Honda Prelude -- Chassis

9/15/2000 5:22:10 PM

OVERVIEW
Perhaps no area of performance defines a driver's car more than its suspension system. Knowing this, Honda engineers redesigned the Prelude's already capable 4-wheel double wishbone suspension system so that it offers even more responsive handling, even greater stability and improved ride quality over its predecessor (the Prelude already enjoyed a reputation as one of the best-handling cars in its class).

PRELUDE 4-WHEEL DOUBLE WISHBONE SUSPENSION
The Prelude uses the latest version of the Honda-designed 4-wheel double wishbone suspension system. Four-wheel double wishbone suspension offers several ride-and-handling advantages over the more-common strut-type independent systems used on many sports cars. In terms of ride, double wishbone suspension offers long wheel travel and low vibration. This is possible because double wishbone uses a separate linkage system to locate the wheels. By contrast, strut-type systems use the damper as a link. Inevitably, there is some "stiction" and binding in the damper when under load and, as a result, a greater degree of road vibration and harshness are transmitted to the body. On double wishbone suspension, Honda engineers can tailor the springs and damper assemblies for ride because the wishbone linkages provide the load path for suspension forces.

Double wishbone suspension also offers several handling advantages as well. Notable among them is its precise control of wheel geometry (hence its almost universal use on high-performance and racing cars). Honda engineers have taken advantage of this trait and tailored the Prelude chassis's roll centers, track, wheel offset, caster and camber in order to give it more linear handling characteristics and better feedback to the driver. The front and rear suspension geometry is designed to minimize front-end dive during braking and squat under acceleration.

Another advantage offered by the Prelude's 4-wheel double wishbone suspension is better outward visibility. Since there are no tall strut towers that would necessitate a high hoodline and cowl, the Prelude has better forward visibility. An advantage, when parking and making tight turns.

Physically, the system consists of two transverse arms (upper and lower) at each wheel. The arms are hinged at their outboard ends to the wheel-hub/upright assembly, and also hinged at their inboard ends to reinforced pickup points on the body. The upper and lower link's inner and outer hinge points form a parallel linkage. Often one, or both, of these transverse links is triangulated, in the interest of increasing its rigidity (hence the wishbone name). On the Prelude, the upper transverse link is wishbone-shaped. The bottom link is a single forged beam, slightly longer than the upper link so that it keeps the outside wheel at the optimum camber angle when cornering. A longitudinal link (leading link) connects each wheel's lower transverse link to the body and provides fore-aft location. A large elastomeric bushing on the forward end of the leading link helps absorb impact harshness from sharp bumps.

BODY RIGIDITY
One characteristic that distinguishes superior-handling automobiles from their lesser counterparts is a rigid chassis, or body in the case of a unit-body vehicle such as the Prelude. A vehicle with less-than-adequate chassis rigidity acts as an undamped spring to suspension inputs. A bump or series of road irregularities can excite a resonant vibration (around 25 Hz) in the vehicle's body capable of feeding energy back into what might already be an overworked spring and damper system. This will, in turn, further compromise the suspension's ability to control wheel motion. As a result, both ride and handling deteriorate.

Additional benefits of a rigid body/chassis include less squeaks and rattles and better panel and door fit over the life of the car.

With the higher anticipated suspension loads generated by the 205/50R tires and 16-inch wheels specified for the Prelude, Honda engineers felt that its handling would benefit from additional body stiffness, so they increased its resistance to bending loads by 55%, and its resistance to torsional loads by 24%.

WHEELBASE
The Prelude's wheelbase was extended by 35 mm (1.3 inches). This serves to increase its stability during straight-line running, as well as when braking and turning.

FRONT SUSPENSION
The double wishbone suspension system used on the front of the Prelude has undergone several revisions intended to enhance its stability under braking, its linearity when cornering and ride compliance. The elasticity and resilience of the elastomeric bushing in the end of the leading link has been revised. The bushing functions as a shock absorber for harsh impact energy (the kind that would be transmitted to the suspension if a tire hit a sharp ridge in the pavement). The new material is stiffer, and eliminates any brake judder or shimmy that might occur under heavy braking.

