Parabolic spring kits represent a genuine evolution of the MGB's rear suspension, replacing the original multi-leaf arrangement with a modern tapered design that addresses the fundamental limitation of the standard springs.
Why Standard Springs Fall Short
The standard MG leaf spring is a collection of steel leaves clamped together, forming a strong arch capable of carrying significant loads. However, the individual leaves each flex differently, generating friction between them that increases with corrosion and age. Interleaving, the thin metal shims placed between the leaves, helps, but the binding force holding the leaves together still means a substantial impact is needed before the spring will begin to move. Small bumps in the road do not generate sufficient force to overcome this inertia, so the shock is transmitted rigidly through the springs and into the car.
This is why the standard MGB leaf spring gives such a characteristically hard ride, and why owners instinctively steer around potholes and manhole covers.
How Parabolic Springs Work
Parabolic springs have a taper rolled into them, thickest at the centre and precisely thinned toward both ends. The thickness and degree of tapering is calculated proportionately to the spring's length and the load it must carry. Manufactured from high-quality chrome-vanadium steel using modern heat treatment techniques, this engineered taper creates a flexing characteristic that can be controlled far more precisely than a multi-leaf spring, with wider operating parameters that allow progressive flexing from lesser road impacts while retaining full load-bearing capability under extreme conditions. The MGB parabolic system uses two leaves.
To eliminate the interleaf friction that plagues the standard spring, an additional crank is formed into the lower leaf, steering it away from the upper mounting spring. The lower leaf moves freely and only contacts the upper spring at either end beneath the shackle mounting eye, meaning interleaf friction is eliminated entirely and both leaves compress and flex independently. The result is a spring that responds far more quickly and smoothly to the road surface, isolating the body from shocks in direct proportion.
Fitting Requirements
Because parabolic springs react much faster than multi-leaf units, they must always be paired with telescopic rear shock absorbers, the improved efficiency of telescopic dampers is required to match the pace of the parabolics. Fitting parabolic springs with the original lever arm dampers will result in a poorly controlled rear end. Each kit includes the tapered springs, U-bolts, fixings, and polyurethane pads. A special feature of parabolic springs is that with only two leaves, shim plates are used above and below the spring to make up the overall thickness of the original six or seven-leaf pack.
This preserves the correct distance to the bottom plate where the shock absorber mounting shackle sits, ensuring full damper travel is maintained. These shim plates also provide a useful lateral levelling facility, if the car leans to one side, shims can be redistributed from above to below the leaves on one side to correct the inclination while maintaining the same overall pack thickness.
Available Kits
Kits are available for banjo axle chrome bumper roadsters from 1962 to 1967, tube axle chrome bumper roadsters from 1967 to 1974, chrome bumper GT, rubber bumper roadsters from 1974 to 1980, and rubber bumper GT. Fast road polyurethane bush versions are also available for tube axle chrome bumper roadster, rubber bumper roadster, and rubber bumper GT applications. Fitting follows the same basic procedure as standard spring replacement using a spring fitting kit, with all fixings fully tightened only when the car is lowered to the ground and the vehicle's weight is on the U-bolts. All nuts should be rechecked after the first few hundred miles.