Hollow stabilizer for vehicle

A hollow stabilizer embodying the invention is made of a curved welded metal pipe. The seam of the welded pipe takes a specific position which is determined by the ratio of the layer thickness of the pipe to the outer diameter thereof and by the ratio of the radius of curvature of the curved section thereof to the outer diameter thereof. In case both ratios are relatively large, the seam is so positioned that it extends along a curve having as small or large a curvature as possible and that an angle of about 30.degree. at most is defined by a line passing the axis of the pipe and the center of curvature thereof and a line passing the axis and the seam of the pipe. In case both ratios are relatively small, the seam is so positioned that it extends along a curve having as small a curvature as possible and that an angle of 45.degree. at most is defined by said lines.

BACKGROUND OF THE INVENTION 
This invention relates to a hollow stabilizer for a vehicle, and more 
particularly to a stabilizer made of one metal pipe and having a torsion 
section to be coupled to a chassis, a pair of arm sections to be coupled 
to a wheel suspension and a pair of curved sections each connecting the 
torsion section and the corresponding arm section. 
There is tendency that stabilizers for vehicles are made of a pipe in order 
to reduce their weight. They are usually made of a seamless pipe, not a 
welded pipe such as an electric-welded pipe. A seamless pipe is strong 
against fatigue and provides a reliable stabilizer. But it is expensive. 
By contrast, a welded pipe is inexpensive. But its seam fatigues more 
quickly than the other portion, particularly when the pipe is repeatedly 
subjected to a large stress. This is why a welded pipe is not usually used 
to form a stabilizer for a vehicle despite its low cost. 
It is an object of this invention to provide a hollow stabilizer for a 
vehicle, which is made of a welded pipe and which is as strong against 
fatigue and as reliable as a stabilizer made of a seamless pipe. 
SUMMARY OF THE INVENTION 
In order to accomplish the above-mentioned object, the invention uses a 
welded pipe and the seam or bead of the pipe takes a specific position 
which is determined by the ratio of the radius of curvature of the curved 
sections to the outer diameter thereof (hereinafter called "curvature 
ratio") and by the ratio of the wall thickness of the pipe to the outer 
diameter thereof (hereinafter called "thickness ratio"). In one preferred 
embodiment of the invention, in case both the thickness ratio and the 
curvature ratio are relatively large, the seam is so positioned that it 
extends along a curve having as small or large a curvature as possible and 
that an angle of about 30.degree. or less is defined by a line passing the 
axis of either curved section and the center of curvature thereof and a 
line passing the axis of either curved section and the seam. On the other 
hand, in case both the thickness ratio and the curvature ratio are 
relatively small, the seam is so positioned that it extends along a curve 
having as small a curvature as possible and that an angle of about 
45.degree. or less is defined by a line passing the axis of either curved 
section and the center of curvature thereof and a line passing the axis of 
either curved section and the seam.

DETAILED DESCRIPTION 
An embodiment of this invention will now be described with reference to the 
accompanying drawings. As shown in FIG. 1, a hollow stabilizer 1 according 
to the invention is made of a welded metal pipe and consists of a torsion 
section 2, a pair of arm sections 3 (only one being shown) and a pair of 
curved sections 4 (only one being shown) connecting the torsion section 2 
to the arm sections 3. To use the stabilizer 1, the torsion section 2 is 
coupled to, for example, an automobile chassis (not shown) by means of a 
connecting member 5 and the free end portions 6 or connecting portions 6 
of the arm sections 3 are coupled to a wheel suspension (not shown). 
In FIG. 1, a curve X-X is the axis of the stabilizer 1, a line Y-Y is the 
center line of the automobile chassis, and O denotes the center of 
curvature of the curved section 4. A line A-A intersects with the axis X-X 
to define the center of the connecting portion 6. A line O-B denotes the 
interface between the arm section 3 and the curved section 4, and a line 
O-C the interface between the torsion section 2 and the curved section 4. 
