Building a car that’s perfectly optimized for every condition is impossible. No segment demonstrates this concept better than off-road SUVs. If you want a placid cabin in an off-roader, you better skip the noisy all-terrain tires; If you want a flat, comfortable ride on the freeway, the vehicle’s ability to soak up massive bumps off-road will suffer. But with more and more focus — and money — concentrated on the overlanding game than ever before, automakers are keen to offer an off-roader without compromise, applying increasingly high-tech solutions to low-tech problems.
Nowhere is this tech better showcased than in Lexus’s new GX Overtrail. The Overtrail — meant to turn the suburban-mom GX into a Rubicon-runner — features Toyota’s Electronic Kinetic Dynamic Suspension System (e-KDSS), a new development that gives the Overtrail more than two feet of wheel travel while allowing the GX to ride flat and smoothly on the highway.
Keep It On The Level
To understand how e-KDSS works, it’s worth explaining the traditional handling trade-offs made to accommodate off-road driving. When turning, inertia causes a car’s body to lean in the opposite direction of the turn. The amount of lean is influenced by spring rates (softer springs compress more easily), vehicle weight, speed, and the vehicle’s center of gravity, among other factors. The further the body leans, the further the center of gravity is pushed to the outside of the corner; At high speeds, or high inertial moments, this can cause poor handling and — if the vehicle is tall enough — rollover crashes. This is a problem.
The simplest solution to tame roll is an anti-roll bar, also known as a sway bar. This is a torsion spring that takes the form of a specifically-shaped tube of metal. The bar connects the left and right sides of a car’s suspension. When one side of the suspension compresses – such as during hard cornering — the torsion bar twists. This transfers force from the loaded side of the suspension to the unloaded side. This fights the vehicle’s inertia, mitigates body roll, and improves cornering grip.
The trade-off for off-roaders is that sway bars — by design — limit suspension travel. More travel improves vehicle balance and the ability to modulate throttle over uneven surfaces to keep all four tires on, or at least close, to the ground. The easiest way to improve articulation and grip is to eliminate the sway bar, which in top-heavy SUVs, destroys on-road performance.
Now you can understand why most off-roaders are terrible at driving down the highway, and most “performance” SUVs aren’t worth much in the dirt.
First, The Obvious Solution
The way some off-road focused trucks solve this problem is simple: disconnect the sway bars in rough terrain and reconnect them on-road. Many aftermarket companies offer quick-release sway bar end links for popular rugged SUVs; This still requires that drivers exit the vehicle, get under their (probably muddy!) truck, and physically tinker with the suspension. This option is reserved for really, really hard-core trail-runners, obviously.
A more advanced solution — and one used in newer trucks, such as the Ford Bronco — is to offer an automated disconnect for the sway bars. The Badlands trim has a button on the dash that electronically decouples the center of the torsion bar, allowing both axles to hinge freely. Above 20 mph, the linkage automatically reconnects for safety. This is a better solution, but one that still requires driver input and technical knowledge.
Under (Hydraulic) Pressure
For more than a decade, Toyota’s offered a fully-mechanical solution which requires no driver input: the Kinetic Dynamic Suspension System (KDSS). On certain SUVs — such as the Lexus GX and 4Runner — a hydraulic line runs the length of the truck. A pressurized piston sits at the center of each torsion bar. When the SUV is on level ground, the two pistons sit in hydraulic equilibrium, and this keeps them locked to their respective torsion bars. The sway bars function normally, and the driver won’t even know the system is installed.
However, when off-roading, wheels are rising and falling independently of oneanother, the pressure differential in the hydraulic system changes. This shift causes the pistons to move relative to one another. The pressure differential allows for the bars to unlock, giving the axles much greater range of motion on undulating terrain. This happens mechanically, without driver input.
This system worked well, but as Toyota itself points out, KDSS has limitations. Specifically, the way the hydraulic loop functioned; The front and rear tires were connected to each other.
“If the [right rear] tire floated, the [front right] tire was also affected,” explained Koji Tsukasaki, chief engineer of the new Lexus GX 550. So how did Toyota improve on KDSS?
E Is Even More Than Hydraulically Can
The newest system is called e-KDSS, and it allows for much more freedom in individual wheel control. e-KDSS replaces the single, connected hydraulic loop with two sets of electronic circuits — one for the front suspension and one for the rear. Each circuit controls hydraulic pistons that either hold the sway bars in place to function normally, or allow them to hinge with the axle movement, thereby letting each wheel swing freely. In normal driving, it functions very similarly to hydraulic KDSS.
However, Tsukasaki-san explains, in the new GX “the [right rear] and [front rear] tires can be controlled independently of each other.”
This allows for more stable performance in a wider range of off-road conditions. e-KDSS typically works automatically, using a combination of vehicle speed, steering wheel angle, and lateral G-force to determine if the SUV is on- or off-road. Then the system locks or unlocks the sway bars as needed. However, in conjunction with the electronic Multi-Terrain System (which allows drivers to select the current surface type), the GX can preemptively unlock just the rear stabilizer bar — a compromise for higher-speed off-roading where stability is still somewhat necessary, but a softer ride is desirable.
The new GX suspension has more flexibility than ever before, and the driver still has no need to intervene directly; the ECU handles it seamlessly. The new GX has over three inches more wheel articulation than the outgoing model with traditional hydraulic KDSS, at 24.5 total inches, so the system is not just more adaptable, but more capable.
Despite the massive amount of suspension travel on offer, the GX still rides as smoothly on pavement as any other luxury SUV. With systems like e-KDSS, the days of needing separate vehicles for commuting and camping might just be coming to a close.
