Lowering vehicle with alternative coil springs

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I´m looking for a way to lower my Tbird.
Moog has this great list of all their springs.
Rockauto lists part # 8314 for front and 8545 for rear application.
The original Ford Specifications form says a front weight of 2495lb and rear weight of 2034lb for my vehicle.

I made a Excel Sheet with potential spring alternatives for lowering the vehicle.
The last column contains calculated installed height of the spring in the Tbird.
Calculation is based on the following formula to receive the final installed height in my Tbird: FreeHeight - (FrontOrRearWeight/2)/SpringRate)
I substract how much the spring will be compressed when installed, from the free height of the spring, to receive the installed height. Front and rear weight are divided by two, because there are two springs in the front and back, not just one. Spring rate tells me how many lb are needed to compress the spring by one inch.
I would say my calculation should be correct.
25428051cu.jpg

So, if I want to lower the car by about 2 inches, I would go with part # 8654 in front and 60230 or 60232 in rear.

Fixed values (with just tiny derivation) are/were Inner Diameter and the End Types of course. Moreover Spring Rate should fit more or less, in case I don´t want to ride a boat or a stiff carriage.

Any objections?
 
I do know you have to raise the spring rate when lowering. How much? I don't know. There are places that can make custom springs. They can look up the factory springs and take what you want from the car to make a one-off set that will do just that.
 
There's more to it than ride height. Differences in spring rates front to rear effects understeer/oversteer. Try to maintain the same ratio in spring rates front to rear to maintain the vehicle handling balance. Increasing the rear spring rate relative to the front decreases understeer. You don't want to go crazy stiffening the rear springs, as you might bring on an oversteer condition, which is not fun to live with.
 
As you can see, it´s not only ride height what I was looking at. Spring rates are pretty much comparable to stock. I also have the Ford Specifications sheet and tried to stay at similar rates.
 
Sounds like you've got a good handle on it. I watched a couple of cut spring lowered cars hippity hop slowly around the cones at the last autocross. Stiffer shocks are needed if you go to much higher that stock spring rate, but probably you still have enough suspension travel left not to need much stiffer springs.
Also I loved taller progressive bumpstops I installed in my old Neon, it was a low car but the bumpstops helped it take big hits without drama and still rode nice the rest of the time.
 
You might want to consider keeping the frequency of the springs close to the factory settings. If the front and rears end up the same you're going to have problems when you hit a bump. Both ends of the car will start moving at the same frequency and you will not like the result. The usual method is to have the rear springs set at a lower frequency than the fronts. Getting the ride sorted out takes a bit of work and you'll have to live with the results.
 
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