I'm covering safe driving in adverse weather conditions in one of my DCPC sessions at the moment and we had an interesting discussion the other day.
Imagine...an artic curtain sider, empty going due North on the M6.
A howling gale is blowing from the East to West.
What action can you take to minimise the risk of being blown over?
My answer, and what I always did was to slow down. The theory being that I only ever went as fast upright as I was prepared to go on my side.
It would appear though, that a vast number of drivers use an entirely different theory and I need to disprove it (or not) logically but in words both myself and they will understand.
Their theory is that the faster you go forwards minimises the risk of being blown over.
Reasons: A bit sketchy but among the reasons I've been given has something to do with forward propulsion being greater than sideways force!
I really need to be able to back up my theory with some facts and a diagram, if it's right!
TIA
Pat
|
But you can see the problem with that theory, can't you? It's not either/or. Doesn't change the sideways force on the trailer in any predictable way, result being it's just going faster when it falls over.
Reducto ad absurdum, does the same theory say it's more dangerous to park up than to be moving?
That theory is in the same category as speeding up when your caravan starts to snake.
Apologies for answering NC's question. Good morning. Just about to set off for my speed awareness course!
|
>> Imagine...an artic curtain sider, empty going due North on the M6.
>> A howling gale is blowing from the East to West.
>> What action can you take to minimise the risk of being blown over?
As it's empty, reduce the side area of the trailer by opening all the curtains.
|
Not always possible on some newer trailers L'es due to the costruction of them.
Some have been known to lose the roof when doing this.
Some firms also prohibit doing this.
Best of luck Manatee, I face 15 of our loudest drivers today, who like most lorry drivers ( me included) know everything:)
Pat
|
I'm not NC either but I have a couple of ideas of what your drivers may be thinking that may help your argument.
- Downforce: F1 and other racing drivers rely on this to keep them on the track at speeds where tyres alone can't provide enough grip, but it requires a lot of speed to produce anything useful. As I understand it, there are corners that would be dangerous at, say, 80mph, that become possible at 110 because of the greater downforce. Do I think a truck, even if it had the right aerodynamic accessories, would produce enough to stay upright in a gale? Frankly, no.
- Caster effect: What keeps a bicycle or a rolling coin upright until it slows down to the point where its inherent instability takes over. But do you ride your bike faster on a windy day to increase stability? I don't think I do. You could do a simple experiment with a coin and a hairdryer; I think the coin still will fall over, and I think a 40-tonner will too, if the wind is strong enough.
Anyway, best of luck.
};---)
|
Well Pat,one of us is telepathic.
Last Sunday this question occurred to me when crossing the Britannia Bridge (A55), there was a negligible cross wind that day (Westerly) - my approach on an exposed section of road like this is to cross as quickly as possible on a bike, I kidded myself that the gyroscopic effects of riding at speed helps keep the bike upright - feels right for me anyway. Fastest across that bridge was a 110mph on a fully faired CB650 many, many years ago.
|
You've hit the nail on the head RP.
We all make out we're not scared in bad road conditions but on ice, I always pondered the problem of does lowering the lift axle mean more tyre=more grip on the ice, and in gales I pondered the above.
What actually happens is that the intense concentration on the mathematics of the problem takes our mind of the 'what if's' that would otherwise come into our minds.
Thanks W de B, that will at least suffice for todays crowd, they spare me no mercy whatsoever;)
*off to pack hairdryer*
Pat
Last edited by: pda on Fri 2 Mar 12 at 08:08
|
off to pack hairdryer*
:-)
|
Having seen film of lories being blown over, while stationary or moving I would have thought everyone is missing an important point.
If you are traveling slower you usually have more time to try and recover a near blow over, and less chance of landing on some poor sap coming the other way.
Also the poor innocent sap has more time and space to try and avoid being flattened.
Last edited by: Zero on Fri 2 Mar 12 at 08:24
|
Is the info on the left of this page any help?
www.forthroadbridge.org/weather
Last edited by: Old Navy on Fri 2 Mar 12 at 08:58
|
The way this is dealt with in theory, and in devising tests in wind tunnels is to construct a vector diagram of the wind velocity vector relative to the ground, the vehicle's velocity vector relative to ground, and then, via taking the resultant of the two vectos, obtaining the velocity of the air relative to the vehicle.
This means that if there is a significant tailwind component to the sidewind, yes, going faster will reduce the wind velocity relative to the truck.
