Physics question (not really flight related)

[houdini]

Here's something I've thought about which has me (and a few others I've talked with) stumped. It may sound silly at first, but if you look at the situation analytically, it will get you thinking.

Suppose you are driving your car on a perfectly flat, straight and level road going due north. There is a constant 30 knot crosswing blowing due west. Obviously, when the car is stationary, the force from this wind isn't strong enough to move the car any distance to the west.

But when you begin to drive north on this road at any appreciable speed, the wind will now have an effect on your car, pushing it to the west which you will have to compensate for by steering into the wind.

My question: Why does the car have to be in motion before the wind can actually push it off its straight line course? After all, the wind exerts the same force against the side of the car both when it is stationary and in motion.

I realize there is a quirk when it comes to static friction coefficients being higher than dynamic coefficients (meaning there's less friction between the car's tires and road when it's in motion) but this is insufficient to explain what is happening in this example.

Anybody have any ideas?

[raptor22]

Basically, you are right about the friction coeffecients, but I'll give it an attempt since something similar happens ina aturn.

The thing is, the wind produces a small amount of flex in the tires fo the car; a sideloading on the walls. This essentially warps the tires in such a way that they pull away from the wind and effectively have changed their heading in an amount proportional to speed. This is irrelevant at zero speed, but becomes worse as you speed up.

Super high performance supercars like the Jag XJ-220 or ferraris often are built with wide tires that have itty-bitty sidewalls in order to reduce the effects of sideloading in turns and other situations.

--Alex

[BMatthews]

Don't forget that the car's steering tends to caster like a shopping cart. So the side force acts on the steering an requires you to exert some effort to counter that.

And the side area distribution of the vehicle makes a difference as well. More area over the rear axle and behind can act like a fin to actually try to turn you INTO the wind. My old truck with the canopy on it did just this. I actually found I had to exert some downwind turning force to stay straight.

The only way to test all this is with a fixed cart and equally distributed side area. And then it will drift.

This happens because of our pneumatic tires. The tread rubber sets up a slip angle to the grip where as it comes into contact with the road and there's a side force at work the rubber distorts to one side. on the other side of the contact patch it mostly comes back. The difference is hysterisis loss. This is how they describe the slippage in a turn more than your wind idea but side force is side force be it from cornering or the wind. The reason we can control our cars in a slip and, in more extreme cases, a pure skidding action is that the hysterisis grows at a controlled rate with some feedback. A lot of the tread support comes from the tire belting design but the "feel" comes from that hysterisis.

Car and motorcycle magazines have had articles on this over the years and thanks to my car and motorcycle intrest I've read more than a few of them. You can probably find it on the net as well. I sure didn't dream it up on my own. I ain't that smart or dedicated.... :D

[raptor22]

Sooo.....basically, i was right. Sideloading warps the tires.

skilz.

--Alex

[Ollie]

BMatthews is right on!