Like all Subaru models, the 2003 Forester comes equipped exclusively with full-time All-Wheel Drive. Subaru is the only carmaker to offer an exclusively All-Wheel Drive model lineup, making Subaru unique in this market.
All-Wheel Drive is itself just one integral part of Subaru core technology, a suite of systems all working in harmony to provide superior levels of traction, handling and overall balance. In addition to full-time All-Wheel Drive, Subaru core technology also encompasses a lightweight horizontally opposed engine, a simple, symmetric drivetrain layout and a long-travel four-wheel independent suspension system.
Subaru’s core technology not only defines the brand but also the driving “feel” of its vehicles. In the Forester, Subaru core technology results in a vehicle that meets all the utility and traction expectations of a small SUV while exceeding expectations of ride and handling quality.
In addition, Subaru core technology forms the basis of the Subaru Active Driving/Active Safety concept, which means designing a vehicle to give the driver a strong sense of control, maneuverability and security.
All-Wheel Drive Built In, Not Added On
All-Wheel Drive forms the heart of the Subaru concept of Active Driving/Active Safety – empowering a driver with the tools to help manage hazardous situations. Subaru All-Wheel Drive works transparently, requiring no input from the driver. There are no switches, knobs, or levers to move – the system is always working, on any road under any condition. In addition, Subaru All-Wheel Drive is robust enough for recreational off-road trail driving in a Forester.
All the elements of Subaru core technology work together to give the driver greater control and confidence, especially in potentially hazardous driving conditions such as rain, snow, slush, or gravel. Subaru engineers have conclusively demonstrated that Subaru All-Wheel Drive makes drivers feel more confident behind the wheel and helps them to drive more proficiently.
Symmetric Drivetrain Layout and Compact Boxer Engine
The Subaru boxer engine is ideal for an all-wheel drive application because it is inherently compact – shorter and lower than V or inline engines. The layout concentrates its mass in a small area and contributes to a lower centre of gravity. Mounting the boxer engine longitudinally (front to back) allows the transmission to be mounted directly behind it. Power travels in a straight, near-horizontal line to the rear differential, minimizing frictional loss. This symmetrical, uniform layout also provides excellent left-right balance. In contrast, an all-wheel drive system in a vehicle with a transversely mounted engine requires additional components to reroute the power from transverse to longitudinal orientation. Such a system adds friction (power loss) and weight.
The 2003 Forester models are powered by the proven 2.5-litre single overhead-cam (SOHC) four-cylinder boxer engine, which produces 165 horsepower at 5,600 rpm and 166 lb.-ft. of peak torque at 4,000 rpm. The engine is equipped with sequential multi-port fuel injection (MFI) with dual-spray injectors and a distributorless ignition system. Low maintenance features include screw-type valve lash adjusters (160,000 kms adjustment interval) and belt-driven camshafts. The engine runs on regular-grade (87 Octane) fuel.
A five-speed manual transmission is standard, and a four-speed electronic automatic (4-EAT) is available as an option. On models equipped with the five-speed manual transmission, the Subaru Hill Holder clutch helps make starting off on an incline easier and safer. The five-speed manual transmission uses a hydraulic clutch for smooth operation and low maintenance. A flexible flywheel helps ensure smooth shift action.
Excellent Weight-To-Power Ratio
While the Forester engine performance figures are by themselves superior to many smaller SUVs, two additional factors contribute to the excellent performance in the new Forester. The engine is tuned to produce much of its torque at lower engine speeds, giving the Forester quick response in everyday driving, and the Forester has one of the best weight-to-power ratios among similar sized SUVs. The lower the weight-to-power ratio, the more responsive a vehicle will feel to the driver.
Optimizing the Forester body structure allowed Subaru to add feature content and still reduce overall vehicle weight by about 40 kg (88 pounds) compared to the previous model (2.5 X model comparison). Using aluminum for components located far from the centre of gravity – including the hood, bumper beams and roof rails – helps move the centre of gravity even lower, contributing to more responsive handling.
Long-Travel Independent Suspension
The Subaru Forester was among the first SUVs to ride on four-wheel independent suspension, a configuration that is becoming more common on large and luxury SUVs. This type of suspension gives the Forester a car-like ride quality, while providing superior handling characteristics.
The four-wheel independent suspension plays a vital role in the effectiveness of the Subaru core technology. The long-travel, low-friction design and specific geometry and spring and shock absorber tuning help keep the four wheels in contact with any road surface (paved or unimproved) under a variety of conditions to make the most efficient use of the All-Wheel Driving System.
The 2003 Forester retains the proven strut-type suspension design for packaging efficiency. This suspension design provides excellent lateral rigidity and geometry, as well as optimal control of bump-steer, and noise, vibration and harshness (NVH). For 2003, the strut units are all new and incorporate internal rebound springs and improved valving to reduce body roll and further refine ride quality. The springs also increase longitudinal stability by reducing dive on braking and lift on acceleration.
The boxer engine allows room for placement of a strong front crossmember directly under the engine, and for equal-length axle shafts. If one axle shaft were shorter than the other, the vehicle would tend to “torque-steer”, a condition that causes it to involuntarily pull to the left or right when accelerating. A liquid-filled rear bushing in each lower control arm helps isolate road noise and vibration.
