Quote:
Originally Posted by Isaac Zachary
Force = velocity^2 x cross sectional area x coefficient of drag x the density of the air.
To find the coefficient of drag you need to know:
- How dense is the air where you're at (usually 1.225 kg/m^3)
- The cross sectional area. That's not just height and width. You have to take a picture of the front of the car from far away and use a bunch of geometry to figure out the actual area.
- The mass or weight of the entire vehicle, including yourself and fuel.
- The rolling resistance, although with enough testing you can kind of figure out both your rolling resistance and coefficient of drag.
- And a way to measure speed and time to figure out deceleration.
Calculate your cross sectional area in meters squared.
Speed up pass a certain speed (i.e. 60mph) on a known flat piece of pavement and throw it in neutral. When the car slows down to that speed (60mph) start the timer. When it has dropped a small amount (i.e. 55mph, 5mph less) stop the stop watch. The more accurate your measuring instruments and techniques are the better the results will be. Several repeated tests are best. Try both direccions on the same road and avearage out the results in case the road isn't perfectly flat.
Now take your speed drop and turn that into meters per second per second (m/s^2). For an example, from 60 to 55mph is about 2.235 m/s. If it takes 1.5 seconds that about 3.353 m/s^2. Now take your weight and turn that into kilograms and multiply that by your m/s^2 to get force in newtons. Subtract any force you believe or know rolling resistance takes up. You could push a pressure operated scale against the car and see how much force it takes to push the car in neutral and convert the number into newtons. Or do a lot of tests at different speeds with different guesses until you find a rolling resistance number that doesn't change much at different speeds.
Take the average speed in m/s (between 55mph and 60mph is 25.7) and square it (multiply it by itself.). Multiply that by your cross sectional area and the density of your air in you area in kilograms per cubed meter (usually 1.225kg/m^3 for an average number) and then divide that into half.
Now take your force in newtons and divide that into the number you just figured out above.
You should now have your coefficient of drag. |
Pardon me, but I think I made a mistake. Take the force in Newtons and multiply that by two. Then take the other number from the velocity squared multiplied by the density of air and the crossectional area, but don't divide into two. Then divide the newtons doubled into that number to get coefficient of drag.