Side Winds

[corganiser]

I've heard that cars/trucks are more affected by side winds the faster they are travelling. Can anybody explain (in non-technical language) why the spped is a factor.

Many thanks

[ian.miller]

I've not heard the fact that the trucks speed is important, but not saying it isn't.

A simple explaination could be that the faster you are travelling the faster everything happens and so the less time the driver has to react if blown across a lane say. Plus there's a certain degree of momentum going on, so blowing a truck off course when it's going at speed means it's harder to correct as well, like when your car over/understeers at speed

[Skids]

The faster a vehicle goes the less friction its tyres experience with the road.

[greenster]

Basically you are adding whatever speed you travel at into the equation. ie if the wind is blowing  at 40mph and you are driving into it at 60mph that wind will represent a 100mph gale.

[greenster]

I've just re-read the above and it's not vey clear.If you are driving at 60 on a calm day the forces on the vehicle are fairly constant. If it is windy those forces vary considerably. If the wind is gusting from say 10mph to 40 or 50, as it can on a windy day then it is as though your vehicle is changing it's speed. The windspeed changes in seconds so it can be as if you are accelerating from 60 to 100mph in a couple of seconds. Not too bad in a straight line but more noticable if the wind comes from an angle. Alternatively imagine walking down the street and bumping into someone, it wouldn't make much difference. Now run into the same person!!

[Jay70]

greenstar, I can understand your explination if you were describing a tail winds effects but I would have thought a head wind which seems to be what your talking about would effect things differently. i.e. would produce no increase in forward speed of the car except for the fact more power from the engine will be needed to power the car at the same given speed as with no head wind. The original question was actually about a side wind.   

[brachiopod]

My guess would be that high winds can deflect you momentarily until you react to correct the movement.

At low speeds, a deflection of a couple degrees (of angle) for a one second will not deviate you from your path by much.

If a gust of wind caused the same angle of deviation for the same period of time at high speeds, it would easily put you across two lanes of traffic before you could correct it.

(Think of the way that 'wiggling' the steering wheel from side to side at 10mph has a minimal effect on the path you travel. Then try exactly the same 'wiggling' motion at 80mph and you would be lucky to avoid spinning or turning the car over.)

I think, therefore, it's largely to do with the fact that even a tiny deflection to a vehicle's path in a gust of wind will cause greater lateral movements at higher speeds.

[slimfandango]

The way to consider the forces is not to think just of vectors, eg wind of speed x coming from direction y, compared to car speed z. You have to take into account the fact that the shape of many cars changes the speed of the air passing over them and thus the pressure bubble around the car. For example, if you have a 40mph side wind, then drive at 60 mph, you have a fairly high apparent wind coming from about 45 degrees. When this hits your car the car distributes the passing air unevenly and in order to maintain constant pressure the air will be accelerated over the top compared to the bottom, creating pressure differentials that will variously lift, bounce, drop, and shift the car.

Having said that, this answer will now probably be banned by Answerbank for some inscrutable reason that shows off their vast powers.

[jills]

I'd have thought Skids answer was simple and correct. 

[Jay70]

Skids and jills: it really depends on the vehicle If a vehicle has good aerodynamics an increase in forward speed should produce more downforce on the vehicle so it will infact have more friction with the road. 

[jills]

Cheers Jay70. x

[akaceefax]

The reasons for speed being an issue are as follows:

most if not all cars generate lift (upwards force) at high speeds and so the tyres have less grip on the road;

there are also reaction times to consider.

this means that side winds will push a car further and the driver has less time to react at high speedrather than at low speeds.