Multi Link Suspension (What is it?)

In this brief article, we will discuss the Multi Link Suspension, its types, advantages, and applications.

A Multi Link Suspension is a type of independent suspension which consists of 3 or more control links. In general, the suspension links of the Multi Link Setup, working in combination, are aimed at constraining 5 degrees of freedom of the knuckle. This would effectively leave the knuckle free in only one direction of motion, i.e., the vertical motion.

In the case of the front suspension, the Tie Rod is not considered as a suspension Link, since it connects to the steering rack, which is a moving part. Hence, for a front suspension, the Multi-Link setup will have the objective of constraining 4 degrees of freedom, leaving the knuckle free to move in 2 directions of motion, i.e., vertical motion and steering motion.

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.

The origins of the multi link suspension can be traced back to the Mercedes C111 Prototype Sportscar which had the first version of a Multi Link Suspension. Later on, in 1982, Mercedes Benz’s new mid-size sedan platform, the W201, introduced the first mass-produced production version of a multi-link suspension.

  • Multi Link suspension is typically used where achieving good ride quality as well as good handling characteristics at the same time are of high priority
  • The design advantage that the Multi Link suspension offers is the possibility to alter a hardpoint (or joint location) in the suspension, without affecting other suspension geometry. An example is the Double Wishbone suspension where, in order to alter the steering bal-joint hardpoint, it would be necessary to change the shape of the control arm. In the case of the Multi-link, this modification can be done by modifying one control arm alone.
  • Using a Multi-link arrangement, it is possible to indirectly constrain a degree of freedom, without necessarily having a control arm in that particular direction. The net effect of links, when working together would achieve the desired constraint.
  • In Off Roading conditions, where wheel articulations are important, a multi link suspension would provide better “Flex” ability as compared to a Double Wishbone setup. The reason is that a multi-link would use 2 extra ball-joints (one for each control arm) and that a control arm, being smaller, does not need the kind of rigidity that a Wishbone arm requires.
  • The effective steering axis of the knuckle can be moved further outboard because of the 4 ball-joints (2 for the upper control arms and 2 for the lower control arms) instead of two in the case of the double wishbone.
  • The Multi-link setup adds complexity and therefore visualization of the geometry would be difficult using a 2 Dimensional representation. The usage of 3D Computer Aided Design softwares becomes essential.
  • Developing a new Multi Link suspension would require comparatively more engineering effort and expense, as compared to an axle beam or a solid driven axle type of setup. 
  • Due to the number of parts and complexity, an independent suspension solution could also result in higher manufacturing costs.
  • The number of maintenance issues and associated costs would increase according to the increased number of parts and different types of joints involved in the Multi Link suspension.
  • The BMW rear Multi Link set-up, called the Z-Link, is a 4 link setup
  • Honda’s multi link setup resembles a double wishbone suspension but with the difference that it has a fifth control arm in addition
  • The Audi A4’s front suspension is a 4-link setup that is similar to the double wishbone. 
  • The Mercedes Benz’s E-Class front suspension, which is quite similar to the Audi A4’s front setup, comes in 3-Link and 4-Link variants depending on whether the car has 4Matic all wheel drive or not. The 3-Link setup is different to the 4-Link setup in that the upper control arm is a wishbone instead of 2 control arms.
  • The Hyundai Genesis has a front 4-link setup, very similar to that of the Audi A4. The front suspension has two upper links, two lower links. The rear suspension is a 5-Link setup wherein, there are two upper links, a lower link, a trailing link, and a toe control link.

The different types of independent suspension are:

  • Front 3-Link
  • Front 4-Link
  • Rear 3-Link (Solid Axle)
  • Rear 4-Link and 5-Link (Solid Axle)
  • Rear 4-Link (Independent)

Front 3-Link

The 3-Link front suspension setup appears similar to a double wishbone, except that there are 2 lower control arms instead of one lower wishbone. There would be 2 lower ball joints instead of one. The advantage of having 2 ball-joints is that the effective pivot point will be at the projected intersection of the 2 control arms. This effective pivot point can be moved further outwards of the body, without having to increase the control arm length (as would have been the case in a Double Wishbone setup). This means that the Scrub Radius can be reduced and adjusted for in order to reduce Tire wear.

Front 4-Link

Like the 3-Link, the 4-Link front suspension setup is almost like a Double Wishbone. The difference is that instead of an upper and lower wishbone, there are 2 control arms in place of each wishbone. This means that there are 2 Upper control arms and 2 Lower control arms to do the same work as the Upper and Lower wishbones respectively.

The multi-link suspension can be found in basically 2 different forms. 

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.

Rear 3-Link (Solid Axle)

The Rear 3-Link is arguably one of the most simple of all suspension setups. It is a suspension meant for a rear Solid live axle. It consists of 2 Trailing arms and a Panhard Rod. Due to this reason, the 3-Link Rear could also be called a Trailing arm suspension. The trailing arms constrain the axle’s longitudinal, Braking/acceleration and roll degrees of freedom. The Panhard Rod would constrain the lateral movement of the axle.

Rear 4-Link and 5-Link (Solid Axle)

The rear 4-Link and 5-Link are rear solid axle suspension setups. The rear 4-Link is a step above the 3-Link, in that, instead of just 2 trailing arms, there are 2 Upper control arms as well. The 5-Link setup is the same as the 4-Link setup, with the addition of a 5th link, namely, the PanHard Rod. In many cases, when designing a rear 4-Link set-up, the need for a Panhard Rod can be eliminated. 

This is achieved by positioning the upper control arms in a ‘V’ shape. The ‘V’ shaped upper control arms, in combination with the lower control arms, can indirectly constrain the axle from sideways movement. The only caveat is that, once the actual control arms are attached through bushings, the combined effect of the bushings in the system must be sufficient to arrest axle lateral movement. In cases where the car is performance oriented, the Panhard rod may not be possible to eliminate due to the excessive lateral forces from the tires.

Rear 4-Link (Independent)

The best example would be the Golf GTI/ Golf R/ Audi A3 Independent Rear Suspension. The 4-links consist of a trailing arm, a lower and upper control arm, and a Toe control arm. This is the performance-oriented version of the MQB platform. Due to this reason, the rear suspension set-up is complicated so as to resist high cornering forces of upto 1g.

Conclusion

In this brief article, we have discussed the Multi Link Suspension, its types, advantages, and applications.

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