Background of the Invention

The present invention is directed to a method for making structural tubes from lightweight composite materials, and is directed more particularly to a method for making internally reinforced structural tubes for use in manufacturing bicycles that have tube and lug frames.

Bicycle manufacturer's have long recognized the advantages of constructing bicycle frames from lightweight composite materials, such as those composed of fibers of graphite, glass, etc. that have been impregnated with synthetic curable resins, typically epoxy resins. Bicycle frames constructed of such materials are desirable both because they are light in weight and because they have a high strength and stiffness. This combination of properties is particularly desirable in specialized bicycles such as racing and off-road bicycles, the former because a lightweight frame increases speed and reduces rider fatigue, and the latter because a strong frame is better able to withstand the stresses and shocks that are associated with riding over rough surfaces.

Bicycle frames that are made out of composite materials are ordinarily of one of two types. A first of these types has a single piece or unitary frame that includes a plurality of relatively long tubular segments, e.g., top tube, down tube, etc., that are connected together by junction structures, e.g., head junction, seat junction, etc.. This type of construction is referred to as unitary because all parts of the frame are cured at the same time in the same mold.

One method for manufacturing a bicycle frame of this type is described in U.S. patent no. 5,158,733 (Trimble).

A second of these types has a multi-piece frame that is made up of a plurality of discrete relatively long tubes and junction structures which are formed and cured separately. These separate pieces are then fitted together and bonded by a suitable curable resin which is cured in a separate curing operation. Frames that are constructed in this manner are commonly referred to as "tube and lug" frames. One example of a frame of this type is described in U.S. patent no. 5,116,071 (Calfee).

Both of the above-mentioned types of bicycle frames may be constructed using a number of different curing methods. One of these curing methods uses a heat-resistant inflatable bladder which is disposed inside of the part to be cured, and which is inflated during curing to compress the composite material against the inside of its curing mold and thereby produce a void-free composite structure. After the part is cured, the inflatable bladder is deflated and removed, leaving a thin-walled hollow part which is ready for final finishing and use.

Another of these methods, known as the "trapped rubber method", used a heat expandable rubber (usually silicone) mandrel which is disposed inside of the part to be cured, and which expands during curing to compress the composite material against the inside of its curing mold. After the part is cured, the part and its mandrel are cooled until the mandrel contracts to a size

small enough to be removed from the part, again leaving a hollow thin-walled part which is ready for final finishing and use.

While tubes constructed by the above-described methods are both relatively strong in relation to their weight, they still occasionally fail during use. One reason is that tradeoffs must be made between the weight and the strength and stiffness of a bicycle frame. More particularly, if the frame is made strong enough to meet all conceivable conditions of use, it will have a weight disadvantage vis-a-vis competing, lighter bicycle frames. If it is made fight enough to provide a weight advantage vis-a-vis competing, stronger bicycle frames, it may undergo a structural failure during conditions of severe use. The making of this tradeoff is comphcated by the fact that the geometry of a bicycle frame makes accurate calculations of optimum structural values extremely difficult, if not impossible. Thus, there exists a need for a bicycle frame which includes structural tubes that are appreciably stronger and stiffer than currently available tubes, but that do not weigh appreciably more than currently available tubes.

Summary Of the Invention

In accordance with the present invention there is provided a method for making composite structural tubes which appreciably increases their strength and stiffness without appreciably increasing their weight. The method of the invention produces such tubes by modifying previously used methods to include additional steps which result in the formation, within the tube, of internal

reinforcing structures that are cured at the same time as the tube, i.e., are co- cured, and are integral therewith, i.e., are free of separately cured seams or joints. These additional steps make possible the making of tubes that have a wide variety of internal reinforcing structures made up of inner walls or septums which extend in bracing relationship to the outer wall of the tube. Such internal reinforcing structures, in turn, allow the construction of bicycle frames which are tailored to exhibit increased strength and stiffness along particular axes and/or in planes in which bicycle frames are usually weak or "soft", and yet which do not weigh appreciably more than conventional frames.

Generally speaking, the method of the invention contemplates the use of two or more heat expandable mandrels, each of which has a cross-sectional shape which is similar to the cross-sectional shape of one of the open spaces or chambers into which the internal reinforcing structure divides the interior of the tube. A round tube which is to be reinforced by a single diametrically disposed inner wall, for example, will include two mandrels, each having a semi-circular cross section. A square tube which is to be reinforced by four inner walls that extend from the center of the square to each of its corners, on the other hand, will include four mandrels, each having a right triangular cross section.

