The reason for the dire rating is what CR considers a major safety risk with the new GX. During the publication's handling tests, the CR drivers experienced significant oversteer (the back end of the vehicle sliding sideways) before the electronic stability control intervened and brought the SUV back under control.
Cornering oversteer is something automakers try to avoid, except on high-performance sports cars. On SUVs with a high center of gravity, this is an especially serious problem because if the vehicle hits a curb, shoulder or other impediment while sliding it's more prone to roll over. During a previous test with the Toyota 4Runner, which shares its basic platform with the Lexus GX, CR didn't experience the same problem. Follow the jump as we examine some possible explanations for the disparity between these two SUVs.
When electronic stability control systems (ESC) were first adopted in the late-1990s, they went on high-end luxury vehicles followed by Sport Utility Vehicles. SUVs have a higher center of gravity than cars and thus an increased tendency to roll over. The rollover tendency is exacerbated when the vehicle is sliding sideways and ESC was found to have a significant mitigating effect on rollovers. As a result, ESC is almost universal on SUVs and crossovers now and will be fully required for all vehicles by 2012.
According to Toyota spokesman Bill Kwong, while the 4Runner and GX share a frame and basic dimensions, there are substantial differences between the two. The Toyota is only available with a V6, while the Lexus has a V8. The Lexus also carries considerably more standard equipment, and as a result the GX is over 600 pounds heavier than the 4Runner. Their suspension setups are also substantially different. Therefore, the two SUVs can be expected to behave quite differently on the road.
According to test engineer Jake Fisher at Consumer Reports, they do two different emergency handling tests. The first is a double lane change to evaluate obstacle avoidance maneuvers. In this test, the GX had no problem at all. The second test is a handling course set up on a paved vehicle dynamics pad. The course has a variety of different types of corners with a total length of just over a mile. The maximum speed through this course is typically no more than 50 mph.
The CR engineers generally do two laps of the course. Over the first lap they drive through at speed, exploring the limits of the vehicle without doing anything unusual. On the second lap the drivers push harder to see what happens in more extreme conditions. One of the tests is too see what happens when the driver lifts off the throttle mid-turn. Drivers of classic Porsche 911s or Corvairs are all too familiar with the results.
Most modern vehicles will simply understeer straight ahead at the limit. The GX, on the other hand, suddenly swung its rear end sideways. The ESC did not immediately intervene and bring it back in line. Instead, it responded late and because of the degree of instability, it brought the vehicle to a complete halt. Just to make sure this wasn't a problem with one particular vehicle, CR procured a second example that exhibited the same behavior. This is particularly surprising because Toyota/Lexus products have a reputation for having overly aggressive ESC systems that tend to intervene early and often.
Fisher also confirmed that the brakes were not applied when the oversteer happened, which rules out the possibility of the electronic brake force distribution (dynamic brake proportioning) applying the rear brakes too aggressively, which could have caused the oversteer.
There are several factors that could be coming into play here. First, Lexus engineers seem to have tuned the GX suspension in such a way that makes it inherently unstable in lift-throttle conditions. This can be done through a combination of spring and damping rates, bushings, tire choices and suspension geometry. It's possible this was done to try to make the handling more responsive and then rely on the ESC to keep things under control. As long as the ESC is doing its job, there's nothing wrong with this, but if the electronics don't do the job, drivers will have a real problem.
On the electronic control side, several things could be at play. It's possible that the engineers simply didn't calibrate the software properly for the characteristics of the GX. When developing vehicles, there are always a set of performance requirements defined with standard tests for verification. It appears that Lexus engineers and their ESC supplier may not have included a mid-corner lift in their test regimen and thus never experienced this behavior. If so, this is a major oversight on the part of Toyota/Lexus. In that case, the ESC could be applying too much brake pressure on the inside front wheel, which could make the problem worse.
Another related software issue is filtering and calibration of input signals. ESC relies on signals such as steering angle, lateral and longitudinal acceleration, yaw rate (the rate of rotation around the vertical axis), brake and throttle position, among others. All of these sensors can experience drift and noise based on vibration, temperature and age. Accelerometers can also be fooled by factors like banked turns. The engineers incorporate algorithms in the control software to zero and adjust these signals if the vehicle is not moving. They also derive signals from each sensor to cross check the others. There is also software that filters out noise from electrical or vibration sources.
If a filter is too aggressive it can result in delays causing the instability to not be detected until it is too late. If a zeroing or self-calibration algorithm is not functioning properly it can have the same effect. It could be a combination of any or all of these factors. Without instrumenting a vehicle, we can't say for sure, but Fisher and Kwong both told Autoblog that Toyota engineers have been in contact with CR engineers in the past two days and are investigating the situation as you read this.
In addition to software, it's also possible that it could be an issue with the ESC hydraulic control unit (HCU). If the HCU can't build brake pressure fast enough, it might not be able to apply enough brake pressure to keep the vehicle under control.
One other possible scenario is that the brake calipers might be knocking back more than expected during repeated cornering. Under lateral loading, the pistons in the brake caliper can be pushed back, which then requires more brake pressure to be applied before braking force is generated. ESC typically compensates for this by pre-charging the brakes, which brings the pad up to the rotor before the brakes are needed. This can typically be corrected with a software calibration change.
So how can Toyota fix this? Most likely it can be corrected by a software update to the ESC. The extent of the software updates is unknown at this time. If it only requires revised calibrations, the fix could come pretty quickly, perhaps within days. If it requires algorithm changes, more extensive testing will be needed and it could take longer. It's also possible although far less likely that Toyota may have to resort to a hardware change that could involve new springs, dampers, bushings or anti-roll bars. Either way, Toyota/Lexus engineers will surely be taking a fresh look at their ESC performance requirements in the future, and until a fix is found, the stop-sale on the GX will continue.