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reprinted from Automotive Engineering, October 1997

New chassis technology in the Japanese Accord

Two versions of Honda's new "world operation" Accord will be launched by the time this appears in print: one in Japan, the other in the U.S.A. Another model on the common architecture will follow in Europe.

The Japanese Accord features three significant chassis technologies:

  • A five-link double-wishbone/Watt's-linkage rear suspension
  • An electronically controlled and electrically assisted power steering with a variable ratio
  • A vehicle stability assist (VSA) system actively employing ABS and traction control to enhance stability.
  • The Ohio-made Accord adopts the new rear suspension, but not the other two.

    Honda presented a significant research contribution by engineer-theoretician Y. Shibahata and his team in 1992 (SAE 923081, The Improvement of Vehicle Maneuverability by Direct Yaw Control). It is called the (beta)-method, and is a revealing vehicle behavioral analytical and control theory that allows an effective analysis of the vehicle in motion. It employs the evaluation standard in conditions ranging from linear to nonlinear extreme driving situations (simply put, those encountered at the physical limit of adhesion between tires and the road surface). Shibahata and his team have produced an advanced yaw control technology, the Active Torque Transfer System (ATTS), which was adopted in the top model in the recently updated Honda Prelude coupe range. Honda is now adding the three new features, born out of that control logic, in the Accord.

    Rear Suspension

    In normal driving conditions, a vehicle must be free from unstable reaction in acceleration, deceleration, and braking, and any variations in its behavior must be minimal. For that, it should be equipped with an optimally design rear suspension.

    As its name indicates, the new rear suspension employs five links. The hub-carrier/spring-shock-absorber mount is located by five tubular links: a trailing link, lower link, lower control link, upper link, and upper leading link. Viewed from the side, the lower trailing and upper leading links form a Watt's linkage, which guides the wheel to travel at a nearly straight rearward inclination. Longitudinal loads imposed by road surface irregularities are thus substantially reduced, according to Honda, contributing to improved ride characteristics. Further, the rear-wheel locus angle remains the same whatever the load.

    The layout, with all outer linkage mounts within the wheel and also the wheel's rearward-inclined travel, minimizes the suspension/wheel's intrusion into the cabin/luggage space.

    The suspension possesses the double-wishbone (short and long arm) type's inherent advantage of optimal camber control. Also, toe-change associated with wheel movement is linear and limited by aligning the virtual axis passing through the trailing and lower link mounting points, and the virtual axis passing through the leading and upper link mounting points, in parallel. This assures improved straight line stability, higher roll resistance, and reduced understeer.

    The suspension's linkage also induces a stable toe-in attitude under cornering (side) force and under braking (rearward) force.

    Each suspension member is positioned to take load from a single direction along the link's axis, allowing the use of a straight and light component.

    Power steering with a variable steering gear ratio (EPS+VGR)

    A vehicle steering system is an essential tool of the man/machine interface, insists Honda. Honda already offers electrically assisted power steering (EPS) in the Acura NSX sports car, introduced in 1990. The choice was logistical as well as technical, to avoid the complication of long hydraulic circuitry between a midship-mounted, engine-driven hydraulic pump and a front steering gear. Honda also employed a pinion-gear-type EPS in the Acty light commercial vehicle that also has its powertrain located under the cargo compartment/truck bed away from the front wheels.

    In the Accord application, the electric motor and ball-screw unit are placed coaxially with the steering rack axis, in contrast to the dual-shaft NSX unit. The configuration allows the adoption of an electric motor powerful enough to handle a front-wheel-drive car's heavier load on the front wheels, according to a Honda engineer. Honda is said to be in negotiation on a patent issue with GKN, which has a similar system. The new electronically controlled EPS provides precise and optimally selected assistance, greatly reduces steering disturbances caused by irregular road surfaces, especially tramlining, as well as torque steer, and produces smoother and natural self-centering action, according to Honda. It also consumes less energy contributing to a fuel economy improvement of 3-5%. Naturally, the EPS retains power assistance should the engine stall.

    The EPS is also programmable, allowing the driver to choose the appropriate amount of assistance, according to driving conditions, by a switch. For the new Accord application, it allows three modes: normal, light, and heavy.

    The new EPS adopts a variable gear ratio, from a standard ratio near the straight-ahead position to a quicker one as more steering lock is sensed. It produces smooth and stable steering feel in such maneuvers as high-speed lane change, and reduces effort in tight turning at low speeds.

    Vehicle stability assists (VSA)

    The fundamental principle of various production vehicle stability systems, such as Bosch's VDC (Vehicle Dynamics Control), Cadillac/Delphi's StabiliTrak, Toyota's VSC (Vehicle Stability Control), and now Honda's VSA (Vehicle Stability Assist), can be explained by Shibahata's method. All these systems add side-slip control to ABS/traction control systems. They handle sudden changes in vehicle behavior, enabling the driver to control the situation. Honda's system is based on the concept of stabilization through control of the front wheels (in a front-wheel-drive car) without reducing the pleasure of driving.

    The VSA uses the ABS/TCS architecture to which a unique electronic control unit and an algorithm are added. Input sources include wheel-speed sensors, yaw-rate sensor, lateral acceleration sensor, steering angle sensor, engine speed sensor, and brake switch. The ECU has main- and sub-CPUs. The system applies an appropriate individual brake, and modulates the engine's torque output as required by the situation.

    In an oversteer situation, the system calculates the driver's intended "target" from lateral acceleration, steering angle, and vehicle speed. If the vehicle's actual yaw rate exceeds the target, the VSA brakes the outside front wheel to reduce yaw, thereby regaining stability.

    In an understeer situation the system intervenes by reducing engine torque output, and if necessary, by braking the inside front wheel. This creates an inward moment, putting the car on the driver's intended line.

    The VSA assists the vehicle in starting on a slippery and split-coefficient surface by applying the brake on the front wheel on more slippery side, and directing engine torque to the other wheel, which has more grip.

    Braking while cornering is a difficult task, disturbing the vehicle's stability. The VSA switches ABS from the normal three-channel, select-low mode (straight-line and cornering at moderate speed) to the four-channel mode, controlling the rear brakes independently. Taking advantage of weight transfer during cornering, the system applies the brake on the outer rear wheel, improving braking performance.