The method of the present invention further contemplates the steps of individually wrapping each mandrel with a plurality of layers of pre- impregnated fiber reinforced material ("prepregs"). These layers may include prepregs of any of a variety of types, e.g., unidirectional tape or isotropic fabric,

or any combination of types, and may be applied with orientations appropriate to the direction or directions along which the tube is to be strengthened. The number of such layers is determined by the desired thickness of the inner walls.

The next steps contemplated by the method of the invention include the assembly of the individually wrapped mandrels into a single multi-mandrel body, and the overwrapping thereof by additional layers of prepregs, the number of such layers corresponding to the desired thickness of the outer tube wall. Ordinarily, but not necessarily, the combined thicknesses of the layers that form the outer wall of the tube will be larger than the combined thicknesses of the layers that form the inner walls of the tube. The latter relationship is desirable because it assures that the weight of an internally reinforced tube is not appreciably greater than that of an unreinforced tube of similar dimensions.

The remaining steps of the method of the invention, namely: packing the assembled multi-mandrel body into a mold; maintaining the mold and its contents at a temperature and for a time dependent on the type of curable resin used; and cooling the multi-mandrel body are conventional, except that, after cooling, each of the mandrels is separately removed prior to the final processing of the tube. Once finally processed, the tube may be used in the

those encountered during off-road riding, which tend to cause the buckling of thin walled tubes.

The method of the present invention may also be used to construct tubes having internal structures with functions that are only incidentally or indirectly related to the reinforcement of the tube. An example of a tube having an internal structure of this type is a bicycle tube that has one or more internal passages for the routing of electrical or mechanical cables. Tubes of this type may be constructed in accordance with the method of the invention by including the additional step of "building out" particular parts of the assembled multi-mandrel body with strips of prepreg that do not wrap around the body. Provided that they are added at the proper stage of construction, and at the proper places, such added strips can be used to produce internal structures having any desired shape and thickness, including regions of variable thickness.

In addition to being directed to the above-summarized method, the present invention is also directed to the internally reinforced tubes that are produced by its use. In particular, the present invention is directed to tubes produced by the method of the invention that are adapted for use in tube and lug type bicycle frames.

Description of the Drawings

Other objects and advantages of the present invention will be apparent from the following description and drawings, in which:

Figs. 1 through 4 show end views of circular tubes that include internally disposed reinforcing structures that have an "open" or "star-like" configuration;

Figs. 5 and 6 show end views of circular tubes that include internally disposed reinforcing structures that have a "closed" or "spar-like" configuration;

Figs. 7, 8 and 9 show end views of internally reinforced tubes having non-circular shapes,

Fig. 10 is an end view of a circular tube having an internally disposed reinforcing structure which is offset from the center of the tube and which defines a plurality of internal passages for the cables used to control the operation of the bicycle;

Fig. 11 is an end view of a circular tube having a relatively thick internal reinforcing structure which includes a plurality of symmetrically distributed passages;

Fig. 12 is an end view of a circular tube which includes an internal reinforcing structure that includes a combination of reinforcing members having "open" and a "closed" configurations;

Figs. 13A through 13F show the steps performed in making the reinforced tube shown in Fig. 1;

Figs. 14A through 14F show the steps performed in making the reinforced tube shown in Fig. 6;

Figs. 15A through 15F show the steps performed in making the reinforced tube shown in Fig. 10; and

Fig. 16 is an enlarged end view of an assembled multi-mandrel body that shows the layered structure thereof in greater detail.

Description of the Preferred Embodiments

Referring to Fig. 1 there is shown an end view of one embodiment of an internally reinforced tube 10-1 that has been made in accordance with the . method of the invention. In the embodiment of Fig. 1, tube 10-1 includes a circular outer wall 12-1 which is internally reinforced by an inner wall 14-1 that is oriented along a diameter of tube 10-1. Although walls 12-1 and 14-1 are referred to separately, as if they were distinct elements, they are actually differently functioning sections of a single piece unitary or monolithic structure, i.e., a structure which has no joints between separately cured parts. The absence of such joints, in turn, allows tube 10-1 to have a strength which is high both in relation to otherwise similar tubes that lack an internal reinforcing structure and with respect to tubes which include an internal reinforcing structure, but which are not as strong as they might be because these reinforcing structures are bonded to the tube in a separate curing step. An internally reinforced tube of the latter type is described in U.S. patent no. 4,900,048 (Derujinsky)

Although, as will be described more fully in connection with Fig. 13, tube 10-1 includes internal layers of a composite material composed of filaments that are bonded together into generally planar pieces by a heat curable resin, these layers are bonded at the same time and in a single curing step. During

this simultaneous or co-curing, these layers lose their separate identities becoming, in effect, commonly encased in a resinous matrix that is free of discontinuities. Because the same is true of the layers in outer wall 12-1 and inner wall 14-1, there also no discontinuities between these structures.

