Split decision

  • Shown in blue are the main components of the five-link set-up: coils, links, transverse rod and dampers.
Date:7 February 2018 Tags:, ,

And the winner in the battle of the suspensions is… it depends. For the moment, comfort means coils.

Simple, reliable and tough, leaf springs have been giving us a softer ride since the middle ages. Particularly if we need to carry a heavy load – as in the back of a bakkie. Which is why there’s bound to be some head-scratching about any decision to ditch a design that’s proven its worth, like Nissan South Africa did with its new Navara double cab.

All new local Navaras are fitted with a multi-link coil-spring rear suspension. The new vehicle is, in fact, the first of its type to sport a coil-spring rear end. To add a little confusion: in time, you will be able to get your Navara with leaf springs. That’s because Nissan designed its next-generation ladder frame chassis to accommodate both coil sprung five-link suspension on high-end models and a more traditional leaf spring set-up for workhorse and mid-range pick-ups.
On the double cabs, ride comfort and handling are what it’s all about, says senior Nissan engineer (insert name) Katako. Not that you should mistake a comfortable ride for a softie approach. “With a pick-up truck, one of the key propositions is toughness,” he says. Okay.

“We implemented the multi-link suspension with the double cabs because of their predominantly dual use character,” says Nissan’s marketing director Kabelo Rabotho. “Comfort is important – people migrate from SUVs because of their lifestyle requirements.”

Why should these people even care about the rear end design?

The Navara double-cab set-up uses heavy-duty coil springs and shock absorbers mounted in front of the solid axle for the main damping. This places the central damping point directly above the rear axle, a double benefit: better ride and handling characteristics as well as heavier load-handling. That translates, says Nissan, into less bobbing of the rear end, especially when unladen, rear wheels that track the front wheels more accurately – especially at speed – and less likelihood of being bounced off line by potholes and mid-corner bumps.

More than one and a half million test kilometres were driven to establish how the suspension interacted with the steering set-up, the wheel and tyre choice and the electronic driver aids. Engineers were able to tweak the damping rates more accurately, too, because bobbing at the rear is controlled more directly by shock absorbers with multi-link suspension, than on a leaf-sprung suspension where the shocks are mounted on different sides of the rear axle.

But… leaf springs have their uses. Especially for heavy loads. And leaf springs have this inherent ability to stiffen resistance as the load is increased. Oh, and besides cushioning the ride, leaf springs hold the rear axle and wheels in position. Simple, yet effective.

On coil springs alone, a vehicle would wobble uncontrollably, so additional locating arms are needed. Five-link suspension has two arms each side that essentially hold the rear axle in place for front-to-rear movement while allowing each rear wheel to move vertically independently of the other, plus a lateral link to control sideways movement. You guessed it: all of this adds complexity – and weight.

Yet there’s no denying the advantage of having each component performing essentially one function. In the case of the coil suspension, the Panhard rod locates the axle – that’s all it does. The damper damps and the links keep things in position. It’s just technically a better solution.

In terms of durability, coil seems to have the edge on leaf. “There is no stress concentration in the parts. With leaf springs, there is contact between leaves. This also generates some noise,” says Katako. From a durability point of view the coil spring set-up has been proven, says Nissan product manager Freddie Louw. “Our own Patrol, Toyota Land Cruiser and Land Rover Defender have been running on coil springs.”

When designing in a more stable ride, one of the key characteristics engineers aim for is lateral rigidity of the suspension. In other words, how much the tyre is displaced relative to the frame. Multi-link suspension components are rigidly mounted to eliminate sideways movement, but allow movement in a vertical plane. You can expect more neutral, more predictable cornering and less waywardness. That’s safer.

If it’s such a good system, why didn’t they do it before? And why don’t the others do it?

Louw takes a moment before replying. “Improvement is the way of the world,” says. “In the previous generation we were the first in the market, not necessarily to come up with new technology, but to apply certain technology. Examples: six-speed transmission, electronic transfer gear operation and McPherson strut front suspension – up to that time it was with torsion bars.

“It is a matter of applying new technology to the segment. I am quite sure that, over time, we will be copied with this as well. The customers are becoming more demanding, especially for double cabs. You have to find solutions to improve ride comfort because ride is one of the big reasons why people tell us they won’t change from a passenger vehicle or an SUV.”

Things will be different for single cab workhorse versions. “Single cab, looking at the customer application, is more likely to be carrying a heavy load. We will introduce these when we start local production, in the near future.”

Why coils
With coil springs and a multi-link rear, it’s a win-win, says Nissan’s Katako. Well, mostly.
1. Improved comfort, through lower friction. “Friction is something that chassis engineers constantly try to decrease. It’s an ongoing fight.”
2. Better handling and vehicle stability. “Usually ride comfort and handling are tradeoffs. Here, you get both.”
3. Optimised performance of spring, stabiliser and damper. “This helps us achieve 1 and 2. That’s because the multi-link components focus on their own specific functions. With leaf suspension, you have to combine these many functions. It is very difficult to optimise one area of performance.”
4. Lower weight. “Weight reduction is key and contributes to better performance and fuel consumption.”

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