Knowledge, as they say, is power. And I can`t think of a time it is more important than when you`re driving a vehicle that can weigh up to 44 tonnes, be over 18 metres in length, has by far the largest proportion of its weight temporarily attached and, when subject to excessive side force, has a tendency to roll over rather than slide sideways. Just a little extra understanding of what`s controlling all that weight will help keep the load secure, the truck upright and you out of the mire. With the right tools you`ll be able to meet any challenge with confidence, argue your case if necessary and act appropriately when something is about to go wrong. So, when it comes to vehicle and load stability, all you need are: a copy of the Department for Transport document, Code of Practice, Safety of Loads on Vehicles, which can be downloaded from the VOSA website; a pair of furry dice; and one of those photocopies of your backside you get by dropping your pants and sitting on the copying machine.
Load restraint and vehicle stability are intrinsically linked - just think of what goes through your mind every time you enter a bend a bit quicker than you intended – and there are laws that govern them both: the laws of physics and the Road Traffic Act. The laws of physics tell you that the force needed to get the load sliding on a trailer or truck bed is proportional to the weight of the load itself. The size of this proportion is determined by the friction properties of the surfaces in contact with one another - wooden pallets, concrete slabs, sheet metal, carried on a wooden bed, all have different friction values. Similarly, the force needed to topple a load is in proportion to its weight; although now dimensions and how they correlate to the height of its centre of mass become important.
The way to make the load less likely to move is to increase the force pushing down on it without increasing the mass of the load itself; in other words, push the load onto the bed with something other than its own weight. And this is where the dfT`s Code of Practice comes into its own: not only does it tell you how to do this (with straps etc) it also represents the official line on how it should be done. In one document the laws of physics and the law of the land are covered: the load is secured properly, so nature can`t shift it, and it`s been done according to government guidelines, so an enforcement officer can`t criticize it.
The Code also has a very telling diagram that shows a truck and the amount of load restraint required, forward, backward and to the sides. The load, it says, has to be secured for a minimum of its full weight forward but only half of its weight to the sides. What the diagram is doing, although not intentionally, is illustrating the relative accelerations an entire truck can endure before it becomes unstable. It`s an interesting fact of the relationship between the forces acting on a body (in this case, a truck) that in some aspects of stability only accelerations need to be considered: the acceleration due to the force of gravity (g) and the accelerations due to forces created as a vehicle brakes or travels through a curved path. The diagram is saying that braking force can be quite high and vehicle deceleration may approach the value of g, so loads have to be secured adequately. However, side accelerations of 0.5g are often enough to destabilize a truck, so a load secured for half its weight is unlikely to move before the truck rolls over.
And it`s the problem of rollover that bring the furry dice into play. The vehicle will be subject to the same accelerations as the furry dice hanging in its windscreen, despite their weights being so different. By measuring the angle that the string the dice hang on makes to the vertical, an estimate of side acceleration compared to the acceleration due to gravity can be made. It`s how, after all, a tilt test is conducted for static stability. A vehicle is placed on a tilting bed and the angle is found at which the upper wheels lift. The angle varies for different types of vehicle, of course, and the test doesn`t allow for the `in use` characteristics of load or other important dynamic considerations. Increasing the height of the centre of gravity, for example, would reduce both the angle and the vehicle`s stability. But from the angle obtained, the basic stability is interpreted as a ratio between the acceleration to the side (trying to pull the vehicle over) and the acceleration due to gravity (trying to keep it upright).
So, if you are unlucky enough to find yourself heading into a bend a little too quickly (because of its relationship with side acceleration, small variations in speed can make a significant difference to stability) and your truck is not equipped with ESP, look at the dice. If they have swung out to more than say, about 25 degrees to the vertical, kiss the photocopy of your backside and say goodbye.
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