Cobra Suspension uses the manufacturer's original design as our reference design. In most cases, the original springs of a modern car already have a progressive effect. We optimize this design even further when lowering a car. In some cases, we choose to give a spring two spring rates, a so-called 'dual-rate'. These are actually two linear springs with different spring rates in one spring, which, in addition to the original progressive effect of the original spring, work separately from each other.
What is progressive?
With a fully progressive spring, the spring rate increases with further compression. So for every next millimeter of compression you have to apply more force than for the previous millimeter. With coil springs, this can be done by making the spring non-linear. Non-linear coil springs are made by varying the pitch (the space between the coils) of the coils or by applying spring steel in a varying thickness. At Cobra Suspension we produce both progressive and linear springs. The two left springs in the picture have a variable pitch (progressive). The two right springs have a linear pitch (non-progressive).
Dual Rate Springs
Cobra Suspension lowering springs apply, if required, a dual rate design, combined with the progressive character of the OE spring. The graph shows the force versus the length of a spring. With these graphs we can also determine the spring rate of a spring. There you can see that at a spring length of 195 millimeters the springs switches to a higher spring rate, the graph becomes more steep. Up to 195mm the spring has a lower spring rate. After 195mm, the spring rate becomes higher. This point varies per spring and per application.
Lowering a vehicle can be done in several ways. This can be done by using a shorter spring, a lower spring rate, or by applying a dual rate spring. Making a spring shorter sounds logical, but the spring doesn't always have to be shorter than original. A spring can't be infinitely short either. A minimum spring length is needed to give enough pre-load between the shock absorbers bottom and top spring seat in unloaded condition. That is the distance between the top and the bottom of the shock absorber or chassis. The spring must not move between the spring seats and must therefore provide enough pre-load.