4.1 Simple visual analysis of 3-Wheeler stability:
A) Center of gravity position:
Consider first a 4-Wheeler as seen from the rear, like here to the right. If the vehicle is in a curve towards the left, for example, we can imagine that a centrifugal force (magenta color) is exerted on the center of gravity (black and yellow circle) of the vehicle-occupants system, while the vehicle's weight exerts a downward gravitational force (cyan color).
Thus, the centrifugal force (magenta) tends to roll the vehicle over towards the right, around an imaginary point (deep blue) under the right tires, while the gravitational force (cyan) holds the vehicle back to avoid rollover.
It's as though the centrifugal force and the gravitational force combined together into a resulting force (black)
exerted on the center of gravity to turn it around this imaginary point (deep blue).
The ratio of the center of gravity height (red) to this half-track (green) thus plays a crucial role in determining the stability against rollover of a 4-Wheeler. Ideally, this center of gravity height (red) should be low like for a sports car, in order to insure a safety margin against rollover. In the case of 'sport-utility' 4X4s, this height is relatively larger than for regular family cars. This explains why these vehicles have a higher rollover propensity.
In the case of 3-Wheelers, another factor comes into play:
Moreover, a 3-Wheeler in a curve can also be subject to a braking or accelerating force that will combine with the lateral centrifugal force, which may further increase chances of rolling over of this 3-Wheeler. For example in the case of the single-front-wheel 3-Wheeler, here above to the right, braking in a curve towards the left will increase chances of rolling over this 3-Wheeler.
So in the case of a 3-Wheeler:
B) Accelerating or braking in a straight line:
When going straight, a 3-Wheeler may be accelerating or braking. Thus:
- It may tip backward while accelerating, as in the case of a two rear wheels 3-Wheeler where the center of gravity
is located too far back,
Summarizing, the 3-Wheeler's center of gravity must be low and close to the two symmetrical wheels, that are alone to
avoid a rollover in curves.
Basically, the center of gravity must be located under a pyramid, as shown to the right in the case of a two-front-wheel 3-Wheeler, to avoid rolling over sideways or tipping forward.
C) Canadian Motor Vehicle Safety Regulations:
"... the height of the center of mass, shown in Figure 1, of a motor tricycle or a three-wheeled vehicle shall not exceed one and a half times the horizontal distance from the center of mass to the nearest roll axis, shown in Figure 2."
So according to this regulation, the center of gravity height (in red) may thus be one and a half times the green line between the center of gravity and the rollover line, as illustrated at the right. The resulting force (black) may thus be aligned over the imaginary point (deep blue) and roll the vehicle over in a curve.
Obviously, this regulation is very large if not too large, since it lets certain insufficiently stable vehicles circulate on public roads.
As a counter part, this new regulation has the merit of bringing order to the world of two and three wheel motorcycle definitions and regulation. Also, while avoiding going too far, there are less chances of killing the touring motorcycle aftermarket, where goodwill manufacturers can continue replacing single rear wheels by two rear wheels, on motorcycles used by goodwill people that use them carefully and do not ride fast.
This new Canadian regulation also stipulates in article 505, that:
"The total weight of a motor tricycle or three-wheeled vehicle on all its front wheels, as measured at the tire-ground interfaces, shall be not less than 25 per cent and not greater than 70 per cent of the loaded weight of that vehicle."
The image at the right illustrates the case of a single-front-wheel 3-Wheeler having its vehicle-occupants center of gravity located at less than 25% of the wheelbase length from the rear wheels. This leaves less than 25% of the weight on the front wheel.
The image below illustrates the case of a two-front-wheels 3-Wheeler having its vehicle-occupants center of gravity located at more than 70% of the wheelbase length from the rear wheel. This leaves more than 70% of the weight on the front wheel.
Even though this new regulation may bring order to motorcycle definitions and regulation, it's nonetheless peculiar that:
Summarizing, there is no reason to treat differently the risk of overturning laterally (rolling)
In both cases:
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