Naked Science Forum
General Science => General Science => Topic started by: Patricia LaRue on 13/10/2009 09:30:05
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PATRICIA LARUE asked the Naked Scientists:
I drive a semi truck & trailer. In high winds, I prefer to slow down to maintain control & stay upright. But many, if not most of my fellow drivers disagree. They insist that "outrunning the wind" is the best strategy. Who's right?
The fully loaded truck is 80,000 lbs. The dimensions are 73' total length for the tractor & a 53' long trailer. The trailer has about 3' of air space underneath (passing air could affect lift). The height is 13'6" for the trailer. The tires at 60 mph have a footprint about the area of a 50 cent piece each. There are 18 tires. Empty, the truck/trailer weigh about 35,000 lbs.
Taking all the factors of weight (downward pressure), wind speed & direction (assuming the wind hits at a 90 degree angle, the variables in the road surface, & any lift that might occur, what is the tipping point of an empty truck vs a fully loaded truck? At what speed will the wind be able to turn a truck over? And is traveling slower or faster going to make it tip over easier. Remember that the faster you travel, the smaller 'footprint' you have.
What do you think?
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Does the wind hit at a 90 degree angle in reference to the road you are traveling? I am asking this since the angle of the apparent wind (= wind the vehicle experiences) will change with the speed of the vehicle.
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Hi Patricia,
You are right. It is not possible to 'outrun the wind'. What your compadres are trying to do is get the journey over ASAP.
If a strong crosswind hits the side of your trailer, it doesn't matter what speed you are doing, if it is strong enough to tip the trailer, then it will. In which case I would rather be going slow than fast when the truck lands on its side.
At low speed you would stand a better chance of turning your unit to try to counter the effect. At speed, as you will know, any sudden change in the attitude of your unit to your trailer could result in jackknifing..... Not a pleasant experience!
Also while in a bend, there is 'G' force to consider. On a left bend, the force will assist any strong gust from left in tipping the trailer. The faster you are going, the greater the 'G' force.
Then there is more chance of overcompensating for sudden strong gusts while driving at speed.
Some curtain sider drivers here in the UK, when travelling unladen, will open and tie back the curtains. While this may alleviate the effect of cross winds, it can also increase the 'lift' by strong winds, thus reducing traction.
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Thanks for the input so far..! Yes, the 90° angle of the wind would be to the direction of the roadway (hopefully the same as what the truck experiences).
I would like to know what amount of windspeed it would actually take to turn a truck over, however. The wind has so much force per square inch at different speeds, this seems like it should be a fairly simple equation. Seems like.. [???]
The safety departments of all the companies for which I've worked all stress safe driving on snow and ice. There are videos and presentations all about that. Absolutely nothing on this subject. I've been a driver for 12 years, and it still amazes me that so many drivers, even the experienced ones like myself, continue to ignore the potential that mother nature holds with just the air around us.
One of the reasons I stay slow in the wind is because of the strong sudden gusts, as you mention, Don. If you jerk the wheel back after such a gust hits, the effects of moving the steer tires suddenly in the other direction can whip through the trailer. Although it's not a direct result of the wind, itself, you will still wind up on your side.
I have quit jobs over this issue in conflict with desk jockey dispatchers who insisted that "everyone else is doing fine.." and I should do what everyone else does. I need some real facts and ammunition. [;)]
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After reading some of the responses above I will say that the angle of the wind, as said, does not matter. A wind strong enough to tip the truck, will tip the truck at any speed.
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I can't help you with this calculation, but I think you will need to let us know the width of the truck. Outside of one wheel to the outside of the wheel on the opposite side of the truck.
For calculation purposes it might be easier to assume the truck to be a immobile 80,000 lbs box with the dimensions you gave (we still need the width) sitting on stilts 3' of the ground. I don't think the foot print of tires would matter a whole lot in regard to tipping forces.
