Understand this: as a leaf spring compresses it flattens out and gets longer. As it gets longer it moves that wheel farther back towards the
rear of the truck. Think of your
outside rear wheel in a turn.
When the
inside rear corner lifts, in that same corner, that leaf spring archs down. Making the leaf shorter. Which moves the wheel
forward in the wheel well.
The leafs are shackled at the rear to allow for this lengthening and shortening. But at the front they are rigidly attached to the frame.
So every time you go around a corner the rear axle kocks in the frame and steers either to the left or the right. Effectively steering the rear of the truck in
the opposite direction that you are steering the front with the steering wheel.
Ok, if that isn't odd enough, what happens when you hit any kind of bump mid-corner?
Well, your rear axle steers
left and right like someone is sawing at the
rear steering wheel whilst you are trying to anticipate and counter act that action to keep the front of the truck pointed around the corner.
You know that feeling when you are going around a corner in the old truck, no load in the bed, fighting the wheel?
That is why leaf springs have gone the way of the dodo...