Calculating spring rate

[DesertT4R]

The following is a basic, but still incomplete, approach to determining spring rate. This approach should account for the added weight of a bumper and winch.

Assume: Shock should be at the midpoint of travel in the static position. (This would provide 50/50 rebound and compression damping travel.)
Assume: 4.5" shock travel (4Runner/FJ coil over front shock); 2.25" midpoint of travel. (Wheel travel is ~2x shock travel.)

Note: Calculations performed on 2011 4Runner with 700 lb/in Eibach front springs, 137 lb front bumper, 87 lb winch.

1. Measure spring length with wheels off the ground.

2. Measure spring length with wheels on the ground.

3. The difference between these two measurements is the static spring/shock compression. In my case this is 1.5".

4. Static load calculation:
[1.5"] x [700 lb/inch spring] = 1050 lb static load on each spring/shock

5. Then, apply the known static load to determine the spring rate that would put the shock in the middle of its travel:
[2.25] x [new spring rate lb/in] = 1050 lb
Solve for the new spring rate: 1050 lbs/2.25"= 466 lb/in

This is obviously too low (it should be around 600 lb/in?). An important variable is missing for the dynamic load on the spring/shock.

Can you provide the missing piece in this calculation? Are the assumptions for midpoint of travel incorrect?

 

[DesertT4R]

If I add 10% for dynamic loading forces to the static load and assume the shock uses 60% of the travel for compression and 40% for rebound instead of 50/50, I would get this:
(4.5 x 40%) x (new spring rate) = (1050 lb + 10%)
New spring rate = 1155/1.8 = 641
This seems more practical. Answered my own question. Sometimes the answer lies within the question.