Shearpin.org - Parabolic Springs

Parabolic Springs

In September of 1997 I stood in the San Diego, California sunshine and contemplated a long list of required repairs for my recent purchase - Eore, the Land Rover. During the previous three weeks he was loaded and run hard in a big loop through Arizona, New Mexico and south through Chihuahua, Mexico's Copper Canyon with a friend named Moe. We had limped home with a bad seal between the transmission and T-case, a weeping water pump, a sagged-out suspension and a steering box that would lock-up at terrifyingly random moments; best friends for life, Eore and I. Standing here, back in the San Diego sunshine, I had my post-travel smile, a stinky, 90-weight oil index finger (from checking fluid levels) and faced a 40 mile daily commute to school. The repairs were difficult to get excited about, but addressing the suspension sag offered the potential to enhance Eore's on and off-road performance.

Parabolic Spring Design

Parabolic springs for Land Rover’s were new in the United States in 1998. The physics of the Parabolic spring dictates that they are thicker in the center of each leaf and taper progressively thinner towards the mounting eyes. The spring uses this parabolic shaped taper to counter the forces at work on an axle. The result is a progressive dampening affect; the spring requires less force for the initial movement and an increasing force as the spring is compressed.

In use, Parabolic springs require less spring-leaves for a given load. The benefits include a decrease in unsprung vehicle weight and an improved ability for the suspension to move. Less leaves means less of the interleaf friction required in traditional leaf-sprung designs to provide a dampening affect. As traditional leaf-springs age and rust, this smooth sliding action degrades and results in an increasingly stiff and jarring ride.

Parabolic Springs In Use

I purchased parabolic springs new in 1998 and have run them continuously to the present day (updated, true in December of 2019). Based on my anticipated axle loads I chose T.I. Console's two leaf front, and the two plus helper-spring rear. With anitcipated loads to include a winch and extra fuel and water - I matched the springs with ARB’s reliable but stiff Old Man Emu shocks (front shock part number N78 w and rear axle part number of N90.)

Eore did suffer a break in the front main leaf - directly in front of the spring plate - in 2003. This was a known weakpoint of early T.I Console production. Without hesitation, I replaced the front springs with @Rocky Mountain Parabolics. The rear set remain the T.I circa 1998.

The effectiveness of parabolic springs continues to be debated on Land Rover forums worldwide. I consider robust discussion to be part of the fun of modifying any vehicle. I have organized my thoughts around the 3 most argued aspects below:

Unpredictable Handling Characteristics – My experience, the springs work the way a suspension should. The body rolls into a corner in a smooth and controlled manner, and then settles for the duration. Emergency steering and braking maneuvers produce predictable and stable results for drivers who recognize the limits of a light rear-end, and the reduced grip of the often fitted mud-pattern tires. In summary - despite load, trailer, or road surface I have always felt planted and in control with this suspension.
Poor Load Carrying Capability - Load carrying ability is important to me. My criteria requires good flex for weekend trails with enough spring to keep Eore off his suspension bump-stops during longer, heavy load travels. For Baja, Mexico, for example, fuel and water will often push payloads to approximately 800 pounds. In this environment, I have found the spring will flex in a controlled, fluid manner throughout the spring range at slow speeds. At higher speeds, like those required for bull dust and deep sand arroyos, the springs provide a smooth and controlled ride and absorb impacts in a progressive manner - without jarring or hammering the bump-stops. I have been impressed with the load carrying performance of Parabolic springs.
Unpredictable Washboard Road Performance –“Washboard road” is the term given to a dirt road with a repeating pattern of small jarring bumps - just like an antique washboard. In Baja, Mexico, for example, you will find 100′s of miles of washboard road. For the sake of your vehicle and mental well-being achieving harmonic speed keeps everything together; this is the speed where your tires skip from peak to peak over the pattern of bumps. Regardless of your suspension, handling and breaking characteristics will be dangerously altered at harmonic speeds. Steering, throttle, and breaking inputs should be done in a slow, smooth, and deliberate fashion. I have found Parabolic springs, coupled with Old Man Emu shocks, function reliably over 100's of miles of this type of road. Whether you are south bound with 800 pounds of gear, or running back to the border lightly loaded - parabolic springs will absorb more abuse then traditional springs while keeping vehicle dynamics manageable. A washboard road safety tip - take frequent breaks, check for loose bolts and fluid loss, and check shock temperatures by cautiously resting your hand on the fluid reservoir.

Parabolic Springs - Final Thoughts

My opinion, Parabolic springs deliver on their promise of a more compliant suspension that maintains the Land Rover's ability to work. When coupled with a quality set of shock absorbers, they are durable and will improve your Land Rover around town, cross-axled on the trail or carrying a heavy load over mixed terrain. Parabolic springs are the type of upgrade that will keep you smiling through the failed water pumps, transfer-case seals, steering box bearings, broken axles - the more mundane Land Rover maintenance and repair tasks.