And if you drive faster you are exposed to the wind less long. The chances of getting caught in an overturning gust of wind are smaller if you are on the road less long. That would speak for getting the journey over sooner (as Don said). Same with lightning: The longer you are exposed - the higher the chances for a strike.
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Being a great mathematician (NOT) I will not comment on Dave's computation.
Just one thing to point out regarding a loaded trailer. Assuming a fully loaded weight of 42 tonnes, you need to take into account the cubic capacity. If the load has a high CC, the centre of gravity will be correspondingly high. While a low CC will lower the centre of gravity.
So if you are loaded with 25t of corn flakes, your centre of gravity will be high, while 25t of steel will mean a lower centre of gravity. A higher centre of gravity will, obviously, mean lighter winds can effect your trailer. You start getting into more tricky situations where loads with a high centre of gravity are concerned. For example, a 5t machine on legs and 2.5m high, may have the bulk of its weight above 1m. Loading 4 or 5 of these could raise the trailer's centre of gravity above the mid height point.
Of course with part loads of say 10 or 16 tonnes, there is also weight distribution to take into account.
To be honest, I do not think it is possible to apply any fixed equation to the problem. Your trailer height and length cannot be used to equate to the weight to arrive at a figure which would determine the required wind speed to push it over, since the centre of gravity is dependant entirely on the type of load.
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I think you should ask the truck manufacturers for their recommendations.
They no-doubt do all sorts of tests to prove the stability of their trucks under different conditions, and will know of any handling quirks (and probably get involved in accident investigations).
My gut feeling however is that slower is safer.
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I know this is an old topic, but I have some interesting information to add. I've been researching much the same thing, but with an aim at cancelling side-wind forces for a motorcycle via "stalling the wing"... using cross-over ducting to cancel lift for a fully-faired motorcycle.
I created a spreasheet that calculated the effective wind speed and angle based upon actual wind speed at a worst-case 90 degree angle from vehicle direction of travel, and vehicle speed.
See the attached PNG file. I can't attach the spreadsheet, it's not an allowed file, so I took a screenshot of the speeds most likely used in driving or riding.
[ Invalid Attachment ]
In it, the topmost row is the vehicle's forward speed. The leftmost column is actual wind speed hitting the vehicle at a 90 degree angle. The data in the cells is effective wind speed and angle.
The upshot is that you do want to "outrun" the wind to minimize the angle at which the effective wind is hitting the vehicle (and thus the force of the wind hitting the vehicle at a close to 90 degree angle), but if you go too fast, the angle will be small enough that the vehicle body will act as an effective wing and produce "lift", which is much worse, since the effective wind speed is always faster than vehicle speed and much faster than actual wind speed.
So, you have to somehow figure out what the "stall angle" of your vehicle is, and attempt to maintain a road speed which is faster than the wind, and that will keep the wind at a shallow angle to the vehicle, but not so shallow that the "wing" is no longer stalled.
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I drove an 18 wheeler for a few years. If you are loaded, you will have less of a chance to turn over than if you are empty. If the wind is hitting the rig broadside, 50 mph wind speed on an empty trailer might turn it over.
Going slower will give you more time to react to the start of a jackknife.
I also have a commercial pilots license. In bad turbulence, in flying, one slows the aircraft to reduce stress on the wings and control surfaces from the turbulence.
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Two different phenomena here. A steady wind doesn't have any effect on a plane except to alter your track over the ground relative to your heading, so you generally maintain the fuel-optimum IAS regardless of wind drift at cruise altitude, but certainly slow down for known or expected turbulence.
Problem with the truck is that it is in contact with the road, which is fine at any speed on a straight road with a 50kt headwind or tailwind, but every so often the road bends, and 50 kt on the beam will produce enough moment to roll some unloaded highsiders, regardless of the forward speed.
There may be a case for driving faster with a gusting sidewind, to prevent lateral "pilot-induced oscillations" from developing, particularly with an articulated truck. But equally, if you have a few mph in reserve, you may be able to pull out of a jacknife.
