One of the things that contribute to an unpleasantly hard ride is spring "stiction" What happens is that the manufacturer places low friction rubbing blocka at the ends of the spring leaves where they rest up against the leaf above. After some miles & years, they wear out & the leaves rub against each other. This gives a lot of interleaf friction & the rust between the leaves does not help. This means that a certain amount of "jolt" is required before the leaves begin to move. Thats stiction.


Datman is right, if the release of energy that is stored in the spring is too slow, on a rough surface, at quick speeds, it feels like the suspension is being "sucked" down. Not much fun.


For stock ride height in a street vehicle, then stock spring rates should be OK, but as the suspension gets lower, the spring rate should be stiffened up a little. The spring rate is the pressure that is required to compress a spring a measured distance. Lets use 1 inch as an example.


If your spring needs 100lb's to compress it 1 inch & you have 4 inches of travel, then it would take 400lb's to cause that spring to bottom out. Lets lower it 2 inches. It would be desireable to make this spring have a new rate of much closer to 200lb's per inch so that it still takes 400lb's of force to bottom out over the reduced distance of 2 inches of travel. Thats why lowering blocks suck & cars lowered this way will bottom out a lot more. The stiffened spring rate makes for a harsher ride, but hey, theres no free lunch. To get a benefit in one area, you pay for it in another.


If you are going to pull the rear springs out during a rebuild [recomended] then have a reputable spring works reset them to the ride height that you select, assuming that lowering is on the agenda, & see if Nissan still supply these friction blocks. They are NLA for 1000's but it's lucky that i bought 2 sets for my sedan many years ago & i still have 1 new set available for my coupe.
If they are not available, then the spring works will more than likely have some generic replacements.


I recomend Urethane bushes when refitting the springs & if the shackle pins are pitted, try to get new ones. The trick is to allow the spring to work with an absolute minimum of friction so that it can do it's job properly. The shock then controls the ride quality by controling the rate at which the spring returns to the static ride height. I set mine so that it returns to ride height from a dip that compresses the suspension about half or more of it's travel in one move. By that i mean that it does not rise above static ride height & drop down again. Do this test at speeds that you are going to drive at MOST of the time.


It seems to have worked well for me. It's not the cheap way, but it is right way to have your car ride & handle well. Do it on the cheap, & you will need to do it all over again to fix it right. Poor man pays twice.
Chris

Damn it 89mm, this was going to be a short reply.