Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to tractor-trailer combinations
and, more particularly~ to streamlining the trailer in a manner so as to
reduce wind drag.
A number of different proposals have been made for reducing
wind drag on mobile articulated vehicles as exemplified by the following
United States patents: 3,711,146 issued January 16, 1973 to Madzsar,
3,971,586 issued July 27, 1976 to Saunders, 3,815,948 issued June 11,
1974 to Alford, 3,243,192 issued March 29, 1966 to Franzel, 3,425,740
issued February 4, 1969 to de Vaughn, 2,118,127 issued May 24, 1938
to Wulle, 2,098,516 issued November 9, 1937 to Pflager, and Canadian
industrial design registration D 12365/59 registered April 29, 1939.
Reference may also be had to an article entitled "Wind Tunnel Tests"
published in the April 1955 issue of Commercial Car Journal. A copy
of this article and copies of the drawings of the aforementioned patents
are attached hereto for convenience of the reader.
In these proposals there is included variously located
deflectors or rounding of corners on the upper, rear and/or front
portion of the box of the traller. While the various designs no doubt
have consiterable effect in reducing wind drag, none have given any
conslderation to the shape of the bottom of the trailer box.
Accordingly, a principal ob~ect of the present invention i8
to reduce wind drag by streamlining the bottom wall of the trailer box.
A further principal ob~ect of the present invention i9 to
proyide a trailer having minimum wind drag characteristics through
the cumulative effect of streamlining the front, top, side, back and
bottom walls of the box of the trailer.
Accordingly, in accordance with one a3pect of the present
invention, there is provided a trailer for a tractor-trailer vehicle
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which has reduced wind drag characteristics and wherein the bottom has
a stepped down portion extending from a first position spaced rearwardly
from the leading end of the trailer to a second position spaced forwardly
of the trailing end of the trailer, wherein the leading end of the
stepped down portion i9 convex tapering upwardly and inwardly in a
direction forwardly toward the leading end of the trailer and wherein
the trailing end of the stepped down portion is concave tapering upwardly
and outwardly in a direction toward the trailing end of the trailer.
In accordance with a further aspect of the present invention
there is provided a trailer having a bottom as defined in the foregoing
and further including a rounded vertical front wall having opposed
ed8es contiguous with the respective side walls, a roof with rounded
corners merging into the respective side walls and the rounded front
wall.
The invention is illustrated by way of example with reference
to the accompanying drawings wherein:
Figure l is a side elevational view of a tractor-trailer having
a trailer providet in accordance with the present invention;
Figure 2 is a top plan vlew of Figure l;
Figure 3 is a graph illustrating the influence of rounded corners
on drag; and
Figure 4 18 a graph illustraClng the effect of the shape of a
trailer~s front wall on wind trag coefficient under different wind
conditions.
Referring to the drawings, there is illustrated in Figure 1
a tractor-trailer consisting of a tractor unit 10 and a trailer unit 20.
The tractor lO is of conventional design having on the roof of the cab,
and illustrated by broken line, a teflector ll commonly used to reduce
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wind drag effect of the trailer. Such deflector, however, is unnecessary
with applicant's trailer design to be described.
The trailer 20 has opposed side walls 21 and 22, a front wall
23, a rear wall 24, a roof or top wall 25, and a bottom wall 26. The
front wall has a vertical portion extending substantially to the height
of the cab and is rounded in top plan view, as seen in Figure 2,
terminating in edges 27 and 28 which are contiguous with the resepctive
side walls 22 and 21. ~Jhile the top plan view shape of the front wall 23
i9 shown as part circular, the preferred shape for minimum drag is that of
part elliptical. The roof has rounded, longitudinally extending corners
21A and 22A merglng with the re~pective side walls 21 and 22 and a rounded
top front 23A merging with the front wall 23. The preferred shape of
rounded top front 23A is elliptical, i.e. at any point in the curvature, a
secticn taken either horizontally or vertically (transverse to the side
,
wall) displays a line of the outer wall which is elliptical.
The front end of the trailer is carried by the tractor and
supported thereon in a conventional manner by a fifth wheel 12.
The bottom wall 26 has a central stepped down portion commencing
at a first positlon designated by the reference numeral 29 spaced rearwardly
fromthetrallingend of the trailer defined by wall 23 to a second position
designated by reference numeral 30 located forwardly of the trailing end
of the trailer. The leading end of the stepped down portion has a stream-
lined part, lndicated generally by the reference numeral 31, which i9 convex
taperlng upwardly and inwardly in a direction forwardly toward the leading
end of the trailer. The trailing end of the stepped down portion is concave
Ln a directLon transversely across the width of the trailer as indicated by
the curved ~roken line 32~, and which concave portion tapers upwardly in
a direction toward the trailing end of the trailer as indicated by
tl-e broken line 32. The stepped down portion provides additional
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interior space for contents in the trailer. Should, however, a flat
bottom floor be desired, the additional space in the stepped down
portion may be used as a separate compartment accessible through a door
(or doors) in the floor and/or side walls of the trailer.
