Difference Between MacPherson Struts And Double Wishbone Suspension

1 Importance Of A Suspension System In A Vehicle

A suspension system of a vehicle is an ecosystem of different parts, like springs, shock absorbers, and linkages that connect a vehicle to its wheels and allow relative motion between the two. If a road was perfectly flat, with no irregularities, suspensions wouldn’t be necessary. But the roads are far from flat, rough and even freshly paved highways have subtle imperfections that inflict forces on the wheels as you drive. A bump on the road forces the wheel to move up and down. The magnitude of this force depends on the size of the obstacle. The car wheel experiences a vertical acceleration as it passes over an imperfection, and without a suspension, all this vertical energy is transferred to the frame, which moves in the same direction. In such a situation, the tires can lose contact with the road completely. Then, under the downward force of gravity, the tires can slam back into the road surface. The suspension system absorbs this energy, and without disturbing the frame and body. This prevents the shock forces from transmitting into the passenger compartment, offering a comfortable ride. Suspensions also play a vital role in reducing rolling, pitching, or vertical movement of the vehicle.

2 How Does A Suspension Work?

When the wheel hits a bump, the shock produced is absorbed by the spring and dampers used in the suspension system. The shock absorber and the spring assembly are usually located between the wheel’s knuckle and the mainframe. The spring absorbs and stores the energy produced due to bumps by compressing its size. After the spring stores the energy, the dampers or shock absorbers start working. Inside a damper is a piston with small holes in it and some pressurized oil. When the spring transfers the energy to the damper, the piston moves through the pressurized oil by using the energy of the spring. The piston has a small orifice through which oil passes to the other side of the piston, generating heat, and successfully converting the energy of the bump in the road into heat energy. This gradual passing of oil eases the jerk and sudden motion of the spring, adding a damping effect.

3 MacPherson Strut Suspension

What are MacPherson Struts?

MacPherson strut is the most common suspension used in modern-day cars. It is an independent type of suspension system that features simple construction and compact design that reduces its manufacturing cost. This makes MacPherson strut the first choice of manufacturers and is today used in almost all front-wheel-drive vehicles. MacPherson strut combines two main functions: it serves as a steering rod and a shock absorber. MacPherson Strut includes a simple assembly of coil springs and a damper/shock absorber built into a single unit. The spring’s job is to store the energy from a sudden jerk from the road, and the damper uses hydraulic fluid to dissipate this energy in the form of heat.

Working Of A MacPherson Strut Suspension

Named after its designer and inventor Earle S. MacPherson, this type of suspension is mostly used on the front wheels of the vehicle, where it serves as a steering pivot and a suspension mounting for the wheel. A MacPherson system has two mounting points. The top mount bolts the strut to the body of the vehicle and the bottom mount connects the strut with the control arm. The spring and damper unit is mounted vertically on top of the steering knuckle and consists of an enclosed cylinder comprising the shock absorber or damper, also known as the strut. At the top, it has a wide collar that cups the coil spring, while the damper shaft runs all the way through the center of the coil to the top of the spring unit. When the wheel moves over a bump, the lower control arm holds the wheels and allows it to steer while moving up and down with the suspensions.

The MacPherson strut benefited from the introduction of unitary construction because its design requires substantial vertical space and a strong top mount, which can be achieved by a monocoque or unibody construction. This also allowed the designers to have more space inside the cabin of the vehicle. The unibody design of the vehicle also distributes the suspension stresses properly making it a viable option for the manufacturers. The strut carries both the coil spring, on which the body is suspended, and the shock absorber, which is usually in the form of a cartridge mounted within the strut. This makes the whole assembly simple and assembled into a unit, compared to other suspension systems, and gives the maximum room in the engine compartment. It is mostly used in front-wheel-drive cars and works in conjunction with anti-roll bars (stabilizer bars or sway bars) for better handling and reduced body roll. This makes the MacPherson strut suspension cost-effective because of which it is widely used in passenger cars.

Components Of A MacPherson Suspension

A complete Macpherson strut assembly comprises an upper mounting point, shock absorber, spring, and a lower control arm. It contains only one lower control arm, so it is also known as a single wishbone system. Each component of a MacPherson Strut is mentioned below with detailed functions and working.

1. What Type of Spring is Mounted on a Strut Suspension System?

Coil springs are found on all MacPherson strut suspensions. A mounting plate welded to the strut acts as the lower spring seat, while the upper seat is bolted to the strut position rod. The coil spring and strut turn with the motion of the steering wheel using a bearing or rubber bushing in the upper mount. Modified MacPherson suspension does not have the coil spring mounted on the strut. While this feature provides a smoother ride under normal driving conditions, the normal MacPherson provides a smoother, more responsive ride over a wide range of driving conditions.

