Tipping Trucks: Does Speed Matter?

[JohnDubYa]

Is a truck easier to tip over in a crosswind if it is traveling at a high speed, or lower speed?

As far as I can tell, it makes no difference. But this doesn't seem logical.

If anyone knows where a similar problem has been worked out, I would love to see it.

 

[Mr. Robin Parsons]

Probably depends upn the angle of the truck, to the wind, inasmuch as it probably works out a little like sailing a boat...and they will sail into the wind, or pretty close to directly into it...it's a lead/clue...

 

[JohnDubYa]

Assume a perpendicular wind.

I don't think the sailboat analogy works, because here we are only concerned with the truck tipping over.

 

[NateTG]

Originally posted by JohnDubYa
Is a truck easier to tip over in a crosswind if it is traveling at a high speed, or lower speed?

As far as I can tell, it makes no difference. But this doesn't seem logical.

If anyone knows where a similar problem has been worked out, I would love to see it.

In practice the difference is probably negligible, but I would guess that the truck is more difficult to tip at a higher speed.

There are two different ways that the truck can react to cross wind: It can either slip, or it can tip. Increasing the forward speed of the truck makes it more likely to slip.

Consider, for example a crosswind that is sufficient to tip a stationary truck. Now, if the truck is going forward so fast that the tires barely hold to the ground. Then, when the crosswind hits, the truck will slip to the side instead of tipping.

 

[Mr. Robin Parsons]

Originally posted by JohnDubYa
Assume a perpendicular wind.
I don't think the sailboat analogy works, because here we are only concerned with the truck tipping over.

OK, but the sailboat has relevance in the real world, but it works nicely here to do it simply...answers are (usually) easier to find...

Also the speed of the truck would be very relevant to the Sailboat example, inasmuch as, it becomes a combination of the wind effects, that topples the truck...as the manner of the trucks generation of wind plays into its "tip'ablity"...and, is it a semi? or attached?
(one piece truck)

 

[JohnDubYa]

But don't you find it counterintuitive that the forward speed has no effect on tipping?

 

[JohnDubYa]

I don't think the sailboat analogy works because, if anything, the sailboat is a more complicated system. Why substitute a relatively easy system for a complicated one?

Rather than a truck, consider a sheet of stiff plastic attached to four tiny wheels so that the plastic extends vertically. Now, is it easier to blow the plastic over if it is standing still, or when it is moving? Assume a perpendicular wind.

 

[Njorl]

A moving truck has angular momentum stored in the tires. Tipping would have to overcome this. This makes it that much harder to tip the moving truck. The engine and drivetrain also store angular momentum, but that vector would not rotate upon tipping, so it does not enter into the problem.

You can treat the situation as if the truck is stationary. Change the wind to a diagonal one, such that the horizontal component is equal to the true wind velocity. I don't see this as making the truck more likely to tip though.

There are practical, non-ideal concerns too. The moving truck is likely to encounter transient effects, like a bump. The energy supplied by that bump might be just the difference between tipping and not tipping. This may nt be important to a physics student, but it is probably important to a trucker.

Njorl

 

[Rappaccini]

There is no such thing as a wind that blows in one direction for more thana few seconds, but, assuming that however the truck moves, the wind remains perpendicular, we have a case...

The work performed by the truck against the wind is

Displacement * Force = Mag(dis)Mag(For)cos(theta)

Theta is the angle between the two vectors. Since the truck is perpendicular, theta is zero and the work is zero.

Therefore, the truck is not having to fight the wind. Moving, it doesn't have to put forth any work to maintain its course.

So the velocity of the truck is of no direct relevance.


Yet, it is of an indirect relevance.
Just because the wind is always perpendicular length-wise does not mean that it has also to be perpendicular height-wise.
If the angle of the wind, height-wise, varies, the truck could be "tipped".
The forces that would prevent this "tipping" are gravity and the friction of the tires.

As the velocity of the truck increases, the friction of the tires to the ground will lesson, and "tipping" will grow likelier.

 

[JohnDubYa]

RE: "A moving truck has angular momentum stored in the tires. Tipping would have to overcome this. "

Which is why my plastic-sheet-mobile has tiny tires.


Ignoring the tires, can we then say for sure that the speed of the vehicle has no effect on its stabilty in a crosswind?

I don't see how a vertical bump affects the problem.

 

[Njorl]

Originally posted by JohnDubYa
RE:

I don't see how a vertical bump affects the problem.

If the truck were on the verge of tipping onto the passenger side, and the bump was only on the driver's side, the truck would tip.

Njorl

 

[Rappaccini]

Ignoring acceleration's and angular momentum's respective effects on the friction of the tires, there is NO relationship, JohnDubya.

I do not think it is helpful for us to consider the bump, which is an upward force, scenario. It only complicates things.

 

[JohnDubYa]

RE: "There is no such thing as a wind that blows in one direction for more thana few seconds,"

You obviously haven't lived on the West Coast. :)

RE: "The work performed by the truck against the wind is Displacement * Force = Mag(dis)Mag(For)cos(theta). Theta is the angle between the two vectors. Since the truck is perpendicular, theta is zero and the work is zero."

Your argument applies to the net work, not the work done by the truck. An airplane can take a perpendicular path into a crosswind, but it does work on the wind because the propellors have to apply a force non-perpendicular to the wind. So does the truck.

 

[Mr. Robin Parsons]

Never driven a truck either, clearly, the truck itself generates wind vectors and velocities that correspond to it's motion relative to the wind, you could easily be generating a vacuum pressure on the other side of the truck because it is moving forward, that might just help tip it real fast...so is it a constant windspeed, and again on a semi, or straight body truck?

