How to specify tender spring rate?

I have been searching how to specify tender spring rates online but didnt find much information. But before we get in more details let me clearly define what I am looking for.

When I talked about tender spring I am NOT talking about helper springs which are really soft and fully compressed when car is on normal ride height. Tender spring has a rate and will be fully compressed only when there is chassis movement so the rate will matter at low compression ratio.

On my case I got an used set of coilover JRZ RS Two which came up with 175lbs tender and 700F/800R lbs. This is more a track setup. I decided to go softer as want to drive the car around ... my plan is 400F/500R which works pretty well with the dampers (already confirmed with JRZ).

What is not clear to is why tender springs were used on the first place?

What is the advantage?

Why we dont see track cars with this setup? Most of the time they use helper springs.

Why not going directly with main spring?

if I calculate the overall spring rate with the tender spring before it is fully compressed I will get 121lbs F / 129lbs R.

Appreciate the support.

"Tender"and "helper" springs are basically the same thing, if in normal use they're collapsed, then that's what they are - the only real potential difference is the spring rate may be higher on the former, and the coil profiles may be differetn, round or flat wound. I don't believe these are what you have.

On the other hand, if the auxilary spring isn't closed at normal ride height, or close to it, it will probably be a dual spring arrangement - and three aprings aren't unusual in long travel off-road setups. This is what i believe you have.

The main advantage is that it allows the use of a sort of progressive spring, without the expense of having them made, and allows fine adjustment of the light, main, and if used, high load/impact spring.

An example may be the front of a race 'buggy' - the larger endurance race type, where a specific spring rate may be optimum for fast driving on the flat-ish ground, but a light spring is also used that allows more droop and an actual spring rate until it closes up for when it's lightly loaded under, say acc'n, or for initial impact absorbtion when landing. In some instances, a third heavy spring may be used for when the suspension is heavily loaded by a bump or hard landing, where the 'normal' spring would completely colapse and shock load the suspension and vehicle's mounting points.

For track cars, there's simply little, or no, point, as a single spring rate usually works best.

With a single 'main' spring, once it's extended, it can't go any further, and if the suspension is able to extend further, the spring will become unseated. back in the day, there wasn't ny alternative to "helper", or "clearance" springs but to have a box of springs in both different spring rates, but lengths as well, to achange for different ride ehights. That last is where the "dual threaded adjuster" type damper is very useful, as it separates the spring seat/damper body relationship from the damper body/chassis relationship.

A pic' would make it simpler to figure out the setup you have.