Vehicle lower section structure

A vehicle lower section structure includes: a suspension member including a side rail extending in a vehicle front-rear direction, the side rail including a front joint portion and a rear joint portion, the front joint portion being disposed on a front side of the rear joint portion in the vehicle front-rear direction; a securing member joined to the side rail at the front joint portion and the rear joint portion; and a stabilizer secured to the side rail by the securing member. The side rail includes a low-strength portion provided between the front joint portion and the rear joint portion and the low-strength portion has a strength lower than a strength of the front joint portion and the rear joint portion.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-174907 filed on Sep. 4, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle lower section structure.

2. Description of Related Art

Japanese Patent Application Publication No. 2010-089549 discloses a stabilizer mounting structure that includes a pair of side members running in a vehicle front-rear direction, a rear cross member running in a vehicle width direction and connecting the pair of side members, a pair of lower arms swingably supported by the pair of side members respectively and respectively supporting a pair of wheels W, and a stabilizer connecting suspensions of the pair of wheels. In this prior art, the side members each have a stabilizer mounting portion protruding upward or downward, and each stabilizer mounting portion is interposed between fixing portions of the side member and the lower arm, and a connection portion of the side member and the rear cross member.

Another related technique is disclosed in Japanese Patent Application Publication No. 2012-091693.

SUMMARY OF THE INVENTION

In securing the stabilizer on an upper surface of an end portion of the rear cross member of the suspension member as described in JP 2010-089549 A, the stabilizer is required not to interfere with a multiplicity of members above the rear cross member. A large stabilizer such as an active stabilizer including an electrically-controlled actuator, therefore, might be difficult to mount.

Also, when a stabilizer having a high roll stiffness is fixed to the suspension member, a load input from the stabilizer to the suspension member increases, so that the plate thickness of the suspension member needs to be increased. The thicker suspension member, however, might be less deformable during a crash, and to be decreased in impact absorbing performance. Thus, a stabilizer having a high roll stiffness might be difficult to mount.

The aspect of the invention provides a vehicle lower section structure that is improved in mountability of a stabilizer while ensuring a desired impact absorbing performance of a suspension member.

A vehicle lower section structure according to an aspect of the invention includes: a suspension member including a side rail extending in a vehicle front-rear direction, the side rail including a front joint portion and a rear joint portion, the front joint portion being disposed on a front side of the rear joint portion in the vehicle front-rear direction; a securing member joined to the side rail at the front joint portion and the rear joint portion; and a stabilizer secured to the side rail by the securing member. The side rail includes a low-strength portion provided between the front joint portion and the rear joint portion and the low-strength portion has a strength lower than a strength of the front joint portion and the rear joint portion.

DETAILED DESCRIPTION OF EMBODIMENTS

A vehicle lower section structure according to an embodiment of the invention will be described usingFIGS. 1 to 15. Arrows FR, UP, and OUT that are shown as appropriate in the figures respectively denote forward in a vehicle front-rear direction, upward in a vehicle up-down direction, and outward in a vehicle width direction. In the following description, the words front and rear, upper and lower, inner, outer, left, and right respectively mean front and rear sides in the vehicle front-rear direction, upper and lower sides in the vehicle up-down direction, inner, outer, left, and right sides in the vehicle width direction, unless otherwise specified.

A vehicle12to which a vehicle lower section structure10according to the embodiment shown inFIG. 1is applied has a suspension member20at a lower front portion of the vehicle. The suspension member20of the embodiment is a casting made of an alloy mainly containing aluminum.

The suspension member20has on both vehicle width direction outer sides a pair of side rails26extending in the vehicle front-rear direction. The suspension member20is formed by laying a front cross member22and a rear cross member24extending in the vehicle width direction, across the side rails26, and has a generally rectangular frame-like shape in a plan view. Four corner portions of the suspension member20are joined to front side members (not shown) arranged at both vehicle width direction side end portions of the vehicle12in the vehicle front-rear direction. InFIG. 1, portions other than the relevant portion of the invention are only shown in an outline and ribs and other configurations are omitted.

As shown inFIGS. 1 and 2, each side rail26of the suspension member20has a stabilizer mounting portion30on a lower surface26A at a middle portion of the side rail26in the vehicle front-rear direction. A stabilizer bracket70is bolted to the stabilizer mounting portion30. The figures other thanFIG. 1illustrate the relevant portion of the left side rail26(on the right side in the vehicle12) shown inFIG. 1. The relevant portion of the right side rail26(on the left side in the vehicle12) shown inFIG. 1has the same structure as that in the left side rail26except that they are symmetrical to each other.

