SPD Tuning Service

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At the end of this article are recommendations for suspension setups. The article is a general discussion of the what and why of Subaru suspension, but starting from the basic ideas about suspension. It may read like glue, but if you have any questions at the end, I will be glad to discuss them with you.

It is very important to understand that the shock rates of a given strut are set to work with the spring rates and the weight of the car. The dampening (shock) rates are then tuned to a somewhat subjective compromise between sport and casual driving for the given spring. This means that a major change in spring rates, REQUIRES a change in the shock valving. The car will bounce up and down the road if you do not and generally be a big disappointment right from the start.

The generally agreed limit for comfortable suspension on a road car is ~150 pound/inch of travel. If you go to a 275lb spring, you have basically no suspension other than the give of the tires below about 60 mph. In addition, the stiff shock rates necessary for these springs cost money also! The LEDA setup, w/springs, to do this work is $1800. This is a pretty steep price to pay for 10 hours of driving on the road with an awful ride quality and ½ hour of neat driving,, again with an awful ride quality! The Leda with standard springs or slightly higher spring rates will do almost as good a job with far greater comfort.

Anyone driving a 275/225 pound gravel spring setup or god forbid, a 400/300 tarmac rally setup, will tire of  it in very short order! You will have a sore back, your arms will ache and if you do not have a 700 driving seat, you will be hanging on for dear life.. An un-reinforced chassis will not take it for too long, either. IMHO, it is much cheaper to set the clothes dryer on fluff and hop in for 10 minutes every time you wish you had spring rates like that on a road car! Our Prodrive and STI version 5 kits are the ideal compromise for a road car. Great handling balance, great long distance comfort..

Before we go any further, for the 2.5RS, the first thing to do is to change the rear 15mm sway bar to the 18mm bar. This cures much of the understeer. Nothing else will make as much improvement. The experieced driver may want the trailing throttle over steer of the 20mm bar, but it is for manual tranmsions RS or improves suspension cars only.   For non-RS owners, changing at least shocks, if not spring rates, is mandatory for sport driving. Stock Imprea's should install the 18mm bar, as car will be a bit twitchy with the 20mm bar.

Second, many people change their spring rates and do not realize that the damping rates of the strut's shock absorbers have been calibrated to control the behavior of the springs. Major changes in spring rates will upset the ride quality AND cause poor handling if the shocks are not also changed to match new spring rates. This is why SPD Tuning Service sells only shock or shock/spring kits, but no spring kits, except for the rigorously tested Eibach units.

Ever see cars "bouncing" down the freeway every time they hit a bump or even a small undulation in the road? They have either cut their springs or purchased a lowered spring kit that takes them over the edge. The energy in their springs is so great that the shock damping rates are simply over powered and the car jerks all over. They went for "the look" and lost their ride.

The third thing is the Impreza should not be need lowered much more than an inch (25mm). If you go lower than that two things begin to happen. One, the car starts scraping the ground in driveways and such and will hit the bump stops frequently, NG (No Good). But, even more important is the front suspension geometry feature called "roll center" is upset, causing poor transition and unwanted camber change in the front end. The car looks good, feels good and is slow around corners because the front tire contact patch is "sub-optimal". (NFG)

For the technically curious, as the car body rolls, it does so on a center point dictated by suspension geometry and ride height. If this center point is too low, the control arm angle relative to the chassis does not change enough to provide camber compensation for roll. Also, if the car is lowed, the initial angle of the control arm becomes flat relative to the ground and is no longer capable of providing camber compensation in the first place! SO, lowering, or raising a MacPhearson strut suspension is counter productive beyond a very small amount. If lowered too much, very large amounts of castor become important and castor is hard to come by in the Impreza without coil over springs. So resist the temptation to lower the car. It will handle better near where it is. Also note that the Legacy Outback has complete raised suspension. Cross members, shocks and even the neck if the steering box have all been extended 30+mm. So even for an 1.5 inch rise, Subaru engineering re-located the pickup points relative to the chassis.

Even the Honda four link front suspension has limits to lowering. Properly lowered Honda's replace the upper link when lowering the car, but too low is too low just the same.

So what to do?

As non-Impreza example, the Mercedes C280 sedan is changed into the Mercedes C280 Sport with very small changes. It has stiffer sway bars, the shocks are calibrated stiffer and like the Honda, they change the upper link in the front suspension. The spring rates are not changed!. The car is much more controllable at high speeds and high rates of yaw (turning quickly). The Mercedes C36 (now C43) has slightly stiffer springs for Europe, but not America. The C280 Sport is "my other car" BTW.

