Semi-elliptic springs, such as those used in Series Landrovers need to have their strength tapered from the axle to the ends, as the bending stress on them increases closer to the axle.
The traditional way of of providing this is to use a series of parallel leaves of constant thickness, but varying length. The advantages (A) and disadvantages (D) of this approach are:-
1. The leaves are all made of the same relatively cheap sheet steel. (A)
2. Multiple leaves means less serious if one breaks. (A)
3. Interleaf friction provides built-in damping (A - less dependent on damper) (D - interleaf friction can vary widely from very little with new, oiled, leaves to very high when very rusty or with worn steps inn the leaves, resulting in a rough ride)
So-called parabolic springs are made from a small number (usually 1-4) of leaves, whose thickness is tapered according to the stress along the spring, following a parabolic law. These few leaves are usually designed to touch the adjoining leaf only at a single point at the end of the shorter leaf. They have the advantages and disadvantages :-
1. Each spring design requires a custom roll to make the profiled sheet of steel that is slit to produce the leaves. Unless made in large numbers, this means they are expensive compared to traditional springs (D)
2. Few leaves means a breakage is more serious (D)
3. Little interleaf friction, means almost all damping is from the Damper, and highly predictable (unless the damper breaks or otherwise fails!) (Mainly A, and the reason for their popularity - good ride!)
4. Because the leaf is thicker over most of its length, the surface stress on the parabolic leaf is higher than on a conventional spring. This demands both better quality steel and a better surface finish than the conventional spring, to avoid stress risers that result in fatigue cracking. Perhaps significantly for the roads many of us drive on, stone impacts can damage this necessary surface finish. (D)
Overriding all these advantages and disadvantages may well be the design of the spring. The conventional Landrover springs are designed to operate at rated maximum carrying capacity (and probably more, as Rover learnt early that many owners routinely overload their Landrovers), whereas the parabolic ones may well be designed for the recreational use of the vehicle where, particularly for the swb, little is carried except a driver and a single passenger. Needless to say, this design will be much more comfortable, but may run into problems if you want to use the vehicle one day top fetch a load of pavers!
John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
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