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ATV

Why is there statistically such a high number of rollover accidents and injuries involving UTV's and ATV's, such as the Yamaha Rhino and the Polaris Razor? The obvious answer to this question is "it is because of their narrow track width and high center of gravity."

Another explanation might be, "Well, the nut behind the wheel caused the accident." But motorcycles are driven by the same thrill seeking individuals as ATV's and UTV's, yet the accident rate on two-wheeled motorcycles is less than that on these four-wheeled vehicles. The motorcycle is certainly unstable in rollover; however, people control them very well, and on extreme off-road terrain. Could one explanation of the frequent and dangerous rollovers of ATV's/UTV's be due to their poor handling? When we say poor handling, we mean that ATV's/UTV's do not respond to steering inputs well. A motorcycle responds to the steering of the handlebars the same way every time. On the other hand, ATV's/UTV's are sluggish to respond at first, and then they snap around. In technical terms, this would be called a transition from severe understeer to severe oversteer.

The fact is that, when examining the basic design of nearly any ATV or UTV on the market, these vehicles usually have two characteristics that break normal vehicle design rules.

The first negative attribute is that the two rear wheels are locked together, meaning they both turn at the same speed. That is OK while driving straight ahead and it gives great traction when in the mud or climbing a sandy hill. However, when trying to turn, the wheel on the outside of the turn must travel farther than the wheel on the inside. Normal vehicles allow the outside wheel to travel farther than the inside wheel by allowing the two driven wheels to turn at different speeds while applying the same torque. In the case of most ATV's and UTV's the wheels turn at the same speed. Therefore, in order to turn the vehicle, the outside tire is dragged around. This results in the vehicle resisting turning with the friction of the rear tires. It wants to keep going straight even when the driver turns the front wheels. And it will respond differently depending on the surface - such as pavement versus sand versus dirt - or how the operator has weighted the wheels by shifting his body weight.

Well, if you keep increasing the steering input it will eventually start to turn, but then the second negative feature comes into play. The rear suspension is designed to be practically rigid in the roll direction or at least very stiff when the vehicle leans the body. The front suspension is softer in roll than the rear, which means that when going around a curve the two front wheels stay on the ground and the inside rear tire tends to lift off of the ground. The good news is that all of that resistance to turning from the locked rear wheels goes away when the rear tire lifts. Now all of that steering that was put in to get it started turning is now really effective and the vehicle begins to turn very quickly. So once the driver gets it turning, it starts turning sharper without the driver doing anything. In addition to that, as it turns harder, the body starts to lean and the rear inside wheel begins to lift. With one rear wheel on the ground and two in the front there is more side friction on the front than on the rear. In this condition the rear wants to swing around, adding to the turning velocity without increasing the steering angle. This all happens much quicker than you can read this.

The human being, as an adaptable animal and a quick learner, usually is able to manage and control the vehicle. This is especially true at slow speeds when errors in judgment will not turn it so quickly causing it to rollover. But as speeds increase, the timing of all of these events becomes shorter and the response of the vehicle goes up and the skill of the driver must be much greater or there will be loss of control and rollover. So the driver is constantly fighting the tendency of the vehicle which is not to start a turn, and then the driver suddenly has to reduce the steering angle to keep the rear end from coming around, all the while shifting his weight, if he is on an ATV, to compensate for these changing circumstances. A UTV has the same suspension as an ATV, but the driver is strapped down, or should be, and cannot affect the handling with his weight shift. A normal, well designed vehicle doesn't present the driver with these challenges. Therefore because of the ATV and UTV odd responses, the driver often finds himself going much faster around a turn than desired; causing the ATV or UTV to roll over or perhaps run into something. Wouldn't it be better to design a vehicle that behaves itself, and allows the driver to enhance already good handling characteristics with his weight shift or rider activity such as the case with an ATV?

The following images demonstrate how a Yamaha Rhino responds in a slalom course, illustrating these unique handling characteristics. Also see Dr. Renfroe's television interview explaining these characteristics.


ATV
ATV
ATV

The point in this illustration is to show that when the driver is in control, when there are no delayed responses, when there is not this transition from severe understeer to severe oversteer, the driver maneuvers through the course without a problem. There is no magic to this performance. This is just the application of standard vehicle dynamics rules that have been understood for at least 80 years.

Although the illustration was with an UTV, ATVs will respond the very same way to reasonable adjustments to their suspension in accordance with accepted vehicle dynamics rules. An ATV or UTV that goes where the driver wants it to go, and responds in a fraction of a second to the movement of the steering wheel or handlebar, will certainly give the driver a much better chance to control his vehicle and avoid collisions and rollovers.

If you are curious about these simple handling principles call the EI Consultants, LLC (a human being will take your call and direct you to Dr. Renfroe.) Just mention that you read our blog "Why ATV's and UTV's like the Yamaha Rhino rollover so often" and he will tell you what these principles are, as well as the secret hidden in plain sight.


EI Consultants, LLC offers a variety of engineering research, consulting, and public services related to the automotive industry, tire failure and design, fire investigations and materials failure. The engineers have expertise in accident reconstruction, automotive mechanical failures; and deposition and trial testimony.

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