WHY SUZUKI SAMURAI’S ROLLOVER

There is no denying it is cute, affordable and fun. The popularity of the Samurai and other sport utility vehicles has literally skyrocketed since the mid-1980’s and every year brings several new competitors into what is easily the fastest growing market segment in the automotive industry today.

In the past however, the Samurai has come under fire for its alleged propensity to rollover. But, the Samurai remains on the market even today essentially unchanged and the long standing question remains. Why does the Suzuki Samurai rollover?

Dynamics of a Vehicle Rollover

If you consider a vehicle as a solid block at rest, a tall, narrow block will more easily tip over than a low, wide block. In a vehicle, the relative stability is given by the static stability factor which is equal to one half the width between the wheels (tread width) divided by the height of the centre of gravity of the vehicle. It has been shown that a vehicle with a higher static stability factor tends to have less rollover propensity than does a vehicle with a lower static stability factor.

However, rollovers seldom occur when a vehicle is at rest. Therefore, the following factors must also be taken into account:

• Wheelbase

• Moments of Inertia

• Tires (size, lateral traction, wear, air pressure, etc.)

• Suspension geometry

• Roll compliance as well as load

• Load distribution

• Vehicle maintenance

• Vehicle modifications

Although these factors are known to influence the rollover propensity of a vehicle, no test procedure has been developed which will consistently and accurately evaluate one vehicle’s rollover propensity relative to another in real world conditions. This lack of test procedure is a direct result of the complex interaction of environment, vehicle dynamics and driver actions which together cause a rollover.

Samurai Rollover Tests

In terms of physical specifications, the Samurai combines several factors which tend to increase rollover propensity. The Samurai is short (80" front to rear wheel), light in weight (2,095 lbs.) and is fitted with highway tires and a stiff suspension. Conversely, the Samurai had the second highest (bet) static stability factor of all sport utility vehicles on the market in 1988. The Samurai’s static stability factor of between 1.08 and 1.13 was only surpassed by the Jeep Wrangler.

With respect to these vehicle specifications, Consumer Reports wrote in July, 1988 "the design itself (of the Suzuki Samurai) is hazardous, combining high centre of gravity, narrow tread width, a short wheelbase and light weight". The Samurai was deemed "unacceptable", based on testing done by the Consumer Union (Publisher of Consumer Reports), while the Jeep Wrangler, Jeep Cherokee and Isuzu Trooper II were found to be acceptable. Subsequent to the Consumer Reports’ article however, questions were raised as to the validity of this testing. In the Consumer Union’s testing, vehicle rollover occurred only at the hands of inexperienced drivers or when outriggers were mounted to the Samurai. The Consumer Union mounted outriggers for rollover prevention but, outriggers have been challenged by a number of automotive experts because they alter a vehicle’s performance. Specifically, outriggers tend to increase the "sprung moment of inertia in the roll mode and in the yaw mode" and hence, rollover propensity. Further questions were raised as to the lack of the Consumer Union’s data other than the Vehicle’s speed at the beginning of the accident avoidance maneuver and if equipment such as automatic vs. manual transmissions affected the results.

Rollover Statistics

Investigation of 113 Suzuki Samurai rollovers using NHTSA’S (National Highway Traffic Safety Administration) FAFRS (Fatal Accident Reporting System) data base provide information about drivers and conditions involved in Samurai rollovers including:

• All fatal "first event" rollovers involved ejection (first event meaning prior to rollover, the vehicle was not hit by and did not hit any other vehicle or object).

• Alcohol was involved in 50% of the rollovers.

• When driver’s age was known, 73% were 25 years old or younger, the oldest driver being 43.

• Limited visibility due to darkness occurred in at least 58% of the rollovers.

• Wet, icy or excessively windy road conditions occurred in 29% of all rollovers.

Similar statistics for failure to use seatbelts, presence of alcohol, driver’s age and adverse road conditions shown in the Samurai rollover data could be representative of accidents involving any number of motor vehicles. Further investigation of the FARS data also reveals that the Ford Bronco II in fact has roughly four times more rollover fatalities per 10,000 vehicles on the road than does the Suzuki Samurai.

Summary

Fatalities (per 10,000 Vehicles) that occur in sport utility vehicles are roughly double those occurring in small passenger cars or small pickup trucks according to the Insurance Institute for Highway Safety. There is little argument over the validity of these statistics. There is also little doubt that sport utility vehicles have lower (worse) static stability factors than do cars. It is also true the Samurai possesses traits such as high centre of gravity, stiff suspension and light weight which tend to increase rollover propensity. There is also significant evidence to suggest that driver error is a primary cause of rollover in a large number of fatal accidents.

By definition, the Samurai and all sport utility vehicles represent compromises. Significant efforts have been made in recent years to lower the centre of gravity, widen the tread width, and provide more "car like" handling characteristics for such vehicles. It still remains however, that you can’t have both a formula one racer and a mud slinging monster truck in a single vehicle. Failure to acknowledge such fundamental handling compromises and modify driving habits accordingly is likely to result in more fatalities as sport utility vehicles are more aggressively marketed for daily transportation.