First, don't read into this until after lunch when you are alert and sharp or, alternately, slightly drunk and dull. Second, I apologize for throwing out a ton of numbers and calculations like this, but simplifying the complex and complicating the simple is what engineering is about.
Often the engineering calculations needed for dynamic applications are somewhat more involved than just a couple of factors. For example, I once went into an in-depth exploration on a mission to have a calculation path to determine the 'most effective' rake and trail for a trike. Starting with a basic set of influencing factors I kept finding layers of secondary and third level factors that would be needed to determine that 'perfect' solution. After listing 18 different factors and extending the formula to a quite complex level, I stopped, although I could foresee another 5 or more factors in the stream to be added. All just to find two base factors: rake degrees and trail distance.
The same is true for determining the 'best' spring load (not addressing 'best' spring rate) for a coil-over spring on your front end design. However, with only a few numbers you can calculate what you are wanting for your springs. So, your planned design photo displays one needed factor - you will be using two springs parallel to each other. If you can provide the dimensions between the three holes in rocker plates (axle, spring mount, downtube/hinge) and the spring angle degrees from vertical, you can yield a good spring load to look for in your setup. Also make sure your anticipated load (the 700 pounds number) on the front end is calculated at the axle point (not at the frame neck). So, gather these numbers:
- Anticipated total load at the front axle
- Number of parallel springs in the design (2 per Triker Don's representative photo)
- Spacing of 3 mount holes in rocker plate
- Angle from perfect vertical of coilover spring
To calculate:
1. Work rocker plate numbers first. With the rear downtube hole be considered the hinge point, the ratio of hinge point to axle hole vs. hinge point to shock hole is your first multiplier factor against final spring load (divide axle hole distance by shock hole distance).
2. Work spring angle next. Subtract spring angle degrees from 90 degrees. Now divide 90 degrees by that first number. That will yield a second multiplier factor against final spring load.
3. Now multiply your 700 pound vertical load times your first multiplier and then that product times your second multiplier.
4. Divide your end answer in number 3 above by the number of springs you will use (likely 2). That will be a close estimate of your spring load needed.
Now comes the 'engineering' part of your design. The final answer may be somewhat high for available candidate springs. You can adjust that final answer quite a bit by varying the hole distances in your rocker plates. By moving the shock mount hole forward you will be reducing the final spring load. Just adjust that measurement in this formula and recalculate to see the effect. You can actually move the spring mount hole forward of the axle hole to lower the spring load requirement to a reasonable range. You can alternately adjust the spring angle to change the final load rate - the nearer to vertical, the lower the spring rate required. This last part of moving and changing the design to get what you think is ideal is what makes engineering such a fun activity.
Gotta go take my meds now, and once I do they will let me out of my straight jacket for 1-hour rec time. It's rough to type on my computer with one hand that I wiggled out the top of my jacket. My life is like NASCAR - go fast, turn left, go fast, turn left - it's just that left turn I have problems with.