Autoblog resident car engineering writer, Sam Abuelsamid, has posted an analysis of the Lexus GX 460 and the recent Consumer Reports test which led to a warning and the current sales stoppage.
Of two emergency handling tests the GX 460 was subjected to, it passed the first, which was a double lane change to evaluate obstacle avoidance. It was the lift-off oversteer test showed problems—here’s how Autoblog interpreted the test performance:
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.
More details on the Autoblog report after the jump.
Abuelsamid identifies five main factors which could have affected its emergency maneuvering performance, mainly involving the GX 460’s Vehicle Stability Control (VSC)—here’s the first possible explanation:
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.
Second, ESC programming for mid-corner lifts (measured by the lift-off oversteer emergency test):
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.
Third, sensor calibration and filtering:
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.
Fourth, the ESC hydraulic control unit:
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.
Fifth, brake calipers:
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.
The entire report is even-keeled and informative—recommended reading if you’re looking for additional insight.
[Source: Autoblog]