2. Damper

A shock absorber or damper is a mechanical or hydraulic device designed to absorb and dampen the vibrations induced in the coil springs while driving over rough roads. A shock absorber or damper uses the valving of oil and gases to absorb excess energy from the spring. Without a damper, the spring will continue to oscillate at its natural frequency, making the ride bouncy.

3. Lower Control Arms

The MacPherson system only uses one lower control arm. It is attached to the steering knuckle via a ball joint and to the frame of the vehicle via rubber bushings. These control arm bushings are made from rubber or polyurethane and they allow the control arms to move up and down freely according to the movement of the suspension and wheels.

4. Strut Mount

A strut mount consists of a rubber stopper, a ring, and a metal part sandwiched between the strut and the vehicle body. It acts as an insulation between the both, absorbs the vibrations and the tire noise, and restricts them from entering the vehicle body. If damaged, a bad strut mount can lead to vibrations, noises from the suspension system, and improper tire wear. While replacing your struts, it is recommended to change your strut mounts or replace the complete strut assembly which includes the strut mounts.

Advantages Of A Macpherson Suspension

1. The MacPherson strut suspension does a great job of keeping the vehicle flat on the road. They don’t take up too much room either, allowing for more compartment space for the passengers. It also offers increased space in the engine compartment, making them favorable for four-wheel-drive trucks and SUVs.
2. The MacPherson suspension has reduced unsprung weight, which makes the vehicle lighter compared to double-wishbone suspensions. This allows the vehicle to accelerate at faster speeds.
3. The cost of manufacturing MacPherson struts is less compared to double wishbone suspensions, which use two control arms. This reduces the production cost of the vehicle, making it affordable.

Disadvantages Of A MacPherson Suspension

1. A MacPherson suspension has a limited camber and roll center. This is why it is not as good as a double wishbone suspension for handling and ride comfort.
2. The MacPherson suspension has a lengthy vertical assembly which makes it tough to lower your vehicle.
3. Since it is attached directly to the body of the vehicle, the transmission of noise, vibration and harshness are more in MacPherson struts suspensions as compared to a double wishbone.

4 Double Wishbone Suspension

What is Double Wishbone Suspension?

The double-wishbone suspension is a type of independent suspension system that uses two wishbone-shaped arms, also known as A-arms. Each wishbone is mounted to the chassis via rubber bushings and features one ball joint at the steering knuckle. The difference between a MacPherson strut and double-wishbone suspension lies in the number of control arms and the shock absorber placement. Unlike MacPherson struts, a double wishbone suspension uses two control arms: upper control arm and lower control arm, with a shock absorber mounted on the control arms. Some popular cars that use double wishbone suspension are Honda Accord, Mercedes Benz, Dodge Ram 1500, etc.

Components Of Double Wishbone Suspension

To understand how a double-wishbone suspension works, you must learn about independent components and their functions. If the automobile faces any sudden bump on the road, the shock is transferred to the A-arms from the tires and finally to the damper where the shock is absorbed.

1. A-Arms Or Control Arms

A-Arms can be termed as the starting part of the suspension system, from where the load transfer takes place. These components hold the wheels of an automobile and are attached to the chassis. The lower control arm connects the steering knuckle to the frame and stabilizes the vehicle by allowing the chassis and the wheels to move in unison while the vehicle is in motion. While the upper control arm connects the top of the steering knuckle to the frame, it is shorter compared to the lower arm to induce negative camber. This arrangement is also known as a short long arm SLA suspension.

2. Shock Absorber With Coil Spring

A shock absorber with a coil spring is mounted on the lower wishbone and connected to the chassis to control vertical movement. Spring is one of the main components of the suspension system that absorbs the forces generated due to bumps by compressing itself. The damper or shock absorber absorbs the energy produced from the spring and converts it into heat energy. Similar to a strut, the function of a shock absorber is to offer a smooth ride and prevents shocks and vibration from damaging the components and vehicle. The type of shock absorber can vary depending on the vehicle and terrain it is used for.

Working Of A Double Wishbone Suspension

A double-wishbone suspension has two control arms, also known as double A-arms: one at the top and one at the bottom with the damper attached to the bottom arm. The upper and the lower control arms have ball joints on ends to allow vertical movement of the wheels while you drive on a rough road. The vertical movement is controlled through the shock absorber and coil spring, which are mounted on the wishbones. This design has a different geometry, as the upper arm is usually shorter than the lower arm. As the double wishbone suspension is compressed, negative camber is introduced, because of the unequal length of the control arms. When the vehicle is on a turn, body roll results in negative camber gain on the lightly loaded inside wheel, while the heavily loaded outer wheel gains positive camber. 