A line D-D intersects with the axis X-X to define the center of the 
connecting member 5. 
When the stabilizer of the above-mentioned structure is exerted with load, 
a bending stress .sigma..sub.B and a torsional stress .sigma..sub.T are 
distributed along the axis X-X as illustrated in FIG. 2(a) and 2(b), 
respectively. Thus, a principal stress .sigma..sub.O composed of bending 
stress .sigma..sub.B and torsional stress .sigma..sub.T is distributed 
along the axis X-X as illustrated in FIG. 2(c). The maximum principal 
stress is exerted on that portion of the curved section 4 which lies in 
plane O-Z which in turn lies somewhere between the interfaces O-B and O-C. 
In plane O-Z, bending stress .sigma..sub.B is distributed in the 
circumferential direction of the stabilizer as shown in FIG. 4(a), and 
torsional stress .sigma..sub.T is distributed in the circumferential 
direction of the stabilizer as shown in FIG. 4(b). Principal stress 
.sigma. is therefore expressed as: 
EQU .sigma.=.sigma..sub.B /2.+-.(.sigma..sub.B.sup.2 
/4+.sigma..sub.T.sup.2).sup.1/2. 
.sigma..sub.B /2+(.sigma..sub.B.sup.2 /4+.sigma..sub.T.sup.2).sup.1/2 
denotes the principal stress applied on that portion of the stabilizer 1 
which is expanded, and .sigma..sub.B /2-(.sigma..sub.B.sup.2 
/4+.sigma..sub.T.sup.2).sup.1/2 denotes the principal stress applied on 
that portion of the stabilizer 1 which is compressed. Hence, principal 
stress .sigma. is distributed in the circumferential direction of the 
stabilizer 1 as illustrated in FIG. 4(c). In FIG. 4(a) and FIG. 4(c), E 
and F designate the points where the maximum principal stresses are 
applied to those portions of the curved section 4 which lie in the plane 
O-Z (FIG. 1), and G and H the points where the maximum principal stresses 
are exerted on those portions of the curved section 4 which lie in the O-Z 
plane. The points E and F are located on curves the curvature of which is 
equal to that (1/R.sub.0) of the axis X-X, i.e. the inverse number of the 
radius R.sub.0 of the curved axis X-X. The point G is on a curve the 
curvature of which is the smallest (1/R.sub.2), and the point H on a curve 
the curvature of which is the largest (1/R.sub.1). Both theoretically and 
experimentally it has been ascertained that principal stress is 
distributed in the circumferential direction of the stabilizer 1 as 
illustrated in FIG. 4. The stabilizer embodying this invention was put to 
a fatigue test. The result of the test proved that those portions of the 
stabilizer which correspond to points E and F fatigued most and that the 
stabilizer was eventually broken at those portions. 
That is, a great principal stress exerts on the curved section 4. And in 
the plane O-Z the maximum stress exerts on points E and F which lie 
respectively on two curves the curvature of which is equal to that of the 
curved axis X-X. From this fact it is well supposed that in any other 
plane perpendicular to the axis X-X of the curved section 4 maximum 
principal stress is applied on the points lying on two curves the 
curvature of which is nearly equal to that of the axis X-X. In other 
words, minimum principal stress is applied on those portions of the curved 
section 4 which lie on the curves of the largest curvature and the 
smallest curvature. 
This invention is based on the above-mentioned facts. It consists in 
positioning a welded pipe in such a manner that its seam 3a which fatigues 
more than any other portion extends along a curve of substantially the 
largest or smallest curvature of all the curves that define the curved 
section of the pipe. In the embodiment of FIG. 1 the seam 3a extends along 
a curve of the largest curvature. This measure taken, the welded pipe can 
provide a hollow stabilizer which is as strong against fatigue as a hollow 
stabilizer made of a seamless pipe. 
