Can the new Navara handle a heavy load?

In last month’s column, Mic van Zyl discussed the load-carrying capabilities of the new Navara with its rather unprecedented choice of rear coil springs. This month, he drills deeper into the issue.

In last month’s article (Drive Out #112) I wrote about how the new Nissan D23 Navara features coil springs on the rear as opposed to conventional leaf springs. This is a first for a mainstream 1-tonne double cab bakkie on the local market. There is some perception that this vehicle would not be able to carry the same load as a bakkie with leaf springs. But is this true? The D23 Navara that was recently launched in South Africa is almost identical to the one on sale in Australia. Nissan South Africa have, however, made a subtle change to the vehicle suspension – our D23s ride about 20 mm higher than the Aussie. I have yet to learn from Nissan SA whether this is related to loadcarrying ability or merely for an increase in ride height.

Let’s do some math

Comparing coil springs to leaf springs, with specific reference to the rear of pickups, there are a number of similarities and differences worth noting. The first is spring rate. This refers to the stiffness and is typically measured in the force required to deflect the spring a certain distance. We used to refer to the poundage of the spring or, more accurately, the amount of force in pounds required to deflect the spring 1 inch. Nowadays, we use a modern formula to calculate the amount of force in Newton required to deflect a spring 1mm: N/mm. In terms of load, 1kg is equivalent to about 9,8 Newtons, which I’ll round off to 1 in 10 for the sake of this article. A 30N/mm spring would therefore require roughly 3kg of force to compress it 1mm. With two springs on the rear of a bakkie, it would require 6kg of load directly on top of the springs to compress them by 1mm, 60kg to compress them 10mm, and 600kg of load to compress them 100mm. Of course, this is a simplified calculation and there are other factors that influence the spring rate. Springs may be linear or progressive in rate, for example. A linear spring will compress by the same ratio as force is applied in a constant manner (as stipulated above). A progressive spring will compress progressively less as load is applied constantly. This is true for both leaf and coil springs. Another factor is spring design. Rear leaf springs on most bakkies feature a twostage design – a number of leafs with a pronounced camber in the upper, first stage and a couple of thicker, flatter loadcarrying or overload blades at the bottom in the second stage. Typically, the spring rates progress between 30N/mm up to 100N/mm, and leaf springs have the advantage of being able to spread this progression over a greater range than coil springs.

Comfort or hard work?

Unfortunately, I did not have the original spring rates for the coils on the rear of the Navara when writing this, so as an example I’ll reference the Ironman 4x4 aftermarket replacement coil springs. A comfort orientated “A” spring is progressive in rate and measures 33N/mm up to 45 N/mm. This is a very comfortable spring and is only slightly up from the standard coils as offered on the Aussie D23. Next up is the “B” spring for occasional load with 33N/mm to 60 N/mm (also progressive). For vehicles carrying a constant load, the “C” spring is linear in rate at 57N/mm. And in cases where the vehicle owner will carry an almost maximum load all the time, there’s the “D” spring, rated at 74N/mm. All the coil springs mentioned above start at a slightly higher rate than the equivalent leaf spring, with the load-carrying springs starting much higher. So if you apply the math to these coils, you’ll see they can undoubtedly take a load. It is my humble opinion that over time more of the popular bakkies should offer coil springs, perhaps as an option, aimed at a more comfortable ride. Don’t write good leaf springs off when it comes to ride comfort, though – one only has to drive a VW Amarok to realise that there are some very clever suspension engineers out there. Nonetheless, kudos to Nissan for taking the plunge with a South African first!

Over time more of the popular bakkies should offer coil springs for a more comfortable ride.