1488 Tractor and trailer skirting

Crosswinds sweeping tangentially underneath the tractor and trailer chassis and between and around the road wheels and axles produce a rise in the drag coefficient. To partially counteract the increase in vehicle drag with increased yaw angle, side skirts can be attached either to the trailer or the tractor or both units. The effectiveness of both tractor and trailer skirting for one particular commercial vehicle is shown in Fig. 14.67(a and b); here it can be seen that with increased yaw angle (relative wind angle) the effectiveness of the trailer skirt peaked at a drag coefficient of 0.07 for a yaw

0.10

Tractor

Trailer

^ skirt

0.08

5 10 15

Yaw angle deg

5 10 15

Yaw angle deg

Fig. 14.67(a and b) Effectiveness of tractor and trailer skirting upon drag coefficient

(a) Low cab without deflector

Crosswind
Crosswind

(d) Low cab + deflector + gap seal + trailer skirt

(e) Comparison of the effectiveness of

Various drag reducing deV^es § 0.7

(e) Comparison of the effectiveness of

Various drag reducing deV^es § 0.7

Yaw angle (y) deg

Fig. 14.68(a-e) Influence of various devices used to reduce drag when vehicle is subjected to crosswinds

Yaw angle deg

Fig. 14.69(a and b) Effects of trailer load position upon drag coefficient

Yaw angle deg

Fig. 14.69(a and b) Effects of trailer load position upon drag coefficient angle of 5°. The skirt effectiveness in reducing the drag coefficient then decreases steadily over an increasing yaw angle until it reaches its minimum of 0.04 at a yaw angle of 20°. Conversely the trailer skirt's effectiveness with respect to the yaw angle rose rapidly to 0.06 over a yaw angle range of 5°; the drag coefficient then continued to rise at a slower rate so that for a yaw angle of 20° the drag coefficient effectiveness reached a maximum of just over 0.09. However when considering attaching skirts to a vehicle there can be a problem with the accessibility for routine inspection and for maintenance of the steering, suspension, transmission and brakes; they can also restrict the cooling of the brake drums/discs.

14.8.9 Comparison of various devices used to reduce vehicle drag (Fig. 14.68(a-e)) A comparison of various devices used to reduce vehicle drag particularly when there are crosswinds can be seen in Fig. 14.68(a-e). The graph shows for a low cab and no roof deflector that the drag is at its highest due to the cab to trailer step and that the drag coefficient rises with increasing crosswind yaw angle (relative wide angle) from 0.48 to about 1.05 over a 15° increase in yaw angle (see Fig. 14.68(a and e)); a reduction in the drag coefficient occurs when a cab roof deflector is matched to the trailer body (Fig. 13.68(b and e)). When a cab to trailer gap seal is attached to the trailer there is a further reduction in the drag coefficient particularly with increasing yaw angles (Fig. 14.68(c and e)), and finally there is even a greater drag coefficient reduction obtained when fitting a trailer skirt (Fig. 14.68(d and e)).

14.8.10 Effects of trailer load position on a vehicle's drag resistance (Fig. 14.69(a and b)) The effects of positioning a container load on a platform container truck considerably influences the drag resistance. This becomes more noticeable with crosswinds (see Fig. 14.69(a and b)), and that with a yaw angle of 20° the drag coefficient without a container was only 0.7 whereas with the container mounted in front, centre and towards the rear, the drag coefficients reached 1.2, 1.6 and 1.7 respectively.

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