Examining multi-link suspension

TRW explains the system on a VW Passat B5

Published:  10 October, 2014

A typical example of multi-link suspension is found on the front axle of the VW Passat B5. The basic elements in this system are two pairs of upper control arms (front and rear), two pairs of lower control arms, stub axle, stabilizer, connecting rods, shock absorbers and springs. All control arms are made of light alloys, and integrated ball joints. The mounting points for the control arms at the chassis allow for certain changes to the geometry of the wheel in line with the specifications from the vehicle manufacturer. These parts may take the form of a metal bushing vulcanized in rubber, which reduces vibration and noise. The design of the rear lower arm mount bushing is more complicated. It is a hydraulic bearing, featuring a membrane containing hydraulic fluid. Well designed metal-rubber bushings enhance the stability of the vehicle when cornering at high speeds and braking.

It's easy to see that multi-link suspension features many more structural components compared to other systems, for example McPherson suspension. This means that multi-link suspension by nature of its design has a higher probability of component failure.

The first step in diagnosing faults should be a visual inspection of specific elements. At this stage we estimate the condition of rubber caps that protect ball joints and metal-rubber sleeves. Finding damage to any of these components means that you have to replace the whole control arm.

The next step is to look for the presence of free play in the ball joints. Here, the best method is to examine the chassis at the test stand. The test stand plates force the wheel to move in all directions, allowing the mechanic to check the individual elements of the system for the presence of play. Damage to the joint can be recognized both by the presence of play and any occurrence of creaking noises while turning wheels or manipulating the suspension. Creaks, or cracks in the dust covers, are an early sign of wear. It means that water has got under the rubber seal, causing corrosion of the ball joint elements. The visual examination on a test stand can also detect any damage to the metal and rubber sleeves. Mechanics must be careful not to be too hasty to declare failure of those elements.

This is especially true for the hydraulic bearings found in the lower rear control arm on a VW Passat B5. Its specific design allows for relatively large arm movements relative to the body and an inexperienced mechanic may interpret it as damage to the sleeve. The condition of a multi-link suspension can also be examined without a test stand.

The examination is carried out by forcing the movement of individual elements using a lever (e.g. tyre lever). This control should be performed when the suspension is under load (from an inspection pit or with the use of a drive-on lift), and when it is unloaded (for example using a two-column elevator). You need to be aware of the strength and direction of the forces acting on individual components. For example, play on the upper arms in the example of the VW Passat B5 is shown most effectively when the car's weight is on its wheels and the steering wheel is turned. Another mechanic can then place his hand on the rubber bushings of the arms and feel the presence of any play in the ball joint. The lower front control arm is best examined on a vehicle which is lifted, by applying upward force on the head of the control arm.

In order to remove the bolt which secures the upper control arms in the example of the VW Passat it may be necessary to use a special tool as this bolt is relatively long and secured tightly in the stub axle. Environmental factors such as water and salt often cause corrosion in this area. The tool allows for the upper control arm removal without the time-consuming disassembly of the vehicle stub axle.

After disassembling the old parts, we move on to the installation of the new ones. First clean the contact surface of the dust cover. If the dust cover is not cleaned water will drain into the ball joint through the capillary effect, and corrosion will follow soon after.

All nuts and bolts must be tightened at the right torque using new nuts. Over tightening the bolt can cause real difficulties when the time comes to remove them. Not only that, a bolt which has been over tightened undergoes more stress than it was designed to handle - which can cause a weakness and compromise the safety of the car.  Bolts which are difficult to reach are more prone to damage, be sure to check whether they need replacing once removed.

It is very important to install the control arm mounting bolts to the vehicle when the suspension is under the load of the car and its wheels are in a natural position. Tightening them on a raised vehicle, when the suspension is not under load causes unwanted torsion strain in the metal-rubber elements.

This causes faster wear and damage to newly installed parts, possibly from the point of the assembly. Care must also be taken not to damage the rubber ball joint bushings.

After the repair an examination of suspension geometry must be performed. A final check of wheel alignment will find any other issues that could have been missed during the inspection (e.g. a slightly bent control arm), as the correct wheel alignment is not possible with these issues. It also ensures even tyre wear, and that the car will behave as designed. When returning the car to the customer, it is good practice to give them an overview of the operation of the suspension system. It is unacceptable to drive onto high curbs or over bumps at high speeds. This rule applies to all types of suspension, but the multi-link suspension system is particularly susceptible to damage resulting from improper use.

Finally, it is a widely held belief that multi-link suspension systems have a high failure rate. While this type of suspension is susceptible damage from improper use, there are a number of other factors to consider. Some believe there are durability issues for parts within the system. This, however, applies mainly to low quality spare parts offered on the aftermarket. Other factors adversely affecting the sustainability of these suspension systems are very bad quality roads and mistakes made during repair and maintenance.

TRW Tech Corner

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