Five hundred issues ago, everything was a trade-off. By increasing suspension height, you gained ground clearance-but your 4x4 leaned like a drunken sailor every time you drove around a corner. The quick steering you wanted for the trail made the vehicle twitch and dart all over the highway. Tires that delivered better traction melted away in no time. You could get much better traction by adding lockers, but then you couldn't steer. If you set up the throttle for nice, light response, it tended to surge in low-range.
Now, all those trade-offs can be practically eliminated. Computers have radically changed the design envelope. The precursor system, ABS, was ho-hum at first, and late making its way into 4x4s. But then manufacturers started developing finer sensors and better coordinated logic. Those systems, originally developed for on-road safety, have now become off-road performance systems. What was merely amusing has now become amazing. And we haven't seen the end of it yet.
Take traction. Early traction control systems required wild wheelspin to kick in, throwing rocks and burning tires. Now we see systems that kick in with just the smallest amount of wheel slip. Working in the background, electronic systems can flash differentials through lock-and-unlock modes, managing as many as three locking diffs at once. With front, rear, and center diffs clicking in and out a hundred times a minute, working the brakes to supply torque to any individual wheel as necessary, an unmodified 4x4 can crawl steadily across steep, slippery terrain. As long as any one wheel has traction at any given instant and throttle is applied, the vehicle moves forward.
With no wheel slip, milder tires can be used. So you tear up the trail less, your tires last much longer, and you can live with the tire on pavement. You are driving elegant, courtesy of modern electronics.
The latest systems, such as Toyota's TRAC, work using magnetic sensors on the wheels that count how much the wheel is rotating, and whether it is moving forward or backward. With that information coming in, a processor checks to see what you are doing with the throttle and transmission-in effect, it determines your intentions-and locks or unlocks a diff, brakes a spinning wheel, or selectively applies ABS pulsing. The instant intervention is no longer required, and the computer flashes back to a routine mode.
We hate to say it, but it's true: no matter how many selectable lockers you have or how fast your fingers might be, there is no way any human being can take similar actions, or make better decisions, on the fly. You cannot manage three locking diffs and wheelspin at all four corners, locking and unlocking, braking and releasing, all in the space of 1 inch. But the computer can do it all day long.
Traction control also reduces the need for extreme gearing. Good, low gearing to crawl slowly upward and roll slowly downhill also results in a 4x4 that takes three gears to reach 25 mph and screams bloody murder on the highway. That reality spawned a wide variety of overdrives, underdrives, and other complex mechanical solutions. Those things will work, but also induce other complexities, such as too-short driveshaft length and driveline component adapter availability.
Now, with electronic traction control and an automatic transmission, uphill progress is slow and relentless, with minimal wheel slip, if any. On the downhill side, another electronic system controls each tire individually for a slow, progressive descent.Meanwhile, your on-road driveability is not compromised.
The Range Rover Sport is a high-performance touring SUV, tuned for the street with low-pro
There are also electronic systems that add the kind of performance that nothing mechanical can address. Take hillclimbs. You're nine-tenths of the way up Suicide Hill when you drop two tires into holes dug by the last guy who just wouldn't give up. Now you're bouncing, kicking up dust, and you have to lift the throttle and set the brake. A hill start from here would be touchy. So you decide to back it down for another try, and looking back, you notice you're just one slip away from a real-world rollcage test that ends up as a video on Four Wheeler TV.
If you had one of the two systems Toyota and Lexus include on the LX470 and other 4x4s, you could breathe a whole lot easier. One system, called Hillstart Assist Control (HAC) automatically holds the vehicle in place to regulate roll-back. So you can pause to try a different line without rolling back. If you do have to back down, another system called Downhill Assist Control (DAC) will regulate downhill speed to 2 or 3 mph. Just put it in Reverse and all you have to do is steer. We had a chance to test the Toyota system on a very steep hillside that started out as dirt and gave way to loose rock. It wasn't a long hill, but it was a very steep, jumbled mess of loose dirt and rock. We were surprised at how well DAC worked, especially in Reverse.
Before electronically adjustable suspensions, a setup for reasonable on-road manners meant
A number of manufacturers have added similar descent control systems into their latest SUVs, some with more capability than others. One system we particularly like appears in the Land Rover LR3 and Range Rover Sport. The Land Rover system, which they call Descent Control, can be adjusted for rolling speeds on the fly, all the way down to 1.6 mph.
Another Toyota system we tested is called VDIM, for Vehicle Dynamics Integrated Management. It's an advanced handling system that integrates brakes, steering, electronic traction controls, throttle, and engine management to prevent skidding. So let's say you're blasting down a long, unfamiliar dirt road. There are little dips, patches of loose gravel, occasional turns, and sometimes a washout obscured by vegetation or terrain. You come up on one a little too fast and whoops! You're sideways, heading for the ditch.
...The value of sway-bar disconnects has long been known. But now the disconnects are elec
With VDIM, the ability to correct a skid is practically a religious experience. We tested it on a loose-gravel road at Toyota's Arizona Proving Grounds. Without VDIM, we skidded all over the place, even at 20 mph. With VDIM, we headed deep into the corners at 35 or 40. Just when you know you're going to die, the truck slows a bit, comes around and goes exactly were you steer it. One guy referred to it as the "Hand of God" system, because what it did seemed like a miracle.
The system works by looking at everything-steering angle, yaw rate, throttle position, and more-and figures out what you are trying to do. And then it organizes a fix. It uses selective braking-maybe just the two brakes on the driver side of the truck, for example-to correct side drift. It might also influence an electronic variable steering system to slow the steering rate to reduce skidding, and tell an electronic "brake by wire" system to maintain traction via ABS pulsing. All of these responses occur at the proverbial speed of light, and are practically transparent to the driver. With remarkably little loss of speed, you steer back onto the road and accelerate toward the next corner, where hopefully you take a decent line this time.
FYI: If it sounds like dirt-roading is fun with VDIM, we admit it is.