Although I have a heavy box-shaped car, I was thinking about the Mercedes G63 specifically. That is a powerful engine on a non-aerodynamic car, what happens if you take it over 100mph and get hit by a gust of 60mph wind? My car gets literally pushed around lanes during heavy winds at highway speeds, has anyone ever even gotten a G63 past 150mph?
In: Physics
It’s because the force of wind is roughly proportional to V^2. (note below)
When your car is going 0 km/h with a 50 km/h wind, the force is ~2,500. (50×50)
Driving at 100 km/h, the wind force from moving is ~10,000. (100×100)
Driving into a head wind of 50 km/h at 100 km/h, the force is now ~22,500. (150×150)
A tail wind of 50 km/h while driving at 100 km/h produces a force of ~2,500 (50×50). Notice that this is the same as the standing-still force. However, it’s a difference of ~7,500 from the driving-at-100 force
Thus, you’ll feel changes in wind much more when moving faster, no matter which direction the wind is blowing relative to your movement.
note: We don’t have a good formula for air friction and it’s kinda non-linear. Some formulas use V^2; some use V^3 or something in between. The formulas are *very rough* approximations, so we use complicated simulations which are *also* rough approximations.
Real answer: because of inertia.
ELI5: if you place a marble on a table and nudge it with your fingertip it moves maybe 2-3 inches in that direction, if you roll it down a ramp but also nudge it the same way it will move many, many more inches in the same direction than if it was sitting still.
Off topic, but this is infinitely less fun (or more fun depending on who you are) when this happens on a motorcycle.
The vehicle will have the same force when it’s parked vs when it’s driving. Ignoring difference in speed of the wind relative to the vehicle depending on whether the cross wind is slightly from the front vs the back.
Some of your movement will be from the flex of the tire. Picture of the part of the tire that’s touching the road staying where it is and the rest of the tire is flexing in the direction the wind is blowing. The next part of the tire that makes contact with the road will be shifted slightly to the side. This is a very small amount but when driving fast, these small amounts add up quickly.
Something else that I believe would happen, that I haven’t seen mentioned, and it would be more pronounced with a taller vehicle like a SUV, is the suspension is going to flex as well. If the wind comes from the driver side, the vehicle is going to lean to the passenger side. The suspension on the passenger side will compress because it has more force on it. That’s going to change the geometry of the suspension and can cause the vehicle to turn a small amount as well.
First we need to understand what forces a wind gust will put on a G63.
The G63 has a smaller front and larger rear, so the rear will experience more force from a crosswind than the front. The net result is a force of *rotation* on the car that makes it rotate *into the wind*. This is an effect called [weathervaning](https://en.wikipedia.org/wiki/Weather_vane). In other words, the car wants to rotate into the wind, since it’s most aerodynamic when the wind is coming head-on.
You can see an extreme example of this [here](https://youtu.be/bdl94J3AxZY?si=vMQN9CWkkKt3KGtH&t=42), note how the car actually starts off *rotating*, with the rear of the car being affected by the wind more than the front; it doesn’t get blown directly sideways like you might have guessed.
Let’s say a G63 is traveling at speed and gets hit by a strong wind gust from the right (the wind is traveling left). As the wind gust puts a rotational force on the G63, it will rotate right, into the wind, by having its left wheels rotate slightly faster than its right wheels. This is easy for the wind to do at speed, since all of the static friction in the drivetrain has already been overcome, and cars are designed for their left and right wheels to rotate at different speeds as they turn.
With the car stopped and in neutral, the static friction in the drivetrain (the wheel bearings, differentials, etc.) will prevent the car from rotating up to a point, but if the wind is strong enough, it will cause the car to rotate into the wind. (For this to work at a stand-still, the differentials would have to allow the wheels to rotate in opposite directions, that’s not always the case.).
If the car is in park, the wheels are prevented from rotating, and the wind will just cause the car to lean, until the wind is strong enough to overcome the static friction between the tires and the road or to flip the car.
