Car instability

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doorknob | 22:25 Thu 10th Jan 2008 | Science
9 Answers
In layman's terms why is a car more unstable in high/gusty wind conditions when driven at higher speeds?


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The amount of side motion attributable to the wind would add up faster since your traveling a greater distance in the same time interval. Likewise correcting for the effects of sporadic changes in wind velocity would require faster and more precise responses.

I think Greg explains it best?
I can't believe I am doing this, but I am going to disagree with mibn2cweus somewhat.

I believe the main reason your car will be more unstable at high speed is because the air has a greater distance to travel over the top of the car than underneath, creating the same aerodynamic affect that also lifts a plane's wings and lifts roofs off houses in storms. To counteract this, racing cars have an upside down aerofoil that pushes the car down to onto the road.
There is also the amplification of any movemenet that occurs at high speed. Ie a small wobble from any source get's amplified, ie say a small pot hole, barely noticable at 30 would cause major concern at 140!
I think mucusbin2meow had it right about sidewinds knocking you off course, and the speed meaning you travel further before correcting.

The aerofoils that F1 cars have is more to do with pushing the tyres down onto the road to keep grip.
I think we need a design engineer on this question - anyone got a phone-a-friend? There must be an effect from the smaller time the wheel is in contact with the ground and there may be an effect from the upward/ downward aerodynamic effect but I reckon there must be a bigger effect from the combination of the forward and sideways wind components. Imagine the car viewed from the top. At speed the air flow around the car will generate drag and turbulence but it will generally be symmetrical, so no tendency to drift left or right. Put in a side wind however and the airflow on the windward side will be pushed against the car but the airflow on the lee side will tend to separate away. I'm no aero engineer but I would guess that the forward speed would have a multiplying effect on any generated sideways component.
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Thank you all, I didn't realise it was that complicated. I like 'loosehead's' amplification version although I'm sure most of the others would have an effect to some degree. It would be interesting to know if there is a definitive answer - Mr Ferrari, are you there?
It has to do with the 'vector of forces'. A car travelling at 100mph has an effective head wind of 100mph. If there's a side wind at 90� of 100mph, the combined effective wind speed on the car is that of a 141.4mph wind at 45�
Strange answers above, I travel over the bridge onto Anglesey a lot on my motorbike which has speed restricted to 30mph in high winds.

My bike is more stable in winds when doing 50+mph, this is because of the gyroscopic stabilization generated by the spinning wheels.

If I go over at 30mph it's dangerous.

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