In this brief article, we will discuss the different Types of Suspension, and their applications.
What are the different Types of Suspension?
Let’s look at the vehicle suspension broadly from the perspective of its basic evolution. In the beginning of motoring history, till the 1930’s, almost all vehicles had Solid axles. The Solid axle suspension was basically known as the ‘Dependent’ Suspension. Later on in the evolution of passenger cars and racing cars came the vehicles that moved from a solid axle to independent wheel setups. The Dependent suspension still exists for SUV’s and pickup trucks, but the passenger car segment has largely moved on to ‘Independent’ and ‘Semi-independent’ suspension setups.
The different types of dependent suspension are:
- Multi-link suspension
- Watts Linkage
Leaf Spring Solid Axle Suspension
The Leafspring-type suspension is the most popular among all solid axle suspension systems. A majority of rear wheel drive trucks and commercial vehicles have a leaf spring rear solid axle suspension.
The most common type of leaf spring is a Semi-Elliptic leaf spring. In recent years, the ‘Parabolic’ type of leaf spring has become more common due to its lower weight and better ride comfort.
The leaf spring started out in the early 20th century as the most common type of suspension and can be found on the front suspension of almost all cars of right upto the 1940’s. Later on, in recent times, the leaf spring setup became more extinct and are found only on Pickup trucks like the Ford F150 and the GMC Sierra.
Multi-Link Rear Solid Axle Suspension
The multi link rear suspension can be of the 4-Link or 5-Link Type. The suspension would consist of:
- 2 upper control arms
- 2 Lower Control arms
- A panhard rod (in the case of a 5-Link type)
The upper and lower control arms take up the braking and acceleration and Roll and cornering loads coming from the tires in the case of the 4-Link setup. In many cases, a 4-link setup would flex under heavy cornering loads for vehicles that are performance-oriented. The 5-Link setup basically evolved to overcome this limitation. The Panhard Rod takes up Lateral inputs from the tires.
A well known example of a current day 5 -link solid axle rear suspension is the third generation (D23 platform) Nissan Navara, also known as the Nissan Frontier.
Watts Link Rear Suspension
The Panhard Rod had the disadvantage that it constrained the axle to move up and down in an arc when looking from the vehicle rear. This would cause sideways movement of the axle from bump to rebound. The sideways movement leads to issues in handling predictability. In order to overcome this limitation the Watts Link was developed. The Watts link did the same job as a panhard rod but by splitting the panhard rod into 3 effective linkages.
This would allow the axle to move up and down in a straight line. Cars that came with a factory fitted Watts Link were the Ford Mustang (2005-2014) and the Dodge PT Cruiser (2001-2007).
An independent suspension is defined as any suspension in which the left/right wheel’s vertical motion does not affect the vertical motion of the opposite side (left/right) wheel. Unlike in the case of the dependent suspension, there is no solid structure that directly connects the left and right wheels.
The different types of independent suspension are:
- Double wishbone suspension
- Multi-link suspension
- MacPherson strut
- Transverse leaf-spring
- Torsion Bar Suspension
Double Wishbone Suspension
Double Wishbone Suspension is probably a more commonly found type of independent suspension. It consists of two wishbone arms and a knuckle or wheel carrier. The Wishbones are mounted to the chassis/body at two mounts each, which are usually flexible cylindrical rubber bushings on the one end. At the other end, the wishbones are attached to the knuckle through one ball-joint each. The Coil Spring is the most common spring element seen on the Double wishbone. The early versions of Double wishbone suspensions had a separate Coil spring and shock absorber mounts. But nowadays, a majority of double wishbone suspensions have a coilover damper strut.
Since the Double wishbone occupies more space, it is more common to find this setup on SUV’s and Pick up trucks. Examples of cars with a Double Wishbone front suspension are Ford F150, GMC Sierra, Ram 1500
Multi Link suspension
A Multi Link Suspension is a type of independent suspension which consists of 3 or more control links. The multi-link suspension can be found in basically 2 different forms. For the front suspension, a multi-link suspension appears similar to a double wishbone, except that there are two upper control arms and 2 lower control arms instead of one upper wishbone and one lower wishbone.
In the rear suspension a multi-link suspension could be either 3-link or 4-link depending on the layout. In a 5-link layout, the control arms would be Upper Control Arm, Lower control arm, Toe Control Link, Trailing Arm. The control arms would usually be attached through rubber bushings at the ends.
A majority of Multi link suspensions have a coilover damper strut, which is an advantage in packaging. A classic example of a 4-Link front suspension is the Audi A4 and the Mercedes Benz E-Class. An example of the rear 4-Link suspension is the Volkswagen MQB platform Golf GTI and Audi A3 rear suspension.
Macpherson Strut Suspension
Since economy cars are the most in number, and almost every economy car has at least a front MacPherson Strut suspension, the MacPherson Strut suspension could be considered as the world’s most common type of car suspension.
The MacPherson strut became more popular with cars that have a front-wheel drive configuration, mostly in the economy segment because of the convenience and space the MacPhsersone strut offered. This did not prevent Rear wheel drive performance cars from adopting the MacPherson front suspension. Some of the notable examples are the Porsche 911, BMW 1, 2 and 3 Series M models and the Mustang GT350.