To settle down the RS2.5 and still have a good ride quality, you do not want to take the spring rates past about 180 in the first place. AND at that rate, getting the shock damping right is critical to enjoying say, Jackson Hole, Wy to Los Angeles, in one day's drive. This is why SPD Tuning Service sells only proven spring/shock kits and only sells finely adjustable Leda shocks for suspension tuning. Many of the Japanese shock/spring kits are just not going to give the results at the track. It is no accident that STi (Subaru's AMG) sells only the Bilstein/Eibach kit and now the new invereted shock STI Version 5. There are many ways to get your suspension settings wrong, but not from SPD Tuning Service.

By settling down, it is meant that the RS have very refined suspension, but its reaction to turn in, pitching and settling down back into a straight line is a bit slow and spongy. This is because of compromises designed to make it a comfortable car. To make the car handle better is to attack these reaction areas, NOT just change the spring rates and cause a poor ride.

Most setups that quicken up the reaction of the chassis and settle the car into and out of corners better cause deterioration in ride quality, no question. But, within limits, the cars are still very fine every day drivers. I run Prodrive suspension, Group N bushings, 19f/20rr ear bar. Those who have ridden with me, find this setup to be an eye opener. My car has a very taunt ride, but is still on the very tolerable and well short of being called harsh..

Here are the setups, in order, that SPD Tuning Service recommends for "fast road" suspension:

Non-RS owners have the option of getting used RS suspension kits or just WRX replacement shocks. The stock suspension springs will be OK with WRX shocks if on a budget. See the "Suspension" section. Most current after market spring sets for the Impreza are too low for good handling.  We offer proven Eibach spring kits to go with replacement WRX shocks

Two changes will make the 2.5RS or a non-RS car with a shock/spring kit  much quicker:

1) 18mm or optional 20mm  rear bar. 18mmstronly recommended for non-RS and automatic transmissions. If you do not know what trailing throttle oversteer is and how to drive it, please get the 18mm bar. The cars go great with either bar, but the 20mm demands experience, attention and smoothmess at high speed. Please judge youself accordingly.

2) 205/55-15 or 205/50-16 performance tires. Budget $120-150 per tire. SPD sells the highly rated Toyo Proxes T-1. Forget the speedo being off a slight bit. Get good low profile tires!

These two changes are the base point for reference. If they are not done, then nothing else matters. If you must run 4-season tires, get good ones. Again, the all season Toyo Proxes FZ4 is not a slouch tire! We would not be selling it if there was any question about its performance.

3) Either purchase a Prodrive Bilstein/Eibach kit, the STI Version 5 kit or go to step 4. The Prodrive kit is a non-adjustable Prodrive solution. It is the best sorted road suspension you will ever drive. Lowered 20mm over stock, with excellent ride quality. None of the other options will ride like this extremely well sorted kit and still offer super "fast road" handling. The STI kit is more aggresive and not a smooth riding as the Prodrive kit, but still goes very well.

4) Increase the shock-damping rate while leaving the spring rate alone. On a budget, we sell the WRX replacement shocks, but the 24-position adjustable Leda shock will give you an education in shock tuning your car. It is worth it to have them just to teach yourself what is possible with shocks alone. The adjustable Leda rides ten times better than the WRX shocks. The Leda are hand made units to order, including ride height for your stock springs. None of the adjustable Japanese shocks compare yet, so SPD Tuning Service does not sell them. Your RS shock units are pretty well figured out for all but the most aggressive driving.

4a) For the budget conscious sport drive we are now offering a Eibach Spring kit that works with the standard shocks and a 18mm rear bar. You may up grade to the stiffer WRX replacement shocks at a later date.

5) Get either Leda springs on the stock spring perch or get the Leda delivered set up for 2.5-inch coil-overs. Then you may install recommended spring rates for front and rear in a number of stiffness settings and adjust the shock rates to suit.

6) Change to hard rubber "Group N" bushings in the suspension. A bit more noise from the back end, but my.my.my what a difference they make in braking distance and turn-in. Requires complete disassembly of the suspension, access to bushing tools (having a large ½" socket selection seems to work) and an bearing press.

Send mail to mshields@spdusa.com with questions or comments about this web site.
Last modified: May 23, 2000