The 3.5 RL is equipped with four-wheel independent double-wishbone suspension with coil springs, four-wheel disc brakes with ventilated front rotors, an Acura-designed Antilock Braking System (ABS) and stabilizer bars in both front and rear. The goals in developing the RL chassis were to increase overall cornering ability and make the response of the chassis to driver input as linear and predictable as possible while at the same time achieving a smooth, well-controlled ride -- and reducing noise, vibration and harshness to a remarkable degree.
Like all Acura models, the RL is designed as a true driver's car, with suspension tuning to give the car nimble and sporty handling while maintaining excellent ride comfort.
LOW-FRICTION DOUBLE-WISHBONE SUSPENSION
The double-wishbone suspension layout of the RL allows precise control of wheel movement while isolating the damper units from the side-loading and resulting friction. The front suspension uses unequal-length upper and lower control arms and a tubular stabilizer bar, while the rear suspension is a combination of a longitudinal trailing arm, three lateral links and a tubular stabilizer. For excellent stability and improved ride quality, added anti-squat and anti-dive geometries were built into the RL suspension. To achieve a compliant ride without compromising handling response or cornering capability, the RL suspension also uses a number of innovations to reduce friction throughout suspension travel. The stabilizer bars use highly efficient ball joints at either end, as well as self-lubricating bushings at the sub-frame mounts. The upper arms of both the front and rear suspension have low-friction ball-joint bushings where they attach to the main structure, reducing overall friction while enhancing rigidity of the suspension assembly. The dampers of the RL have an advanced Progressive Valve design for increased damping control at low damper speeds while retaining quick, fluid response to large impacts. Special Teflon seals within all four damper units reduce friction even further for improved ride quality.
LOW ROLL CENTER
To give the RL a stable, predictable feel during hard cornering, while improving overall lateral-acceleration capability, the suspension geometry was designed with lower roll centers than previous Acura luxury sedans. The roll center of the front suspension is 59 mm above ground level and 70 mm above ground level at the rear. This has resulted in increased cornering performance, from 0.77 g to 0.80 g, as well as more predictable response at the limits of adhesion.
LIQUID-FILLED REAR TRAILING ARM BUSHING
To reduce road noise and harshness, the bushing at the front of the rear-trailing arms is a liquid-filled design, instead of the more conventional rubber-damped bushing.
VEHICLE STABILITY ASSIST (VSA) SYSTEM
Building on Acura's long history of advancing the science of automotive handling stability and control, the 3.5 RL comes standard with a state-of-the-art Vehicle Stability Assist (VSA) system. Developed in cooperation with Bosch, this four-channel system orchestrates the throttle and each of the RL's four disc brakes independently to seamlessly integrate traction control, anti-lock braking and stability enhancement processes under widely divergent road and driving conditions.
By monitoring input from six vehicle sensors, the VSA can calculate a predicted range of vehicle response while constantly monitoring the vehicle's actual response and the driver's control inputs. If the actual response is outside of the predicted response range -- as when cornering forces exceed the tire's performance, for example, VSA automatically intervenes with corrective action. In the case of oversteer during hard cornering (which might lead to spin), VSA applies braking to the outside front or rear wheel to help counter the unintended yawing tendency. On the other hand, if understeer becomes pronounced, VSA applies braking to the inside front or rear wheel to help turn the RL back onto the intended course.
Unlike some vehicle stability systems, which can seem intrusive to enthusiast drivers, the Acura VSA is calibrated to add to stability and predictability, without stifling the driving enjoyment of the RL's carefully engineered chassis and double wishbone suspension. An indicator light flashes on the instrument panel while the system is actively stabilizing the vehicle. A cockpit switch is provided to disable the vehicle stability and traction control aspects of the system while leaving the antilock braking system fully functional.
Large, power-assisted four-wheel disc brakes are standard, with ventilated rotors in front for minimal fade and excellent heat dissipation. By fine-tuning the leverage ratio of the brake pedal and the response characteristics of the power-brake vacuum diaphragm, the RL engineering team achieved reduced stopping distances while making brake response more linear and intuitive. Combined with the "low-creep" torque converter, these refinements also help make it easier to hold the RL at a stop.
ANTI-LOCK BRAKING SYSTEM
The 3.5 RL has a four-wheel Anti-Lock Braking System (ABS) that is integrated into the Vehicle Stability Assist (VSA) system. The ABS system operates in three-channel mode during straight-line braking (apportioning braking power to the front wheels independently and the rear wheels in tandem), and then automatically switches to four-channel mode when the VSA system senses high cornering forces. This allows the ABS system to take advantage of weight transfer to the outside of the turn during cornering and apply higher brake pressure to the outer rear wheel for braking performance.
The ABS enhances steering control during hard braking. Speed sensors at each wheel send signals to the ABS control module; if the system detects impending wheel lockup, it first holds, then reduces hydraulic pressure to the affected wheel, letting it regain traction before full braking resumes. The ABS function is also highly effective on split-friction surfaces, in which one side of the vehicle has significantly less traction than the other.
In the highly unlikely event of an ABS failure, a warning light is illuminated on the instrument panel. In such a case, the control module cancels the ABS function, allowing the standard braking system to operate normally. The system used in the RL is a design that weighs 1.3 kg less than the previous-generation ABS system.
The traction control aspect of the Vehicle Stability Assist (VSA) system works just as seamlessly. When wheel slip is detected under acceleration, differential braking and engine controls are called into play to modulate the amount of power put to the ground. In situations where the drive wheels are on surfaces with different friction properties, such as when one drive wheel is in water while the other is on dry pavement, the system applies braking action to the wheel that's slipping, allowing the tire with better grip to move the vehicle. In addition to independent control of the front wheel brake, the system can also reduce total engine power output to keep wheel spin in check.
ROTARY-VALVE, VARIABLE POWER-ASSISTED, RACK-AND-PINION POWER
The speed-sensitive variable power-assisted rack-and-pinion steering uses a rotary valve that senses torque applied to the steering wheel and a vehicle speed sensor to vary power assist accordingly. The system adjusts steering assist for easy steering in parking-lot conditions and quick, precise steering feel at speed. The RL utilizes a magnesium-cast steering wheel for lighter steering feel, and a dynamic damper in the steering shaft for reduced wheel vibration.
WHEELS AND TIRES
The RL is equipped with P215/60R16 MXV4 Plus Michelin tires on 7.0 JJ x 16 cast-aluminum alloy wheels to maximize handling while minimizing road noise. The tires of the RL are of a special low-rolling-resistance design, which yields outstanding ride characteristics and improved fuel mileage while achieving excellent handling response and overall grip. The tires are an All-Season design, for enhanced traction on wet roads as well as on dry pavement.