In all other cases though, increasing the vehicles speed will increase the wind velocity relative to the truck, and increase the aerodynamic force acting on the vehicle.
I do tend to agree that in the vast majority of cases, it's a sensible and pragmatic solution to slow down.
I hope I'm not too late to have been of use!
|
|
That makes sense in a marine environment too. Where it is called relative wind speed, you must add or subtract your speed to or from the true wind speed (depending on its direction) to get the wind speed felt.
Last edited by: Old Navy on Fri 2 Mar 12 at 09:08
|
|
Depending of the size of the ship you slow down in severe weather.Smaller vessel you have to power into the sea in storm the waves will take you.
|
Thanks NC, not too late at all.
I think I like the idea that it has to be a 'seat of the pants' decision taken at the time.
It's what good drivers always go by anyway;)
Pat
|
|
Now I do do seat of the pants ! Dynamic risk assessments and all that !
|
You do seat of the pants on a bike all the time!
Pat
|
>> You do seat of the pants on a bike all the time!
Except of course when you are flying over things or doing wheelies.
Or a highside.
|
>> I think I like the idea that it has to be a 'seat of the
>> pants' decision taken at the time.
What about if you're "going commando"?
|
>> >> I think I like the idea that it has to be a 'seat of
>> the
>> >> pants' decision taken at the time.
>>
>> What about if you're "going commando"?
>>
Just the seat then!
|
|
As an ex commando, i can assure you that, unlike the Jocks, we did and do, wear undergarments!
|
>> This means that if there is a significant tailwind component to the sidewind, yes, going
>> faster will reduce the wind velocity relative to the truck.
But isn't the sidewind component you're worried about the component perpendicular to the truck, which isn't going to change no matter what speed you're doing?
Eg. 50mph wind at 60 degrees to the side of the truck has a 25mph perpendicular component trying to blow it over, independent of the truck's speed?
And of course as Z says, if/when it does blow over, better for all concerned that the truck speed is low.
|
>> But isn't the sidewind component you're worried about the component perpendicular to the truck, which isn't going to change no matter what speed you're doing?
Sorry, I could only type out the first part of the answer because I had to be elsewhere at 9 this morning.
The value of doing the vector analysis is that it allows both the magnitude and the direction of the relative velocity vector to be found.
The problem is that as the direction of the relative velocity vector changes, the position of the centre of pressure on the vehicle changes, and the aerodynamic flow patterns can suddenly jump from one configuration to another, with an associated jump in the vehicle reaction forces and moments.
However, some of the results of testing show some interesting results - for cars, for example, the side force coefficient reaches a maximum when the angle of the relative velocity vector is 60 degrees - NOT 90 degrees as you might initially think.
Another interesting point though is that the vehicle's response to the wind in terms of reaction forces and moments is proportional to the relative velocity vector's magnitude squared. Which means that if you are driving along close to the vehicles limit of stability, it doesn't take a large gust to make the vehicle unstable.
|
Pat,
This doesn't answer your question, but there may be something of interest to you. tinyurl.com/787gfqj
|
he vehicle's velocity vector relative to ground, and then, via taking the resultant of the two vectos, obtaining the velocity of the air relative to the vehicle.
That's what I do every time the wind blows (I think)
|
Thanks L'es, I found that site the other day but I wish they would still send them to us FOC as they used to.
I download and print them to hand out to the drivers and it was this
>>Lower speeds improve vehicle handling in high cross winds<<
that prompted the lively discussion.
I try really hard never to say 'Well, it's worked for me for 30 years' and try to embrace their ideas and solutions too, and then come up with a logical theory.
It stops me getting old...I hope!
Pat
|
Forgot to say that there are some excellent booklets on L'es link that are worth a read for everyone too.
Pat
|
Pat.
I remember being told the plates fitted to the front corners of trucks were there to aid stability in side-winds.
It would therefore appear the manufacturers have done some research into this. Couldn't you approach them?
|
|
If it's a curtain sider, and empty, removing the curtains would reduce the probability of its being blown over.
|
>> If it's a curtain sider, and empty, removing the curtains would reduce the probability of
>> its being blown over.
>>
You're 5.1 hours too late!
Last edited by: L'escargot on Fri 2 Mar 12 at 12:00
|
My instinctive feeling is that it depends whether the wind is gusting or not.
