Aerodynamic Drag and Rolling Resistance

[gohan]

What are the Aerodynamic Drag and the Rolling Resistance, in layman's words?

What are the typical numbers of CdA/m^2 and N/m^2 for early, middle and late 911?

Best Regards

[masraum]

Aerodynamic drag is the force created by the car forcing itself through the air opposite in direction to the motion of the car. One of the most important factors for drag is frontal area. My understanding is that the Bellows bumpers cars were around .45 for Cd. The 964 with it's under tray and redesigned parts was around .32, and I think since the 964 things have stayed between .30 and .35. I believe the early cars were closer to the late cars in the mid to low .3 range.

As for rolling resistance, I have no idea, and I think it would depend on the tires.

[island911]

Aerodynamic Drag & Drag Coefficient are 2 different, but related things.

Aerodynamic Drag is a total drag . . .Drag Coefficient is drag per frontal area.

A VW bus has a lower Drag Coefficient than a VW bug, but higher total aero-drag (more frontal area) .

[island911]

. . . you can measure the combination (Aerodynamic Drag and Rolling Resistance) by doing a "Roll-down" test.

This is where you map (plot) the deceleration from coasting on a flat road. (or coast both ways on an almost flat road, and average the two. . .may account for wind, that way too)

If you point a camcorder on the (accurate) speedo, you can easly plot the deceleration (speed/time). Then, throw in the weight (mass) of the car, for the rolling & drag force combo. (F=ma)

edit; . . and the 'force thru distance' is work (HP) . . .the area under any section of the ploted curve (integral) will be equal to the distance traveled for that section.

That's my shot at a layman explaination.

[tryan]

very well put island. the coefficient ( a empirical number ) is a farse in advertising unless they give you the frontal area. this Cd number compensates for flows ( turbulent or laminar ) on the top, sides and rear of the vehicle and will actually be different at different speeds, air density,windows up or down , how clean the car is, ect.


to be truthfull, frontal area x the coefficient would be more representive of true drag.

performance boats are texturing the bottoms to induce turbulent flow at the running surface to decrease drag. (golf ball therory)

[Randy Webb]

Cd is useful because it is a figure of merit, tho true it can't be used alone to extimate gas mileage, etc.

Rolling resistance is a function of tires, and of wheel bearings, etc. to a lesser extent.

from Ludvigsen:
p. 630 & Ch. 16:
up thru 1968: Cd = 0.380; frontal area = 17.9 ft2
1969: Cd = 0.410; frontal area = 18.4 ft2
an increase in total drag of 8% - due to wider tires and wheels

PAG did not have wind tunnels when it designed the original 356 or 911. In the late 70's and 80's the use of numerical computer models allowed big increases in understanding fluid flow. The basic analytical equations, however, have never been solved. A mech. eng. text will help with understanding this are, or there are some books on birds that are more conceptual. E me if interested.