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What are the effects on the vehicle in terms of roof loads and safe driving?

Our topic today is “What are the effects on the vehicle in terms of ceiling loads and safe driving?” :

A fine analysis that you won’t find anywhere else.

The forces acting on the vehicle can be basically divided into static and dynamic forces. Static conditions are only available when the vehicle is stationary. Due to the fact that the vehicle is in constant motion; We can also call the effects that cause variable loss of balance under different conditions, due to the effects of momentum, centrifugal force, suspension system and rotational effects from the engine to the wheels, and sometimes the braking effect, etc. Since dynamic effects are all mass-related effects, we can think of them as acting from the combined center of gravity of the vehicle.

Combined center of gravity: It is the center of gravity calculated by taking into account all the non-fixed additional equipment, goods and people loads inside the vehicle other than its own weight.

Dear off-road and nature enthusiast friends, we are together with another sharing of our Off-road Basic Technical Information article series. The information I will provide; These are concentrated, technical-based, lived experiences, and some of the details that will find a name in the off-road literature for the first time. With the hope that it will benefit the readers.

Any additional load placed on the vehicle will shift the combined center of gravity of the vehicle upwards if it is placed at a location above the calculated center of gravity of the vehicle when empty. Conversely, the additional load placed below the vehicle’s curb center of gravity shifts the vehicle’s combined center of gravity downwards.

The basic rule of balance-rollover is that the higher the center of gravity is, the easier it will be to roll over, and the lower the center of gravity, the harder it will be.

In order to better understand the subject, it will be useful to read our article where is the balance in offroad and what is its importance .

Denge off-road‘un neresindedir ? Off- Road Araçlar Denge ve önemi nedir ?

Now let’s come to the vehicle specific that will serve as an example for our analysis.

Our vehicle is 190 cm wide from the outside of the tire to the outside of the tire,

Vehicle height is approximately 190 cm,

Our vehicle weight is ~2000 kg,

The additional ceiling test load placed on the vehicle is placed at a height of 200 cm from the ground,

The additional buffer level placed on the vehicle is placed at a height of 70 cm from the ground,

We assumed that the vehicle’s own curb center of gravity was approximately 95 cm above the ground.

ceiling loads; It can form roof tent, roof basket, roof integrated canopy, roof rack, etc.

Buffer level loads are; It can create iron bumper, crane, buffer water tank etc.

In our analysis, we will basically examine the positive and negative effects of an additional 100 kg weight on the vehicle on the bumper or roof position, statically and dynamically.

Positive effect: The effect that helps to stay in balance

Negative effect: The effect that causes the vehicle to lose its balance.

In other words, what would be the difference between the 100 kg weight that we will load additionally to the vehicle at the bumper level and the roof basket level?

According to these answers, we will discuss how we should regulate our driving habits when we put equipment such as roof basket, roof tent, roof shade, roof rack in our vehicle.

For ease of calculation, some measurements have been rounded to more understandable numbers.

The direction of dynamic effects may vary depending on the factor causing the effect. In terms of ease of calculation and the direction in which we think that the moment arm of the effect is at the highest value, we have taken as a basis in our case study.

In addition, since this is an evaluation analysis, assumptions were made in line with the explanations below.

Provided that the F force is the function of the mass constituting the 100 kg weight; will act on the vehicle as a function of momentum, moment of inertia, centrifugal force, braking effect, suspension system, etc. For a 100 kg weight, even though the mass is constant, it may affect the vehicle with different force values at the roof basket level or the bumper level depending on what is caused by the effect. That’s why they are called F1 and F2.

If we enter a curve with a radius of 30 m at a speed of 60 km/h (16,667 m/s);

Centrifugal force to which an object weighing 100 kg will be subjected;

(100 kg) / (9.81 m/s 2 ) * (16,667 m/s) 2 / (30 m) = 94.39 kg. In our example calculation, we can accept this as approximately 100 kg and ignore the situation that the position difference will create and accept F1=F2=100 kg.

Our notation; “+” for all effects trying to stabilize the vehicle and “-” for all effects trying to overturn the vehicle.

