I understand that a tender isn't going to work some magic and suddenly make things exactly how I want them. I suppose I just don't fully understand what the different spring setups are going to do for me. I was hoping someone like Myles would find his way in here and either tell me I'm way off or that I have half a clue.
Broken down, my thought is this...
A helper spring is really soft. Say its 80lb/in and has a stroke of 1.5". So when the car is at static ride height, that spring is bottomed out and absorbs 120lbs of the corner weight. That means the other ~700lbs is go into the main spring. A 550lb/in main spring is going to compress 1.25" under that load. Static compression of the suspension is then 2.75". With a useable stroke of 4.1" for the main springs minus the 1.25" static compression, you end up with 2.85" of compression travel before you bottom out the spring. You also can't forget that you've taken up 2.75" of the damper's total stroke.
If I use a tender springs with a stiffer rate (280lb/in) and a 1.61" stroke I now only have 370lbs that are eaten up by the main spring. So the main spring is now compressed 0.67". With the same spring stroke, I now have 3.43" of compression travel.
Does that make sense or am I way off in left field? I know its not quite that simple. Springs in series are a softer total rate, and the tender isn't going to compress 100% before the main starts to compress. But the differences in stiffness means the tender should take most of that static compression. With the right stiffness tender it should see 100% compression under static load. Then its just a case of fine tuning main spring length to achieve the desired stroke.
All of that said, it really depends on where the suspension should be sitting in terms of required compression and rebound travel. How much travel do I want / need? Do I want to set it up such that static ride height provides equal compression and bump travel, or do I bias it one way or the other?