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Post by the light works on Aug 22, 2014 17:56:05 GMT
people continually disparage small cars with the threat that the slipstream from a truck will "blow it off the road"
however - I think the situation is actually more noticeable the opposite way around - a truck with a lot of "sail area" will actually feel MORE disruption from the encounter than the small car will.
the question is, is this a telegenic topic, and how can we test it?
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Post by wvengineer on Aug 23, 2014 0:55:42 GMT
I will defantly agree that out of the car I have owned, my Subaru Outback and Dodge Grand Caravan are/were much more affected by crosswinds than my traditional cars.
I wonder if a lot of it could be in the car design. A smaller car, sure has a smaller cross section to be affected by wind, but lower wight, smaller tires, looser suspension, etc. I remember a few years back esting mid sized cars and taking a TOyota Camry out. First trip was a mid-line with a I4 engine. I remember going over a hill that was windy that day and getting blown around a bit. Later for fon, the wife and I took out o Sport version with a V6 engine for fun. It was out of the budget, but hey, why not? I remember taking it on the same course and not getting blown as bad on the hill. I wonder how much having a bit more weight in the engine, wider tires and stiffer suspension helped to reduce that sway.
As far as testing, my first guess would be to set the car on a test track and put load cells (The type that cops use to check trucks to see if they are over weight at traffic stops) under each tire and then run a large truck next to it at speed. A large gust of wind on the side of the car will push it sideways, rocking the car on the suspension, changing the weight distribution. it will shift the weight to the outer wheels. By looking at the weight distribution as the semi passes, you can measure how much force it being exerted on the car by the wind.
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Post by the light works on Aug 23, 2014 5:06:05 GMT
I will defantly agree that out of the car I have owned, my Subaru Outback and Dodge Grand Caravan are/were much more affected by crosswinds than my traditional cars. I wonder if a lot of it could be in the car design. A smaller car, sure has a smaller cross section to be affected by wind, but lower wight, smaller tires, looser suspension, etc. I remember a few years back esting mid sized cars and taking a TOyota Camry out. First trip was a mid-line with a I4 engine. I remember going over a hill that was windy that day and getting blown around a bit. Later for fon, the wife and I took out o Sport version with a V6 engine for fun. It was out of the budget, but hey, why not? I remember taking it on the same course and not getting blown as bad on the hill. I wonder how much having a bit more weight in the engine, wider tires and stiffer suspension helped to reduce that sway. As far as testing, my first guess would be to set the car on a test track and put load cells (The type that cops use to check trucks to see if they are over weight at traffic stops) under each tire and then run a large truck next to it at speed. A large gust of wind on the side of the car will push it sideways, rocking the car on the suspension, changing the weight distribution. it will shift the weight to the outer wheels. By looking at the weight distribution as the semi passes, you can measure how much force it being exerted on the car by the wind. I like that, and it is a really easy test to transfer to other vehicles. however, you ahould also drive the car past the semi and check how it affects the semi. in the motorhome, I went through thunderstorms on the leg I was running wednesday - and I had to do quite a lot of correction to stay in my lane. but I also had to correct every time a car passed me I was also thinking about accelerometers. I don't think after the abominable snowplow episode ANYTHING would literally blow anything else off the road - but it would be interesting to test what weight and shape classes are most affected. |
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Post by wvengineer on Aug 23, 2014 11:54:49 GMT
One thing that might make for an interesting variation on the testing in addition to things like car size & type, wheels, suspension, is how much will opening windows affect how wind blows a car around.
One thing I have noticed is that any car that has a softer, more passenger focused (luxury) suspension tends to have worse control in crosswinds. However, stiffer, sport or work suspension tends to be easier to control.
One test might be to outfit a car with a number of side to side accelerometers which you can drive around, and have a several wind machines setup on one part of the test track so you have a known wind force to make your measurements. A variation here would be to put the wind machines on a trailer towed behind a small car (if practical and safe) so you can have a mobile wind source.