If the wind can be relied upon to blow from a constant direction with a constant force, then perhaps it would be safer to go as fast as possible. By analogy, if you are in a small boat trying to counter a strong sideways tidal force, you have more chance of avoiding being swept away if you go at full speed.
But in a real situation of an unpredictable cross wind, the amount of counter steering you need to do to balance the wind will be liable to vary, so there is a strong likelihood of being caught out with insufficient steer, or too much if the wind suddenly slackens or changes direction. All these split-second changes will be more dangerous the higher the speed.
And if you get it wrong it will be much more difficult to counter a swaying swerving oscilation if going faster - exactly like the caravan snaking example.
|
>>?If the wind can be relied upon to blow from a constant direction with a constant force, then perhaps it would be safer to go as fast as possible. <<<
This is obviously correct, because as the speed increases, journey time decreases. At the limit, we have a zero time journey, hence the probability of the lorry falling over tends to zero.
Analagous to the old conundrum of how to cross a truly blind junction.
The only flaw, of course, is that magnitude of the 'nondesired outcome' increases!
|
>> You're 5.1 hours too late!
Supersnail!
|
|
.
Last edited by: L'escargot on Fri 2 Mar 12 at 12:18
|
|
I thinl L'Es was going to explain that some creatures don't have this problem.
|
|
Not snails, though - they're just high-sided slugs.
|
>> .
>>
Les's contribution got blown over.
|
|
If truck tyres had suction cups on the tread (like us gastropods) the trucks wouldn't blow over.
|
If trucks mostly have stability problems if running with empty trailers (or low weight cargo like cereals).... then how about designing a trailer with integrated water ballast tanks. Fill them up when trailer is light/empty and empty the ballast tanks when you're taking a full load.
If the depots had the infrastructure for this no water need be wasted as it could be put back in the tanks at the depot.
Probably a barmy idea I know. But if there as an accident involving these trucks there'd also be a ready supply of water for any fires.
|
>> Probably a barmy idea I know. But if there as an accident involving these trucks
>> there'd also be a ready supply of water for any fires.
And you have just solved the souths drought problem.
|
>> how about designing a trailer with integrated water ballast tanks. Fill them up when trailer is light/empty
Increased weight = increased fuel consumption & wear = increased emissions.
Operators won't like the extra cost, no-one will like the extra emissions :(
|
|
Maybe not - but then you'd find two of them side by side on the A1. Imagine the tailback!
|
>> Maybe not - but then you'd find two of them side by side on the
>> A1. Imagine the tailback!
You dont need to imagine the common place.
|
What about having laterally-positionable stabiliser wheels? You just click on where you want them to be, and that's where they'll be whether you can see them or not!
;-)
|
|
Was that your usual lettuce at lunchtime, l'Es?
|
Another problem with going faster is that the involuntary lane change when you are blown across the road happens too quickly, happened to me one night donkeys years ago in that dip just before jct 9 M1 Northbound, went from inside to middle lane and i couldn't do a thing about it.
That water tank idea is brilliant.
|
>> That water tank idea is brilliant.
Is it? Somebody should patent and sell it :-) I will call it iWater
|
>>You could do a simple experiment with a coin and a hairdryer;
I missed this earlier. This can't tell you anything useful about how a truck reacts to a sidewind - there's no similarity between the dynamic systems. The whole of the mass of the coin contributes to its gyroscopic stiffness - where for atruck, gyroscopic effects are negligible.
|
I wasn't really suggesting it would, NC. I was trying to imagine what factors that increase with speed Pat's drivers might be thinking would help them if they went faster.
The hairdryer was just a bit of fun - but if a lot of gyroscopic stiffness (new term to me) can't save a coin from a hairdryer, it seems fair to think that speeding up to increase a relatively tiny gyroscopic effect won't help keep a truck upright in a gale. I realize - as I'm sure Pat and her drivers do - that, unlike a coin or a bicycle, a truck doesn't just fall over unless something pushes it.
Anyway, I was only filling in till you got here.
}:---)
|
>>The hairdryer was just a bit of fun
Quite so
- but if a lot of gyroscopic stiffness (new term to me)
A fairly stark demo of this uses a bicycle's front wheel (removed from the bike). If you try to hold the axle of the stationary wheel horizontal using just one hand, you'll struggle to apply enough moment to stop the wheel drooping. If, however you spin the wheel up first, it's really easy.
|
More fun if you do it while sitting in a revolving chair.
;>)
|
Help Number_Cruncher please. - Manatee