While our example vehicle is stationary and level, it will be “+” as it tries to keep both its own weight and additional loads in balance.

While the vehicle weight and additional loads will create a “+” effect when the vehicle is in a flat position and in motion, the force arising from the centrifugal force, for example, will create a “-” effect.

If the “-” dynamic effect that the vehicle is exposed to exceeds the “+” effect that tries to keep the vehicle in balance, the vehicle will roll over.

If we consider that the vehicle is not in a straight position and is leaning to the side in off-road conditions, this rollover may occur at much smaller values of “-“ effects.

Now, let’s do our numerical example for the situation where the vehicle’s center of gravity is inclined enough to align with the outermost point of the tire, which is the tipping point of the vehicle. The assessment will be made only for the additional load.

In this case, the distance of the weight of the vehicle to the vertical axis passing through the tipping point is 0. That is, the force arm of the moment of the vehicle curb weight acting in terms of overturning or balancing is 0 cm. Naturally, if L = 0 in the formula M (moment) = G (weight) x L (arm of force), then M = 0. In this position where the effect of the vehicle weight is 0, the effect of the additional load placed on the roof and bumper level of the vehicle appears as follows;

For static effects only:

For ceiling level loads,

M(-) = 100 kg x (-0.75 m) = -75 kg-m to overturn the vehicle

For buffer level loads,

M(+) = 100 kg x 0.15 m = 15 kg-m to keep the vehicle in balance

If the bumper level and the roof level are on the vehicle simultaneously with an additional 100 kg load,

-75 + 15 = -60 kg-m overturning moment, the vehicle will overturn.

Only if the roof level is an additional 100 kg load on the vehicle, the vehicle will overturn with an excess of -75 kg-m tipping torque.

For static and dynamic effects:

For ceiling level loads,

(As explained above, F1=F2=100 kg is assumed)

M(-) = 100 kg x (-0.75 m) + 100 kg x (-2.10 m) = – 285 kg-m to overturn the vehicle

For buffer level loads,

M(-) = 100 kg x 0.15 m + 100 kg x (-1.15 m) = – 100 kg-m to overturn the vehicle

If the bumper level and the roof level are on the vehicle simultaneously with an additional 100 kg load,

(-285) + (-100) = -385 kg-m overturning moment, the vehicle will overturn.

Only if the roof level is an additional 100 kg load on the vehicle, the vehicle will overturn with an excess of -285 kg-m tipping moment.

As can be seen from the numerical example, additional loads at the ceiling level will cause the vehicle to tip over earlier, both in static condition and under dynamic effects, especially if the vehicle is in a lying position.

While additional loads at the bumper level have a positive effect on the stability of the vehicle in static condition, the dynamic interaction may cause the vehicle to topple even for loads at this level.

Conclusion and Suggestions:

Almost none of us use the vehicles we use in the field in their original form. The most basic modifications to off-road vehicles are vehicle upgrades and tire upgrades. These modifications already shift the center of gravity of the empty state of the vehicle up, bringing a negative situation in terms of balance. In addition to these, any additional load we put on the roof level will cause the vehicle to topple earlier, especially in the side position.

The rider loves his vehicle and loads it with as much equipment as possible. Some of these equipment are visual and some are functional. E.g; inoperative snorkel, inactive dummy radio antenna are some of them. If we accept that we will not be able to drive a raised vehicle with MT – XT tires at high speed, even on asphalt, theoretically, the snorkel, which only negatively affects the vehicle’s aerodynamics, will not have a life-threatening negative effect.

However; Any additional load placed on the roof level adversely affects vehicle safety. So don’t put any really useless heavy equipment on the ceiling level just for the sake of it.

If you need actual use on the vehicle roof; If there is equipment such as baskets, spare wheels, tents, roof racks, awnings, etc., it is vitally important that we drive by being aware of the negative effects of these.

As the vehicle gets higher and/or narrower, or as the 100 kg basket load, which we accept as a reference, increases, the effects will increase negatively.

Best Regards,

22/05/2020

Süleyman ATILGAN

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You can view other articles professionally prepared by Süleyman Atılgan, which include off-road basic techniques and off-road driving advice, below.

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