Lift-off oversteer

Lift-off oversteer (also known as snap-oversteer, trailing-throttle oversteer, throttle off oversteer, or lift-throttle oversteer) is a form of oversteer in automobiles that occurs while cornering when closing the throttle causes a deceleration, causing the vertical load on the tires to shift from the rear to the front, in a process called weight transfer. This decrease in vertical load on the rear tires causes a decrease in the lateral force they generate, so that their lateral acceleration (into the corner) is also decreased. This causes the vehicle to steer more tightly into the turn, hence oversteering. In other words, easing off the accelerator can cause the rear tires to lose traction, with the potential for the car to leave the road tail first.

Causes and countermeasures

This type of oversteer is often more pronounced in rear-engined cars and cars with swing axle rear suspension. Rearward centers of gravity (such as older Porsche 911s) enhance this effect, though technically any vehicle can experience lift-off oversteer. Various suspension enhancements, such as a Weissach axle, Passive rear wheel steering, or a multi-link suspension, can limit a vehicle's tendency to oversteer in this situation.

Dynamics

The graphs to the right show the simulated effect of lifting off the throttle in the middle of a left turn. The transients in the first couple of seconds are due to the test, in which a step steer input (wheel angle) is applied at 0 s and held constant throughout. The steady state cornering is at constant speed, with a lateral acceleration of 0.45 g approximately until the throttle is released at 6 s.

The yaw rate plot shows the oversteer due to the rear wheels losing traction - after an uncomfortable jerk at 20 deg/s, the vehicle spins sharply in the direction of the turn. The lateral acceleration also spikes to 0.6 g and levels at about 0.55 g, meaning that the radius decreased (i.e., the turn tightened). The side forces on the outside wheels increase and the inside rear (LR) wheel even lifts off the ground, a common occurrence.

In the 1940s, the Nazis described the Tatra automobile as the Czech Secret Weapon due to its treacherous and deadly lift-off oversteer handling characteristics.

Attorney and consumer protection activist Ralph Nader described this type of handling in his 1965 book Unsafe at Any Speed.

Lift-off oversteer is often exploited in motorsport - particularly on loose surfaces (e.g. in rallying) - as a method of cornering faster as, when controlled, it has the effect of turning the nose into the apex and the car slightly sideways which allows early application of power on exiting the corner.

On April 10, 2010, Consumer Reports magazine rated the 2010 Lexus GX 460 SUV a "Don’t Buy: Safety Risk." A panel of four automotive test engineers determined that the vehicle was subject to excessive lift-off oversteer during a standardized evaluation for emergency handling. The test simulates scenarios such as transiting a highway exit ramp while traveling with excessive speed.[1] On April 19, Toyota recalled the GX 460 and the Toyota Land Cruiser Prado for a software update to the electronic stability control to correct the issue.[2]

See also

References

  1. ↑ Consumer Reports (2010-04-13). "Don't Buy: Safety Risk--2010 Lexus GX 460". Cars Blog. Consumers Union. Archived from the original on 2010-04-15. Retrieved 2010-04-14.
  2. ↑ CNN Money http://money.cnn.com/2010/04/19/autos/lexus_gx460_recall/. Retrieved 2016-06-14. Missing or empty |title= (help)
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