Interesting experiment I just performed

[fumesucker]

I ran some baseline aerodynamic drag tests on our 94 Altima today in preparation for doing some aero mods like blocking the grill..

I used my digital camera in video mode to record my speedo as I coasted down from 70 mph indicated to 65 mph indicated (my speedo reads almost exactly 10% fast since I have smaller diameter than stock tires).. I would like to have used a greater speed differential but finding a flat straight road here is quite difficult.. I had to drive nearly ten miles to get to the 3/4 mile stretch I used.. And then I had to wait for traffic to clear.. I made three runs in each direction and averaged the results.

I then counted the frames on the resulting video (15 FPS) and came up with a time for the coast down.

Anyhoo.. I came out with a deceleration of 1.49 feet per second squared..

Then I used the curb weight of the car and added my weight and then calculated the force applied to the car to decelerate at that rate and came out with 139 pounds..

Further calculations show that works out to about 22 horsepower to maintain 60 mph..

Then I went to this website:

http://www.fbody.com/calc.htm

And used a fuel calculator there to discover that 22 hp takes about 1.833 gallons of fuel per hour..

This works out to 32 mpg.. Which is pretty damn close to what I get at 60 mph..

Pretty cool, I think.. It definitely gives me an idea that I'm in the ballpark as far as my calculations go..

[hybriDatsun350]

Interesting calculations. You've totally lost me, but it sounds like you got some good info from it!

[Hateful]

you're better at math than I ever was. I just look to see if I've coasted any further in the same speed range starting from the same place.My test doesn't give any hard numbers but it does show if it's better or not. There are alway variables like wind, temp and rain that fuzz things up a little.

[fumesucker]

The math really isn't that hard, just basic grammar school stuff really.. It's a question of knowing what numbers to multiply, add and so forth and in what order.

Speed in feet per second = mph * 1.47

In my case my coast down was from 63 to 58 mph (speedo is 10% off remember, so at 70 indicated I'm doing 63).

63 - 58 = 5 mph

5 * 1.47 = 7.43 feet per second change in speed..

Acceleration = speed change / time to change speed..

In my case it took an average of 4.89 seconds to slow down so the acceleration was:

7.43 / 4.89 = 1.52 feet per second squared.. (rounded to 1.5)

Since the acceleration due to gravity is 32 feet per second squared then the force required to slow your car is:

Force = Mass of car * (your acceleration/gravity acceleration)

My car weighs 3000 lbs with me in it.. (look up the curb weight for your car)

3000 * (1.5/32) = 140.6 lbs

Your average speed during the coast down = low speed + ((high speed - low speed)/2)

58 + ((63-58)/2) = 60.5

I rounded the numbers off to 60 mph and 140 lbs..

One horsepower is 550 foot lbs per second..

60 mph * 1.47 = 88.2 feet per second. (rounded to 88)

88 feet per second * 140 lbs = 12320 ft lbs per second

12320 / 550 = 22.4 horsepower

If you were to do a coast down from 65 mph to 55 mph and time it, that would work out just right.. I just can't find enough flat, straight road to do it.

[dosco]

Originally Posted by fumesucker

The math really isn't that hard, just basic grammar school stuff really.. It's a question of knowing what numbers to multiply, add and so forth and in what order.

Note that this is the total drag, which is aerodynamic and mechanical drag.

Which is good, in that it will get you your total force requirement and total fuel consumption.

On the other hand, you have to do more work to figure out how to separate the two.

[fumesucker]

Originally Posted by dosco

Note that this is the total drag, which is aerodynamic and mechanical drag.

Which is good, in that it will get you your total force requirement and total fuel consumption.

On the other hand, you have to do more work to figure out how to separate the two.

Yeah, I know, but you need a quite long flat, straight road for that.. And around here that is unobtainium.

[monroe74]

fume, that's very clever. Especially using your camera and counting frames. Nice way to get a precise result without running out and buying special instrumentation. And I appreciate the way you clearly explained the math. I hope you do more stuff like this.

[fumesucker]

Originally Posted by monroe74

fume, that's very clever. Especially using your camera and counting frames. Nice way to get a precise result without running out and buying special instrumentation. And I appreciate the way you clearly explained the math. I hope you do more stuff like this.

Thanks..

My dad was about _this_ far away from being a genius at math and drilled me and drilled me to try to get me up to his level.. He used to live in India as part of the British Raj before WWII and the currency there is sixteen annas to the rupee.. He could add, subtract, multiply and divide base sixteen in his head faster than I can do base ten with a calculator, and this was long before computers made calculating hexadecimal (base sixteen) a common thing in the US.

I'm so old I even know how to use a slide rule..



There was quite a bit of variation in my measurements, it really is necessary to take multiple measurements and average them for accuracy. To be honest, I was surprised at how well the results came out. The reason I did the horsepower to fuel mileage conversion was to check my numbers.

Tomorrow I have the car for the day and I'm going to run tests with the grill blocker in.. There is monster hole down close to the ground on the Altima, I intend to block it completely and see what happens.

A good part of the fun of doing stuff like this is sharing with others who are interested.. I don't have anyone IRL who gives a damn about much of anything I'm really interested in so I do a lot of posting and reading online.

An air dam is definitely in the works too..

[monroe74]

Slide rules were around when I was a kid, but I was enough of a slacker that I managed to not learn how to use it.

[landspeed]

Fumesucker - when you look at the VIN number of your car, does it have 'U13' in it? I think that is a U13 Bluebird. Is yours the more conservatively styled sedan or then more sloping one? (the sloping one has the 'boot' sloping downwards).

You will be able to get huge mileages from that car. I drive a U12 Automatic (the previous generation to your car), and am currently getting 30mpg+ (recent tanks have been well over 30mpg). I also see you have a good choice of colour for your car