Just a few comments on the 5L design philosophy as prompted by PaulJ, trying to avoid marketing and not making direct comparisons to the lovely ARP4.
Leaf springs in themselves add roll stiffness by the middle, where the axle is clamped, twisting relative to their ends in roll. This can not be ignored. So total rear roll stiffness with leaf springs is a combination of this twisting stiffness of the leaf springs plus the vertical stiffness of the leaf springs. See, leaf springs are actually quite clever!
So, if the leaf springs are removed and the same rear roll stiffness is required then the wheel rate (stiffness) from the new coil springs needs to be higher than that once provided by the leaf springs, so as to replace the roll stiffness previously provided by the leaf springs twisting. So ride will be harder.
The 5L uses two pairs of links that incorporate rubber bushes plus a Panhard rod. These links are unequal and nonparallel and so together with the Panhard rod they 'fight' in roll, there being too many constraints. (No restriction in bump when both wheels go up/down together.) But as there are rubber bushes in the top links these deform progressively to control roll stiffness, whilst still allowing small single wheel compliance to cope with degraded poor road surfaces. Thus a low initial spring rate (lower than any std Morgan leaf spring) can be used to give good ride, but then to cope with big bumps and heavy loads the springs are also very progressive to stiffen up when loaded. This stiffness progression then usefully also adds to the roll stiffness.
This configuration was developed to achieve good ride from a low initial single wheel rate but with a roll stiffness that significantly builds the harder the car is cornered.
I ran my car with the current spring rate but with prototype rubber bushes that were way too soft and it was nearly undriveable around medium speed corners with so little roll stiffness, like being at sea!
OK there is a lot more to it than this but PaulJ did hint the the 5L was "designed in a different way", and this is some of the background.
ALSO
I too was pleasantly surprised how the steering of cars fitted with 5L seemed to sharpen, as mentioned it must be that the car will respond to the steering input better with the rear axle far better located.
I hope this helps explain some of what was done to develop the 5L over three years with various cars and thousands of miles.
"Each to their own", "horses for courses", "pays yer money and makes yer choice", all as ever. Peter.
