The post below was written when I owned an '06 WRX and had gotten myself educated on the problems that can be encountered when "going too low." The Forester sits considerably higher to start with and with springs like the Swifts, the problems described below will not be an issue. If, however, you are swapping in Impreza struts and use Impreza lowering srpings, read on!
The following is based on my reading of accounts from those "in the know" about Impreza suspensions - specifically, those who race or autocross the Impreza - and on my own first-hand experience.
First things first, some definitions;
- an alignment term that indicates the tire's vertical inclination.
When viewed from the front or back of the car, negative camber would look like this - / \ - the tops of the tires tip in towards the center of the car. From a strictly handling POV, negative camber is a good thing - as the car leans into a corner, the outside tire (the one bearing the most load) maintains better contact with the road with some negative camber.
Positive camber looks like this - \ / - the tops of the tires tip out away from the center of the car. Positive camber isn't good for much of anything, especially handling.
Toe (in or out)
- another alignment term that indicates how the tire is "tracking."
Toe out is when the distance across the front of the tires (side-to-side) is greater than at the rear of the tires. When viewed from above
the car, toe out looks like this - \ / . Toe out on the front of the car makes the steering feel more sensitive - the first tiny turn of the steering wheel makes for very quick response. It also can make the steering feel "nervous" or "twitchy" at higher speed.
Toe in is just the opposite. When viewed from above, it looks like this - / \ . Toe in makes the steering feel less responsive but more stable, which is desireable for a street driven car - especially if you spend a lot of time on the highway.
Toe in or out places a lot of stress on both tire and steering, and when too much is present, will cause accelerated wear on both. Only tiny amounts of toe are typically used (if any at all) - no more than a fraction of a degree or a millimeter or two difference in measurement between the front and rear of the tires. "Zero toe" makes both tires track perfectly straight down the road.
refers to a suspension's tendency to change camber as it compresses. Some suspensions have a "greater curve" than others, and the Impreza front suspension has a very noticeable one.
As the front suspension of the Impreza compresses beyond a certain point, camber moves in a positive direction
. In other words, as the suspension compresses over bumps or hitting the brakes, the tops of the tires move out away from the center of the car. This is due to the "geometry" of the suspension, and the relationship between the three pivot points that allow the suspension to articulate - (1) where the lower control arm secures to the chassis; (2) the ball joint, where the control arm connects to the spindle/strut assembly and (3) the top hats, where the top of the struts secure to the chassis.
It's called a camber "curve" because the curve is actually a plot (on paper) of how much the top of the tire tips out ("goes positive") versus the amount the suspension compresses. In our case, the more the suspension compresses, the faster the camber goes positive - the "nasty" part of the camber curve. This happens at about 1.5 inches of "drop" from stock ride height for a WRX, and about 1 inch for the WRX/STi. Ideally, you want springs that don't lower the car to this point.
At stock height ("ugly wheel gap"), as the suspension begins to compress, the change in camber is only very slight. However, the more you "drop" the car, the higher up the camber curve you go and the faster the camber goes positive with only a small amount of compression. So with a "lowered" car, even though the static camber can be set negative with an alignment (tops of the tires tipping inward), this negative camber is very quickly offset by the Impreza's rather nasty camber curve as the suspension compresses.
So with an Impreza that's been lowered too far;
When making a right turn, when viewed from the rear
the front tires look like this - \ \
When making a left turn, like this - / /
When hitting the brakes and having the front of the car "dive," like this - \ /
Not so good, eh? And in order to avoid "bottoming" the excessively lowered suspension you want to cut the bumpstops too, allowing even greater compression?
The only way to deal with these tendencies on a lowered car (and get back down lower on the "less nasty" part of the camber curve) is to modify the suspension geometry by "relocating" the ball joint and by neccessity, the steering linkage to avoid bump steer with the relocated ball joint/modified suspension geometry. See post #67 on page three of this NASIOC thread by Tom Hoppe of www.6gunracing.com
But it's even worse than just a camber problem.
Take a look at the front suspension of your Impreza and you will see that the steering linkage attaches to the front
of the spindles. What the steering linkage does in the context of this conversation is serves as "brace" between the front of the spindles so that they must always have a set distance between them at the point where the steering linkage attaches to them. But what happens is that when the suspension compresses, camber moves positive, which means that distance between the front spindles where the steering linkage attaches gets smaller. But since the steering linkage is a set length, the linkage holds the front of the spindles at the set distance while the rear of the spindles move in as the suspension compresses, resulting in what is called "dynamic toe."
So as the suspension compresses, not only does camber go positive, but toe goes "out." And the lower the front of your Impreza is, the faster and to the greater degree this happens!
What to Do
If "the lowered look" is important to you, here are some points that you should consider as part of any lowering job;
1. Avoid springs that lower too much. A wheel center-to-fender lip measurement in the front of greater than 14 inches is desireable (a 1.5 inch drop in the front yields 14 inches). Springs that drop the front more than the back will accentuate the weight-forward bias of an Impreza.
2. Adjustable top hat plates, aka "camber plates." These will allow you to dial in a lot more negative "static" camber. Without plates, you will not get much negative camber with an alignment due to where you now are on the camber curve. Plates allow you to move the top pivot point in toward the center of the car and get more negative camber. Camber will still go positive as the suspension compresses, but the extra "negative static camber" will help offset that.
3. Get heavier sway bars. By reducing body roll, camber will not change as much when cornering.
4. Use "stiff" lowering springs with a higher rate. Springs that are advertised to "lower your car and maintain good ride quality" are usually the worst offenders of the problems described above. Of course, now you will need to uprgrade your struts as well to handle the heavier "lowering" springs.
5. Can you say "coilovers"? Yes, coilovers in combination with plates, heavy bars and adjustable lateral links in the rear are by far the best way to go. Just be prepared for the kind of ride that will result, and the amount of $$ you will need to spend to do the job right.
6. The Subaru "SPT Pink" springs I am now using are not promoted as "lowering springs," but rather "performance springs." They do lower the car a small amount but have a substantially higher rate than the stock springs for less body roll while cornering. Based on my experience with them, they have lowered the car a touch (14-3/4 inches wheel center to fender lip measurement front, 13-3/4 rear), greatly improved handling (compared to stock), maintained reasonable ride quality/suspension travel all while avoiding the worst of the the camber curve/dynamic toes issues described above. For the non-STi WRX or NA Impreza, SPT Pinks + upgraded sway bars = a great handling car without spending a ton of $$ or creating serious suspension compromises. Do you now see why they (or any spring with minimal drop/higher spring rate) are a good choice if you don't want to go with #4 above?
I will not go into the other "lowering issues" such as ride comfort and reduced ground clearance. Those are a bit more subjective - what is "harsh and bouncy" to one person may be perfectly acceptable to the next. All
lowering springs will result in a harsher ride, some worse than others. Only you
can decide if the ride is objectionable or not, or whether the "lowered look" is worth the problems that could arise -
Lowering your Impreza requires some careful consideration. Doing the job correctly is much more than just throwing on a set of "lowering springs" and getting an alignment if you want to avoid the compromises/problems described above.
Some good NASIOC threads worth reading from beginning to end;