|MadSci Network: Physics|
Hello there. The simple answer is because of the difference in the size and shape of the vehicle
You may have heard of Sir Isaac Newton (a very famous scientist who lived between 1643 and 1721); well he set out 3 laws which helped to explain how and why objects moved. In his first law he pointed out that you need an overall force acting on an object to make it move faster or slower or in a different direction, BUT NOT to move at a constant speed in a straight line.
Of course this doesn't always tally with what you see happen in everyday life. For example, if you stop pedalling your bike you slow down; to keep moving you must keep pedalling. But of course the unseen force of friction is at work and your pedalling force is balanced out by the friction force.
When you want to turn a corner you have to turn the handlebars AND then you rely on the force between your tyres and the road (imagine what happens on an ice-covered road!)
So traffic going round corners needs a force to make it change direction - we call this a centripetal force. But the bigger the object the higher up is the centre of gravity of the object. This increases the chances of it toppling over as it turns into the corner. So a light vehicle can corner at up to 40 to 45 kph without danger of toppling over but a heavy vehicle can only do so at up to 20 kph.
Hope this helps.
from Keith, your Mad Scientist in Wales, UK.
Try the links in the MadSci Library for more information on Physics.