-friction liners, and the mounts were closer together. These two traits prevented it from acting as an anti-roll bar, and caused it to be heavier and less reliable than the modern one-piece unit. | |||
| It is true that in the Corvette C4 and subsequent generations, the motion of one wheel deliberately impacts the motion, or more accurately the instantaneous spring rate, of the other wheel. However, this is common to all “independent” suspensions that use anti-roll mechanisms. | It is true that in the Corvette C4 and subsequent generations, the motion of one wheel deliberately impacts the motion, or more accurately the instantaneous spring rate, of the other wheel. However, this is common to all “independent” suspensions that use anti-roll mechanisms. | ||
Revision as of 10:29, 8 December 2006
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.
Leaf springs on the Corvette
The Corvette suspension has unequal length double wishbones, or A-arms, for all four wheels, and half-shafts in place of a solid rear axle. This allows independent articulation for each wheel. A-arms are considered the ideal arrangement for performance cars because they keep the wheel perpendicular to the road, regardless of chassis movement. All modern Ferraris use this system.
Traditionally, a coil spring is mounted between the chassis and each lower A-arm. The coil compresses in proportion to the spring rate when the A-arm rises, and it is this resistance against compression that suspends the car.
GM has equipped the Corvette with two one-piece fiberglass composite leaf springs in place of coils. They run transversely across the width of the car, mounted in two places equidistant from the centerline. Each end is bolted to the bottom of an A-arm such that when the A-arm rises, the leaf pulls it down, again in proportion to a known spring rate. In this way, four coils are replaced with two leaf springs.
Because both coils and leafs in these configurations act only as simple springs and are not required to stabilize the wheels, their function is almost identical.
Top Gear once joked that as trucks use leaf springs, the Corvette must handle like one. In reality, the Corvette's leaf springs afford certain advantages and disadvantages.
Advantages of transverse leaf springs
- 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.
- 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 Corvette leaf has ever been replaced due to fatigue failure.
- 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 Corvette, the leaf spring acts as an anti-roll bar, allowing for smaller and lighter bars than if the car were equipped with coil springs.
Disadvantages of transverse leaf springs
- 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.
- Susceptibility to damage. Engine fluids and exhaust modifications like cat-back removal might weaken or destroy composite springs over time. The 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.
The leaf spring as an anti-roll bar
The extent to which a leaf spring acts as an anti-roll bar is determined by the way it is mounted.
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 tight 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.
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 the suspension compresses and the end of the leaf is pulled up, the center of the leaf spring between the two mounts moves down. This in turn reduces the spring force on the wheel on the opposite side of the car. In this way, the leaf acts like an anti-roll bar.
When both sides of the suspension compress, the center length between mounts forms a U, with each wheel contributing. When one wheel moves up while the other moves down, the leaf is forced to make an “S” shaped bend. One wheel resists of the motion of the other, again like an anti-roll bar. The C4 engineers had hoped that the resulting anti-roll rate would be sufficient to eliminate the need for conventional anti-roll bars. It was not, but those required were smaller and lighter.
The C2 and C3 Corvettes from 1963 until 1983 also used a transverse leaf spring with two mounts, but it was constructed of multiple steal leafs with plastic anti-friction liners, and the mounts were closer together. These two traits prevented it from acting as an anti-roll bar, and caused it to be heavier and less reliable than the modern one-piece unit.
It is true that in the Corvette C4 and subsequent generations, the motion of one wheel deliberately impacts the motion, or more accurately the instantaneous spring rate, of the other wheel. However, this is common to all “independent” suspensions that use anti-roll mechanisms.
For more information, please see the external link below.
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.
- Open-wheel cars are too low to use them; the leaf spring must span the width of the car, and to do that, it 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.
Coil spring conversion
Anecdotal evidence suggests that most Corvette owners are satisfied with the car’s suspension performance. Of those that are not, a small percentage have converted to coil springs by Penske or otherwise. The chassis has all the necessary hardpoints already; the conversion is just a matter of removing the leaf spring and installing coilover shocks in place of the existing shocks.
Suffice to say, however, the stock suspension is plenty capable. The stock 2006 Corvette Z06 posted a time of 7:42.99 around Germany’s tremendously challenging Nürburgring racetrack. Only a handful of cars in history have gone faster; neither the Ferrari F430, nor any Lamborghini, are among them.
Composite transverse mono-leaf spring
In addition to the Corvette, a composite transverse leaf spring has been used on the Volvo 960. Many small European cars such as the Fiat 128 used transverse steel springs in similar fashion.
External links
- http://www.carbibles.com/suspension_bible.html
- http://www.autozine.org/technical_school/suspension/tech_suspension1.htm
A scan from page 44 of The Newest Corvette, a 1984 book by Michael Lamm:
- http://temp.corvetteforum.net/c4/gcrouse/Suspension/fiberglass_spring.gif
- http://www.circletrack.com/techarticles/ctrp_0602_leaf_springs/