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I am a truck driver in Colorado, USA. I drive through all kinds of conditions but some of the most dangerous are in Wyoming where wind speeds can sometimes hit 50-75 mph (80-120 kph). I have been doggedly pursuing an answer to this same question. I have even written to professors in Physics at a University... but not a single person has answered this seemingly simple question. We know the sail area of a a trailer as above described to be 477 sq. ft.....and the other dimensions she described are accurate. We need to have a formula where we can plug in numbers to evaluate risk/safety. the variables are the tractor/trailer configuration..... the weight of the load...if the center of gravity for the load is low, mid level or high (based on the subjective opinion of the driver.....The speed of travel.....The wind speed at 90 degrees angle of the trailer.....forget about any other angle than perfect perpendicular wind force. If someone would create an application I could use on my phone to plug in these variables....I and many other truckers would pay for it.
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As I wrote before, there are far too many variables to come up with an equation which would be meaningful.
While some of these variables would be fixed (according to the trailer construction) others would be set by the load, thus remaining constant for that particular journey while others would remain variable throughout the journey.
Trailer fixed variables would be:
Length of trailer.
Trailer deck to roof height.
Rigid or curtain sides or tilt. Flatbeds would add more variables to the load variables.
Anti underrun design: none, bare bars or enclosed bars. I don't know about the US, but in the EU anti underrun is compulsory.
Trailer aero-dynamics.
Number of axles.
With or without rear-steer.
Pneumatic adjustable suspension.
The towing unit would also add variables dependant on its aerodynamics and position of the 5th. wheel, both of which will effect airflow over, under and around the trailer. The height of the 5th. wheel, relative to the rear axles of the trailer would set the trailer attitude variable. However, this attitude variable of the trailer and its height from the road will be further effected by the load distribution.
Next come the load variables:
Total weight of load.
Front - back centre of gravity.
Height of centre of gravity.
While the total weight of the load is easy to determine, The centre of gravity could be extremely difficult, if not nigh on impossible to determine, especially with a mixed freight load. Since the centre of gravity would be one of the most important variables to establish, any error in this factor would render any equation worthless.
If you were to load 23t in Denver and drop off 4t in Laramie, then a further 3t in Casper before continuing to your final destination in Buffalo, your total weight and your centre of gravity will change at each drop off point. Add to that a 2t load from Douglas and a further 4t load from Kaycee and you'll spend more time calculating than driving.
Lastly are the journey variables and these can change from second to second and yard to yard.
Wind speed.
Wind direction.
Road direction.
Turbulence caused by the lie of the land and even the bow wave of other vehicles.
G force exerted through bends.
One variable here would not only be constantly changing but changing so fast and to such a degree, not to mention totally unpredictable is wind speed. While the wind speed may be fluctuating at +/- 10mph around 40mph, there can always be a 'freak' gust of 60+ mph.
As a professional driver, I'm sure that like me, you know the 'feel' of your truck. You can only use your experience to tell you when conditions are becoming such that you need to slow down and be ready for anything.
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I'm sorry, but you are over thinking it! Truckers don't need an answer that is exact...we just need a "ballpark" answer to help us evaluate the situation that we are currently experiencing.... forget adding/dropping partial amounts....forget road direction and wind direction....just figure on an absolutely perpendicular wind force. For instance... I know from experience that a 35mph wind can blow over an empty 53 ft. trailer that is attached to a truck......but what if the trailer has 25,000 lbs. of freight?....What is the wind speed that will blow it over?...and how do you figure it? We aren't mathematicians... and we know the formula won't be an exact numerical statement.....even our Bills of Lading that show a stated weight are commonly fabrications given to garner a specific price for shipping......rarely do any of us actually get to go over a scale other that the ones at ports of entry.....all we do is a best guess....If I think my weight is "x" lbs....how much wind speed in miles per hour will it take to blow me over if I am standing still?