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Post by the light works on Aug 23, 2014 13:29:18 GMT
One thing that might make for an interesting variation on the testing in addition to things like car size & type, wheels, suspension, is how much will opening windows affect how wind blows a car around. One thing I have noticed is that any car that has a softer, more passenger focused (luxury) suspension tends to have worse control in crosswinds. However, stiffer, sport or work suspension tends to be easier to control. One test might be to outfit a car with a number of side to side accelerometers which you can drive around, and have a several wind machines setup on one part of the test track so you have a known wind force to make your measurements. A variation here would be to put the wind machines on a trailer towed behind a small car (if practical and safe) so you can have a mobile wind source. I would say put them on a light truck chassis if you wanted to do that. - or a parade float chassis. (since those spend a relevant portion of their lives with no bodywork) |
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Post by wvengineer on Aug 23, 2014 23:42:13 GMT
My main concern about placing the wind machine(s) on a trailer would be the blowers blowing themselves off the road if they are powerful enough. You would need to make sure that the trailer has some weight to it.
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Post by the light works on Aug 23, 2014 23:52:26 GMT
My main concern about placing the wind machine(s) on a trailer would be the blowers blowing themselves off the road if they are powerful enough. You would need to make sure that the trailer has some weight to it. that was my concern, too - which is why I recommended a truck chassis, rather than a trailer - one less point of articulation for things to go strange with. *note: by light truck, I meant as opposed to tractor-trailer, rather than lightweight. |
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Post by silverdragon on Sept 2, 2014 8:35:35 GMT
Depends on the trailer, the load, and what is passing.
Light trailer curtain sider, its common to open the curtains in windy conditions to stop the trailer getting blown about if its empty.
If the trailer is full, it aint moving by traffic disruption alone.
In High winds, if passed by a large vehicle, that large vehicle will change the winds hitting the trailer, that may cause handling anomalies.
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Post by the light works on Sept 2, 2014 15:04:52 GMT
Depends on the trailer, the load, and what is passing. Light trailer curtain sider, its common to open the curtains in windy conditions to stop the trailer getting blown about if its empty. If the trailer is full, it aint moving by traffic disruption alone. In High winds, if passed by a large vehicle, that large vehicle will change the winds hitting the trailer, that may cause handling anomalies. wind shear can be significant, but I am thinking more of just the bow wave hitting an enclosed trailer. |
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Post by silverdragon on Sept 5, 2014 6:54:04 GMT
There isnt any great "V" shaped bow wave in front of a truck.
Its more a "U" shaped wave, as it bends round the truck.
Air is better at being compressed at low speeds, this is why in UK, Trucks travel at 55mph, and dont blow each other off the road?.....
Same truck at 90, you are going to feel that bow wave if you get too close...
Perhaps that needs testing.... the same as the train suck thing, what would a vehicle passing at moderate speed do and again at high speed.
I cant see much of a problem at moderate 55mph speed or below.
