Corvette leaf spring

Corvette leaf spring commonly refers to a type of independent suspension which utilizes a fiber-reinforced plastic (FRP) mono-leaf spring instead of more conventional coil springs. It is named after the American sports car Chevrolet Corvette^[1] for which it was originally developed and first utilized.^{[2][3][4][5][6]} A notable characteristic of this suspension configuration is the mounting of the mono-leaf spring such that it can serve as both ride spring and anti-roll spring. In contrast to many applications of leaf springs in automotive suspension designs, this type does not use the spring as a locating link. While this suspension type is most notably associated with several generations of the Chevrolet Corvette the design has been used in other production General Motors cars as well as vehicles from Volvo and Mercedes-Benz. Fiat and Zastava produced cars with a similar configuration using a multi-leaf steel spring in place of the FRP mono-leaf spring. Several manufactures and suppliers have since researched, adapted and utilized variations of this suspension design.

Design

The C5 Corvette's rear suspension

This suspension configuration is independent and utilizes control arms to define the motion of the wheel as the suspension is compressed. The coil springs are replaced with a single, FRP spring which spans the width of the car. As in independent suspension systems using coil springs, and unlike the common leaf spring supported Hotchkiss rear axle, the suspension kinematics are defined only by the control arms. Because the movement of one wheel is not determined by the position of the other this is an independent suspension type.

As in a coil spring suspension design, the FRP mono-leaf spring supports the weight of the vehicle. However, the FRP leaf springs differ from steel coils and traditional steel multi-leaf springs in a number of significant ways. The FRP plastic springs have 4.3–5.5 times the strain energy storage per weight as compared to steel.^[7] This results in a lighter spring for a given application. The single FRP mono-leaf front spring used on the fourth generation Corvette weights 33 percent of the weight of an equivalent set of coil springs.^[8] FRP spring weight savings as compared to conventional leaf springs in similar applications are even greater. The third generation Corvette offered an optional FRP mono-leaf spring as an alternative to the standard multi-leaf steel spring. The 22-kilogram (48 lb) steel spring was replaced by a 3-kilogram (7 lb) mono-leaf spring.^[9] Volvo claims a weight savings of 5 kilograms (10 lb) by using a FRP spring in the rear suspension of the second generation XC90 as compared to designs using coil springs.^[10] In addition to reduction in vehicle weight, the weight of the spring is partly unsprung mass.

The relative sliding movement of the leaves of a multi-leaf steel spring results in stiction based hysteresis with respect to spring compression. This stiction reduces suspension compliance and can compromise both ride quality and handling.^[11] Lacking individual leaves, the mono-leaf spring avoids stiction.

FRP springs are advertised as having exceptional cycle life and corrosion resistance.^[8] A GM test comparing the third generation Corvette springs found the multi-leaf steel springs failed spec after 200,000 full travel cycles. The replacement FRP leaf spring showed no loss of performance after two million full cycles.^[9]

Packaging is cited as both an advantage and disadvantage of the transverse FRP leaf spring as compared to coil springs depending on the application. The FRP spring is typically packed low in the suspension resulting in a low center of gravity. It also allows manufactures to avoid tall spring mounts thus resulting in a flatter load floor about the suspension.^[10] James Schefter reports that as used on the C5 and later Corvettes the use of OEM coilover damper springs would have forced the chassis engineers to either vertically raise the shock towers or move them inward. In the rear this would have reduced trunk space. In the front this would have interfered with engine packaging. The use of the leaf spring allowed the spring to be placed out of the way under the chassis and while keeping the diameter of the shock absorber assembly to that of just the damper rather than damper and spring.^[12] However, in other applications such as race car designs, the need to span the width of the vehicle result in significant design limitations. Coil and torsion springs present better packaging options for racing applications. FRP springs also have limited availability and selection as compared to coil spring.^[13] Higher cost has also been cited as a disadvantage when compared to coil springs on production road cars.^[14]

Properties

FEA model of a leaf spring under load. The initial, unbent shape of the spring is shown as a silhouette box. An upward deflection on the right side of the spring results in a smaller upward movement on the left side.

A notable advantage of the FRP transverse leaf springs when supported with widely spaced, pivotable mounts is the ability to supplement or replace the anti-roll bar. Typically springs that provide a sufficient ride rate need a supplemental spring (the anti-roll bar) to increase the suspension roll rate. The coupling of the two sides of the transverse leaf spring across the vehicle results in an anti-roll bar like behavior. Corvette engineers have cited this property as enabling the use of a lighter anti-roll bar^[9] and even eliminating the rear anti-roll bar on some versions of the seventh generation Corvette.^[15]

When either wheel is deflected upward, the center span of the spring (the portion between the pivotable mounts) deflects downward. If both wheels deflect upward at the same time (displacing the suspension in bump) the center section bends uniformly between the pivot mounts. In roll only one wheel is deflected upwards. This tends to try to form the center of the spring into an "S" like curve. The result is the wheel rate of one side of the suspension depends on the displacement of the other side.^[8][9][13] The extent to which the spring acts as an anti-roll bar depends on the distance between the pivot mounts and the rigidity of the pivot mounts.^[8]

A transverse leaf spring with a central rigid mount. The two spring halves are effectively isolated. Movements of one half of the spring do not affect the other half.

A simplified flat, rectangular spring illustrates this principle. Deflecting the right side of the spring results in the left side rising. In comparison, a rigid central mount (2nd and 3rd generation Corvette and other cars) shows no movement on one side when the other is deflected.

Applications

A number of manufactures have produced vehicles or concepts utilizing independent front or rear suspensions supported by transverse leaf springs.

• Chevrolet Corvette: C4-C7, 1984–present(front and rear).
• GM W platform cars: first generation (Lumina, Grand Prix, Regal, Cutlass Supreme).
• GM E platform cars: (Eldorado, Toronado, Riviera, Reatta, Allante).^[16]
• Volvo 960 (from model year 1995) renamed to S90 (sedan)/V90 (wagon) from model year 1997 to 1998.^[17]
• Volvo XC90 (second generation)^[18]
• Mercedes-Benz Sprinter vans (front only, Hotchkiss in rear)^[19]
• Smart ForTwo Mk1-Mk3 (used with MacPherson Struts)^[20]
• Volkswagen 1-litre car, prototype^[21]
• Indigo 3000, 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.^[22]
• The Fiat 128 used a similar system with a more traditional mutli-leaf steal spring but configured with two pivots to provide the anti-roll effect described earlier.^[23]
• Transverse leaf spring and solid axle front suspension of early Ford cars.

Recent patents and related research

Several automotive companies have researched suspension designs using a transverse composite leaf spring supported in a fashion similar to that of the Corvette.

• Ford Global Technologies, 2006 patent #7029017, Wheel suspension for a motor vehicle with a transverse leaf spring.^[24]
• Porsche AG, 2000 patent # 6029987, Front Axle for a Motor Vehicle. Describes a strut suspension system supported by a transverse leaf spring system largely the same as that used by the Corvette. The Porsche patent mentions the beneficial stability effects of this arrangement^[25]
• Honda, 1992 Transverse leaf spring type suspension patent #5141209^[26]
• DaimlerChrysler, 2004, patent #6811169, Composite Spring Design that also Performs the Lower Control Arm Function for a Conventional or Active Suspension System^[27]
• ZF released a concept rear suspension design in October 2009 using a composite spring based rear suspension. The strut based suspension uses a transverse leaf spring to function as both ride and anti-roll spring. The ZF concept differs from the system used on the Corvette by using the leaf spring as one of the suspension links.^[28]

See also

• Leaf spring

References