The stabilizer mounting portion30of the side rail26has a front joint hole32(seeFIG. 3A) and a rear joint hole34(seeFIGS. 3C and 5) that are spaced apart from each other in the vehicle front-rear direction.

As shown inFIGS. 1, 2, and 6 to 8, a front end portion72and a rear end portion74of the stabilizer bracket70that secures a stabilizer100(seeFIG. 6) are respectively bolted to the front joint hole32(seeFIG. 3A) and the rear joint hole34(seeFIGS. 3C and 5) of the stabilizer mounting portion30.

As shown inFIG. 6, the stabilizer100in the embodiment is an active stabilizer including a stabilizer main body102and an electrically-controlled actuator104. The stabilizer100connects suspensions of a pair of left and right wheels not shown and inhibits roll of the vehicle12.FIG. 6shows the same portion as inFIG. 2but also illustrates variable components including the stabilizer100arranged on the side rail.

As shown inFIGS. 8 and 9, the stabilizer bracket70, an example of a securing member, includes a bracket lower80and a bracket upper90. As shown inFIGS. 8B and 9, the bracket lower80is formed by a U-shaped body portion86and flange portions82,84extending in the vehicle front-rear direction. The bracket upper90is formed by a body portion96and flange portions92,94extending in the vehicle front-rear direction, and has a hat shape in a side view.

A stabilizer bush76is provided between the body portion86of the bracket lower80and the body portion96of the bracket upper90. The flange portions82,84are respectively aligned to the flange portions92,94to integrate the bracket lower80and the bracket upper90. The integrated flange portion82and the flange portion92form the front end portion72and the integrated flange portion84and the flange portion94form the rear end portion74.

As shown inFIGS. 2, 9, and 10, a portion of the stabilizer mounting portion30on the side rail26of the suspension member20where the front joint hole32(FIG. 3A) is formed and the front end portion72of the stabilizer bracket70is bolted is called a front joint portion42. Similarly, a portion of the stabilizer mounting portion30where the rear joint hole34(seeFIGS. 3C and 5) is formed and the rear end portion74of the stabilizer bracket70is bolted is called a rear joint portion44. A portion of the stabilizer mounting portion30between the front joint portion42and the rear joint portion44is called a middle portion46.

Here,FIG. 3A,FIG. 3B,FIG. 3C, andFIGS. 3D and 4are sectional views respectively taken along the line3A-3A,3B-3B,3C-3C, and3D-3D inFIG. 2, each illustrating the side rail26with the stabilizer bracket70removed. The line5B-5B inFIG. 5Aand the line7-7inFIG. 6are drawn at the same position as that of the line3C-3C inFIG. 2.FIG. 5Ais a plan view of the side rail26with the stabilizer bracket70removed,FIG. 5Bshows the same portion as that inFIG. 3Cbut also shows a gearbox mounting portion60to be described later, andFIG. 7shows the same portion as that inFIG. 5Bbut also shows the stabilizer bracket70, a gearbox122to be described later, and other components.

A thickness t1of the front joint portion42shown inFIG. 3Aand a thickness t2of the rear joint portion44shown inFIG. 3Care greater than a thickness t3of the middle portion46shown inFIG. 3B. In other words, the front joint portion42and the rear joint portion44are stronger than the middle portion46. That is, the middle portion46has a lower strength. In the embodiment, the thickness t1of the front joint portion42is the same as the thickness t2of the rear joint portion44. Accordingly, the thicknesses t1, t2, t3are in a relationship of t1=t2>t3.

As shown inFIGS. 2, 3B, 9, and 10, at a vehicle width direction outer area of the middle portion46in the stabilizer mounting portion30is formed a thick rib portion48(FIG. 3B) as an example of a reinforcing portion. The rib portion48is formed along the vehicle front-rear direction such that the rib portion48connects the front joint portion42(FIG. 3A) and the rear joint portion44(FIG. 3C) as shown inFIG. 2. Here, as shown inFIGS. 9 and 10, the rib portion48has a concavely curved profile protruding upward in the vehicle up-down direction.

A thick portion50as inFIGS. 3D and 4is formed on a rear side of the rear joint portion44of the stabilizer mounting portion30as shown inFIG. 2. A concave portion52is formed on a vehicle width direction inner side of the thick portion50, and a stepped portion54is formed on an inner side of the concave portion52. The stepped portion54is positioned on a vehicle front-rear direction rear side of the gearbox mounting portion60to be described later, in the vicinity of the gearbox mounting portion60as shown inFIG. 2.

