Classic/Antique Car Repair: Replacing Coil Springs on 1972 Buick Centurion, pontiac street, suspension geometry

Question
Hi,

Thanks in advance for answering this question.

I have a 1972 Buick Centurion with a 445 engine. It has new sensatrac shocks and new bushings on the front end.  I have the original factory build sheet, which includes "firm ride" suspension--heavy duty springs.  

As I have a bad back, I am interested in the smoothest, most cushioned ride in this vehicle. Hence I am considering replacing the "heavy duty" coil Springs with standard coil springs. Both types of coil Springs were original equipment on the car and are available from Moog.

Here are my questions: Over time, did the old heavy duty springs get stiffer as they aged, or softer as they sagged?Do you think replacing the old "heavy duty" coil springs with new standard coil springs (moog 5270F and 5455R) will result in a more cushioned ride and less vibration? Will they cause a lot of increased body roll?

Thanks again,

Scott

Answer
First some of the answer will be opinion and some will be fact so please do not give me a bad rating on the feedback for my opinion based on experience. The heavy duty springs from the factory had a higher rate, meaning stiffer, and also were shorter making the car a bit lower. As they age the ride got progressively harder. Now putting the standard springs under the car will raise it giving it more spring travel to absorb bumps better, softer ride, but will change the steering and suspension geometry a bit. Body roll as we have learned since your car was built is more a function of sway bars than springs. There are after market sway bars that will flatten the cornering out without effecting the ride quality. I did this on my Pontiac street rod and made a good handling lady out of the car with a smooth ride.
Brad