1446Rear end tail extension

Windtunnel investigation with different shaped tail models have shown that the minimal drag coefficients were produced with extended tails, see Fig. 14.33(a and b), but this shape would be impractical for design reasons. Conversely if the rear end tail is

Large mass

Small trailing

Large mass

Small trailing

(a) Downturned nose profile
Small mass air flow overhead

(c) Upturned nose profile

Fig. 14.26 (a-c) A greatly exaggerated air mass distribution around a car body for various nose profiles

(c) Upturned nose profile

Fig. 14.26 (a-c) A greatly exaggerated air mass distribution around a car body for various nose profiles cropped at various lengths and curved downwards there is an increase in the drag coefficient with each reduction in tail length beyond the rear wheels.

14.4.7 Underbody roughness (Fig. 14.34(a and b))

The underbody surface finish influences the drag coefficient just as the overbody curvature, tapering, edge rounding and general shape dictates the drag resistance. Moulding in individual compartments in the underfloor pan to house the various components and if possible enclosing parts of the underside with plastic panels helps considerably to reduce the drag resistance. The underside of a body has built into it many cavities and protrusions to cater with the following structural requirements and operating

Fig. 14.27(a and b) Influence of forebody bonnet (hood) edge shape on drag coefficient

Radius or chamfer (mm)

Fig. 14.27(a and b) Influence of forebody bonnet (hood) edge shape on drag coefficient

Bonnet (hood) slope

8 Windscreen rake angle

Bonnet (hood) slope

Bonnet slope angle (a) deg

Fig. 14.28(a-c) Bonnet slope and windscreen rake angle versus drag coefficient

40 50

Windscreen rake

Bonnet slope angle (a) deg

Fig. 14.28(a-c) Bonnet slope and windscreen rake angle versus drag coefficient

40 50

Windscreen rake

Fig. 14.29 (a and b) Effect of roof camber on drag coefficient

Roof camber (h/l)

Fig. 14.29 (a and b) Effect of roof camber on drag coefficient

0*2

Fig. 14.31 (a and b) Effect of rear side panel taper on drag coefficient

Fig. 14.31 (a and b) Effect of rear side panel taper on drag coefficient

Diffuser angle (P) deg

Fig. 14.32(a and b) Effect of rear end upward taper on drag coefficient

Diffuser angle (P) deg

Fig. 14.32(a and b) Effect of rear end upward taper on drag coefficient components: front and rear wheel and suspension arch cavities, engine, transmission and steering compartment, side and cross member channelling, floor pan straightening ribs, jacking point straightening channel sections, structural central tunnel and rear wheel drive propeller shaft, exhaust system catalytic converter, silencer and piping, hand brake cable, and spare wheel compartment etc. A rough under-body produces excessive turbulence and friction losses and consequently raises the drag coefficient, whereas trapped air in the underside region slows down the air flow and tends to raise the underfloor pressure and therefore positive lift force. Vehicles with high drag coefficients gain least by smoothing the underside. The underfloor roughness or depth of irregularity defined as the centre line average peak to valley height for an average car is around + 150 mm. A predictable relationship between the centre line average roughness and the drag coefficient for a given ground clearance and vehicle length is shown in Fig. 14.34(a and b).

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