Holding the technological edge

Feb. 24, 2000
The Lexus LX 470 captured judges' attention and high marks on scorecards.

The LX 470 provides sure stops that give full confidence to the driver. JIM DUNNE, BEST RIDE JUDGE

While not normally considered as an off-road vehicle, the Lexus 470 played in the mud with the best of them.

The theory proposes an imaginary shock absorber suspended in air. The imaginary shock absorber is inactive against any force applied from the ground but effectively activates a damping force against body vibrations.

This sport ute racked up scores higher than any of the other 26 vehicles tested. Judges called the LX 470 ride smooth and luxurious, similar to relaxing in your living room. So what gives Lexus the edge? In a word — technology.

An adaptive variable suspension (AVS), semiactive shock absorber control system uses sensors to read information concerning road surfaces, braking, acceleration, and steering. Shock absorbers continuously adjust among 64 settings in a few milliseconds. Also, drivers select from 16 different ride preferences via a switch on the dashboard. A damping force control system makes suspension adjustments according to the roughness of the road. This system is based on a so-called "skyhook theory" (see diagram).

According to Lexus, the system independently controls the front and rear shockabsorber damping forces. Here, a suspension-control ECU calculates thevertical relative speed between thebody and wheels and adjusts actuators to optimize ride.

Judges liked the AVS. One commented that over a particularly challenging section of the course, the LX470 "handles the frame twist as thoughdriving over small bumps in the road."

Working in conjunction with AVS is automatic height control (AHC), a load-leveling system with three settings. In high mode, the system automatically lowers the vehicle to normalride height at speeds above 18.6 mph. In lowmode, the vehicle drops further to let people inand out easier. Sliding resistance-type controlsensors sit in both front wheel housings and atthe center of the crossmember above the rearaxle. The sensor — basically a brush sliding on aresistive substrate — gives a resistance readingproportional to the shaft's rotational angle.

Sophisticated technology for the 2000 LX 470 includes vehicle skid control (VSC), brake assist, and four-wheel active traction control (ATrac). VSC compares vehicle motion to driver inputs from the steering wheel, throttle, and braking. Hydraulically controlled solenoids independently manage each wheel brake under the direction of a skid-control ECU. If necessary, the VSC applies individual wheel brakes and/or reduces engine power to help the driver regain control.

A-Trac also controls engine power output and applies the brake to a slipping wheel, so drive force normally lost through slippage goes to the remaining wheels. The effect is similar to that of a limited-slip differential.

The skid control ECU also steps in during emergencies. Depending on the pedal pressure and vehicle speed it sees from ABS sensors, the ECU might help the driver slam on the brakes. If necessary, it boosts brake pressure by actuating a special hydraulic pump.

While this system aims to help, a few judges had issues with the brake assist, claiming it impaired their control of the vehicle. However, Lexus engineers claim this system, along with VSC, are there to give control to drivers under normal driving conditions. Unusual conditions, such as the figure eight dirt track that our judges experienced with the LX 470, ventures into the realm of extreme driving. The safety systems kicked in to counteract perceived unsafe driving conditions.

The big Lexus ute even has electronics that consider the roughness of the road before deciding whether or not to intervene with braking. ABS has an electronic brake force distribution (EBD) function. EBD uses the ABS control unit, recognizing the correct brake force distribution between front and rear wheels while considering driving conditions. For example, ABS can operate in low-range 4WD with the center differential locked.

Sensors monitor road conditions in concert with wheel speed. As roads grow rougher, intervention from ABS decreases. Developers say an independent front suspension featuring a long wheel stroke contributes to a smooth ride. Control arms are positioned for a lower roll-center height during cornering for stability. Also, the front suspension has anti-dive geometry to avoid brake-induced pitching. The rear suspension consists of four-link coil springs with lateral rods.

The upper arm mounts relativelyhigh, reducing the lateral force on thearm from the road during cornering.Also, the location of the ball jointpressed into the arm minimizes theoffset between the ball joint centerand upper arm center, reducing thetorsional force to the upper arm. Along lower arm gives a long suspension stroke and sits high to preventroad interference. It has a closedcross-section to minimize possibledamage from the road. Also, as withthe upper arm arrangement, the balljoint minimizes torsional force to thelower arm.

Torsion bars mount above the frame side rails for protection during off-roading. Torsion bar springs secure to the frame through rubber bushings in anchor-arm brackets.

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