 

[JohnDubYa]

Let's assume a single truck, (say), a UPS truck.

I don't see how the velocity of the truck would create a relative vacuum on one side of the truck as opposed to the other.

 

[Mr. Robin Parsons]

Let's assume a single truck, (say), a UPS truck.
I don't see how the velocity of the truck would create a relative vacuum on one side of the truck as opposed to the other.

Yup, OK, Uhmm, I have driven lots of trucks, the wind flows around the truck in an alternating manner, windspeed shifting higher/lower, changng from side to side, Heck drove a salt truck, dropped salt onto the road, so I could easily see the wind patterns behind the truck as the salt followed it, it makes like an S pattern behind it as it shifts from one side, high speed, to the other side, high speed...

When driving, it is around 55 MPH that the air (molecules in the atmosphere) stops being 'split apart' (no, not 'atomically' split) and the vehicule starts pushing air in front of it...this too, would make a difference...

Anyone want to test this out?

 

[JohnDubYa]

How about taking a box and pulling it horizontally under water?

 

[Mr. Robin Parsons]

How about taking a box and pulling it horizontally under water?

Well sounds really good but relevant to the reality of the, well "real world" it would teach us much less, brute forces of physics rule in absolute conditions hence stationary will tip first, in Non-real scenario of "absolutism of condition"...but I suspect, in the real world, the moving truck would tip first, dependant upon conditions...more likely to be subjected to more 'opportunities' then the stationary one...

Have Fun!

 

[Mr. Robin Parsons]

Also it seems that every time I attempt to introduce what would be the "distinguishing factors" between a truck, moving, and not, you attempt to eliminate them as having consequence...which amounts to limiting/constraining the example to the point that it will no longer be a relevant question! soooo...

 

[jdavel]

JohnDubYa,

I have to agree with Mr. Robin Parsons in the preivous post. You're making your question such a frequently moving target, it's too frustrating to try to answer it. You started with a truck, then it became a sheet of plastic with tiny wheels, then it turned back into truck (UPS). What next, a rocket engine on the roof pointing sidewise that comes on evertime the truck stops?

How about restating your question based on all the changes you've decided to make since your first post, and then agree not to change it again for awhile.

 

[JohnDubYa]

We need to simplify. I don't want to introduce factors such as the trailer hitch, hub caps, and such. That is why I introduce the flat sheet of plastic on tiny wheels. But everytime I simplify the model, everyone wants to reintroduce complicating factors.

If we can'f figure out the simple sheet of plastic (and I think we have), then we can't even begin to answer the question when we introduce the angular momentum of the wheels. I introduced the UPS truck because everyone seemed to be demanding that we begin the discussion with an actual truck. The UPS truck is the simplest truck I know because it is one piece and very flat-sided.

First you solve the spherical cow. Then you introduce the hoofs.

By the way, I am fully convinced of the answer so I think we can drop the discussion. Thanks for the input to everyone.

 

[Averagesupernova]

I can't believe no one has asked or mentioned if the truck is traveling in a PERFECTLY straight line.

 

[JohnDubYa]

Well, I did stipulate that the wind hit the truck at 90 degrees.

 

[Mr. Robin Parsons]

...yes, but left off so many other needed details that, well, the answer you have accepted is void of the needs of reality...although I would easily accept that simplicity, in forming questions, does help in finding answers, but sometimes it simply isn't prudent...

 

[RayRoc]

I know that this is an old topic but it has merit in analyzing a dynamic system.

First consider the OP question, is the trucks velocity relevant? It sure is!

If we have a wind angle of 90 degrees across the road, and 0 movement of the truck then the force exerted on the truck would be at its maximum. We can represent this as F:

F = .5 * p * v^2 * Cd * A

p = density of the air
Vw = velocity of the wind
Vt = velocity of the truck
Cd = Coefficient of drag
A = area as seen by the wind

Lets modify this basic drag math by applying the forward velocity of the truck

F = .5 * p * (Vw * (1/(Vw+Vt)/Vw))^2 * Cd * A

Now consider this example:

A truck is parked at a truck stop. The wind is a 60 Mph (88 fps), the truck has a 50 ft trailer on it, the trailer is 8 ft high, giving an area of 400 square ft. the coefficient of drag is 1.05 and the air density is .00237 slugs per cubic foot.

F = .5 * .00237 * 88^2 * 1.05 400 = 3854 lbs of force perpendicular to the trailer.

if the same truck is moving at 75 mph(110 fps) with the same 60 mph crosswind

F = .5 * .00237 * (88 * 1/((88 + 110)/88))^2 * 400 = 761 lbs of force perpendicular to the trailer.

One important note: If the pitch of the trailer relative to the wind incident is less then the "stall" point of the trailer, the trailer will exhibit strong forces due to its behavior as an airfoil.

This is an approximation based on relative wind, the actual processes involved are a lot more complex. But it does show that the forward travel of the truck is relevant to the tipping of the trailer.

 

[Bob S]

There probably is no difference between a stopped or moving truck in a crosswind, as long as the driver's reaction to it is not considered. In the western United States, crosswinds along highways tend to be stronger in valleys, and can catch a driver off guard. If a truck is hit by a crosswind say from the right, and the truck starts drifting to the left, the driver corrects this by steering back to the right. His reaction time is delayed by up to a second or two, and when he does correct by steering to the right, there is an extra outward (to the left) centrifugal force that adds to the crosswind force, thus making it more likely that the truck will tip.