New for 2003, Subaru enhanced the steering response in the Forester by employing a quicker variable-ratio power rack-and-pinion steering system (16.5-to-1 versus 19-to-1 ratio for the previous model).
All Forester models come with standard 16 x 6.5-inch wheels (aluminum alloy on the 2.5 XS model) and 215/60R16 94H Yokohama Geolander G 900 M+S all-season tires. The standard Yokohama tire was specifically developed for the new Forester and features lower rolling resistance and a lighter weight than the previous generation tire. All models include a standard full-size spare tire mounted on a steel wheel and stored underneath the rear cargo deck.
Forester Offers Two Types of All-Wheel Drive
The 2003 Subaru Forester comes with one of two types of full-time All-Wheel Drive, depending on the transmission selected. Forester models equipped with the five-speed manual transmission feature 50/50 Full Time All-Wheel Drive with a viscous coupling centre differential. Forester models equipped with the four-speed electronic automatic transmission (4EAT) feature a different system called Active All-Wheel Drive, which uses an electronically managed continuously variable transfer clutch.
Continuous All-Wheel Drive: Simple and Effective
In Subaru vehicles with the five-speed manual transmission, 50/50 Full Time All-Wheel Drive uses a viscous-coupling centre differential mounted inside the transaxle case. Resembling a canister, the viscous coupling contains a series of opposing discs attached to the front and rear output shafts, surrounded in a type of silicone fluid.
In normal operation, power is distributed equally between the front and rear wheels (50/50 power split). Slippage at the front or rear wheels causes a rotational difference between the front and rear discs in the viscous unit which, in turn, shears the fluid. The shearing action heats the fluid, causing it to thicken (like a mixer thickens cake batter). As the fluid thickens, power transfers from the plates rotating faster (the slipping wheels) to those rotating more slowly (the wheels with the best traction). When the slippage ceases, all the discs rotate at the same speed, restoring the 50/50 power split.
The Continuous All-Wheel Driving System is simple, compact, and virtually invisible in operation. Its traction benefits not only add a significant margin of safety on slippery or unpaved roads, but also enhances dry-road handling.
Active All-Wheel Drive
Forester models equipped with the four-speed electronic automatic transmission (4EAT) employ Active All-Wheel Drive, which continually optimizes power distribution in response to acceleration, deceleration, cornering force and wheel slippage.
Active All-Wheel Drive uses an electronically managed continuously variable transfer clutch housed in the transaxle tailshaft. The transmission control module (TCM) controls the All-Wheel Drive multi-plate transfer clutch.
Active All-Wheel Drive constantly adjusts power split depending on many input factors. In the unlikely event that the front wheels slip, the TCM signals an increase in hydraulic pressure on the transfer clutch, reducing slippage of the plates and transferring the power to the rear wheels.
Intelligent Control
With Active All-Wheel Drive, the TCM monitors input from speed sensors on the front and rear output shafts and also takes input from the throttle position and transmission. All of these factors cause the TCM to select a software map that determines how aggressively it manages the power split.
Subaru Active All-Wheel Drive varies the power split according to driving conditions. Under most conditions, 60 per cent of the engine torque is transferred to the front wheels and 40 per cent to the rear; this is to match the vehicle’s weight distribution. Under hard acceleration, however, or in the case of loss of traction at either the front or rear wheels, the transfer clutches instantly receive higher pressure to distribute torque evenly between the front and rear wheels (50/50).
Wheel speed differences occur in normal driving, not just in slippery conditions. A front/rear wheel speed difference of up to 20 per cent tells the TCM that the car is simply cornering. Any difference greater than 20 per cent will signal front wheel slippage, and the TCM will then transfer power to the rear.
Limited-Slip Rear Differential
For even greater traction capability, the 2003 Forester 2.5 XS comes equipped with a viscous limited-slip rear differential. If one rear wheel starts to lose traction, the differential automatically diverts power to the other wheel. A viscous limited slip differential not only enhances traction on slippery road surfaces, but it’s also a handling aid. For example, as weight transfers to the outside wheel while turning, the inside wheel can lose traction. The viscous limited-slip differential would in that case transfer power to the outside wheel, which has greater grip.
Anti-Lock Braking With Active All-Wheel Drive
All 2003 Forester models (and all 2002 Subaru models), come equipped with standard four-channel, four-sensor anti-lock brakes (ABS) to help preserve steering ability under hard braking by preventing wheel lock-up. On a vehicle equipped with automatic transmission, when the system engages, the transmission selects third gear to reduce the unpredictability of engine braking, and thus the possibility of wheel lock-up. It also maintains hydraulic pressure on the AWD system so as to keep all wheels rotating therefore improving maneuverability.
The new Forester also features Electronic Brake Distribution (EBD) for added stability under vehicle braking. This is the first time this feature has been offered on a Subaru vehicle in North America. The EBD system was first introduced on the STi model of the Subaru Impreza WRX for the Japanese domestic market.
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