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The case of the maligned Montero

The case of the maligned Montero

Recently, Consumer Reports (CR) gave the 2001 Mitsubishi Montero Limited a "Not Acceptable" rating. The reason given was a tendency of the Montero to tip up on two wheels or roll over during a short-course double-lane-change emergency-avoidance maneuver.

Any vehicle will roll under the right conditions, but when considering vehicles for purchase, we are interested in which vehicles might have a greater tendency to roll over than others. SUVs and pickups have been rightly criticized as being more susceptible to rollover. According to statistics of the National Highway Traffic Safety Administration (NHTSA), rollover as a percentage of single vehicle crashes is 32% for SUVs, 2.5 times that for cars. More than 60% of SUV deaths were associated with rollover in 1999, compared with 23% in cars.

In single vehicle accidents, rollovers are caused by a number of things, traveling straight ahead across a steep slope, sliding into an obstacle such as a curb, side winds, or turning too quickly. In the CR test the vehicle is driven straight ahead, quickly turned into a left lane as though to avoid an obstacle in the road, and then quickly turned back into the original lane as though to avoid oncoming traffic.

Rollover due to turning results from inertial forces. The centripetal acceleration is v2/r, where v is the speed and r is the radius of curvature of the path. A free body diagram shows that the centrifugal force F tending to tip the vehicle over one side is W x v2( r x g), where W is the weight of the vehicle and g is the acceleration due to gravity. This produces a moment about the point of tipping of F x h which is offset by the moment due to the weight of W x t/2, where h is the normal height of the center of gravity (CG) above the ground and t is the track width.

For the vehicle to tip, the moment produced by the centrifugal force must be greater than the moment due to the weight. Thus, for tipping to occur, t/(2h) & v2/( r x g). If the CG is higher or the track width is smaller, a vehicle is more likely to tip. Since SUVs and pickups are taller, their CG is higher off the ground, and the ratio t/(2h) (the static stability factor (SSF) is smaller than for cars. Thus, they are more likely to tip or roll over than cars. Putting significant weight in a car-top carrier would also increase the tendency to roll over. The NHTSA has recently implemented a Rollover Resistance Rating (RRR) of vehicles based on the SSF.

The above analysis assumes that the vehicle is a rigid body, and that tire forces are adequate to prevent skidding. Often, the tire frictional forces are not large enough to cause tipping and the vehicle will skid. Mitsubishi has quoted a SSF of 1.15 for the Montero, which gives it a NHTSA 3 star RRR rating, similar to many other SUVs. However, as CR pointed out, the SSF does not account for several critical factors that could make significant differences in emergency handling, such as non-rigid body effects associated with the suspension, tire properties, steering response, or the presence of a stability control system. Mitsubishi did redesign the Montero for 2001 including body and suspension changes.

Mitsubishi claims records show no reports of complaints, and is critical of the CR test maneuver and the methodology, quoting an NHTSA conclusion that CR's procedures "do not have a scientific basis and cannot be linked to real-world crash avoidance needs or actual crash data." Partly, this has to do with CR's use of human test drivers and the possibility for driving variations. This raises interesting questions about the nature and extent of evidence required before safety concerns are raised. If you are interested, you can refer to the following websites, and, for more information and to make your own decisions.

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