[ 3-Wheeler Index ] [ Up one level ] [ 4.1VisualAnalysisOfStability ] [ 4.2LogicBehindMathematicalAnalyses ] [ 4.3Stability-FrontDrive3Wheelers ] [ 4.4Stability-RearDrive3Wheelers ] [ 4.5SummarySafetyMargins ]

## 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).
We can thus easily understand that if the center of gravity height (red) is greater than the half-track (in green) (the half distance between the two wheels seen from the rear), the resulting force (black) will be aligned over the imaginary point (deep blue) and will thus roll the vehicle over in a curve.

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:
As can be seen for a 4-Wheeler on the illustration at the right, the 4-Wheeler rolls over around a line (blue) corresponding to the imaginary point (deep blue) of the previous illustration.
But in the case of a 3-Wheeler, the vehicle rather rolls over around a line (blue) going from the unique wheel to one of the two symmetrical wheels. We can immediately see that the green line between the center of gravity and the rollover line is thus shorter than in the case of the 4-Wheeler, even though the center of gravity height, the length and the track of the 3-Wheeler are the same as those of the 4-Wheeler.
The center of gravity height (red) is thus proportionately greater, which reduces the safety margin against rollover in curves.

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:
- The center of gravity height should be low in relation to the half-track, like for a 4-Wheeler.
- But the center of gravity's position also has importance: The farther it is from the two symmetric wheels towards the single wheel, the shorter is the distance from the center of gravity to the rollover line, which reduces the safety margin against rollover of the 3-Wheeler compared to the 4-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,
- Or, while braking in the case of a two front wheels 3-Wheeler illustrated at the right, it may roll around the blue point under the front wheels and tip forward.

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.
But this center of gravity must not be too close to these two symmetric wheels, to avoid tipping backward or forward.

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:

These new Canadian Motor Vehicle Safety Regulations of 2003, stipulate in Standard 505 that:

"... 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:

- There is no 'mechanical' reason to treat differently these two types of 3-Wheelers: The first could 'merit' 30% of the weight on its unique front wheel. Or the second could 'merit' 75% of the weight on its two front wheels.

- In each of these two cases illustrated above, the vehicle-occupants center of gravity is located below the pyramid, so that the single-front-wheel will not flip backwards when accelerating and the two-front-wheel will not tip forward when braking.

Summarizing, there is no reason to treat differently the risk of overturning laterally (rolling)
and the risk of flipping backwards or tipping forward.

In both cases:

- It seams more appropriate to consider overturning, flipping or tipping points or axes.

- And to insure an adequate ratio between the vehicle-occupants center of gravity height and the horizontal distance between the center of gravity and these points or axes, instead of a weight percentage on the front wheels.

3-Wheeler Index ] Up one level ] [ 4.1VisualAnalysisOfStability ] 4.2LogicBehindMathematicalAnalyses ] 4.3Stability-FrontDrive3Wheelers ] 4.4Stability-RearDrive3Wheelers ] 4.5SummarySafetyMargins ]

 The content of this site remains the sole property of Pierre M. Ethier. All rights reserved. All information on this site is presented as is for the benefit of readers. Great efforts have been made to cover many aspects of 3-Wheelers and make texts and illustrations as clear as possible. But It remains the sole and full responsibility of the reader to apply or test any recommendation, theory, suggestion…made. Thus, Pierre M. Ethier cannot be held responsible for any misunderstanding or any consequences resulting from using this information. Send any question, remark, joke, complaint or suggestion to pierre.ethier@clevislauzon.qc.ca or to Pierre M. Ethier, 2120 Du Foulon, App. 8, Sillery, Quebec, Canada, G1T 1X4