The trailing end of the trailer is carried by a plurality of
wheels 33 attached, by way of spring~, to the trailing end of the trailer
in a conventional manner and such wheels are located rearwardly of the
trailing end of the stepped down bottom wall. As clearly seen in
Figures l and 2, the rear wall of the trailer merges into the side walls
through rounded corners and into the top wall through a rounded corner.
A trailer of the foregoing design is intended primarily for
hsuling wood chips but obviously may be used for other loads. The shape
of the trailer with the stepped down bottom wall increases the volume
capacity permitting increased load sizes. Since chips are typically
not a dense load, existing trailer units used for such purpose are often
well below legal weight limits resulting in higher costs/unit than the
trsiler of the present design. The streamlinlng of the present trailer
decreases wind drag considerably resulting in improved gas mileage,
higher travel speeds, lower maintenance costs and in smaller engines
being requlred in the tractor to pull the traller at the same speeds.
There 18 also considerably improved truck performance under cross-wind
conditions. There are perhaps other intangible benefits which include
reduced nolse levels, lmproved truck handling, less driver fatigue, less
spray and dust behind the trailer and increa~ed safety.
The reduction in dra8 occurs in several areas and these have
a cumulative ef$ect. The reduction in dra8 occurs from a reduced gap
separatlon between the traller and the tractor because of the rounded front
wall on the trailer~ The falred or rounded nose on the trailer top at
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the leading end reduces the drag as does also the rounded corners at the
rear and the lowered or stepped down trailer bottom with smoothing at
the front and the cancave recess immediately ahead of the rear wheel
assembly.
The rounded front wall provides improved manoeuverability over
square front trailers with similar king-pin locations. The trailer can
swing through more than 270 degrees without contacting the tractor, at
no increase in steering effort.
The reduction in the gap between the tractor and trailer permits
,: 10 addltional load weight to be transferred onto the tractor, since the
load~s centre of gravity is transferred forwardly. This will produce
better handllng, improved traction and smoother riding.
From an aerodynamic point of view, there are six major areas
that are considered critical, namely the gap between tractor and trailer,
the slope of the faired trailer top, the shape of the faired trailer top,
the rear corner radii, the frontslope of the bottom compartments, and
the trailer surface.
Minimization of the gap between tractor and trailer is of ma~or
importance in minimizing aerodynamic drag. The table below illustrates
the effect of separation on the wind averaged drag coefficient (a term
; which is believed well known in the art) for a square front trailer.
SEPARATION
i 13 0.88
` 23 0.91
33 O.g4
43 0.98
53 0.99
63 1.00
`` 73 1.00
~ 83 0.98
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.,
In the above table, separation is the space between the tractor cab and
leading end of the trailer in inches and CD is the wind averaged drag
coefficient. In the present design the separation proposed is approximately
12 inches but, if desired, this can be decreased considerably.
The surface roughness of the trailer has some effect on aero-
dynamic drag. For example, exterior structural posts and corrugated
sheet metal walls have 10~ percent more drag than interior posts with
corrugated metal walls. The present trailer uses a smooth material, such
as fibreglass reinforced plywood for surfaces with interior posts.
The rounding of trailer corners reduces aerodynamic drag, as
is lllustrated by the graph in Figure 3 wherein there is plotted CD
(wind averaged drag coefficient) versus the radius in inches of the rounded
rear wall corners. Rear corner rounding with an 18 inch radius has
been found to reduce drag by 11 percent.
The shape of the faired trailer top portion 23A is important.
Figure 4 graphically illustrates the effect of shape on the drag of a
tractor-traller unit under different angles of wind. ~lliptical sections
tin vertlcal and horizontal plsnes) have been found to be more effective
than flat, concave, or concave-convex surfaces, especially as yaw angles
increase. The graph ln Figure 4 is sy~metrical and thus only the right
half has been shown.
In the foregoing, the term "rounded" is intended as defining a
surface with a continuous smooth curvature which includes part circular,
elllptical and other such shapes. In the preferred form the front wall
23 and top front wall 23A are prefersbly of elliptical shape and the
r0ar wall corners part circular with a radlus of at least 6 inches. The
wind averaged drag coefficient CD is for average wind conditions in
Canada at a truck travel speed of 55 mph.
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