The thickness t4of the thick portion50is less than the thickness t1of the front joint portion42(FIG. 3A) and the thickness t2of the rear joint portion44(FIG. 3C), but greater than the thickness t3of the middle portion46(FIG. 3B). That is, the thicknesses t1to t4are in a relationship of t1=t2>t4>t3. The thick portion50is wider than the rear joint portion44(FIG. 3C) and thus has a larger cross-sectional area than the rear joint portion44.

As shown inFIG. 6, a tie rod end boot110is placed on a lower side of the side rail26of the suspension member20, on a vehicle front-rear direction rear side of the stabilizer bracket70and the stabilizer100. A rack parallel-type electric power steering120is placed on an inner side of the tie rod end boot110, on a vehicle width direction inner side of the side rail26.

As shown inFIG. 4, the thick portion50has a thickness (height) that does not interfere with the tie rod end boot110.FIG. 4illustrates the tie rod end boot110at the highest position within a movable range (a position nearest to the side rail26).

As shown inFIGS. 2 and 5A, a gearbox mounting portion60is provided on a vehicle width direction inner side of the rear joint portion44in the side rail26of the suspension member20. The gearbox mounting portion60has a circular shape in a plan view. The gearbox mounting portion60is formed to expand downward in the vehicle up-down direction as shown inFIG. 5Band has a mounting hole62at the central portion of the gearbox mounting portion60as shown inFIGS. 2, 5A, and 5B. As shown inFIGS. 5A and 5B, an arc-shaped boundary area between the rear joint portion44and the gearbox mounting portion60is called a boundary portion64.

FIG. 14illustrates a rear joint portion344and a gearbox mounting portion360in a side rail326of a suspension member320of a vehicle lower section structure310according to a comparative example of the embodiment. An arc-shaped boundary area between the rear joint portion344and the gearbox mounting portion360is called a boundary portion364.

A level difference L1between the rear joint portion44and the gearbox mounting portion60in the embodiment shown inFIG. 5Bis less than a level difference L2between the rear joint portion344and the gearbox mounting portion360in the comparative example shown inFIG. 14B(L1<L2). Accordingly, the boundary portion64between the rear joint portion44and the gearbox mounting portion60in the embodiment shown inFIG. 5Bhas a radius of curvature R1that is larger than a radius of curvature R2of the boundary portion364between the rear joint portion344and the gearbox mounting portion360in the comparative example shown inFIG. 14B(R1>R2). The boundary portion64in the embodiment shown inFIG. 5Bhas a thickness L3that is greater than a thickness L4of the boundary portion364in the comparative example shown inFIG. 14B(L3>L4).

As shown inFIG. 7, the gearbox122forming the rack parallel-type electric power steering120(seeFIG. 6) is bolted to an upper surface of the gearbox mounting portion60(also seeFIG. 5) in the side rail26of the suspension member20. An alternator124is placed on an upper side of the gearbox122.

Next, operation and effect of the embodiment will be described.

Compared with the front cross member22and the rear cross member24of the suspension member20, the side rail26has a small number of components arranged in its surrounding area. Also, as shown inFIG. 7, the alternator124is placed on an upper side of the side rail26. Thus, the stabilizer100is fixed through the stabilizer bracket70to the lower surface26A of the side rail26of the suspension member20where the stabilizer100is less likely to interfere with other components, thereby improving mountability of the stabilizer100(flexibility in mounting the stabilizer100).

The improved mountability of the stabilizer100enables the employment of the active stabilizer including the electrically-controlled actuator104as the stabilizer100like in the embodiment (which means an active stabilizer can be mounted easily). The rack parallel-type electric power steering120can also be mounted.

At the same time, as shown inFIGS. 2 and 3and others, the middle portion46in the stabilizer mounting portion30of the side rail26, between the front joint portion42at which the front end portion72of the stabilizer bracket70is joined and the rear joint portion44at which the rear end portion74is joined is thinner and thus weaker than the front joint portion42and the rear joint portion44.

Accordingly, when an impact load is input to the side rail26in the vehicle front-rear direction, the side rail26is deformed around the middle portion46. This ensures a desired impact absorbing performance of the side rail26even when the stabilizer bracket70, which might prevent deformation of the side rail26, is joined.

Thus, the embodiment improves mountability of the stabilizer100(flexibility in mounting the stabilizer100) while ensuring a desired impact absorbing performance of the suspension member20.

On the vehicle front-rear direction rear side of the rear joint portion44in the side rail26is formed the thick portion50that is thicker and stronger than the middle portion46(seeFIG. 4). The formation of the thick portion50causes a stiffness differences between the middle portion46and the thick portion50, and thus, this allows the side rail26to be deformed around the middle portion46further easily to improve the impact absorbing performance of the side rail26.