In high winds, all sensible transport (Us truckers) either slow down or get off the bloody road.#
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Post by the light works on Sept 5, 2014 14:16:39 GMT
There isnt any great "V" shaped bow wave in front of a truck. Its more a "U" shaped wave, as it bends round the truck. Air is better at being compressed at low speeds, this is why in UK, Trucks travel at 55mph, and dont blow each other off the road?..... Same truck at 90, you are going to feel that bow wave if you get too close... Perhaps that needs testing.... the same as the train suck thing, what would a vehicle passing at moderate speed do and again at high speed. I cant see much of a problem at moderate 55mph speed or below. In high winds, all sensible transport (Us truckers) either slow down or get off the bloody road.# but you are still off the point of what happens with your box truck when you are sitting at an intersection and a Smart goes past in the other direction at 55 MPH. how does that compare to what happens if it is the Smart that is stopped and you moving? |
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Post by silverdragon on Sept 6, 2014 16:24:46 GMT
There isnt any great "V" shaped bow wave in front of a truck. Its more a "U" shaped wave, as it bends round the truck. Air is better at being compressed at low speeds, this is why in UK, Trucks travel at 55mph, and dont blow each other off the road?..... Same truck at 90, you are going to feel that bow wave if you get too close... Perhaps that needs testing.... the same as the train suck thing, what would a vehicle passing at moderate speed do and again at high speed. I cant see much of a problem at moderate 55mph speed or below. In high winds, all sensible transport (Us truckers) either slow down or get off the bloody road.# but you are still off the point of what happens with your box truck when you are sitting at an intersection and a Smart goes past in the other direction at 55 MPH. how does that compare to what happens if it is the Smart that is stopped and you moving? "Might is Right"... The larger Box truck will produce more air movement, thats a "Known", of course. My heavy truck will not waiver under the waft of a slight breeze from a Smart car, but at the other end of the scale, a pedestrian at the edge of the pavement will feel the air move substantially?... The heavier the vehicle in actual weight and axle weight (the two may be different) decides how the vehicle may react. A Large truck at speed passing very close to a large truck standing still, if the stationary one is empty, the trailer will wobble a lot. If is just the Cab unit, no trailer, even though that makes the gross weight lighter, the cab unit will not wobble at all, because it has a greater axle weight. Therefore, in that hypothesis, A Small heavier car will not be affected as much as a large empty van, even if the small heavy car is not as heavy as the large van?... (Confused yet?... read on...) This has much to do with the maths bit equation that determines the area of Bodywork against weight.. the more area, the more the wind will affect, its effectively a sail area thats reacting to the breeze?... Go back also to the empty trailer, if that is a flatbed, no sides at all, even when empty, passing traffic will not wobble it much. So to put this to the test, I can explain it with a simple thought experiment. Take one sail ship in a good strong breeze... whats it speed?... Now raise some sail, is it different?.. of course, it has more surface area reacting to the wind. The difference between the small vehicle and large vehicle moving is how much air has to be displaced to let it move, and where does that air go. |
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Post by the light works on Sept 6, 2014 16:32:02 GMT
but you are still off the point of what happens with your box truck when you are sitting at an intersection and a Smart goes past in the other direction at 55 MPH. how does that compare to what happens if it is the Smart that is stopped and you moving? "Might is Right"... The larger Box truck will produce more air movement, thats a "Known", of course. My heavy truck will not waiver under the waft of a slight breeze from a Smart car, but at the other end of the scale, a pedestrian at the edge of the pavement will feel the air move substantially?... The heavier the vehicle in actual weight and axle weight (the two may be different) decides how the vehicle may react. A Large truck at speed passing very close to a large truck standing still, if the stationary one is empty, the trailer will wobble a lot. If is just the Cab unit, no trailer, even though that makes the gross weight lighter, the cab unit will not wobble at all, because it has a greater axle weight. Therefore, in that hypothesis, A Small heavier car will not be affected as much as a large empty van, even if the small heavy car is not as heavy as the large van?... (Confused yet?... read on...) This has much to do with the maths bit equation that determines the area of Bodywork against weight.. the more area, the more the wind will affect, its effectively a sail area thats reacting to the breeze?... Go back also to the empty trailer, if that is a flatbed, no sides at all, even when empty, passing traffic will not wobble it much. So to put this to the test, I can explain it with a simple thought experiment. Take one sail ship in a good strong breeze... whats it speed?... Now raise some sail, is it different?.. of course, it has more surface area reacting to the wind. The difference between the small vehicle and large vehicle moving is how much air has to be displaced to let it move, and where does that air go. ok, you did finally get down to sail area - althoug with that, density is also a factor. (I know, weight and axle weight are also related.) as far as axle weight - say you take one of our large chip trailers - both tractor and trailer have tag axles to save on tires when running empty - I am interpreting your statement about axle weight to be that if you take the same truck, it will be more affected by wind gusts with the tag axles down than with the tag axles up - because the axle weight is less. not sure that is accurate, because their is also the factor of contact - two more sets of tires, and air springs, should add more suspension stiffness to counteract windage. (my understanding is the suspension on the tag axles is air springs with some sort of return spring, so when you vent the pressure from the air spring it lifts the axle) |
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Post by silverdragon on Sept 6, 2014 16:50:06 GMT
Its a Grip thing also.