A transverse leaf suspension has the leaf spring transversely mounted and clamped at the middle, leaving the two free ends that can bend along with the wheel movement. In the Corvette rear suspension, the transverse arrangement had a lateral control arm as well. The shock absorber has mounting provisions on the knuckle, quite similar to the double wishbone arrangement.
The transverse leaf suspension is quite rarely seen on cars nowadays. It was more prevalent in Triumphs, Heralds and Fiats from back in the 1960’s. It can be found on the front suspension of the Utility Van Iveco Daily, sold in Europe.
Torsion Bar Suspension
Torsion Bar suspension uses the twist resistance of a rod and translates that into the vertical motion of the suspension. One end of the Torsion bar is anchored to the chassis, while the other end is fixed to the lower wishbone pivot. The shock absorber lower mount is either on the Lower wishbone or the Knuckle.
Some well-known and high selling cars that had a Torsion Bar type of front suspension are the Volkswagen Beetle (upto 1966), early (1940’s) Porsche Racing prototype cars and GM Light-duty 4-wheel drive trucks like GMC Sierra and Chevy Silverado.
What is the difference between an Independent and a dependent Suspension?
To understand the concept of an independent suspension it is essential to know what a dependent suspension is. In the earliest forms of 4 -wheeled vehicles, the front wheels, and the rear wheels were connected by the front axle and the rear axle respectively. The axle was a solid beam that had the knuckle attached at each end, in the case of the front suspension. In the case of the rear, the axle consisted of 2 tubes connected at the center to a differential. The earliest form of suspension for the front and rear axles was the leaf spring type. Because the left and right wheels were connected rigidly, the vertical motion of the left and right wheels was always coupled. In other words, when the left wheel went upwards over a bump, it pushed the right wheel to go downwards.
Most modern cars have an anti-roll bar in the front suspension and in the rear suspension too. Though the anti-roll bar connects the left and right side wheels, it doesn’t make one wheel dependent on the other. Rather, the Anti-roll bar causes one wheel’s motion to affect only the wheel rate (stiffness) of the other side suspension.
Advanced Suspension Systems
Traditionally, the more commonly used Primary Spring elements have been the Leaf Spring, the Coil Spring and the Torsion Bar. All of these are essentially manufactured from high tensile “Spring-Steel”. Steel suspensions do come with certain drawbacks such as:
- The ride height is not adjustable during driving. It could only be modified by actually taking apart.
- The ride quality is difficult to balance with handling. A softer spring would lead to problems in handling
From these drawbacks, there emerged a need to have a more sophisticated suspension system that was dynamic in nature, meaning that adjustments could be made as per the road and driving conditions. Some of the most common advanced suspension design approached were:
- Pneumatic suspension
- Hydro-pneumatic Suspension
- Active Suspension System
The “Pneumatic Suspension” commonly known as “Air Suspension” is a system where the coil springs are replaced by Air Bags made of reinforced Rubber which are pressurized using air coming from a compressor within the vehicle.
The advantage of this system is that ride height is adjustable through air pressure variation. The other advantage is that ride quality could be decoupled from handling since the air bag system is self-adjusting and can harden or soften as per the given driving condition. The air suspension comes with leveling valves with sensors that can adjust the air spring pressure. Most of the luxury sedans currently available in the market come with air suspension as either a standard or an option. Some examples are the Mercedes Benz E-Class, S-Class and the Audi A8.
A Hydro-Pneumatic suspension is a bit more advanced as compared to the air suspension. The hydro-pneumatic suspension combines the advantages of the hydraulic and pneumatic systems.
- Hydraulic fluid being incompressible in nature transfers loads
- Gas or Air, being compressible in nature, can absorb loads
The main innovative component in the hydro-pneumatic system is a ”Sphere”, which contains a flexible membrane that separates the Oil and Gas acts as the primary spring element. The shock absorber is replaced by a hydraulic actuator that can raise and lower the suspension as per the system pressure.
The car maker that pioneered this technology is Citroen, which introduced this system in the 1960’s and still uses it in current models like the C5. Other vehicles with a similar system is the Lexus LX570 luxury SUV.
Active Suspension System
An Active Suspension System is probably one of the most advanced forms of suspension systems currently available in passenger cars and SUVs. An active suspension is defined as an Electronically Controlled suspension system in which system parameters like the Damping settings and Ride Height are adjustable automatically.
Each car manufacturer has its own proprietary Active suspension system. The “Active Body Control” system by Mercedes Benz is a system that has a pneumatic suspension and Electronically adjustable dampers that will continuously self-adjust as per the signals it receives from the sensors at the 4 corner suspensions. The system will adjust individual air bag ride heights in order to maintain the body as flat as possible.
Another example is the Audi A8 Predictive Active Suspension. The A8 predictive active suspension comes with a range of driver modes that range from ‘Comfort Plus’ to ‘Dynamic’, that changes the suspension response (among other things like throttle response and gearshift) to behave either ‘Soft’ or ‘Sporty’, among other settings. The Anti-Roll Bar is electronically controlled by actuators to automatically self-adjust in roll-stiffness in response to the road profile. The road profile is captured through a camera at the front.
In this brief article, we have discussed the Multi Link Suspension, its types, advantages, and applications.