Time some more on preload.

Since the base formula is pretty straight forward we need to look at what the preload effects. The preload poundage as discussed above compresses the spring stack from 24.250 to 23.250 and in this case 1” for 68 pounds which is the combined rate or the dual rate spring set up. We also know the crossover was set at 4” with the spring stack at 24.250 which is it’s free length with ¼” or .250 spring spacer and two 12 inch springs.
Two of the effect which have taken place is the cross over collar distance from the spring spacer has changed and the force acting on the spring stack went from 0 to 68 pounds.

What effects does this have on the shock?

Since we’re looking at poundage and shaft travel let’s look at those. First let’s look at the effect on the crossover point (poundage). As discussed before each spring moves a certain amount for the combined rate of the two springs so I’ll skip the math and go for the answerer. The Main Spring (150) has been compressed.454545 of an inch and the Tender Spring (125) has been compressed.545455 of an inch for a total of 1 inch. Since the crossover poundage is based on the movement of the Tender Spring and the cross over point which is now closer than before at zero preload we must find the new crossover poundage.
Before the preload change we were at Zero so we took the 4 inch setting and times it by the Tender (125) and came up with 500 pounds.

After the change we take the .454545 of the Main Spring length change and subtract it from the .545455 of the Tender Spring length change, which is 0.09091 on an inch then we subtract it from the 4 inch we had at zero and come up with 3.90909 inches then we times by the Tender Spring rate of 125 and come up with 488.63625 and then we must add the preload poundage which is the force the shock must first over come before shaft travel can begin. When we add the 68.18 to the 488.63625 we come up with 556.81625 of COP or Cross over poundage.

Now for the flip with the 150 over the 125 we can do the same math which I will save you from since I have a cheat sheet or spread sheet to help me. Even with the 68.18 pound per inch combined spring rate remaining the same the new COP is 681.8182. What a huge difference in poundage just from a flip and one inch of preload.

Next will look at the Shaft Travel for both examples and then bring the Static Sag into it.