Baiting the Fishhook
Consumers are seeing stars in NHTSA�s new dynamic rollover test, but what does the Fishhook Maneuver really mean to vehicle rollover safety?
�Keep the rubber on the road� � that old worn out cliche from the CB radio craze of the 1970s couldn�t be more appropriate for today�s truck-crazy Americans.
According to National Highway and Traffic Safety Administration (NHTSA) studies, while only 3 percent of all accidents involved rollovers, nearly 33 percent of those accidents resulted in fatalities. And, no surprise to anyone, a taller truck or SUV with a higher center of gravity is more likely to end up on the roof than a low slung sports car.
Statistics show that in fatalities by type of crash in 2001, rollovers accounted for 61 percent in SUVs and only 22 percent in passenger cars. Rollover gained national attention early this year as NHTSA announced the results of its first dynamic maneuvering test, nicknamed Fishhook. Up to that point, a vehicle�s rollover resistance, or static stability factor (SSF), was calculated by measuring the track width and the height of the center of gravity.
The Fishhook maneuver subjects a vehicle to a high-speed collision avoidance maneuver that consists of a sharp left turn (-270 degrees) at speeds between 35 and 50 mph, followed by an overcorrection of 540 degrees to the right. A vehicle is considered to have tipped if both inside wheels lift off the ground two inches or more. NHTSA combines this rating with the SSF into a combined rollover resistance rating using a system of one to five stars (five being the best) to rate the vehicles.
Since safety star ratings often play a roll in vehicle purchases, should we be concerned that OEMs will look for technologies that will help them earn the highest possible rankings. And what actually does Fishhook bring to the safety party?
| NHTSA�s Fishhook Maneuver is a dynamic test designed to measure a vehicle�s propensity to roll over. While there is no way to make a rollover-proof vehicle, suppliers are developing chassis systems to help keep vehicles from rolling over.
�It�s important to understand what NHTSA�s done in terms of creating a star rating,� says Brian Murray, manager Innovation Center at Delphi. Murray has a chart that plots all vehicles along a line that represents NHTSA�s Static Stability Factor. He points out that vehicles like sports cars are all clustered tightly at the high end of the curve (five star) while SUVs tend to cluster at the lower end of the curve based on SSF.
�SUVs are all over the map here because their rollover propensity really depends on a lot of things besides their static stability factor,� Murray adds, �how their suspension works, how much body roll you get.� Murray says that the purpose of the Fishhook test is to try and understand why the SUVs are spread out all over the lower end of the chart. He also doesn�t think that throwing a lot of technology at the Fishhook test will do much to improve a vehicle ranking.
�If you have a three star vehicle you can maybe move it to a four star vehicle if it�s right on the line. That�s really all the benefit you get from that dynamic test. So when you see the rankings coming out at NHTSA, they should look pretty similar to the static stability factor,� Murray says. �The Fishhook test is really an open loop test, where they (NHTSA) specify the steering wheel input and where the car goes is not controlled at all,� says Phil Healy, chief engineer of Advanced Technologies, Continental Teves NA.
�At times, it takes up a lot of area. When you do the Fishhook maneuver it may cover more than several lanes of traffic.� Healy thinks that a handling test would be more realistic, a test where you actually have to follow a certain path. But he says that NHTSA hasn�t figured out how to do a handling test without affecting the driver. �They�re very thorough on making sure the driver doesn�t have any affect on the test,� Healy adds.
Delphi, the innovator of rear-steer systems for GM�s pickups, is currently developing a rear-steer system for passenger cars. Rear steer can be integrated into the chassis system to improve the vehicle�s handling.
|TRW�s Active Roll Control (ACR) system uses hydraulic actuators on two or four wheels to eliminate body roll on vehicles with a high center of gravity, like SUVs.
Vehicles with a higher center of gravity usually experience more body roll, which can make the vehicle easier to tip.
Systems like Delphi�s Active Stabilizer Bar and TRW�s Active Roll Control (ACR) help reduce roll rate and roll angle making the vehicle easier to handle.
Both systems use a hydraulic actuator that�s attached to the roll bar. During a turning maneuver, lateral acceleration and steering angle are measured and a control unit adjusts the hydraulic pressure in the actuators to stiffen the roll bar, reducing body roll.
�The Fishhook test does simulate what happens when you really get the body active,� says Murray. �By reducing the body roll, you�re increasing the ability of the vehicle to sustain lateral acceleration.�
Murray says that an active stabilizer bar can actually put energy into the body to counteract what the driver�s doing. �You can also think of it as effectively increasing the track width. Because the track width shrinks when the vehicle is body rolling and if you can keep it flat, then you can keep it spread out.� �We had a lot of discussion at TRW to see what technologies could be used for rollover avoidance and mitigation and we came to the conclusion that there really is no a silver bullet,� says Aly Badawy of TRW.
Badawy says that statistics show that 90 percent of rollovers occur when the vehicle leaves the road � either deliberately or through a sudden loss of control. �Keeping the vehicle on the road is the key.� �We feel that handling technology is the foundation for vehicle dynamics control,� says Murray. �If you think about technologies like our Quadrasteer rear steer system � those are technologies that can fundamentally change the way a vehicle will handle and make it more predictable for a driver in an emergency maneuver.�
Delphi and Continental Teves are currently developing active front steer systems for passenger cars. On both systems, steering input is electronically modified so the road wheels turn at a different angle than the steering wheel input. This can be done without any input from or feedback to the driver.
