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.
NEW
Yes, it's a real multi-link. A lower link, trailing link, and toe link--all in a line pretty close to the same plane--act like a virtual lower control arm. The shorter cast (because it's curved) upper link swings in a tighter arc than the lower link, assuring a gain in negative camber under roll. The short toe link gives passive toe-in under roll.
OLD
The previous model's rear suspension is a good ol' MacPherson strut design. The toe link is nearly the same length as the lower link, so toe-in under roll is minimal. This is a simple and straightforward rear suspension.
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.
Here is the 2008 Impreza's rear suspension from the side. The difference in angle between the upper and lower control arm pivot axis contributes to the new car's anti-squat characteristics. The shocks are now mounted on the rear of the upright and are much shorter, taking up less space and not intruding into the cargo area. Obviously, the new base-model Impreza 2.5i has rear drum brakes. Although drums are not the desired stoppers for performance driving, we preferred the new car's firmer and easier-to-modulate pedal feel.
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.