elliptic spring
A curved leaf spring of different styles.
elliptic spring: a curved leaf that bends both ways
An elliptic spring is a composite leaf spring formed by stacking two curved steel plates or blades together, each shaped like a segment of an ellipse. The two leaves are usually clamped at the center and work as a pair, with one leaf curving upward and the other downward under load. This symmetrical arrangement allows the spring to compress and extend more smoothly than a simple cantilever leaf, distributing stress more evenly across both blades and providing a progressive spring rate.
The term "elliptic" refers to the curved profile of each leaf, not the overall shape. When unstressed, each blade follows roughly a quarter-ellipse arc. Under vertical load, the blades bend toward a more vertical orientation, then rebound. This geometry is why elliptic springs appear in vehicle suspensions, carriage axles, and industrial machinery where controlled deflection and energy absorption are required. The springs are typically made from medium-to-high carbon steel or alloy steel, heat-treated to withstand repeated flexing.
Common variants include the full-elliptic spring, which uses two complete curved leaves mounted back-to-back, and the semi-elliptic or half-elliptic spring, where a single leaf is anchored at one end and left free at the other. The full-elliptic type was standard on horse-drawn carriages and early automobile suspensions because it handles both bumps and body roll efficiently. Semi-elliptic springs are lighter and cheaper, making them widespread in lighter vehicles and equipment.
Real-world performance and failure
Elliptic springs typically deflect 50 to 200 millimeters under normal vehicle loads, depending on blade thickness, span length, and number of leaves stacked. Fatigue cracking usually starts at the center clamp or at stress concentrations where leaves meet. Corrosion and loss of clamp tension are the most common field failures. Wear between leaves generates noise and reduces effective stiffness over time.
The design fell out of favor in automotive suspension during the 1980s as manufacturers adopted coil springs paired with dampers, which offer better isolation and tuning. However, elliptic springs remain in use in agricultural equipment, trailers, and light commercial vehicles where simplicity, durability, and low cost outweigh the need for refined ride quality. They are also found in precision machinery where predictable spring rate and low hysteresis are valued.