Previously, the pickup points for the upper A-arm were anchored to the body with a ball joint on the body side and brackets on the end of the A-arms straddling the ball joint. The new mounting is similar to the one introduced on the 1996 Civic. The end of the A-arm is hinged in a broad-based bracket (two per arm) attached to the body. The new bracket distributes force over a wider area, and is very resistant to flex and camber, caster and toe distortion.

The Prelude front suspension also has a relatively low roll center: 2.2 inches (58 mm) above the ground. This helps minimize any suspension-jacking effect and track (width) changes. Minimizing suspension jacking improves steering feel and linearity when cornering, and minimizing track changes reduces road sensitivity and tire friction from side forces when traveling over uneven surfaces. When cornering, the inner wheel also remains closer to perpendicular (relative to the ground plane) throughout a greater range of travel, which improves tire adhesion. A hollow 1-inch-diameter stabilizer bar helps minimize body roll.

REAR SUSPENSION
Honda engineers have also revised the Prelude's rear suspension in order to improve its handling precision and ride. In addition to lengthening the new Prelude's wheelbase by just under an inch and a half, they changed the rear transverse arm length to better optimize camber angle with body roll. As a result, when cornering, the rear wheel remains at a more optimum toe angle (where it generates the most cornering force). In order to more effectively aid in camber control, the diameter of the rear anti-roll bar has been increased (0.90 in.) and all its joints use low-friction rod-end type ball joints. The rear-wheel bearing was also increased in size, resulting in a stiffer bearing mounting. This helps minimize any steer-effect in the rear suspension during transient maneuvers, such as braking and turning.

HONDA PROGRESSIVE VALVE SHOCK ABSORBERS
The latest version of Honda Progressive Valve (HPV) shock absorbers is used on the Prelude and significantly contributes to its smooth ride and precise handling characteristics. These gas-pressure dampers use a stacked disc-valve arrangement that yields easily and progressively to high-velocity damper piston movement, such as those created by bumps or road impact; however, the same velocity-sensitive valves present more resistance to the small, low-velocity movement associated with body transient motion at high speed.

WHEELS AND TIRES
The Prelude and Type SH models feature 205/5OR tires on 16" x 6.5 jj aluminumalloy wheels.

A space-saver spare tire is stowed under the trunk floor.

POWER STEERING SYSTEM
The Prelude uses a power-assisted rack-and-pinion steering system with a 15.75:1 ratio (15.61 for the Type SH model). The power-assisted system is steering-torque sensitive and senses the amount of force (torque) created between the tire and the road as the wheel is steered. As the force increases, the system increases the amount of power-assist accordingly.

The previous power system used on the Prelude had a fixed program that progressively reduced boost as speed increased. One advantage of the new system is that, regardless of speed, an increase in effort (for example, if one wheel were to encounter a rough surface) is countered by a quicker increase in boost.

The power-steering rack uses a rotary valve and torsion-bar system that replaces the 4-way valve and pinion holder used in the past. In the new system, the torsion bar twists in proportion to steering effort. As it twists, it progressively opens hydraulic valves that increase boost. The new system greatly reduces power steering noise, rattle and kickback. Larger orifices and reduced oil-seal friction serve to reduce overall resistance in the system, resulting in a light, linear feel to the steering.

The power-steering pump is a large-capacity vane type that operates quietly and responds quickly to movements of the steering wheel. The vanes create a smaller pressure fluctuation and the pump body incorporates a flow-control valve that supplies optimum pressure. The system has two relief valves: one valve maintains a constant return pressure from the pump, while the other responds to pressure fluctuations from the steering system itself. The result is faster response (or boost) to steering loads.

Roller bearing-equipped constant velocity joints (tripod joints) permit a larger steering angle and smaller turning diameter. The turning diameter is 18.04 feet (5.5 meters) curb to curb. Turning diameter for the Prelude Type SH is 18.70 feet (5.7 meters) curb to curb.

BRAKING SYSTEM
The Prelude features 4-wheel disc brakes and ABS as standard equipment. The front discs are 11 inches (282 mm) in diameter x 0.9 inches (23 mm) thick and the rear discs are 10 inches (260 mm) x 0.4 inches (10 mm) thick. The front discs are also ventilated for improved cooling and fade resistance. The discs have a bright, corrosion-resistant finish and the calipers have a gold finish.