FIG. 5 shows the relation between the repetition number N of load tests and 
the maximum principal stress .sigma., the former plotted on the horizontal 
axis and the latter plotted on the vertical axis. A solid line M 
represents the fatigue characteristic of a hollow stabilizer made of a 
manganese steel (JIS SMn443) seamless pipe having a wall thickness of 2.3 
mm and an outer diameter of 17.3 mm. Marks "o" denote the fatigue 
characteristic of a hollow stabilizer made of a manganese steel (JIS 
SMn443) welded pipe of the same dimensions which has its seam positioned 
along a curve on which point G lies and which has the smallest curvature. 
Marks ".DELTA." denote the fatigue characteristic of a hollow stabilizer 
made of a manganese steel (JIS SMn443) welded pipe of the same dimensions 
which has its seam positioned along a curve on which point E lies and 
which has the same curvature as that of the axis X-X of the curved 
section. As FIG. 5 clearly shows, the stabilizer with its seam positioned 
on a curve on which point G lies is substantially as strong against 
fatigue as the stabilizer made of a seamless pipe of the same material and 
the same size. By contrast, the stabilizer with its seam positioned on a 
curve on which point E lies turns out to be far less strong against 
fatigue that the stabilizer made of a seamless pipe of the same material 
and the seam size. 
In the plane O-Z, principal stress is distributed in a different manner in 
the circumferential direction of the stabilizer 1 according to the 
thickness ratio m(=t.sub.0 /d), where t.sub.0 is the wall thickness of the 
stabilizer 1 and d is the outer diameter thereof or the curvature ratio 
c(=R.sub.0 /d) of the curved section 4. As shown in FIG. 6, the smaller 
the ratios m and c are, the closer is that portion of the section 4 to 
point H, which is exerted with the maximum principal stress. This will be 
evident when curves .sigma..sub.1 to .sigma..sub.4 shown in FIG. 6 are 
compared. 
A number of hollow stabilizers were made of seamless pipes having an outer 
diameter d of 24 mm, a layer thickness t.sub.0 of 2 mm and, thus, a 
thickness ratio m of 0.083. The stabilizers are put to repeated fatigue 
tests. Upon the tests it was found that the maximum principal stress was 
applied on that portion of the curved section which was spaced from point 
E toward point H by an angular distance of about 30.degree. to 40.degree.. 
It was also found that the smaller the curvature ratio c was, the greater 
was the maximum principal stress. 
The fatigue tests suggest that if a hollow stabilizer is made of a welded 
pipe having a thickness ratio m and a curvature ratio c both relatively 
small, the seam of the pipe should better be positioned at or near point 
G. That is, the seam should better extend along a curve having as small a 
curvature as possible. More specifically, as shown in FIG. 3, it is 
desired that the seam be positioned at point G or be spaced from point G 
by an angular distance of .alpha./2 or less, where .alpha. is about 
90.degree.. On the other hand, if a hollow stabilizer is made of a welded 
pipe having a thickness ratio m and a curvature ratio c both relatively 
large, it is desired that the seam be positioned at point G or H or be 
spaced from point G or H by an angular distance of .beta./2, where .beta. 
is about 60.degree.. 
Therefore, according to this invention, in case the stabilizer 1 has a 
thickness ratio m and a curvature ratio c both relatively large, the seam 
is so positioned that it extends along a curve having as small or large a 
curvature as possible and that an angle of about 30.degree. or less is 
defined by a line passing the axis X-X and the center O and a line passing 
the axis X-X and the seam. On the other hand, in case the stabilizer 1 has 
a ratio m and a ratio c both relatively small, the seam is so positioned 
that it extends along a curve having as small a curvature as possible and 
that an angle of about 45.degree. or less is defined by a line passing the 
axis X-X and the center O and a line passing the axis X-X and the seam. 
Thus positioned, the seam is not exerted with the greatest principal 
stress, which is applied instead on the other portions stronger against 
fatigue. Although it is made of a welded pipe, the hollow stabilizer 
according to this invention is substantially as strong against fatigue and 
thus as reliable as a hollow stabilizer made of a seamless pipe and is 
less expensive than a hollow stabilizer made of a seamless pipe.