The progressive rear springs are composed of end coils that are much
smaller wire diameter than the main coils. These thinner coils sag over
time at a faster rate than the much thicker middle coils. The front
springs have all thicker coils and sag less.

This is also why adding rear spacers to make up for the sag results in a
harsher ride than if the springs weren't sagged. The thinner end coils
are what made the rears progressive and without the thinner coils being
active anymore the spring is too stiff.

Mark

Steven Shelton wrote:
> My van rarely squeaks.  I just assumed that, because it took a 1 1/2"
> spacer to level the van, the springs are compressed and the range of travel
> is  reduced.  Of course, I also assumed that since the engine is in the
> rear the weight distribution would be toward the rear also.  That
> apparently is not the case.  Why, then, does the rear sag in so many
> vanagons and not the front?
>
>
> On Tue, Nov 13, 2012 at 10:03 AM, Steven Shelton <shelton4@gmail.com> wrote:
>
>> I used 1 1/2" spacers to cure a serious saggy butt problem on my 1985
>> camper.  The van looks much better, but the lack of spring travel affects
>> the ride, especially over rough terrain.