Linking the past and the present

Many films feature the ‘under the car’ trope, where a main character, such as Indiana Jones, ends up on the underside of a moving vehicle. Next time you watch Raiders of the Lost Ark, keep an eye out for the real star of the show, the Panhard rod, says Nick Jordan, head of engineering and quality at Pailton Engineering.

If you’re anything like me, when Indiana Jones narrowly escapes death by climbing underneath a speeding truck, you will have noticed the vehicle was equipped front and back with leaf spring suspension systems.

Previously popular in horse drawn carriages, leaf springs are still widely used in heavy vehicles owing to their simplicity and load bearing capacity.

However, since the early 20th century, vehicles have changed drastically. As roads have improved and vehicle speeds have increased, suspension systems have had to evolve to provide a smoother ride.

Smooth operator

For example, coil springs can absorb road bumps and irregularities more effectively than leaf springs. The spring compresses and expands smoothly during jounce and rebound, which helps dampen the vibrations and shocks transferred from the road surface to the vehicle through the tyres.

While they are perhaps not as robust as leaf springs, in lighter vehicles or in extreme conditions, coil spring systems can offer superior performance. Using rubber bushings can further dampen vibration, reducing shocks and impacts coming up through the tyres.

However, the deflection in the rubber bushings and suspension linkages introduces the potential for lateral movement of the axle in a coil spring system. The driver would be able to feel this as imprecise steering and reduced handling capability when turning.

In a leaf spring system, the front and rear connections of the leaves limit lateral movement, but a coil spring suspension needs a lateral location mechanism.

The French company Panhard et Levassor introduced the Panhard rod in the early 1900s to address the issue of lateral axle movement. A near horizontal bar connecting the axle to the chassis, it prevents side-to-side motion. A simple but revolutionary solution that enhanced stability and control.

There are two main requirements when designing placement of a Panhard rod. First, it must be as long as possible. Because one end is fixed to the chassis and the other end is attached to the axle, the up and down motion of the axle will actually be converted to an arc by the rod. A short rod would have a steep arc and introduce dreaded lateral movement — the longer the rod, the flatter the arc and the less noticeable the effect.

Secondly, the rod must be as close to horizontal as possible, so that it performs consistently during cornering. That’s because if the bar is under tension when the vehicle turns left, it will be under compression when the vehicle turns right, and vice versa. If not level, the chassis end of the bar will be driven either upwards or downwards, depending on the direction of forces on the wheels. The driver might feel this as ‘bump steer’.

Changing a vehicle’s ride height, such as by upgrading the suspension system or adding heavy cargo, can increase the angle of the Panhard rod and pull the axle over to one side compared to the chassis. This can make the steering pull to one side or the other, and unevenly wear tyres. Using an adjustable Panhard rod can help realign the axle.

Since its invention around 100 years ago, the Panhard rod remains relatively unchanged. It was still relatively new technology in the setting of Raiders of the Lost Ark. Had the truck been equipped with one, Indiana and the henchmen he was fighting would have had a much more pleasant journey.

Though simple, a quality Panhard rod can be relied upon to enhance your vehicle's performance for thousands of miles. For more information on Panhard rods, get in touch or visit our Panhard rods page..