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How to measure frontal area?

Posted: Sat, 30 May 2009, 01:27
by Paul9
Hi people,

As we know the frontal area of a vehicle influences drag. My question is how do I measure frontal area? I can easily measure the distance from the roof to the bottom of the car and multiply by the width of the car to get the frontal area - correct? However doesn't the angle of the windscreen and the bonnet influence the amount of drag?

I am lost as to how simply height times width eaquals frontal area? Doesn't the angle of the dangle have something to do with it?

Thanks in advance
Paul

How to measure frontal area?

Posted: Sat, 30 May 2009, 02:02
by Tritium_James
The drag on the car is frontal area * Cd. Cd is the drag coefficient, and depends on things like the windscreen angle, airflow through the radiator, turbulence on the wheels, etc, etc. So reducing either the drag coefficient OR the frontal area will reduce your total drag.

How to measure frontal area?

Posted: Sat, 30 May 2009, 02:04
by acmotor
http://en.wikipedia.org/wiki/Drag_equation

Frontal area is the cross sectional area. i.e. max dimension w x h that you are pushing through the air. The Cd takes care of the other surfaces like windscreen.
Image

edit: made the link work !

How to measure frontal area?

Posted: Sat, 30 May 2009, 02:05
by acmotor
Snap. Beat me to it ! Image

How to measure frontal area?

Posted: Sat, 30 May 2009, 02:49
by Paul9
Sorry people but I am still confused. I thought my drag coefficient was dependent on my frontal area (among other things) but your replies seem to say drag coefficient is separate from frontal area. I note acmotor's red suzi has a frontal area which is almost "flat on" (perpendicular) to air onrushing. Another car may have the same frontal area but be shaped like the front of an mx5. Simple logic says the two cars will have different drag coefficients?

Or are you saying drag coefficient is something I have just got to look up somewhere?

I apologise for still missing the point.
Thanks
Paul

How to measure frontal area?

Posted: Sat, 30 May 2009, 03:48
by acmotor
Paul9, the drag coefficient is a measure of how easily an object of a certain frontal area moves through the air, not a measure of the actual drag.
An object with the same frontal area (or cross sectional area at its largest h x w part) can have an 'aerodynamic shape' such that is has less resistance to moving through the air. A square box being bad Cd=1

basically...
Aerodynamic drag = drag coefficient (Cd) x frontal area

There are other factors of air density and velicity to consider for complete calculations. see link below.

Yes, red suzi has a bad Cd, maybe 0.45
A prius had a good Cd, 0.26
Both have around the same frontal area so I dream of low drag like the prius !

If you wish to reduce aerodynamic drag then either reduce the frontal area or make the vehicle more aerodynamic..... The tear drop (round bulb front and thin tapered tail) is the best aerodynamic shape. Cd=0.04
The departure shape is more important than the approach shape. So the go-fast sportscar wedge shape is not actually very good Cd.


Does this help more....

http://en.wikipedia.org/wiki/Drag_coefficient

Does that help ? Image

How to measure frontal area?

Posted: Sat, 30 May 2009, 17:12
by Paul9
Thanks tons acmotor - the concept, with yours and the others help, is starting to become clearer!
Thanks again
Paul

How to measure frontal area?

Posted: Sun, 31 May 2009, 18:33
by Thalass
Here's a method for measuring frontal area. You should be able to do the same thing in photoshop.

Also, if you're in the mood to improve things: 65+ vehicle mods for better fuel economy (many of which are probably illegal in australia - like removing the mirrors)

How to measure frontal area?

Posted: Sun, 31 May 2009, 20:42
by Squiggles
One of our cars is a 2005 Honda Jazz that came new with some crap Yokohama tyres that lasted 18,000km (even had the alignment checked but it was good). With these tyres we got 8.1 l/100km, all city driving.

Replaced the tyres with some Michelin Energy XM1+, 10,000km later there is nice even wear probably more than 75% of tread left and the car is getting 7.8 l/100km. That seems like better than 3% improvement to me, so maybe just the tyres alone can add 2km+ to your range.

How to measure frontal area?

Posted: Sun, 31 May 2009, 23:16
by Thalass
Pumping your tyres up close to the max sidewall pressure (which is on the tyre), and buying low rolling resistance tyres, I think they have lots of silicon in them or something, should all help.

How to measure frontal area?

