Coefficient of friction and Frontal area

[Chris More]

Does anyone have some ballpark numbers for the Coefficient of friction value and the front area value (ft or m) for an average FF?

Lets say non-DB1 vehicles with side radiators.

I just need to get something close to throw into some equations I'm working on.

Thanks,
Chris

[David Ferguson]

Chris,

Most modeling is done using real-world data, and iterating until you can get results from your simulation that model the real world. Frontal area is something you can figure out yourself, but a number like 10 - 12 sq feet might be a good starting point for a FF.

Not sure what you Cf number represents, so I can't even guess.

[Mike Ahrens]

Chris,

My guess is you meant to ask for a Cd...Coefficient of drag.

[DaveW]

IIRC, the CD for an otherwise aerodynamically smooth open-wheeled (depends a lot on the size of the tires, suspension configuration, etc.) car is in the 0.35 to 0.60 range. Good closed wheel #'s are ~ 0.2 to 0.6, depending on aerodynamics (open or closed cockpit, wings, spoilers, etc.). for reference, a cube with one face perpendicular to the air flow has a CD of 1.0.

These are from memory, so I am probably not very accurate...

[Frank C]

Sometimes it is easier and more meaningful to use the product of the drag coefficient, C(sub)D, and the frontal (projected area), A. It is their product that one actually uses in calculating drag force and that really matters for the car. In addition, since the force is actually what is measured in wind tunnel (or coast down) tests, the drag coefficient is just backed out using the area. Someone else may have better numbers for the drag coefficient - it is considerably above the close closed wheel car values cited above.
- Frank C

[Chris More]

opps! I meant drag, not friction... that phrase was on the tip of my tongue. My formula uses Cx for the drag coefficient when I think most other formulas describe it as Cd.

What's the method for calculating Frontal area? It seems like one way of doing it is placing a light in front of the car level with ground. Have the light at the mid point in the height of the vehicle and then with the lights off project a shadow on a board directly behind the car. Trace the outside and find the areas of the shadow. Finding the area of the shadow probably would be complicated and time consuming and I'm not even sure my idea would even be correct. I'm just pulling it out of air, but I'm sure one of you can set me straight. I don't want to spend a lot of time on this because I have other more important things to look into. I just need to set a few variables for some calculations.

Chris

[James Hakewill]

I've been using Cd=0.5 and A=1m2, and rho = 1.19 km/m3 in the good old metric system, and this seems to match reasonably well with real data.

drag = 1/2 x rho x Cd x A x v2 .. where v is in ms-2, rho in kg/m3, A in m2

The 'Race and Rallycar Sourcebook' by Alan Staniforth quotes a for an early 80's FF - Cd = 0.525, A = 1.54m2, but this seems to me to be rather high.

James

[R. Pare]

The Cd's for open wheel cars ARE rather high, and when you add wings, they go even highet - uip to .8+ at times. Closed wheel cars have been as low as .11 (for the Mercedes C111 if I remember right).

[Frank C]

I guess I was thinking wings! Joseph Katz's "Race Car Aerodynamics" tabulates (p. 50):
1.07 - 1983 Generic (who were they?) F1, no sidepods
1.06 - 1987 March Indy
1.11 - 1991 Penske PC20, high downforce
0.74 - 1991 Penske PC20, speedway
1.397 - 1992 Galmer G92, high downforce
0.669 - 1992 Galmer G92, speedway

Hucho's "Aerodynamics of Road Vehicles" (4th Edition, p. 394) has a graph of C-drag versus C-lift for a generic open wheel car with and without wings and side pods and different skirt heights showing a drag coefficient a little below 0.5 without wings.

If James' Cd-A product matches data, I'd use that.

- Frank C
Cx is used for drag coefficient mostly in Europe

[Dave SanF 50]

Guys, I dug up the following data for frontal area:
ADF MkII 1351 in^2
Crossle 35F 1331 in^2
Lola T-540 1255 in^2
PRS RH01 1302 in^2
Van Diemen RF79 1354 in^2
Zink Z-10 1339 in^2
This was published in "Sports Car', Oct 1979. Super Test written by Paul Pfanner. No explanation as to how it was measured.

Also note, a flat plate perpendicular to the airflow has a Cx of 1.00 IN THEORY. In actuality, it's Cx is more like 1.17 because of turbulent flow around the edges.

another notepen wheel vs fender cars
"As wheels rotate on the road, their motion generates a breakdown (technically, a separation) of the adjacent airflow. Tests have shown that wheels and tires of an ordinary passenger car generate added drag of 0.005 or less. But for cars whose ratio of wheel/tire area to frontal area is significant (read: open wheeled race cars), it's necessary to simulate rotation" or have the wheels actually rotate in a wind tunnel because of the big increase in drag caused by the rotating wheels. The disruption of laminar flow around the wheels. R&T 8/1982

One final tibit: Frank Dernie of Williams (also in R&T8/82) "We find if you reduce your F1 car's drag by 5% the driver comes in and can't tell the difference - he's not interested. But if you can increase the car's downforce by 5% he gets all fired up, says it feels fantastic and drives like a hero."

Hope you all had a pleasant Mother's Day.
dave jalen