Drag coefficient?

[Turbonutter55]

How do I work it out? I know it's a number less than 1, but otherwise what exactly is it?

[fpv_gtho]

I think without a windtunnel its next to impossible to just "figure out". As to what it is, its something along the lines of the aerodynamic efficiency of the car's frontal area......but dont hold me to that...

[derekthetree]

the drag coefficient is a dimensionless number that defines how well an object can move through a fluid. it can be bigger than 1 and is determined experimentally. it is dependant on many factors some of them very complex like boundary layers and various types of drag

i did a project on theoretical modelling in car design last year and had to research all about stuff like this. check out his site for more info

http://www.engineeringtoolbox.com/21_627.html

[jcp123]

I've always wondered what mine is. Heard somewhere it's around .45.

[Matra et Alpine]

I've always wondered what mine is. Heard somewhere it's around .45.

Man (upright position) 1.0 - 1.3
Ski jumper 1.2 - 1.3
Skier 1.0 - 1.1
Parachutist 1.0 - 1.4

It all depends how BIG you are

or did you mean your car ----

[jcp123]

*sigh* Yeah I meant the car, smart@$$ hehehe

[Matra et Alpine]

*sigh* Yeah I meant the car, smart@$$ hehehe

Couldn't resist adding some humour

Anyway, you can usually find the Cd of a car by searchign long and hard if it's anything like decent it will have been published. Finding it for 'ordinary' can be a little more difficutl.
post the car, someone might know.
Calculating it is possible but you need to invest in $50,000 of software and spend about 500 man hours entering the data

[KnifeEdge_2K1]

.30 is a good cd, but when determining drag you have to find the surface area as well and take it into consideration

a truck with cd of .26 will still have more drag then a motorcycle of cd .45 (i dont know drag coefficients of motorbikes so i got a random number)

intrestingly though is on a motorbike you can get amazing fuel efficiency by drafting a big truck or van, same idea as in racing but instead you stay behind the car and let the vortex suck you in creating a pulling force on your bike really cool

the same idea can be implemented on a car but since the car has so much more mass the increase in efficiency will be hardly noticible, and it requires you to get pretty close behind the truck/van which is dangerous

[jcp123]

My dad got 50mpg drafting a truck in his Passat wagon...usually it gets ~25-28 on the freeway. It was amazing.

[KnifeEdge_2K1]

lol man u serious? is it diesal ?

dude ur dad has leet skills, i drive too fast on motorways

its understood that 55mph +- a few miles per hour gives best fuel economy

too bad that no one in toronto understands the 100km/h speed limit on the 401 (name of our main highway) and all drive at 120-130, im even worse occasionally hiting 150, i get better economy in the city then on highway sometimes lol (which still isnt good, if you've seen my other threads/posts ud know im getting 21-22 mpg in a corolla ... i should be getting 28-32)

[gregvl52]

In one issue of Car & Driver, they won a Toyota-sponsored fuel efficiency competition by drafting behind a big van which they had added plywood extensions to the whole way.

[jcp123]

lol man u serious? is it diesal ?

dude ur dad has leet skills, i drive too fast on motorways

its understood that 55mph +- a few miles per hour gives best fuel economy

too bad that no one in toronto understands the 100km/h speed limit on the 401 (name of our main highway) and all drive at 120-130, im even worse occasionally hiting 150, i get better economy in the city then on highway sometimes lol (which still isnt good, if you've seen my other threads/posts ud know im getting 21-22 mpg in a corolla ... i should be getting 28-32)

No they don't sell the diesel Passat here, ours had the V6. He can drive for economy when he tries, his lifetime average for the car was almost 23mpg. Now that he drives the Mach 1 though, he thoroughly disregards speed limits.

That Passat got its best economy at about 42mph, it'd get over 40mpg cruising steadily on a level road at that speed. 62mph (my dad's favorite freeway cruising speed in the Passat) was about 25-26mpg.

[Stik]

Well one method i use to make rought estimates about the drag coefficient is by knowing the car's maximum power and considering that the maximum power is being used at max speed then you have all the "ingredients" to estimate the Cd.
Power(W)=Velocity(m/s)*Force(N) and
Force(N)=0.5*pho(kg/m^3)*Cd*A(m^2)*Velocity^2(m^2/s^2)
Power(W)=(HP)*745
Cd*A=Power/(pho*0.5*V^3)
pho=1.2 depends on altitude and weather, etc

Then you'll have to find the frontal area of your car(that´s what is normally used) but the most important thing isn't fiding the Cd but the Cd*A

Example: My dad's minivan has 120Hp and a top speed of 180Km/h
Considering that the top speed is reached using full power we have
V=180(km/h)/3.6=50(m/s) (don´t forget using the right units)
Power=120*745=89400(W)

then Cd*A=89400/(1.2*0.5*50^3)=1.192 m^2
You see that area is only used to make the Cd dimensionless, because the area can be any as long as that when you make calculus to find the aerodynamic force you use that same area.

For example, at 120Km/h you only need to use very litle power.Let's see:
V=120Km/h=33.333m/s
Cd*A=1.192 and pho=1.2
then Power=0.5*pho*Cd*A*V^3=26500(W)=35.6Hp
Remember these are only rought estimates just to give a picture.

One other method you can use is putting your car in neutral in a descent and knowing the grade(? =inclination angle!? ) ,the car's top speed at that descent and the car's weight you can find the Cd*A. This time i´m gonna be faster in my explanations:

There are two forces acting:the air against the car and gravity. At max speed and constant grade (?) they match each other so:
0.5*pho*Cd*A*V^2=MassCar*G*Grade (Aprox. formulae(?))
Pho=1.2, MassCar is in Kg G=9.8 m/s^2 and Grade is a percentage
then Cd*A=MassCar*G*Grade/(0.5*pho*V^2)
I used this formulae(?) to find the Cd*A of me and my bike and the results are pretty acceptable.

Just to finalise another example but in a climb. We want to find the power needed to do a climb with constant inclination and constant speed:
V=120Km/h=33.333m/s
Grade=4%
MassCar=1700Kg
Cd*A=1.192
G=9.8
Force=AeroForce+"WeightForce"
Force=0.5*pho*Cd*A*V^2+MassCar*G*Grade
Power=Force*V=>Power=0.5*pho*Cd*A*V^3+MassCar*G*Gr ade*V
Power=0.5*1.2*1.192*33.333^3+1700*9.8*0.04*33.333
Power=48700W=65.4Hp
Well we see that a small inclination requires more 30Hp to maintain the speed.

Hope it helps, and sorry about my english!
Fell free to correct me if you find errors!

Cheers!

[RacingManiac]

In one issue of Car & Driver, they won a Toyota-sponsored fuel efficiency competition by drafting behind a big van which they had added plywood extensions to the whole way.

Honda Insight I believe it was, they drove behind a Ford Excursion(lol @ the irony) with the rear 3 piece door open and sealed with plywood extension and he was literally like 3 inch away from the rear bumper of the Ford. They got I think 120mpg....by driving like that @ 55mph....

[Renesis]

then of course theres the speed biker who pedaled somewhere along 145mph, partly because he pedaled behind a specialized truck that removed drag... but we've all heard that story before..