The vehicle12to which the vehicle lower section structure10of the embodiment is applied was subjected to various crash tests (frontal crash and small-overlap crash), and an example of results of the tests is shown inFIG. 15. A dashed line SS in the figure indicates broken areas. Like this, it has been observed that in the suspension member20of the embodiment, the side rail26is deformed and broken around the middle portion46to ensure a desired impact absorbing performance. Although broken areas and broken shapes and others varied with conditions of the crash tests, the side rail26was observed to be deformed and broken around the middle portion46to ensure a desired impact absorbing performance in any of the tests.

As shown inFIG. 4, while the thickness t4of the thick portion50is less than the thickness t1of the front joint portion42(FIG. 3A) and the thickness t2of the rear joint portion44(FIG. 3C), the thick portion50is made wide to ensure a cross-sectional area that is needed for a desired strength. Thus, the thick portion50will not interfere with the tie rod end boot110even when the tie rod end boot110moves in the vehicle up-down direction. That is, the configuration ensures mountability of the tie rod end boot110while improving the impact absorbing performance of the side rail26.

As shown inFIGS. 2, 3B, 9, and 10, at the vehicle width direction outer area of the middle portion46in the stabilizer mounting portion30is formed the rib portion48that runs in the vehicle front-rear direction and connects the front joint portion42(FIG. 3A) and the rear joint portion44(FIG. 3C). The rib portion48increases the strength against the load input by the stabilizer100via the stabilizer bracket70into the side rail26in the vehicle up-down direction.

Thus, the embodiment ensures the stabilizer mounting portion30a desired strength against the load (road inputs) input by the stabilizer100via the stabilizer bracket70into the side rail26in the vehicle up-down direction during travel on a bad road, while inhibiting prevention of the deformation of the side rail26caused by an impact load in the vehicle front-rear direction (which means while ensuring a desired impact absorbing performance of the suspension member20). In other words, the side rail26has an improved ability to withstand vertical bending deformation caused during travel on a bad road.

Next, a description will be give of the vehicle lower section structure310of the comparative example to which the embodiment is not applied.

The vehicle lower section structure310in the comparative example shown inFIGS. 11, 12, and 13has a stabilizer mounting portion330on the side rail326of the suspension member320.

In the stabilizer mounting portion330of the comparative example, a front joint portion342to which the front end portion72(seeFIG. 2) of the stabilizer bracket70is bolted, the rear joint portion344to which the rear end portion74(seeFIG. 2) is bolted, and the middle portion346between the front joint portion342and the rear joint portion344have the same or almost the same thickness, and are thinner than the corresponding portion in the embodiment. Also, a portion350of the stabilizer mounting portion330on a rear side of the rear joint portion344(a portion corresponding to the thick portion50(FIGS. 3D and 4)) has the same or almost the same thickness as that of the front joint portion342, the rear joint portion344, and the middle portion346.

FIGS. 11, 12C, and 13Cillustrate a bending deformation of the side rail326due to the load (road inputs) input by the stabilizer100(seeFIG. 6) via the stabilizer bracket70into the side rail326of the suspension member320in the vehicle up-down direction during, for example, travel on a bad road, and also illustrate an example stress distribution at the bending deformation. Higher density of the dots means higher stress. Portions subjected to great deformation and high stress are called a high-stress portion HA, a high-stress portion HB, and a high-stress portion HC. InFIGS. 12C and 13C, the amount of the vertical bending deformation of the side rail326is illustrated in an exaggerated manner for ease of understanding.

Portions of the side rail26in the suspension member20of the embodiment that correspond to the high-stress portion HA, the high-stress portion HB, and the high-stress portion HC are illustrated inFIGS. 2, 5, and 10.FIG. 10corresponds toFIG. 11of the comparative example and illustrates stress distribution at the vertical bending deformation.

The high-stress portion HA of the embodiment has the rib portion48(seeFIGS. 2, 3B, 9, and 10) formed in the vehicle front-rear direction to have increased strength over that in the comparative example. Similarly, the high-stress portion HB has the thick portion50to have increased strength over that in the comparative example (seeFIGS. 3D and 4). The increased strengths suppress the vertical bending deformation in the high-stress portion HA and the high-stress portion HB in the side rail26in the suspension member20of the embodiment during travel on a bad road, and reduces the stress as shown inFIG. 10(see and compareFIGS. 10 and 11).

