Praxis Air System Available for the WRX

[ArthurS]

At a price of 3,450.00$, this is supposed to be the best suspension setup available. Can adjust on-the-fly for touring, sport, or track modes and allows you to adjust the dampaning.

If I had the bucks, I would so go this route.

http://www.tirerack.com/suspension/s...+Perf+Susp+Sys

[sperry]

For that much, I'd get the adjustable STi struts, lowering springs, complete STi link and control arm kit, and save $500.

Also, what happens to the suspension geometry when you change the ride height? You'd need automatically adjustable links to compensate for the ride height changes, or you'll have to make compromises on handling somewhere.

[AtomicLabMonkey]

That system looks pretty interesting - the components look fairly high quality, and I like that they included a compressed air dryer; I wouldn't normally expect to see details like that in an aftermarket automotive system. Like they said, that will prolong the life of the pneumatic components and should make the system more reliable in extreme weather conditions.

The only real red flag I see with it is the ride height change in each different operating mode. Switching from everyday "Touring" mode to the "Track" mode is a 50mm(1.96 inch) drop in ride height - and that sounds fairly extreme to me for an on-the-fly change. The alignment settings of all 4 wheels will change quite a bit going through that range of motion, possibly more than could be effectively compensated for by running a very conservative street/Touring alignment (alignment settings will often change to the agressive side as a cars ride height drops, like gaining a lot of static negative camber and toe-out). This would be vehicle-specific though, and depend on the suspension geometry of the particular car it was put on.

I would actually be more concerned about the change in suspension geometry itself that can occur from lowering a car 2" from its factory height. Intuitively you'd think lowering the car would make it faster due to the resultant lower Center of Gravity, which should decrease load transfer between the inside and outside tires during cornering. Again, you'd have to look at different model cars as individual cases, but in general especially for cars with strut-type suspensions, as you lower the inner pivot points of the control arms (which you're doing by lowering the car) the roll center heights at each end of the car can drop. The roll centers can very easily drop more than the C.G. does, and you could actually end up with an larger roll moment than you started with... which then requires stiffer springs and swaybars to cure the increased body roll and you end up with less suspension compliance over bumps and less total grip than you started with. Not to mention it can have a bad effect on the camber curves of the wheels (camber angle vs. vertical suspension travel) and make the car bumpsteer like crazy.

Well, I've been rambling again... so, what's the point of all this? (Sorry, I really didn't mean to lecture... feel free to smack me around) Just that slamming the car down isn't necessarily going to be a benefit - it can actually has the potential to really muck up the alignment and factory suspension geometry and make the handling worse.

[sperry]

Yeah, that's what I was getting at Austin, I just don't know the nitty gritty.

As far as lowering WRXs... I hear the max you should lower it it to the point where the lower control arms are parallel to the ground when sitting static. Any more than that, and you have to start running to high of a spring rate, like Austin said.

[AtomicLabMonkey]

Originally Posted by sperry

Yeah, that's what I was getting at Austin, I just don't know the nitty gritty.

As far as lowering WRXs... I hear the max you should lower it it to the point where the lower control arms are parallel to the ground when sitting static. Any more than that, and you have to start running to high of a spring rate, like Austin said.

There's an important caveat to that too; when trying to figure out if the LCA's are parallel to the ground, the geometry is determined by drawing a line through the middle of the lower ball joint, and connecting it perpendicular to the centerline of the LCA pivots... a lot of the time control arms are stamped or cast and the shape can be misleading from just looking at it.. it might *look* parallel at a glance underneath the car but the inboard pivots could actually be below the ball joint centerline, which would generally be bad.

[ArthurS]

I guess these air systems were designed on the WRX....so you would think that the company took many of these items into consideration. My opinion is they are to expensive...but it may be a effective tool for those that have the cash.

[sperry]

Originally Posted by ArthurS

I guess these air systems were designed on the WRX....so you would think that the company took many of these items into consideration.

No matter how well designed the system is, if you change the ride height the toe will be wrong unless the control arms are also automatically lengthened to compensate. Since there are no servos attached to the control arms, using these struts *will* pull the car out of alignment. So you have to pick an "optimal" ride height, any deviation from that will affect the toe, i.e. any change in ride height is a compromise.

[BOO]

Originally Posted by sperry

For that much, I'd get the adjustable STi struts, lowering springs, complete STi link and control arm kit, and save $500.

Also, what happens to the suspension geometry when you change the ride height? You'd need automatically adjustable links to compensate for the ride height changes, or you'll have to make compromises on handling somewhere.

So Scott,
What you are saying is This set up that you speak of Comes installed from the factory on an STi?? Or are they an Upgrade even for an STi ?

[sperry]

Originally Posted by BOO

So Scott,
What you are saying is This set up that you speak of Comes installed from the factory on an STi?? Or are they an Upgrade even for an STi ?

STi has a set of 4 way adjustable struts available as an aftermarket item for Subarus. They do not come as factory equipment on any cars. From what I've read, setting 1 of 4 is a little softer than the normal STi struts, and setting 4 of 4 is harder than hell (i.e. race stiff).