Quote:
Originally Posted by Xringer
I'm wondering about crosswinds. If a crosswind is hitting the side of the car,
90deg off the direction of travel, I can't understand how it will hurt MPG...  |
Because you will have to drive with your wheels turned into the wind to maintain a straight heading. To drive in a straight line the force vector generated by your tires must exactly oppose the force vector generated by your aerodynamic drag. If you're traveling straight at 50 MPH and there is a 10 MPH cross-wind perpendicular to your travel, your vehicle sees an effective wind of 51 MPH at an angle of 11.3 degrees from your direction of travel. You must then turn your wheel into the wind to generate lateral force to oppose the cross-wind. This increases your rolling resistance since you will be traveling straight even though your wheels are slightly turned (operating at a slip angle). Since lateral tire forces apply perpendicular to the tire, any time you turn your tires the force that pushes your car sideways also opposes its forward motion by pointing slightly rearward.
Heavy truck manufacturers evaluate the aerodynamics of their trucks in wind tunnels by setting the truck at an angle to the air flow. I can't remember what angle they use, but they've determined the statistical wind profile that vehicles are effectively always at something like an 8 degree angle to the wind or something like that.