Originally Posted by
Chocula
For simplicity sake, assume you have two 10 lb/in springs. Assuming you apply a 10 force to one of the springs, it will compress 1 inch. Now stack them, and apply that same 10 lb force. The bottom spring now has 10 additional pounds on it so it compresses 1 inch. The top spring also has 10 additional pounds of force on it so it compresses 1 inch. This means your stack compressed 2 inches from 10 pounds of force which is a 5 lb/in equivalent.
This is why a longer spring that is wound exactly the same as a shorter one will be softer. Each coil deflects the same amount for a given load, but you have more coils on the longer spring so your total deflection is greater.
Using your example above and applying 10 pounds of force, you get the 1 inch from the 10 lbs/in spring and 0.5 inches for the 20 lbs/in spring for a total of 1.5 inches which you correctly calculated as 6.67 lbs/in.
Where dual rate springs react differently that simply using a single spring is when the softer spring exceeds its ability to compress linearly (overly simplified: it gets flattened and can't compress anymore), then any additional force is only compressing the stiffer spring. In the 10/20 lbs/in example you get 6.67 lbs/in until the 10 lb spring fully compresses, then you get 20 lbs/in. It is not quite that linear in the real world, but this should give you an understanding of what is going on.
-Chocula
Right. We referred to that as coil bind, but we always brought the secondary collar in to activate the lower (primary) spring before the upper (secondary) spring hit coilbind.
From the dual spring sets that I have seen, they are making the upper spring heavier than the lower, essentially negating the possible benefits of the upper spring coilbinding and working on the lower spring alone. That's why I don't see the advantage to it in this application- yet. The ones I had were from Pro line, maybe they changed them or someone else will come up with something better. Either way, until a true position sensitive damping method is created that is consistent to setup and use, I don't think there's an advantage to it as the sudden transition to a firmer rate will drastically unbalance your damping characteristics at that point in the suspension travel as the car will be oversprung for the available damping.