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Rollover Behavior
NHTSA vs. ATI ROLLOVER TEST
The ATI Reversed Steer Protocol and the NHTSA NCAP Rollover test (variously called "NHTSA Fishhook" or "Road Edge Recovery Maneuver") are similar in that both use reversed steer with roll rate feedback. However, their basic aims are different: the ATI protocol is an engineering test intended to simulate an on-road highway rollover accident while producing data on vehicle characteristics important to rollover. The NHTSA is a Go/No-Go test intended simply to discriminate among vehicles for consumer information. "Relatability" to the real-world highway environment is not as important in the NHTSA test as the ability to discriminate between vehicles in obtaining "Star Ratings" for consumer information.
The standard ATI is run at 50 mph with steer angles incremented from 90 degrees to 270 degrees. 50 mph is a typical highway speed which is vehicle independent, and with steer angles above 270 the loaded "outside" front tire tends to suffer bead unseating. Bead unseating is almost never found in on-road highway accidents, and steer of 270 is considered to "push the limit" of panic steering in non-test situations. (Human capabilities are much greater, and in test situations extreme steer and steer rates are common (12). The ATI test is intended to simulate on-road rollovers, and too-large steer angles may also produce too-large path deviations. With incremented steer, plots can be made of lateral acceleration, yaw velocity, sideslip, roll angle, etc. to study vehicle trending toward rollover. If rollover occurs, the protocol provides small variation in test speed to determine precise thresholds. The protocol also provides optional additional testing at incremented speeds at constant steer to plot motion "partial derivatives" - variation with steer at constant speed and variation with speed at constant steer.
The NHTSA test has regulatory aspects. in which all vehicles from minicars to large vans must be tested under the same conditions For that reason the NHTSA test uses constant steer angle with speed increased incrementally from 35 mph to 50 mph. The lower test speeds are considered safer than 50 mph, especially for the larger vehicles. For fairness between vehicles, the test steer angle is set at 6.5 times the steer required to obtain 0.3g of lateral acceleration at 50 mph. This is intended to compensate for differences between vehicles in steering ratio and understeer gradient. It sometimes occurs that a vehicle will not tip up at the 6.5 ratio due to tire force saturation: in these cases a ratio of 5.5 is tried, sometimes successfully. To avoid bead unseating tubes are used in test tires: this protective measure causes no significant effect on test results.
In NHTSA testing some vehicles exhibit severe bouncing on the outriggers, which is another reason to restrict test speeds. This behavior has not been seen in ATI testing: the difference is possibly due to outrigger design. The NHTSA outriggers are relatively stiff transverse beams attached to front and rear bumper mounts. These can act as undamped springs when a vehicle tips up "hard" onto them. ATI uses one center-mounted outrigger on each side, with air cylinders fitted to soften the strike-down and absorb energy, and they may prevent the severe bouncing experienced by NHTSA. On the other hand, the ATI outriggers permit 4 or 5 degrees of additional roll angle after "outrigger down".
The ATI protocol is limited to tip-ups with the driver on the "low" side, to avoid back injury to the driver when the vehicle "falls" at the end of a tip-up run. The NHTSA test must be run in both directions, which is incentive to limit tip-up roll angles.
ROLL BEHAVIOR NEAR TIP-UP THRESHOLDS
The ATI Reversed Steer Protocol has enabled precise study of roll behavior at the tip-up threshold. Below the threshold the roll angle increases to an asymptotic value or (more usually) a slight overshoot, then settles to a steady-state value. Just above the threshold the roll angle tends to follow an identical trace, but then "takes off" to a high value. At the threshold the roll can go either way, like a card standing on end. In the tip-up case the inflection point where roll angle begins to turn upward can be considered the time of rollover.
In all cases the peak roll angle before tip-up is significantly greater than that found in steady-state tests, because of the time lag between the "excess" roll angle and the take-off point.
The ATI Reversed Steer Protocol and the NHTSA NCAP Rollover test (variously called "NHTSA Fishhook" or "Road Edge Recovery Maneuver") are similar in that both use reversed steer with roll rate feedback. However, their basic aims are different: the ATI protocol is an engineering test intended to simulate an on-road highway rollover accident while producing data on vehicle characteristics important to rollover. The NHTSA is a Go/No-Go test intended simply to discriminate among vehicles for consumer information. "Relatability" to the real-world highway environment is not as important in the NHTSA test as the ability to discriminate between vehicles in obtaining "Star Ratings" for consumer information.
The standard ATI is run at 50 mph with steer angles incremented from 90 degrees to 270 degrees. 50 mph is a typical highway speed which is vehicle independent, and with steer angles above 270 the loaded "outside" front tire tends to suffer bead unseating. Bead unseating is almost never found in on-road highway accidents, and steer of 270 is considered to "push the limit" of panic steering in non-test situations. (Human capabilities are much greater, and in test situations extreme steer and steer rates are common (12). The ATI test is intended to simulate on-road rollovers, and too-large steer angles may also produce too-large path deviations. With incremented steer, plots can be made of lateral acceleration, yaw velocity, sideslip, roll angle, etc. to study vehicle trending toward rollover. If rollover occurs, the protocol provides small variation in test speed to determine precise thresholds. The protocol also provides optional additional testing at incremented speeds at constant steer to plot motion "partial derivatives" - variation with steer at constant speed and variation with speed at constant steer.
The NHTSA test has regulatory aspects. in which all vehicles from minicars to large vans must be tested under the same conditions For that reason the NHTSA test uses constant steer angle with speed increased incrementally from 35 mph to 50 mph. The lower test speeds are considered safer than 50 mph, especially for the larger vehicles. For fairness between vehicles, the test steer angle is set at 6.5 times the steer required to obtain 0.3g of lateral acceleration at 50 mph. This is intended to compensate for differences between vehicles in steering ratio and understeer gradient. It sometimes occurs that a vehicle will not tip up at the 6.5 ratio due to tire force saturation: in these cases a ratio of 5.5 is tried, sometimes successfully. To avoid bead unseating tubes are used in test tires: this protective measure causes no significant effect on test results.
In NHTSA testing some vehicles exhibit severe bouncing on the outriggers, which is another reason to restrict test speeds. This behavior has not been seen in ATI testing: the difference is possibly due to outrigger design. The NHTSA outriggers are relatively stiff transverse beams attached to front and rear bumper mounts. These can act as undamped springs when a vehicle tips up "hard" onto them. ATI uses one center-mounted outrigger on each side, with air cylinders fitted to soften the strike-down and absorb energy, and they may prevent the severe bouncing experienced by NHTSA. On the other hand, the ATI outriggers permit 4 or 5 degrees of additional roll angle after "outrigger down".
The ATI protocol is limited to tip-ups with the driver on the "low" side, to avoid back injury to the driver when the vehicle "falls" at the end of a tip-up run. The NHTSA test must be run in both directions, which is incentive to limit tip-up roll angles.
ROLL BEHAVIOR NEAR TIP-UP THRESHOLDS
The ATI Reversed Steer Protocol has enabled precise study of roll behavior at the tip-up threshold. Below the threshold the roll angle increases to an asymptotic value or (more usually) a slight overshoot, then settles to a steady-state value. Just above the threshold the roll angle tends to follow an identical trace, but then "takes off" to a high value. At the threshold the roll can go either way, like a card standing on end. In the tip-up case the inflection point where roll angle begins to turn upward can be considered the time of rollover.
In all cases the peak roll angle before tip-up is significantly greater than that found in steady-state tests, because of the time lag between the "excess" roll angle and the take-off point.
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