The shape, camber, of the spring is not parabolic but semi-elliptical and sometimes
referred to as parabolic by mistake.
Parabolic tapered springs may not be confused with linear tapered springs (every length step is tapered with the same value forming a wedge). Although linear tapered springs will look the same, at first glance, they will not be as good as the paradoxically tapered springs since the stress inside the leaf is not constant (evenly distributed) but peaks at a certain given point, where it is obviously more likely to break.
The reason why the parabolic spring is more comfortable is simple but before I explain this I like to explain what happens with a (multi) leaf spring when it moves under force. When a multi leaf spring compresses the leafs will become longer since they are in a
Simi elliptic curve when they are without load and will go straight when they reach maximum load. So the leafs will tend to slide against
each other when the spring compresses and thus it will cause friction. This 'interleaf friction' needs a
certain amount of force to contra act this interleaf friction before the spring compression
(inward) movement starts.
This means that when you drive over a bump the spring will first have to overcome the interleaf friction and then it will start to move inward. By then the bump 'shock' has been transferred through the chassis and body giving the occupants bouncy ride.
Ideally a spring should have only one parabolic tapered leaf. This then eliminates all interleaf friction at once, however, the stresses inside the leaf are very high so when the extreme axle movements of off-road driving are working on this single leaf spring the stresses will be too much. Therefore a 2 or even 3 leaf spring set is required to divide these stresses evenly. These extra leaves will be similar in length and tapering. They will only touch at the center and at the leaf ends. By putting our HST polyurethane blocks between the leaves at the leaf ends they will act as spacer and friction reducer in one, preventing the leaves from touching and reducing the interleaf friction.