As found on the incomparable Imperialclub.com website, a website who's value is nearly priceless and worthy of monetary donation (note to self: send in that check) this page in the technician's booklet describes the geometry with which the live rear axle operates: http://www.imperialclub.com/Repair/Lit/Master/214/Page05.htm
Of note to this thread, it specifically defines what happene when springs are installed that remain arched when the car body's weight is put on them. This effect takes away from the lateral stiffness that a flat leaf provides, allowing more sway. The original question of Steve B.'s was about going with HD springs or standard. When I had my '72 Newport Custom, it had the optional A32 Trailer Towing Pkg that had the HD suspension which merely incorporated an extra leaf. The assembly was pretty flat as intended.
I understand that a good result can be had by just renewing the one longest leaf in the existing leaf assembly. I haven't tried that but it might be worthy of investigation.
That technician's booklet section link is a really good read for those who may have thought leaf springing was in some way antiquated or of lesser quality to the competition's coil / track bar setup. It's so good, here's the link again... http://www.imperialclub.com/Repair/Lit/Master/214/Page05.htm
Happy understeering! EricLebanon, Oregon
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