Good suspension has many attributes, one of which is quietness. But does your suspension squeak and rattle? Here’s why and how to fix it.
SOME MAY SAY it doesn’t matter how much noise your suspension makes, it’s a 4×4, so, who cares. But that’s wrong, because unnecessary noise masks the noises you do want to hear such as the other occupants (sometimes), UHF radio transmission and, of course, problems elsewhere in the car. The term used for ‘noise’ in vehicles is NVH, or noise, vibration and harshness.
The average touring 4×4 has lots of extra noise compared to a stock vehicle due to the modifications. Pretty much everything makes noise; tyres, snorkels, roofracks and the like. So you don’t want any more noise coming from the suspension. Noise is also a factor in driver fatigue, something touring offroaders need to be wary of as we frequently drive long distances.
So why does the suspension make noise, and how is it fixed?
Suspension has two main parts – the spring and the damper.
The spring sets the ride height and absorbs the bumps in the road. But a spring by itself has a major problem because once compressed it will then extend, then compress, then extend, then compress… a bit like a rubber ball bouncing on the ground. That’s why the other major suspension component is a damper (also known as a shock absorber) which damps out that bouncing effect.
Dampers work in a similar way to a coffee plunger – imagine a rod with a circular disc at the end pushing through a cylinder filled with fluid, except there’s oil under pressure instead of coffee. The disc has holes in it, and the larger the holes, the easier it is to move the disc and its rod up and down. The speed of the rod moving up and down is known as the ‘rod speed’, and it’s not proportional to vehicle speed. Even at fairly low vehicle speeds, corrugations tend to produce high rod speeds thanks to the rapid up-and-down movement of the wheels relative to the body, whereas high-speed cornering will be a lower rod speed as vehicle’s weight transfers from side to side.
The diagram below shows how a basic twin-tube damper works:
A major part of suspension damper tuning is all about figuring out how much resistance the damper should provide as it compresses or extends, and there’s a very complicated series of valves and other devices to precisely tune those rates for all sorts of terrains, vehicle weights, driving styles, and more.
One of the tuning dilemmas is to work out how to design a damper so it offers little initial resistance to a high-speed rod movement and greater resistance to a low-speed rod movement. One way to do that is a shim.
A shim is another disc, but it’s made of thin metal and is designed to bend when fluid flows past it, and the faster the fluid flow, the bigger the bend. This is a useful tuning feature as it means that at low speeds there’s quite a bit of resistance as the shim’s diameter is large, but as rod speeds pick up the shim’s effective diameter becomes smaller and therefore offers less resistance.
Suspension is also a major factor in NVH, both directly and indirectly. Poor suspension leads to a harsh ride and consequent noise effects on the rest of the car. For example, suspension that reaches the limits of its travel will hit bumpstops and that’s not comfortable nor good for the car.
But even when the suspension is delivering the perfect ride and handling it can be noisy. As the rod in the damper moves up and down, forcing fluid to flow through the shims and valves, noise is generated, and the damper can transmit vibrations through to the car’s chassis. Vehicles with a separate chassis design (read more here) are inherently insulated from these effects as the body sits on rubber mounts separate to the chassis, but the early breed of unibody (monocoque) vehicles are a problem.
Practical Motoring spoke with Kristian Ristell, Ironman 4×4’s director of suspension, who said, “unibody bodied vehicles generally lack ability to isolate the passenger cabin from NVH when compared to separated chassis vehicles. This issue was a particular concern with Mitsubishi Pajeros from the year 2000 onwards and is also a factor in the Suzuki Grand Vitara and some Jeep models.”
Those vehicles were some of the earliest unibody off-roaders. Since that time carmakers have improved their designs and vehicles released over the last few years are less prone to these noise issues. This is a wagon-only problem, as all utes are separate-chassis designs.
According to Kristian, “one specific source of suspension noise which has been a concern for some time is the ticking of the shims in the shock absorber over high rod speed movements in the suspension. Most shock absorbers make some noise as the shims open and close at high speed, but this is not normally detectable in most vehicles except those with unibody constructions and/or a poor NVH suppression system.”
Basically, as those shims flex up and down they make noise, and that’s not good. Kristian says the solution was to “develop a new piston and shim stack that didn’t generate any noise at all. We successfully resolved the issue beginning with Pajero and we have now applied the same update to all unibody vehicle models, including the Grand Vitara.”
To test out the theory, we asked Ironman 4×4 to fit the revised design fitted to our Grand Vitara for a test over all the usual terrains from bitumen to low-range work, and we can confirm it is an improvement. It doesn’t fix the Grand Vitara’s inherently poor NVH, but every little bit of noise reduction helps.
There are lots of other ways suspension can make noise.
Leaf springs make noise as they rub against each other, the bushes connecting things like swaybars to the body can screech, and to minimise noise it’s important not to neglect general maintenance and lubrication of worn components – everything from dampers to bushes. Adding other accessories can change noise too, for example, underbody protection can reflect noise or change its tone. There are a few cases where adding accessories helps; one is fitting heavy drawer systems that absorb noise, another is a heavy-duty aftermarket sound system!
In the case of our Grand Vitara, we also discovered an unusual problem, a slight knocking noise, but that was a much easier fix. Kristian told us that “it appears that during original installation the end of the coil was resting hard on the spring seat step and preventing it from seating on the flat plate, we rotated the coil anticlockwise slightly to separate the coil end from the step, which stopped the noise. We’ve not had this issue before, it appears it was just an awkward coil seating position, mainly on the left side”.
Of course, as soon as you get rid of one noise other noises which existed before but you didn’t notice start to make themselves heard. Then you get used to a certain level of noise and think it’s ok, but when you drive a quieter vehicle or get rid of a couple more irritating noises you realise just how much better life can be.
