I understand why some manufactuers use progressive rated front and rear springs, and other use linear rates. What is the purpose of having a linear and progressive rate like the stock wrx springs and Eibach? (ie linear front and progressive rear)

Just Curious!

 

I would guess linear in the front would get you better steering feel, and would make it more predictable... this precision would be less important in the rear, so they opt for more comfortable progressives there

 

The reason many aftermarket springs are "progressive" may not at all be what you would expect. Spring makers pass it off as a functional advantage, but in reality it is needed in order for them to provide the desired combination of spring rate and vehicle trim height while having enough spring seat pressure at full rebound to keep the spring from becoming dislodged.

As a suspension is "lowered" on an unmodified strut, the total wheel travel from jounce to rebound remains the same and jounce travel is traded for rebound travel, i.e. if the jounce/rebound travel is 100/100 from the factory and the vehicle is lowered by 30 mm, then the travels are then 70/130.

And lowering springs typically are designed with an increased spring rate compared to stock, which reduces the amount of spring travel corresponding to a given change in applied load. This results in a shorter spring free (unloaded) length.

In order to counter these compounding effects, lowering springs often utilize a "coil-on-coil" variable rate design. This type of spring has a main section with one rate and another section with a much lower rate. The section with the lower rate has a reduced spring pitch and corresponding coil to coil clearance. As the spring is compressed, the lower rate section progressively goes solid, stacking the coils one atop the next, effectively locking out their contribution to spring rate. Thus the rate varies with the remaining number of "active" (non-locked) coils. This spring design offers the ability to simultaneously provide increased spring rate (stiffer) and reduced check load (lower) while maintaining spring free length and thus ensuring sufficient seat pressure in full rebound for spring locating.

This being the primary reason for use of variable rate is often evidenced by the low rate section being fully or nearly fully closed out at the curb height position, resulting in little or no rate change from curb height position to full jounce.

There are two distinct disadvantages to "coil-on-coil" variable rate springs, and this is why you will seldom if ever see a factory spring of this type. 1) Noise - the spring coils coming in and out of contact with one another results in unavoidable spring slap noises. 2) Durability - the contact pressure and abrasion between contacting spring coils results in very short spring coating life and springs, due to their high operational stress levels, cannot typically tolerate corrosion for long before failing. This is why, in an effort to reduce these effects, springs of this type often have polymeric sleeves adhered to them where coil contact occurs (such as the light grey pads on pink STI springs).

A linear rate spring simply does not offer the same design freedom, thus the spring maker is limited in how much spring rate and check-load can be changed from factory values.

You'll notice that "coil-over" type struts typically have two springs on them, which serve the very same purpose as a "coil-on-coil" variable rate spring. The lower rate spring is often wound in flat wire to further reduce its stack height.

If you are going to use "coil-on-coil" variable rate springs, be sure to inspect them often and replace them when the spring coating is damaged, especially if you use them on salted roads in winter.

 

WOW!

Thanks for the info. That's a lot of research you've done!

 

blue5spdwrxwgn: I hope it's alright with you, I posted your write-up on another board I frequent, and gave you credit.

thread found here:
http://www.clubprotege.com/forum/sho...352#post267352