The outer end of the control arms or wishbones is attached to the steering knuckle. In older designs, the knuckle has a kingpin for horizontal radial movement and rubber bushings for vertical hinged movement. However, newer designs have a ball joint at each end that allows free movement of the wheels. The control arms are made from steel to withstand high impact and bending force and possess superior dynamic characteristics for good load handling capabilities. This makes double-wishbone suspension favorable for trucks, SUVs, off-roading vehicles, and racing cars. The double wishbone suspension provides the engineer with more design choices and it is fairly easy to find out the effect by moving each joint, so the kinematics of the suspension can be tuned easily and wheel motion can be optimized.

Advantages Of Double Wishbone Suspension

1. The double wishbone suspension has horizontal upper and lower control arms that increase the negative camber. As a result, your vehicle has better on-road stability because the tires are more in touch with the road. 
2. The parallel linkage between the upper and the lower control arm helps keep wheels parallel to the car’s vertical line throughout the suspension travels. Your wheel alignment remains consistent as well as your steering. 
3. The double wishbone design allows easy tuning of the suspension parameters for optimizing ride and handling performance.

Disadvantages Of Double Wishbone Suspension

1. A double wishbone suspension system is a lot more complex than the MacPherson suspension, making it costlier.
2. More components in suspension design increase the service time and are heavier compared to a MacPherson strut system.
3. Due to several components in the system, if one part gets worn out or fails, then the whole suspension can stop functioning properly. As the owner, this leads to increased maintenance and repairs cost. For example, the cost of replacing your shock absorber can range somewhere between $300 to $700.

5 Difference Between MacPherson And Double Wishbone Suspension

Here are a few key differences to understand when comparing double wishbone suspension systems against MacPherson systems:

1. The MacPherson system relies on struts, which makes the vehicle cabin more spacious as they have a compact construction compared to a double wishbone suspension.
2. Wishbone suspension is usually installed at the rear of a vehicle, whereas the MacPherson system is mostly used in the front, especially in front-wheel drive vehicles. A system is recognized as a double-wishbone when there is a wishbone configuration in the front, as well as in the back, of a vehicle.
3. Wishbone systems keep vehicles more stable and offer better steering and handling as well. It keeps the wheels aligned and offers better performance compared to MacPherson struts.
4. The MacPherson system doesn’t have an upper control arm, which allows it to absorb individual impact at each of the front wheels, and gives a smoother ride experience to the passengers. But this also reduces the car’s stability in corners.
5. The presence of an upper arm makes the double wishbone suspension arrangement more rigid and increases the vehicle’s ability to handle challenging areas like hilly or curvy terrain. Therefore, a double wishbone system is widely used on independent front wheel suspensions of off-road vehicles. Refer to our how to choose an off-road suspension guide for more.
6. The complicated design and more components involved in construction make the double wishbone suspension systems more expensive than MacPherson struts. Double wishbone suspensions are mostly used in sports and high-end luxury vehicles. Whereas the simple and cost-effective design of MacPherson strut suspension makes it the first preference for all budget and mid-segment vehicles.

If you are a vehicle enthusiast and can spend a little extra for better performance, double wishbone suspension is a better option. But if you are looking for an affordable setup with less maintenance, then MacPherson should be your choice. Choosing a correct suspension system is an art because it controls how your vehicle handles and performs on different terrains. This article not only gets you acquainted with the key differences between double wishbone and Macpherson strut suspension but also the advantages and disadvantages of each type. This will help you understand both the suspension in contrast with each other and make a sound decision while purchasing one for your car, truck, or SUV.

6 FAQs

What Cars Have Double Wishbone Suspension?

The double wishbone suspension system is a type of suspension that provides great driving performance and abrasive handling. Some high-performance, sporty, and luxury cars are the most common ones to have a double wishbone suspension system.

What is SLA Suspension?

A short long arms (SLA) suspension is a type of double wishbone suspension. It is an independent suspension design for automobiles that uses two wishbone-shaped arms to locate the wheel. The upper arm is usually shorter than the lower link, which can be an A-arm, an L-arm, or a pair of tension/compression arms. The shock absorber and coil spring mount to the wishbones to control vertical movement. Double wishbone designs allow the engineer to carefully control the motion of the wheel throughout suspension travel, controlling such parameters as camber angle, caster angle, toe pattern, roll center height, scrub radius, scuff (mechanical abrasion), and more.