Yes wear, tag axles are raised for cornering wear, two axles going round a corner drag the tyres sideways, raising axles stops that wear, and gives more grip to the axle doing the work, so, less likelihood of it getting blown by wind.
But not that much..... its what happens with a huge side wind to the wagon as a whole that gets troubling....
The large sail area above the axle will determine the force collected that acts on the tyres, if there is no weight, it will "Lift" one side and remove grip under the tyres, its almost tilting the vehicle over, the suspension does not have that ability. the tyres will just lift up.
Putting all the force on one side of the suspension, that does play havoc with your wardrobe.....
Once the wind gets a big volume of air UNDER the trailer, then you have whats known as a "Box Kite".....
This doesnt happen often on the roads.
Its not exactly side winds when one vehicle passes another, in different directions, its a headwind on one side of the wagon.
Overtaking, mostly the wagons pass good and slow, if I am at 55 and passing a slow moving wagon doing under 25, I will pass either very wide, or slow down a bit.
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Post by the light works on Sept 6, 2014 17:01:54 GMT
Its a Grip thing also. Yes wear, tag axles are raised for cornering wear, two axles going round a corner drag the tyres sideways, raising axles stops that wear, and gives more grip to the axle doing the work, so, less likelihood of it getting blown by wind. But not that much..... its what happens with a huge side wind to the wagon as a whole that gets troubling.... The large sail area above the axle will determine the force collected that acts on the tyres, if there is no weight, it will "Lift" one side and remove grip under the tyres, its almost tilting the vehicle over, the suspension does not have that ability. the tyres will just lift up. Putting all the force on one side of the suspension, that does play havoc with your wardrobe..... Once the wind gets a big volume of air UNDER the trailer, then you have whats known as a "Box Kite"..... This doesnt happen often on the roads. Its not exactly side winds when one vehicle passes another, in different directions, its a headwind on one side of the wagon. Overtaking, mostly the wagons pass good and slow, if I am at 55 and passing a slow moving wagon doing under 25, I will pass either very wide, or slow down a bit. if wind shear is going to cause you to skid out of control, there's something wrong with your tires. (or you have bigger problems than just whether to have your tag axles down or not.) |
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Post by silverdragon on Sept 7, 2014 7:03:04 GMT
How do you define wind shear.....
If its a light gust, no problem.
We get 60mph gusts here on exposed stretches of motorway quite regularly.
Barton High Level over the M60 is shut to high sided vehicles frequently.
But then its not a skid problem, its a blow-over, as I said, if you get wind under that trailer, it becomes a box-kite.....
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Post by the light works on Sept 7, 2014 14:28:56 GMT
How do you define wind shear..... If its a light gust, no problem. We get 60mph gusts here on exposed stretches of motorway quite regularly. Barton High Level over the M60 is shut to high sided vehicles frequently. But then its not a skid problem, its a blow-over, as I said, if you get wind under that trailer, it becomes a box-kite..... for this exercise it is the pressure wave generated by a vehicle in motion, assuming 55 MPH as a good average difference in speed between a stopped vehicle and a vehicle passing in the opposite direction. - with effect judged as perceptible motion from the driver's seat of the stopped vehicle. |
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Post by silverdragon on Sept 8, 2014 8:48:27 GMT
Ok, so, the parameters we wish to work with....
1) its stationary vehicles at the side of the road, and test a line of vehicles there to see what size/weight ratio is affected most.
