In your travels or even in your company journey management plans are you talking to staff about HIGH WINDS and the chance a semi could blow over in front or on top of them. As I was travelling this AM, I was reminded as I watch another semi lead over like a large fat pig, that the force of high winds can move mountains or at least large trucks. And as we waited for the police and fire people to assist I wondered out loud do people truly understand the risks not always regulated by law!
So, there’s two things to think about here – how much force is needed to tilt the truck and how far you can tilt it before it topples over. IMPACTS…DRIVERS OF LIGHT WEIGHT OR HIGH PROFILE VEHICLES MAY EXPERIENCE CONTROL PROBLEMS. LOOSE OBJECTS LIKE GARBAGE CANS AND LAWN FURNITURE ARE LIKELY TO BLOW AROUND. WEAKENED OR BROKEN BRANCHES ON TREES MAY COME DOWN.
A WIND ADVISORY MEANS THAT WIND GUSTS IN EXCESS OF 45 MPH ARE EXPECTED. WINDS THIS STRONG CAN MAKE DRIVING DIFFICULT…ESPECIALLY FOR HIGH PROFILE VEHICLES. USE EXTRA CAUTION.
So how does it create a force?
Well air still have mass and density. It’s pretty small, much less than water or metals, which is good because otherwise, humans would all fall into the atmosphere. But this means that when the air is moving, it has momentum. Imagine putting your hand in a jet of water. as it hits your hand and slows down, you experience a force as the momentum of the water is transferred to your hand. So, if we stop a fast-moving air, for example, when it hits a side of something solid like a semi/van. box trailers , it will transfer momentum and there’ll be a force pushing the semi/van. box trailers over.
But what determines how fast the wind needs to be moving to tip the semi/van. box trailers over?
The wind isn’t the only force. There’s also gravity pulling down on the semi/van. box trailers , resisting the urge to tilt. For the semi/van. box trailers to start to lean over, the wind needs to be fast enough that enough momentum is transferred to overcome gravity. Once the wind gets strong enough, the semi/van. box trailers will shift over onto two wheels. If the wind keeps blowing at the same speed, the semi/van. box trailers will topple. But if the wind drops off a little bit so that the forces match again, the semi/van. box trailers can stay on two wheels indefinitely, which is very cool, but it will be a little bit terrifying to see it coming along the motorway.
I imagine semi/van. box trailers driving on two wheels must be very unstable. Surely, they fall over eventually?
Only if the wind speed increases again. Every object has a centre of mass and as long as that is over its base, it won’t topple. For most people, theirs is just behind their belly button. Think of your feet as a base. If your belly button sits above that base, your stable. But as soon as you lean out, you start to fall. That’s actually what walking and running is. Overbalancing so that you start to fall in a direction and then moving your legs fast enough to keep from toppling over. As soon as the wind has tilted the semi/van. box trailers enough that its centre of mass is on the wrong side of the two wheels, then gravity is now pulling the semi/van. box trailers in the same direction as the wind is pushing and it will topple over. Probably, quite spectacularly.
If you’re a semi/van. box trailers driver, what could you do to stop your truck blowing over?
Well if you can, fill it with stuff. The more you’re carrying, the heavier the truck is and the larger the force, and then higher the wind speed is needed before you topple over. For an empty semi/van. box trailers like the one Marco was driving, you can drive in winds up to about 60 miles per hour. But if you increase the weight with cargo then you can manage much higher winds.
So, the speed you travel at isn’t so relevant. It’s how much your truck weighs and how large an area it presents to the cross winds that matters.