Special Features of the ATV
In many ways, ATVs are very different from any other kind of vehicle you might drive. The special features of the ATV help explain some of the serious safety hazards, so understanding them will make you a safer ATV operator. You will also be able to use this information when explaining ATV safety to others on your operation. The special features are:
- Large low-pressure tires
- A high center of gravity
- Fixed rear axle
- Limited protection for the operator
The Tyres
The ATV’s big balloon-type tires are designed to be very good on rough terrain. On the other hand, they are very bad on pavement. Like driving a truck with under-inflated tires, they affect the steering. In part, this is why it is not legal to drive an ATV on a highway. In addition, the tires create a much more bouncy ride than other vehicles, which adds to the instability by exaggerating the ups and downs.
The tyres are intended to have very low air pressure. The manufacturer’s manual will say exactly, but it’s usually between 2 and 6 pounds per square inch (psi), which is much lower than the 35 to 40 psi required by most vehicle tires. With such low pressure, a regular tire gauge is not accurate enough, so a special smaller-scale gauge will be supplied with the ATV. Always use the right gauge.
On ATV tyres, even the difference of 1 psi will make a difference. The tyre pressure must be the same on all four tyres. If it’s not, the ATV will pull towards the soft tyre. Always check the tire pressure before each ride, and top up the air if it’s low.
The High Center of Gravity
All vehicles have a point called the center of gravity, which is the point where all parts of the vehicle balance each other in all directions. In addition, all vehicles have an imaginary boundary at ground level, usually a square or a triangle, called the stability baseline. If the center of gravity moves outside the boundary of the stability baseline, the vehicle will tip and roll over.
Where are these located on an ATV?
Each ATV model is a little different. But in general, the center of gravity of a parked ATV can be estimated:
- about six or more inches above the center of the axle
- about six or more inches rear of the halfway point between the two axles.
The stability baseline can be estimated by imagining the point where each wheel touches the ground, and drawing a line through each of those four points, to form a square. The center of gravity of the ATV itself stays the same. But if you add other weights to the ATV (the body weight of an operator, and heavy gear on the carrier racks), the center of gravity includes those weights, too. The new center of gravity is higher, and it may also be further forward or backward, or off to one side. The typical utility model ATV weighs about 600 pounds by itself, so the additional 200 pounds of an operator and gear can significantly raise the center of gravity. (Lightweight recreational ATV models that advertise a high ground clearance will, of course, have an even higher center of gravity).
Therefore, when the ATV is fully loaded, travelling across a slope, the center of gravity is dangerously close to the stability baseline.
In this situation, running over a bump may be enough to cause a rollover. Or, in another example, consider what happens to the stability baseline when the ATV makes a sharp turn. The four points of wheel contact with the ground no longer form a square – they become more like a triangle. If the ATV is then loaded with gear over the front axle, the center of gravity is higher and further forward.
When the front wheels are sharply turned, the stability baseline at the vehicle’s front end is dangerously narrow – so narrow that the center of gravity can easily move outside the boundary of the stability baseline. The result will be a rollover.
When you explain to family members or employees why they should never carry a passenger on a single-operator ATV, be sure to point out that the vehicle’s center of gravity is already high enough with just the operator’s body weight. The addition of a passenger makes an ATV dangerously unstable.
The Fixed Rear Axle
Most ATVs have a fixed or solid rear axle. This is a significant difference from other vehicles, which use a differential mechanism to allow the two wheels to turn at a different rate when going around a curve. When the ATV comes to a curve, you steer the handlebars, and the front wheels turn in the direction of the curve. But both back wheels, because the ATV has no differential, want to keep going straight. Therefore, when you turn, the force is pulling the ATV out of the curve. The whole ATV tries to skid outwards, or roll over away from the curve. This is more noticeable at higher speeds.
The safe operation of the “driver-active” ATV depends on your body movement. To overcome the outward roll of the ATV, you must shift your upper body into the direction of the curve. In effect, this brings the center of gravity back inside the stability baseline. And to overcome the straight-ahead movement of the rear wheels, you must force the inner rear wheel to slip a little more than the outer rear wheel.
This happens when you brace your outer foot hard on the footpeg as your upper body leans inwards. Of course, these movements are affected by the distribution of the load on the ATV. An unevenly distributed load makes the ATV much more difficult to control.
The operator’s movements are another reason why you should never carry a passenger (unless the ATV is specially designed for one). The long seat on an ATV is intended to allow the operator to move freely, and play a driver-active role in controlling the vehicle. Carrying a passenger would interfere with this necessary movement.
Very Little Operator Protection
Most workplace vehicles have some kind of built-in protection for the operator: it may be the reinforced cab of a truck, or the rollover protective structure (ROPS) of a tractor or heavy equipment. The ATV has nothing of the kind. There is no ROPS on an ATV, no cab, and very little shielding.
What keeps the operator safe is a combination of preventive measures (good operating skill, and well-maintained equipment), and if something goes wrong, protective gear and clothing. Even in a simple upset on rough ground, you can suffer cuts, puncture injuries, and broken bones. In a rollover, you may be pinned under the machine. In any kind of ATV accident, there will not be time to protect your head, if you’re not already wearing a helmet.
Head injuries in ATV accidents are serious. Operators have been struck by falling rocks, by low-hanging brush, or have been thrown to the ground. Wearing a helmet will not prevent the accident itself. However, the helmet can make the difference between a fatal head injury and survival after the accident. Or, the helmet can make the difference between a permanent brain injury and recovery from a less-serious head injury. This choice is in your hands every time you ride the ATV.
