Frontal Area

[butelja]

When drag coefficients (Cd's) for a road vehicle are quoted, how is the frontal area calculated? Is it:

a.) The "true" frontal area where the area below the air dam is not counted. This frontal area would not be affected by raising or lowering the ride height.

or

b.) The projected frontal area, where the car is artificially assumed to extend from the ground up to the roof line. This frontal area would increase with increasing ride height.

 

[sciguyjim]

The site below has some explanation of how frontal area is calculated. It sounds like it follows your choice "a". That's my guess too. The air below the car isn't dragged along with the car (much) and some cars have panels below the chassis which cover up unevenness and help the air flow more smoothly. The higher the car rides above the ground, the more inaccurate method "b" would become.

http://www.bgsoflex.com/airdragchart.html

 

[carnage]

I think the flow of air under the vehicle would have a similar effect as the air flow through the engine compartment, so the higher the vehicle the greater the drag.

 

[106197]

The frontal area is the shadow of the car when projected on a wall. So the frontal area of the tyres do count. When the car is lifted, the frontal area becomes bigger due to the tyres. But the complete drag will not always increase. In the calculation for aerodynamic force stands out the value for Cd of Cw(EU). This value can not be calculated. It is a coefficient to compensate for body structure, ground effect and pressure differences around the car. This value can only be measured in a controlled area such as a windtunnel.
So: Cd is not the same as frontal area...
F = (1/2)*rho*v^2 * A * Cw

 

[patprimmer]

Have I missed something here.

I thought drag was Cd X frontal area.

It should be done at the specified ride height to accurately take ground effects into account, as they will vary considerably with ride height.

One major effect I can see from ground effects is the pressure build up under the floor as the air entering the engine compartment through the grill and bumper vents will be forced down by the firewall, into the airstream entering below the bottom edge of the bumper, front pan or spoiler.

As the amount of air entering the engine compartment will be relatively constant, it will impose a lesser effect on the air under the car in %age terms if the car is higher.

The part of the tyre exposed in the front on siloette should be considered frontal area.

By raising the ride height you will exposure more of the tyre. This will also alter the Cd of the total car, as well as increasing frontal area, as the tyre on it's own must have a much higher Cd than a modern car body on it's own.

I have no idea how car companies do it, but if it is not well controlled by a very good realistic standard, they will arrange the variables to enhance the apparent results, like they did with the SAE horsepower figures pre 1980's.

Regards
pat

 

[GregLocock]

CdA is not a certified number so in principle they could pick the best result they ever saw in testng and advertise that. They tend not to because competitors put cars into wind tunnels and would use it against them.

A lower ride height usually results in lower CdA because there is less air going under the car, even if it is being sheared more violently.

To be honest most of the time a production car is in a windtunnel these days it is for cooling, HVAC, wind noise, and yaw coefficient work, straight CdA is not that sensitive to the small changes that we are allowed to make to the body by the stylists (who have hopefully worked to the guidelines the aero guys gave them).


About 10% of the drag of a current car is due to the flow through the engine compartment.


Incidentally the reason I refer to CdA is that you really get Cd by dividing the 'drag area' by A, but the drag area is about equal parts skin friction and recovery drag (the only bit that Cd really works for) and a bit less interference drag.

Cheers

Greg Locock

Cheers

Greg Locock

 

[patprimmer]

Greg
Thanks for the explanation.
Sorry I was so cynical, but I have been involved in both useing and developing data, so I have seen a few dodgy tricks used to obtain a desirable result in the past.

I guess recent laws, and increasing tendencies to litigation are forceing everyone to be more realistic and transparent.

Re a previous discussion, we have some of our bronze filled nylon away being P/V tested right now. Once we have "realistic" results, I will ask our guy in Melbourne to contact you


Regards
pat