Moreover, the radius of curvature R1of the boundary portion64of the high-stress portion HC in the embodiment shown inFIG. 5Bis larger than the radius of curvature R2of a boundary portion364in the comparative example shown inFIG. 14, and the thickness L3of the boundary portion64in the embodiment shown inFIG. 5Bis greater than the thickness L4of the boundary portion364in the comparative example shown inFIG. 14B(L3>L4). Accordingly, the boundary portion64of the high-stress portion HC in the embodiment has increased strength over the boundary portion364of the high-stress portion HC in the comparative example. This inhibits the vertical bending deformation of the high-stress portion HC in the side rail26in the suspension member20of the embodiment during travel on a bad road to reduce the stress as shown inFIG. 10(see and compareFIGS. 10 and 11).

Furthermore, the embodiment allows the stabilizer100(active stabilizer) including the electrically-controlled actuator104and the rack parallel-type electric power steering120to be mounted as shown inFIG. 6, while ensuring the boundary portion64the desired strength as shown inFIG. 5. In addition, the embodiment ensures a clearance into which an alternator mounting tool for mounting the alternator124is inserted as shown inFIG. 7.

The stresses applied to the high-stress portions HA, HB, HC in the side rail26in the suspension member20of the embodiment (seeFIG. 10) have been observed to be reduced from the stresses applied to the high-stress portions HA, HB, HC in the side rail326in the suspension member320of the comparative example (seeFIG. 11) by 48%, 25%, and 48%, respectively.

Because the deformation in the side rail26of the suspension member20is inhibited like this and the rib portion48ensures the stabilizer mounting portion30the desired strength as described above, a stabilizer100having a high roll stiffness can be used. That is, the embodiment can use a stabilizer100having a high roll stiffness while ensuring a desired impact absorbing performance of the suspension member20.

The use of the stabilizer100having the high roll stiffness improves the roll stiffness of the vehicle12. It has been observed in simulation that the vehicle12having the suspension member20to which the high roll stiffness stabilizer100is secured increases roll stiffness over that of a vehicle having the suspension member320of the comparative example to which a low stiffness stabilizer is secured, by about 1.4 times.

The suspension member20of the embodiment is formed by a casting of an alloy mainly containing aluminum. This makes it easy to form the thin middle portion46, the rib portion48, the thick portion50, and the thick boundary portion64that have been described. Even though the rib portion48, the thick portion50, and the thick boundary portion64are formed at the areas respectively corresponding to the high-stress portion HA, the high-stress portion HB, and the high-stress portion HC to ensure the desired cross-sectional areas (strength), the embodiment still meets production technique requirements (casting requirements) of the suspension member20.

The invention is not limited to the embodiment.

While in the embodiment, the rib portion48is formed in the vehicle front-rear direction on the vehicle width direction outer side of the middle portion46in the stabilizer mounting portion30of the side rail26of the suspension member20to connect the front joint portion42and the rear joint portion44, the vehicle lower section structure is not limited to the configuration. For example, a separate reinforcing member may be joined to connect the front joint portion42and the rear joint portion44.

While in the embodiment, the entire middle portion46between the front joint portion42and the rear joint portion44in the stabilizer mounting portion30of the side rail26of the suspension member20is made thin to be weakened. However, the vehicle lower section structure is not limited to this. For example, the middle portion46may be made thin partially to provide a low-strength portion between the front joint portion42and the rear joint portion44. Alternatively, the middle portion46may be weakened by methods other than by reducing the thickness. For example, the middle portion46may have one or a plurality of holes to be weakened.

While in the embodiment, the thick portion50is formed at the high-stress portion HB on the rear side of the rear joint portion44of the stabilizer mounting portion30, the vehicle lower section structure is not limited to this. For example, a separate plate member may be joined to the high-stress portion HB.

While in the embodiment, the stabilizer100is secured to the lower surface26A of the side rail26of the suspension member20via the stabilizer bracket70, the stabilizer100may be secured to the upper surface of the side rail26via the stabilizer bracket70.

While in the embodiment, the technique disclosed in the invention is applied to the suspension member20at the lower front portion of the vehicle12, the technique disclosed in the invention may be applied to a rear suspension member provided at a rear portion of the vehicle12. In this case, the thick portion50is provided on a vehicle front-rear direction front side of the front joint portion42so that the side rail26is easily deformed and broken around the middle portion46by a rear crash.

While the embodiment is a suspension member20that has a generally rectangular frame-like shape in a plan view, the technique disclosed in the invention is not limited to this. For example, the technique disclosed in the invention may be applied to a suspension member that includes a cross member and side rails on the right and left of the cross member to have a generally H-shape in a plan view.

Because the positions of the high-stress portions HA, HB, HC generated during, for example, travel on a bad road vary with the shape of the suspension member, the rib portion48, the thick portion50, and the thick boundary portion64may be formed appropriately at the positions of the high-stress portion HA, the high-stress portion HB, and the high-stress portion HC, respectively.