Power assist for the system comes from a dual-diaphragm vacuum servo with 8-inch and 9-inch vacuum servo units. Using two diaphragms in tandem gives the system strong, progressive boost, with good pedal modulation.

For, the brake-pedal lever ratio has been revised in order to give the pedal a shorter stroke and a firmer feel when braking.

THE PRELUDE TYPE SH WITH ACTIVE TORQUE TRANSFER SYSTEM
Historically, the Prelude has served as a technology demonstrator for promising Honda automotive technologies (many of which, such as double wishbone suspension and 4-wheel steering, have been designed to enhance vehicle handling and stability). The Active Torque Transfer System on the Prelude Type SH is the latest example of this practice.

Much like the fly-by-wire control systems used in modern aircraft, Active Torque Transfer uses a microprocessor to process information from driver inputs (Honda calls this "Feed-Forward Control") as well as vehicle status data from various sensors (Feedback Control). It then uses this information to augment the driver's control of the vehicle, thereby optimizing vehicle control and stability when turning.

With Active Torque Transfer the Prelude Type SH exhibits a significant increase in handling control and steering response and practically no vehicle understeer and oversteer. This is especially noticeable if the driver alters the cornering dynamics of the Prelude Type SH by lifting off the throttle, braking or applying more power while cornering. The Prelude's cornering line will not change in response to lateral acceleration and speed, and its handling response will remain neutral and linear.

The Prelude Type SH Active Torque Transfer System is not like a traction-control system that limits torque to a tire already overworked by the demands of drive traction and lateral acceleration. Active Torque Transfer steers the Prelude Type SH and increases drive torque to the outside wheel in a turn. Normally, a steered-wheel vehicle, such as an automobile, must depend entirely upon the phenomenon of tire slip to generate the lateral acceleration that makes it yaw and turn. By rotating the outside wheel faster than it would normally rotate in a turn, Active Torque Transfer adds an additional steering assist to this yaw moment in much the same way a tracked-vehicle such as a tank or bulldozer turns.

The system consists of the actual drive-torque distribution unit called a Moment Control Unit (MCU), the microprocessor-based Engine Control Module (ECM) and various sensors located throughout the car. The sensors send vehicle attitude information to the ECM, including right- and left-wheel speed data, steering-angle information, gear ratio and engine torque data, yaw (turning) rate and lateral acceleration (G-sensor). The ECM then uses this data to determine how much torque should be applied and how much faster the outside wheel should rotate. It then commands several electrically controlled hydraulic-solenoid valves in the MCU to open and close. The solenoid valves send hydraulic oil to whichever clutch is controlling the outside wheel in a turn (the clutch for the inside wheel is disengaged). When this occurs, drive torque from the inside wheel is transmitted via the differential to the engaged clutch and then to a planetary gear set in the MCU that multiplies its rotational speed.

The additional rotation is then fed back through the differential to the outside wheel. This turns the outside wheel at a faster speed than it would normally turn in a corner, resulting in the creation of a yaw (turning moment). Depending on conditions, the MCU can raise the outside wheel speed by as much as 15% and distribute up to 80% of the drive force to the outer wheel and 20% to the inner wheel.

The MCU also contains a hydraulic pump, oil filter and hydraulic fluid. The system is fail-safe, in that if for some reason it should stop working, the Prelude Type SH will revert back to the handling characteristics of a normal Prelude. The system will also not engage until its hydraulic fluid has reached operating temperature. A warning light on the instrument panel alerts the driver when the system is not operating.

The system is very compact, and fits under the engine, between the differential and the left and right driveshafts, where it adds only around 40 pounds to the vehicle's weight. The only periodic maintenance it needs is replacement of the hydraulic fluid at about the same interval as an automatic transmission's-- around 100,000 miles.

Honda has designed and built both 2WD and 4WD Active Torque Control Systems and is the first company to present it as a completed system for mass production, regardless of drive system. Together, the two systems have generated 99 new patents.

PRELUDE TYPE SH FRONT SUSPENSION
In order to take advantage of the greater steering feel and linearity offered by Active Torque Transfer system, Honda engineers revised the Prelude Type SH's front suspension. In place of the normal single-axis compliance bushings located in the suspension leading link, Honda engineers added a ball-type bushing. This bushing reacts in a more linear fashion to drive-torque input.