2) the passing truck will be a average box trailer Semi, 60ft trailer, to maximise air displacement, 13ft high or more, air spoilers, make it a good average truck.
3) the passing wagon will be at Highway/Interstate legal speeds, 55mph max... unle3ss you know there is a legal maximum higher for "Semi" articulated vehicles?.. its definite no to that over here.
3.5) Additional, if on a closed road, take the limiter off, and blast past at as fast as you can just for the last run to see what you can get..... some trucks drive "Dirty", they can disable the limiter, this may be a way to show how dangerous that is, and why?... public education into WHY we have speed limits for trucks that are lower than Cars.
4) no weight (luggage) other than as supplied in each parked vehicle... of course no persons, but Buster may be allowed in the drivers seat, and thats it.
5) the wagon to make several passes closer and closer and the movement of each vehicle noted.
6) the test to be done on a calm day, no existing side wind above a gentle breeze, acceptable local climate conditions.
My suspicion is that you will be taking door mirrors off before you get any movement of the parked vehicle from where it was parked...
It may wobble, but the tyres wont be scraped sideways.
Other variables....
Can this be done in the rain?... I suspect a rain shower may show the forces involved and how the vehicles are pushed/pulled by the wind?...
I also suspect Venturi effect.....
As the Big truck passes, it will not "Blow", but "Suck" the car at the side of the road.
I know this to be true, I have felt the pull of a truck passing when stood at the side of the road.
So you have an initial bow wave that may rock the car away from the truck, but then a venturi suck that will rock it back towards the truck.
I am also quite sure that Wing Mirrors will be the highest damage, may I suggest that the Truck be fitted with guards to make sure the mirrors snapping off dont get into anywhere important?.... like Tyres?....
I WANNA DRIVE THE TRUCK...............
Why?...
Because I can not in any way tell you how much I have been tempted....
I have passed a line of cars parked on a do not park zone many times, its SUPPOSED to be an access road, but they park there, It makes that road very narrow, especially when one truck is leaving as another arrives?..
And how often am I tempted to just take a wing mirror or two off as I go past?...
Of course, I am doing that road at between 10 and 20 mph, max, often you stop to let other trucks pass.
Professional pride says six inch is too close to the mirrors... but how close can I get, can I just prune a few mirrors with no other damage?...
Final suggestion, break out that snow plough again.
Please.
Please please please please?....
I cant tell you how much I enjoyed seeing a double line of vehicles "Parted" by a big plough in the last series....
How many can you do before it stops the truck?...
Can you put a bet on how many will be done?.. 30, 40, 50, 60......
My suspicion is with the right truck and 1,000 horses pulling, it wont be stopped.
Anyone else want to add to this?....
Side test... Type of Wagon, skirt or no skirt.
On UK roads we are seeing an increase of trucks with side skirts and other aerodynamic additions.
Especially the long distance motorway high speed "Trunkers" that spend all their life on Motorways.
Does that make them slip through the air better, thus causing less disturbance?....
Its supposed to be good for fuel economy, there is a lot of evidence to support this, but does it make a difference to what we are testing.
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Post by the light works on Sept 8, 2014 14:04:39 GMT
In Montana, the daytime speed limit on non-freeway roads is 70 MPH, period. - they believe the driver should be able to judge for himself or herself what speed is reasonable and prudent for the conditions. (the reason for the limit is to stop motorheads from making Montana a destination for stupid driving)
otherwise, the maximum speed test would be a crossing speed of 110 MPH (each vehicle closing at 55 MPH, which is the average rural road speed)
my reason for testing with one vehicle at a standstill is that you don't have to try to isolate road bump from the aerodynamic effects.
and still keep in mind - you are still thinking of whether the truck will blow the car off the road - I am thinking of whether the truck will feel it more than the car.
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Post by silverdragon on Sept 9, 2014 11:49:51 GMT
Two vehicles passing head on.... close?...erm.... I think dedicated "Stunt" drivers for that?....
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