The big news regarding the 2008 Impreza was the re-designed chassis and resulting suspension changes. Until now, though, no one really knew how the sophisticated rear multi-link system would compare to the old MacPherson strut set-up of previous generations. WRX fans have been rejoicing, because multi-link can only mean better performance; on paper, a double A-arm suspension would have an advantage in terms of camber gain. But is the new suspension really all it's cracked up to be?
We were able to take a brief look at the undercarriage of a new 2008 Impreza sedan, side-by-side with a 2007 Impreza wagon. Doing some quick measurements, as well as back-to-back street drives, we have a general idea of how the new suspension is going to stack up. Even though we didn't have time to go over the car with a plumb bob, angle finders and precision measuring devices, the educated eyeball can impart a good amount of information in a short time.
Rear Suspension
Due to their ability to keep tires on the tarmac for more grip under cornering roll, multi-link designs are usually coveted by enthusiasts over more mundane sedan suspension systems, like the economical and pedestrian MacPherson strut. Initially, we were puzzled why Subaru put a multi-link in the back of the Impreza at all. Nose-heavy, understeer-prone all-wheel-drive cars are usually hurting for front grip. Putting a multi-link in the back seems a little counterproductive for performance, as it tends to increase dork-safe-but-racer-annoying understeer.
Our take is this: Subaru did it to offer a car with a wider, flatter trunk or rear hatch area, since the shocks no longer reach all the way up into the rear shelf sheetmetal. With no intrusion of MacPherson struts into the shock towers, the trunk (or rear cargo area) can be one vast plain of flatness, better to store luggage, race tires or bodies. Doing away with shock towers also means that the rear seats can be pushed closer to the wheel wells. There are perhaps some safety reasons too, as it is possible to design more crush space from side and rear impacts with a multi-link set-up. Or maybe it's as simple as Subaru trying to emulate its archrival-the new Mitsubishi Lancer/Evo X, which (obviously) has Mac struts in the front and a multi-link in the rear.
The Impreza's multi-link suspension has its geometry arranged for greater negative camber gain under roll compared to the old car's MacPherson struts. This keeps the tire tread flat to the ground, rather than allowing it to tilt up onto its outside edge, thereby reducing the contact patch. With this footprint maintained, grip is improved during cornering when compared to the old strut set-up.
Some multi-link set-ups, like the one found in the back of the Scion tC, have little negative camber gain, but this suspension has a good amount. With shorter front lower links, the new Impreza suspension will also toe-in the rear wheels passively under roll. Both measures tend to increase rear traction and thus understeer, since the front remains the same and provides stability when cornering. And that's what the 2008 Impreza feels like on the road. One of the things in the old STI that made cornering interesting was its penchant for corner-exit power-on oversteer. Perhaps these changes will subdue that tendency, although they will probably affect mid-turn rotation as well, something experienced drivers may find annoying on the track.
We also noted, through inspection of the control arm mounting angles, that the new Impreza's rear suspension has more anti-squat geometry than the old set-up. This will reduce the amount of annoying torque-induced squat under acceleration, familiar to anyone who has drag-launched an STI with a street suspension. This also means the suspension will become more bound up under torque load, increasing resistance to movement.
More anti-squat can mean a greater tendency toward oversteer when on the gas, because the bind will act like an increase in suspension stiffness when the throttle is applied. Hopefully, this may cancel out some of the understeer-inducing effects of greater negative camber gain and passive toe-in; not too noticeable in our putt-putt base Impreza, but we reckon it will be a large factor in the chassis dynamics of the more-powerful upcoming STI.
Subaru has used small rubber bushings at the pivot points. This is good, because it reduces overall compliance and, under load, helps keep tires pointed where Subaru's engineers intended. We've become rather dismayed at the huge, squishy rubber bon-bons many late-model suspension systems are sprouting in an attempt to reduce noise, vibration and harshness. We also noticed that although our basic non-turbo test car has rear drum brakes (unlike the previous generation), bosses for brake discs and calipers are on its uprights. Likewise, holes and braces are present to allow subsequent addition of anti-roll bars. However, please note that this is a pre-production car and these useful appendages may disappear from showroom vehicles. Ironically, our 2007 base Impreza had a small rear bar.
Front Suspension
The front suspension on the previous-generation Impreza and WRX had some unpopular traits. Its steering was plagued with a numb on-center feel and a poor return to center when cornering. High-power modified WRX models have a hint of torque steer and a large amount of anti-dive/lift geometry. This leads to a non-linear build-up of understeer when trail braking and when on the throttle at corner exit. Compared to the turn-in of its rival, the Lancer Evolution, the WRX felt considerably less sharp.
Subaru's engineers have been working on these issues. At a glance, the front suspension appears to be identical to the old car (the tried-and-true MacPherson strut design), but there have been many subtle changes in front-end geometry-the most notable being more positive caster and a greater steering axis inclination angle.
Both changes increase straight-line stability, on-center feel and steering return to center. Increasing positive caster also improves turn-in and contributes to negative camber gain as the steering wheel is turned off-center. In addition to improving on-center feel and straight-line stability, increasing the steering axis inclination angle moves The Dave Point closer to the center of the front tires' contact patch, reducing the tendency for torque steer and steering pull when accelerating or braking over two surfaces with different levels of traction.
The most amusing thing is that all the changes Subaru has adopted are already popular modifications performed by top tuners on the current Impreza. We wish the new multi-link rear suspension had found its way to the front, but realistically, any multi-link would be difficult to package, due to the width of the boxer engine.
It also appears that the new car has less anti-lift and anti-dive incorporated into the front suspension geometry, as the front lower control arm is more parallel to the ground than the older car. This change is well known in the aftermarket, featured in many popular anti-lift kits that relocate the rear pivot of the lower control arm, reducing anti-dive and increasing positive caster at the same time.
Driving Impressions
Due to our limited time with the new car, our driving impressions were confined to a short street drive-not the place to conduct handling evaluations. So we had to be content with some mild seven-tenths driving to get more of a general feel of the differences. As we expected, the new car has greatly improved steering feel. The center is less numb, with more definition and the steering wheel centers itself crisply when recovering from a turn. There is not a hint of torque steer, but, with a naturally aspirated motor, there isn't as much torque to induce any. Turn-in response is more immediate-nervous almost, on such soft suspension and hard tires. But so far, a general thumbs-up.
On the throttle, the new car exhibits very little squat. This was the complete opposite in the previous-generation Impreza, as that car had less dive under braking and more rear squat under acceleration. Although neither base car have anywhere close to enough power to break the rear tires loose, the new one feels more connected and solid in all attributes, except for an annoying tendency to dive under braking. On a non-suspension-related note, the new car also has an improved braking feel with a harder, higher pedal and better modulation. But the older Impreza, with a slightly stiffer suspension, still feels like more like a driver's machine compared to the new Impreza's high-precision, but soft econobox feel.
These suspension changes are a good step forward for overall performance and safer driveability for the masses. The aftermarket can easily address brake dive with stiffer springs and improved dampers. Changes to the front-end geometry basically mimic what top suspension tuners are already doing to the old car, making for a much better base platform from which to build. In the rear, we can expect the new suspension to improve traction and the aftermarket will quickly find interesting ways to optimize it for performance driving and motorsport.
We can't wait for the upcoming STI to see what the changes feel like with some real power behind them. A new project WRX or STI might even be in the works for us some time soon. If that's the case, then we'll really wring the all-new Impreza by the neck and give you a blow-by-blow account of what works and what doesn't.
Photo Gallery: 2008 Subaru Impreza Suspension - Sport Compact Car Magazine
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