How do we know if we have the correct spring rates;
To begin with we need to understand tha the spring in a drag car has only two functions.
Support the weight of the car up to the starting line.
Give as smooth of a ride aas possible while going down the track.
Possibly a third function is to support the additional weight during weight transfer.
If you can not adjust lower spring adjustment enough for there to be 75% maximum, to 62.5% minimum of the total shock travel inside the shock while car is sitting at rest with all weight in it including driver, you have the wrong springs for the car.
Usually a 2900 to 3000 lb car will want between a 130# to a 145# rear spring on it, but that is never absolute. I know Outlaw 3,000 lb cars working well with 150# springs even. You can not go by just the rear weight to guess what springs you need. Too many other thing figure in, such as the exact shock mount placement in relation to center of axle, Shock mounting angles if any,
You let the car tell you what it wants and you can do that without ever going down the track. At all times you want to keep enough compression available for car to sink on shocks some when front weight is transfered to rear by wheelstands and/or accelleration forces which shove a bunch of weight to the rear even if no wheelstand. You also need enough compresion to allow a good smooth ride down the track. Bottoming or topping a shock out is a sure fire way to unload tires and if at high speed to crash.
Support car on all 4 wheels so you can get under it. Disconnect anti_roll bar on one side (if equipped) just to be sure it is not interfering. adjust the bottom spring adjusters all the way even on both sides, doesn't matter right now how high or low. Measure how much of the shock ram is sticking out of the shock. If more than 50% of the total travel is outside the shock, you need to adjust spring adjusters down so that you will only have appx 25% to 37.5% of the total shock travel sticking out. If you can't and at all the way down on adjusters, that means the spring is too stiff. If you have adjusted the lower spring adjusters as high as you can and you still can not get at least 37.5% to 25% of the total shock travel to stick out of the shock, that spring is far too light and the shock will bottom out while lanuching and while going down track when it encounters any sort of bumps.
The more rear travel the shocks have available the bettter it is to tune. At one time people were using shocks with 5 inches total travel in the rear. That does not allow much room for any error. 6 inches is far better and over 6 inches is even better. Problem is that the longer travel shocks do require more room to mount.
I will use the 6 inch travel shock as my example;
Ideal will be 1.500 to 2.25 inches of ram inside the shock I can not accept anything else if I am tuning the car.
This tolerance does allow it to work with more than one spring rate spring becuase of the lower spring mount adjustability.
lol
I do believe I will be adding this into my 4 link article at Racing Junk. I did touch on it but not this much.
Hope this helps you.
Oh' A ladder Bar car will need to hit the tires a little harder than a 4 link car will becuase of the difference in how they work.
I see that you are also expermenting with power adders etc. Know this, no matter how crazy and backwards it wiill sound. The more you increase power to the rear wheels the less suspension action you want front and rear.
Ed