A spring is rated in lbs per inch for deflection. Leaf springs come with a designed arch in them. When you add load the spring will deflect based on the load. It also grows longer, thus you need shackles at least at one end.
As the spring ages (wears out due to constant overload) the arch becomes less so with the same weight it deflects about the same amount, but it is starting from a flatter arch so the back end sags.
Adding leafs increases the spring rate (stiffness) so even though the main spring has less arch the spring rate goes up and the spring pack deflects less. So the back end comes up. Of course the rig rides harder
Re-arching springs is normally a waste of time and money as most places do it cold, ie just rebend the sucker big time. Re-arching hot and re-tempering will restore the spring to almost new. But there are VERY few places that can do this.
Re-arching cold and adding extra springs will last longer but eventually it will sag too as the extra leafs are caring a lot of the load.
Best is a new spring pack with the correct capacity, airbags would be next best.
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