[Here’s another example of a semi getting blown around](https://www.youtube.com/watch?v=bj-dDQ9UJ7U), note how it’s causing the whole vehicle to rotate into the wind and the driver is correcting for it by steering in the same direction as the wind is going.
Another factor that comes into play is moment of inertia. As the car is moving it is already at a point where it can be directly affected by external forces such as drag.
When you are moving your tires do not have as much traction.
I was driving to my favorite ski resort Lutsen Mountains in Minnesota on Hwy 35, it was cold and very windy with snow the previous night.
About Barnum MN. the wind pushed my Jeep Unlimited X from the right lane to the left lane. I had passed over some ice and that big sail of a Jeep just moved.
Most of the other answers pointed out how strong winds are able to more easily push you off course at higher speeds by changing the car’s direction. However, I’ll take a second to talk about how the wind can blow the car *sideways* without changing the car’s direction.
The main force that stops your car from moving sideways is the friction between the wheels and ground. This friction depends on the force of contact between the car and the ground, which is basically the weight of the car that pushes down onto the road. The heavier the car, the more the resulting friction, the better the grip, and the less likely it is to slip.
However, this friction also depends on whether the car is moving or not. The friction is strongest when the car is stationary (static friction), a little less when the wheels of the car are rolling perfectly (rolling friction), and at its lowest value when the wheels experience some slipping on the surface of the road (sliding friction).
When the car is standing still, the only force acting on it is its own weight, which is all supported by the tires, so we have the situation of the maximum weight (which should actually be the contact force between the wheel and the ground), as well the maximum friction coefficient due to it being static friction.
When the car starts moving on the road, while the weight remains the same, the wheels shift to experiencing rolling friction, which is less than static friction, so the car has less grip with the road, but not by much. So at low speeds, cars don’t easily lose their grip on the road.
However, roads are not perfectly flat. Curves, slopes, bumps and other imperfections on the road can all cause one or more wheels to temporarily lose contact with the road or not rotate at the right speed relative to the speed of the car. If the speed of the wheel is not ideal, it will take a short moment to speed up or slow down to the correct speed, during which the wheel is slipping against the road and so experiencing sliding friction, which can sometimes be only half as strong as rolling friction. Even worse is if the wheel loses contact, leading to a moment when the car only has three wheels to maintain grip with the ground, which can cause it to slide more easily.
On top of that, the car itself vibrates – sometimes because of imperfections on the road, but more often because of the vibration of the engine itself. As it vibrates, the contact force between the wheels and the ground also varies at the same rate of the vibration, leading to some moments where it experiences stronger than average grip, and some moments where it experiences weaker than average grip.
Finally, all cars experience something called *aerodynamic lift*. This happens because the speed of air that passes below and above the car isn’t equal, which can lead to the air around the car pressing up or down on the car. This is the same force that acts on the wings of airplanes to keep them in the air. In general, with the shape of modern cars, the overall lift force is upwards, which reduces the apparent weight of the car and causes a lower contact force with the ground, leading to less friction. This lift depends on the speed of the car. At low speeds, this lift force is almost negligible and you’d never notice the effect. However, at higher speeds, this lift can significantly decrease the apparent weight of the car, causing the wheels to have almost no grip with the road. It’s for this reason that formula one cars have those strange shapes with wing-like structures and large spoilers – these all provide *negative* lift that pushes the car back down so that it keeps its grip with the road and doesn’t just fly away.
In summary, as you speed up, your car’s wheels can lose grip with the road because:
* Wheels experience less friction when rolling than when static
* Unevenness of the road can cause your wheels to lose contact or slip on the road
* Vibrations in the car can cause the grip to vary as well
* Faster speeds can cause the car to experience lift like airplane wings
It pushes the car the same amount, you’re just not driving it which results in you not noticing it.
If you are still and I push you an inch you move an inch.. if you are walking your first step is an inch off and your second is 2 inches etc. My push altered your course, so the faster you were moving the more it alteres your path.
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