�I like to look at it as improving the driver�s performance rather than taking control from them,� says Murray. �We haven�t experimented very much with technologies that would grab the steering wheel out of the driver�s hands. That would seem to be pretty intrusive.�
Healy says that Continental Teves is working on a front wheel steer system that incorporates electric steering into both front end control and lane departure systems. Bringing the technologies together and creating synergies make it more cost effective.
�You may pay a little extra for electric steering but you can pair it with two different systems,� says Healy. �One advantage is that it�s more than just replacing hydraulics with electric motors. You actually get some additional functionality out of it.�
Delphi has demonstrated an active rearsteer system that�s mounted on a Jaguar SType. Active rear steer cannot only enhance the vehicle�s performance due to its highly tunable software but it can also be integrated into the chassis control system and used as a first line of defense in keeping a vehicle under control. �The combination of the two is quite powerful,� Murray says, �because the vehicle is already being controlled by countersteer before the brakes need to come on.�
�The brakes are your last line of defense,� Murray says. �If you�re driven to use the last line of defense first then you�re kind of missing out on everything that might keep you from getting there in the first place.�
| ESP II, Continental Teves stability control system, integrates electric steering for better performance.
TRW is currently developing a rear-steer system that is built from the same components as its electric front steer system. Besides allowing for greater maneuverability of larger vehicles, the system can also be integrated into the chassis system and used to control the vehicle. Just about every vehicle on the road these days has some kind of electronic stability system or traction control system that utilizes the �last line of defense� when braking is needed to keep the vehicle under control.
Brake-based systems like TRW�s Vehicle Stability Control (VSC), Continental Teves Electronic Stability Control (ESC) and Delphi�s Electronic Stability Control (ESC) are designed to keep the vehicle moving in the driver�s intended path by maintaining the vehicle�s energy at a level that�s appropriate for the driver to control. The systems utilize yaw sensors, lateral acceleration sensors and steering input sensors along with the ABS system to apply braking to specific wheels and, in some cases, reduce engine power to control the vehicle when it�s experiencing understeer, oversteer or a loss of traction that could result in a spin.
Further modifications can be made to these systems to further enhance their ability to control the vehicle, keeping it from getting sideways and possibly rolling over.
Continental Teves ESC II and TRW�s VSC integrate active steering intervention making the vehicle easier to control at critical limits. �You actually improve the ESP performance by adding the steering,� says Healy.
�One of the issues you always have is when you�re on a slick coefficient stop where you have two wheels on the gravel and two on the pavement. Physically, the ABS system will ramp up the braking on the high coefficient side to give the driver time to react to it and put the steering in. If you have electric power steering you can put it in immediately and actually stop quicker on that kind of surface.�
�VSC coaches the driver in an oversteer situation to countersteer much sooner and in the right direction and level,� says TRW�s Badawy. �Oftentimes the drivers reaction time is significantly delayed and well out of phase with the activity that�s happening with the brakes. The driver can actually cause problems because they�re not in line with what is actually occurring.� With added software and sometimes added sensors, these systems can be tweaked to help prevent rollover.
�It�s important to understand that you can do the algorithms that help prevent rollover,� says Murray. �You can approach that two different ways. One is what we�ll call a feedback estimation approach where you�re trying to understand how close the vehicle is to maybe rolling over. And you would typically use additional sensors for that. �Alternatively, you can use a model of how the vehicle would likely behave and you can predict, without additional sensors, that the vehicle might roll over based on what the driver�s doing.� TRW�s Gen 1 uses a VSC based system with added software that takes into account the roll dynamics of the vehicle. It measures lateral acceleration, yaw rate and focuses on lateral dynamics but does not calculate the roll dynamics of the vehicle.
GEN 1+ uses the same sensor base as the VSC but incorporates vehicle roll dynamics software. Gen II adds a roll-rate sensor which gives you a direct measurement of the vehicle roll dynamics. �There are OEMs of ours that are requesting to jump straight to Gen I+,� says Danny Milot, chief engineer, New Products NA, �due to the fact that they can reduce the cost of having the extra sensor.�
Milot says there is some debate on whether or not the roll sensor gives you all that much more benefit. �Some customers are willing to take on that additional cost believing there is value and others haven�t bought into that.� Continental Teves� ARP (active rollover protection system) relies on the modeling of the vehicle and activates when it recognizes maneuvers that might cause that particular vehicle to tip up.
�These technologies are trying to recognize an imminent rollover and use the brakes in such a way that you could do a very quick, fairly hard application of the correct brakes to gather up that rollover whether it be a Fishhook maneuver or some other kind of drastic maneuver,� Healy says.
Ultimately, the Fishhook test will improve rollover safety but the goal of safety systems suppliers is to make safer vehicles, not to ace the test. �If you really wanted to pass the Fishhook test,� jokes Healy, �you could put a lot of understeer in the car and put on tires that would slide. A car with an awful lot of understeer might pass the Fishhook test very well but might not be a car that responds from a maneuverability standpoint.