4WD MODIFICATIONS - TECH TORQUE
The leaf spring has been with us since horse-drawn wagon passengers demanded better ride quality and the long-serving leaf spring is still the principal springing medium for the rear ends of light commercial 4WDs.
Before we get into the technicalities of different leaf spring types, here’s a little background.
You could have any suspension you liked on your pre-1970 4WD: as long as you wanted leaf springs. Sure, there was a choice: you could have front springs with forward-mounted or rearward-mounted swinging shackles, but rear springs were all rear shackle types.
The Range Rover introduced coil springs in 1970, but the world was slow to follow.
The choice remained limited to coil-sprung Rangie or leaf-sprung everything else until the Pajero was launched in 1983, with independent, wishbone front suspension.
The fact that leaf springs are still the most commonly seen light commercial 4WD rear suspension is testimony to the fact that the leaf spring has done its job well, since the early Jeeps and Land Rovers bounced off the production lines.
Another factor is the definition of ‘commercial vehicle’ in some markets, requiring truck-like springs to be fitted if tax breaks are sought.
The most commonly seen leaf spring is the
conventional leaf pack, consisting of parallel curved leaves, with the same or varying thicknesses, tightly clamped together.
Often, on the rear ends of utes and light trucks, there’s a secondary leaf or two that come into play only when the spring is compressed by load or suspension action.
The leaf spring has many attributes that appeal to 4×4 designers. The spring itself is easy to make and can be repaired in the field. Its design, with successive leaves clamped together, is simple and easy to make in varying spring rates and load capacities: “if it breaks, put in another leaf”.
Best of all, the leaf spring is a spring and a damper in one unit. Because a leaf spring has its leaves clamped together there’s friction between them and this friction acts as a partial self-damper.
However, that self-damping action means that when the wheel hits a sharp bump the spring pack takes some time to flex, because it has to overcome internal friction. By that time the vehicle chassis has moved upwards from the severe bump action.
That’s why the conventional leaf spring has the worst ride quality of all 4WD suspensions and that’s why most 4WD makers have incorporated softer-riding designs.
The cheapest alternative is separating the leaves with wear-pad ‘spacers’, to eliminate most of the interleaf friction and that’s seen on some 4WD ute rear ends.
A development of that idea is the taper-leaf spring.
A taper-leaf spring pack looks different
from a conventional spring pack, because the leaves are thicker at their mid-points, where they clamp to the axle tube, than at their fore and aft extremities. This design puts additional strength near the centre point of the spring and also ensures that there’s no interleaf friction between adjoining leaves.
Taper-leaf springs for 4WDs were first used on late-model MQ Patrols in the 1980s, in an effort to improve ride quality, but the effort had limited success, because the more flexible springs weren’t damped by quality shock absorbers.
The 1990s Ford F250 4WD had taper-leaf springs, which is why Ford Australia had to adopt dual shock absorbers on the rear axle, to make up for damping lost by the move to softer-riding leaves.
Taper-leaf springs and parabolic springs are more flexible than standard leaf spring packs and can benefit from progressive bumps stops, rather than the hard rubber blocks that are fitted as standard.
Parabolic springs are versions of the taper-leaf
spring, with an important, invisible difference. Obviously, they’re not called ‘parabolic’ because they look like a parabola: they’re semi-elliptic like most springs, so what’s the odd name mean?
The ‘parabolic’ reference refers not to the shape of the finished spring, but to the mathematical process that assesses the stress levels and degree of taper throughout the spring length.
We’re indebted to Heytsee Automotive in the Netherlands, a global-scale maker of parabolic springs, for input in the following explanation.
If you’d been paying attention to your high school maths teacher you probably remember quadratic equations and also recall that graphs of quadratic functions have a parabolic shape. These graphs help solve simple maximisation-minimisation problems without having to resort to calculus.
The parabolic process is used in solving widely differing max-min problems, including stock market fluctuations: the Parabolic SAR is a technical indicator used to determine the price direction of an asset, and is also known as a ‘stop and reverse system’.
It’s interesting that the same mathematical principle can be employed for solving such different push-pull applications as a ‘bull’ and ‘bear’ markets and springs that flex up and down.
So, why is the parabolic spring an improvement on the basic taper-leaf spring?
A parabolic spring is a leaf, or a set of leaves that are tapered in a parabolic way rather than a linear way, as is the case with a simple taper-leaf spring.
From the middle, where it is thick, to the ends, where it is thinner, the tapering steps down in a parabolic manner. For every millimetre of its length the spring tapers by a parabolically-calculated
value, ensuring that the stress inside the spring is evenly distributed throughout its entire length.
In contrast, a taper-leaf spring is shaped
like two long wedges that taper from a thick centre to thinner ends. This linear tapering means that the stress inside the leaf isn’t constant and
evenly distributed, but peaks at a certain point, where it’s more likely to break.
The diagram at right shows a taper leaf spring calculation formula.
In theory, a spring needs only one parabolic tapered leaf, to replicate a similarly-rated multi-leaf spring pack. However, the stresses inside the leaf can be very high, during extreme axle movements of off-road driving, so a two- or three-leaf spring set is required to spread these stresses evenly.
These extra leaves are similar in length and tapering, and touch only at the centre and the leaf ends. Polyurethane blocks between the leaves at the leaf ends can act as spacer and friction reducer in one.