Posted: Mon, 01 Jun 2009, 21:20
by 300zxev
Changing your driving style effects your fuel consumption (and range for an ev) ... I have a 2003 1.5 Honda Jazz and consistantly run 5.8L/100 no aircon & 6.3L/100 with aircon.

How to measure frontal area?

Posted: Mon, 01 Jun 2009, 23:26
by Squiggles
300zxev wrote: Changing your driving style effects your fuel consumption (and range for an ev) ... I have a 2003 1.5 Honda Jazz and consistently run 5.8L/100 no aircon & 6.3L/100 with aircon.


What sort of driving do you do in your Jazz? My wife does around 7000km a year in hers and it rarely gets above 60kph. In fact it would do a large percentage at around 50kph as most of our suburban areas are limited to that. I should add that it is a CVT auto, and most trips it does would be under 10km. Even when I take it for a drive and deliberately go light on the throttle it won't get much under 7 L/100.

On the other hand my 2.0 TDI Golf gets 6.2L/100 even if I am not driving efficiently. It is easy to get it well below 6 and does about 4.2 on the highway, can easy get 1100km out of a 45L tank of fuel on a highway trip. I can't figure why anyone would buy a petrol power machine with these things available.

How to measure frontal area?

Posted: Mon, 01 Jun 2009, 23:30
by Squiggles
By the way my Golf tells me it uses 0.2L per hour to run the Air Con.

Can any of the clever chaps out there convert that to Watts consumption?

How to measure frontal area?

Posted: Wed, 03 Jun 2009, 22:26
by 300zxev
Mine is also a CVT ...
My Driving in the Jazz is about 50kms per day.
10Kms @ 100km/h
10Kms @ 80km/h
30Kms @ 60 or less km/h
I have driven down the coast one day when there was a fair ammount of traffic which brought the speed down to 80km/h the whole way ... I averaged 4.3L/100 for the whole trip.
If I do the same trip at 100-110Km/h I'll only get 5.3L/100
I try and keep the revs 2000 or below whenever I can.
Oh, and I modified the air intake ... but whatever gains I had from that, I lost by putting mags and wider tyres on.

How to measure frontal area?

Posted: Wed, 03 Jun 2009, 23:16
by fuzzy-hair-man
Squiggles wrote: By the way my Golf tells me it uses 0.2L per hour to run the Air Con.

Can any of the chaps out there convert that to Watts consumption?
I'm bored so I'm going to have a very rough bash at it... and then everyone can point out where I went wrong Image

So 200ml of petrol
Petrol is 737g per litre apparently, so 737/5 = 147.4 grams
According to this petrol has 48,000 kJ/kg but the ICE is something like 25% efficient so the air conditioner only uses 25% of that, the ICE is wasting the rest (I'm assuming that you want to know what powered e-motor might be needed to power the AC?)

so 0.147 x 48000 = 7056 kJ used by the ICE to power the AC per hour.

then converting kJ to watts using

this gives 1960 or 1.96 kW per hour.

BUT if we assume the ICE is only converting 25%(the figure I read last night, not sure how it applies to your Golf) of that to mechanical power so it's 1960 Watts x 0.25 = 490 Watts per hour or 0.49kW / hour.

Now you're going to tell me your Golf is diesel aren't you? Image Image

How to measure frontal area?

Posted: Thu, 04 Jun 2009, 00:40
by Tritium_James
Your units are a bit off. 7056 kJ = 1960 Wh   So 7056 kJ per hour = 1960 Wh/h = 1960W. Multiply by 25% efficient = 490W. Not W/hour.

That answer sounds about right. I seem to remember that the max power on a big A/C like in a Commodore or Falcon is 2kW or so. So cruising once you have it at the right temp would be about half or less, and the A/C on a Golf is probably smaller again.

How to measure frontal area?

Posted: Thu, 04 Jun 2009, 01:10
by Squiggles
fuzzy-hair-man wrote:

Now you're going to tell me your Golf is diesel aren't you? Image Image


Yes :)

How to measure frontal area?

Posted: Thu, 04 Jun 2009, 01:19
by Squiggles
How dumb am I, here I was thinking that Gasoline, Petroleum and Petrol were all the same thing! But they are listed with three different calorific values.

How to measure frontal area?

Posted: Thu, 04 Jun 2009, 01:26
by antiscab
petrol is a (poorly defined?) mixture of hydrocarbons around C8H18.
unless you define the proportion of each hydro carbon in petrol, there will be variation in the properties of each source.

Matt