Independent Suspension
Independent suspension /in-di-pen-duhnt suh-spen-shuhn/ noun (uncountable)
Independent suspension is a type of vehicle suspension system that allows each wheel on the same axle to move vertically, reacting to bumps in the road, without affecting the opposite wheel. This is the engineering concept that finally allowed a car's wheels to deal with their own problems individually, rather than dragging the other side into the argument. It replaced the crude "live" axle, a solid beam that connected both wheels like a pair of conjoined twins, ensuring that when one hit a pothole, the other was unpleasantly jolted in sympathy. The move to independent suspension was a great leap forward for ride comfort and road holding, separating sophisticated cars from agricultural carts.
The Full Story of Independent Suspension
For the first few decades of motoring, most cars were built with solid axles front and rear, a design inherited directly from the horse-drawn cart. This was a simple, strong, and cheap solution. At the rear, this "live" axle was a heavy steel tube that housed the driveshafts and was usually suspended on a pair of equally ancient leaf springs. Its great drawback was that the two wheels were permanently linked; a bump on one side would tilt the entire axle, upsetting the composure of the car and the fillings of its occupants.
The first great leap was the widespread adoption of Independent Front Suspension (IFS) after the Second World War. This immediately improved steering response and ride comfort, and even humble British cars like the Morris Minor featured a sophisticated torsion bar IFS. The rear of the car, however, remained a stubborn holdout for the old ways.
Engineering an independent system for the driven rear wheels was a far more complex and expensive challenge. The definitive British solution to this problem arrived in 1961 with the Jaguar E-Type. Jaguar’s Independent Rear Suspension (IRS) was a work of art. It was a complete, self-contained module built onto a subframe, which could then be bolted into the car. It featured coil springs, inboard disc brakes, and cleverly used the driveshafts themselves as part of the suspension linkage. This design was so effective at providing both superb handling and a cosseting ride that Jaguar would use it for the next three decades on all its cars.
While Jaguar perfected the high-end solution, other British companies brought IRS to the masses. Triumph developed a simple but effective swing-axle and later semi-trailing arm system for its small sports cars, giving them a level of agility that their live-axled rivals at MG could not match.
The eventual dominance of front-wheel drive, pioneered by the Mini, made independent rear suspension much easier to implement. With no need to transmit power, a variety of simple, lightweight, and space-efficient designs became possible. Today, some form of independent suspension is used on almost every passenger car, a quiet confirmation that the cleverness of engineers at companies like Jaguar had pointed the way forward.
For The Record
What is a "live axle"?
It is a solid beam or tube that connects the two wheels on an axle and also contains the driveshafts that transmit power to them. When one wheel hits a bump, the entire axle tube tilts, affecting the other wheel. It is simple and strong, but poor for handling and ride quality.
What are "inboard" brakes?
This is where the brake discs are mounted on the final drive unit, next to the differential, rather than out by the wheels. Jaguar's famous IRS used this system. It reduces "unsprung weight," which improves suspension performance and ride quality, but it also makes the brakes much more difficult and expensive to service.
Is a "De Dion" axle a type of independent suspension?
No, it is a clever halfway house. It uses a solid tube to keep the two wheels parallel, like a live axle, but the differential is bolted to the car's chassis, and drive is sent to the wheels via separate driveshafts. This makes it much lighter than a live axle and improves handling, but the wheels are still not truly independent of each other.
Why did it take so long for the rear suspension to become independent?
On a rear-wheel-drive car, it is much more complex and expensive to engineer an independent system that can handle the torque from the engine while also controlling the wheel's movement. For many years, a simple live axle was a far cheaper and easier solution for manufacturers to produce.
Do all modern cars have fully independent suspension?
Most do. However, many smaller, cheaper front-wheel-drive cars still use a non-independent "torsion beam" or "twist-beam" rear axle. This is a simple, cheap, and space-efficient compromise that works well enough for everyday cars but is less sophisticated than a true multi-link independent setup.