Torsional rigidity

Torsional rigidity /tor-shuhn-uhl ri-jid-i-tee/ noun (uncountable)

Torsional rigidity is the measure of a vehicle's chassis resistance to twisting or flexing forces, typically measured in Newton-metres per degree of twist. This is the engineering term for how much a car's body refuses to bend when you drive it enthusiastically into a corner. A car with poor torsional rigidity is built on a foundation of jelly, its chassis twisting and contorting over bumps, making the suspension's job impossible. For decades, many British sports cars, particularly the open-topped variety, had all the structural integrity of a poorly assembled tent. Achieving high torsional rigidity is the secret to making a car feel solid and handle predictably.

The Full Story of Torsional Rigidity

To understand why torsional rigidity matters, picture a car entering a corner. As the driver turns the wheel, immense force is placed on the front outside suspension. In a car with a weak, flexible chassis, this force does not just compress the spring; it physically twists the entire car. The front corner lifts, and the car's body contorts diagonally, lifting the opposite rear corner. The suspension cannot do its job properly because the points it is bolted to are moving. The car feels vague and unpredictable because the very platform the driver is commanding is constantly changing shape. This was the unfortunate reality for most early cars built on a simple ladder frame.

This problem was magnified tenfold in the classic British sports car, the roadster. Chopping the roof off a car is like taking the lid off a shoebox; the structure loses almost all of its inherent stiffness. The result was a phenomenon known by the deeply unglamorous name of "scuttle shake." This was the visible and unnerving flexing of the front part of the car, where the dashboard, windscreen, and steering column would wobble and shudder over bumps, seemingly independent of the rest of the vehicle. Stories abound of classic convertibles where parking with one wheel on the kerb would twist the body so much that the doors could not be opened. This was often passed off by enthusiasts as "character."

The quest to solve this problem drove some of Britain's greatest engineering innovations. Colin Chapman of Lotus understood the importance of a stiff chassis better than anyone. His spaceframe design for the Lotus Seven used a web of interconnected triangles, an inherently rigid shape, to create a light and stiff platform. His masterstroke was the backbone chassis of the Lotus Elan. This was a single, deep steel beam that ran down the middle of the car, providing all the necessary torsional stiffness by itself. A pretty, non-structural fibreglass body was then dropped on top. The result was an open-topped car that was famously rigid, a key reason for its sublime and legendary handling.

The ultimate solution for mass-produced cars was the monocoque, where the entire steel bodyshell is welded together to form a single, stiff box. The fixed roof of a saloon or coupe is a critical part of this structure, which is why a convertible version of a modern car still requires a huge amount of extra, heavy bracing underneath to compensate for the loss of the roof. Today, engineers use powerful computers to design bodyshells with astronomical rigidity figures, creating the solid, secure-feeling cars we now take for granted. The wobbly chassis, once a hallmark of the British sports car, has been engineered into extinction.

For The Record

How is torsional rigidity measured?

The standard method involves clamping the chassis at one end (usually the rear suspension points) and then applying a measured twisting force (torque) to the other end (the front suspension points). The amount of twist, measured in degrees, for a given force gives the final figure in Newton-metres per degree.

Why is a convertible less rigid than a coupe?

Because the roof is a critical structural element. A fixed roof connects the pillars and completes the "box" of the passenger compartment, making it highly resistant to twisting. When you remove the roof, you lose this structural lid, and the chassis is much more prone to flexing, like an open shoebox.

What is "scuttle shake"?

It is the name given to the unpleasant vibration and flexing felt through the dashboard, steering column, and windscreen of a convertible with poor torsional rigidity. When the car hits a bump, the front of the body, or "scuttle," can be seen and felt visibly shaking.

Did the Lotus Elan's backbone chassis solve this?

Yes, brilliantly. The deep, rigid central backbone beam provided all the necessary torsional stiffness, meaning the open-topped fibreglass body didn't have to do any structural work. This is why the Elan was famously stiff and handled so well, unlike most of its contemporaries, which flexed and wobbled.

Is more rigidity always better?

Generally, yes, up to a point. Immense rigidity allows the suspension to be tuned with great precision. However, for some applications like off-roading, a small amount of chassis flex can actually help the vehicle maintain traction over uneven terrain. For a road car, however, stiffer is almost always better.