Because of the high structural integrity resulting, from its reinforced, monolithic structure, and because of lightness of the composite material used in its construction, tube 10-1 of Fig. 1 is ideally suited for use in bicycle frames which are of tube and lug construction. In such frames, tubes of the type shown in Fig. 1 may be used in any one or more of the long tubes that are disposed between the tube junction members that connect the tubes together. When so used, the tubes cause the frame as a whole to exhibit generally the same desirable combination of properties as the tubes themselves. It will be understood, however, that the method of the invention, and the internally reinforced tubes constructed by its use, are not restricted to use in bicycle frames, and may be used more generally in any structure which requires a combination of high strength, high stiffness and fight weight.

Referring to Figs. 2, 3 and 4, there are shown end views of tubes constructed in accordance with the method of the invention which utilize internal reinforcing structures having configurations different from that of Fig. 1. More particularly, Figs. 2, 3 and 4 show tubes 10-2, 10-3 and 10-4, respectively, which all include internal reinforcing structures in which a plurality of inner wall segments form at least one intersection within the interior of the tube. Fig. 2, for example, shows a tube 10-2 which includes a

diametrically disposed inner wall segment 14-2 which intersects and bridges secantially disposed inner wall segments 16-2 and 18-2 to form a reinforcing structure having a cross section similar to that of an I-beam. Fig. 3 shows a tube 10-3 which includes three generally radially disposed inner wall segments 14-3, 16-3 and 18-3 which intersect at the center of the tube to form a reinforcing structure having a Y-shaped configuration. Fig. 4 shows a tube 12-4 which includes four generally radially disposed inner wall segments 14-4 through 20-4 which intersect at the center of the tube to form a reinforcing structure having a cross-shaped configuration. Because the internal reinforcing structures shown in Figs. 1-4 have different reinforcing properties along different axes, different ones thereof may be used in different parts of the bicycle frame, depending upon the directions in which the frame needs reinforcing or stiffening.

In spite of their apparent differences, the internally reinforced tubes of Figs. 1-4 have a number of common features which make it possible to construct all of them using the method of the invention. One of these common features is that each of the tubes shown in Figs. 1-4 has a septimated interior structure in which one or more inner walls or inner wall segments partition the interior of the tube into a plurality of elongated chambers having longitudinal axes that are generally parallel to one another. Another of these common features is that all of intersections between the inner wall segments and between the wall segments and the tube are free of separately cured joints and therefore preserve the desired continuity of the tube structure. Thus, in spite

of their different appearances, the differences between the tube configurations shown in Figs. 1-4 are insubstantial from the standpoint of the practice of the method of the invention.

Referring to Figs. 5 and 6, there are shown tubes which include internal reinforcing structures which take the form of closed, generally polygonal spars. In Fig. 5 this closed polygonal spar includes four secantially disposed inner wall segments 14-5 through 20-5 which together form a square. In Fig. 6, the internal reinforcing structure includes three generally secantially disposed inner wall segments 14-6 through 18-6 which together form an equilateral triangle. Except for the spar-like configuration of their internal reinforcing structures, the tubes shown in Figs. 5 and 6 are generally similar to the tubes shown in Figs. 1-4, and the remarks made in connection with the latter figures will be understood to be equally applicable to the tubes shown in Figs. 5 and 6.

Referring to Figs. 7, 8 and 9, there are shown internally reinforced tubes which have a non-circular shape. Fig. 7, for example, shows a tube 10-7 having an outer wall 12-7 which forms a square and is internally reinforced by a plurality of inner wall segments 14-7 through 20-7 which have a cross-shaped configuration. Fig. 8 shows a tube 10-8 having an outer wall 12-8 which forms a triangle and is internally reinforced by three inner wall segments 14-8 through 18-8 which have a Y-shaped or star-like configuration. Fig. 9 shows a tube 10-9 having an outer wall 12-9 which forms a hexagon and is internally reinforced by three inner wall segments 14-9 through 18-9 which have a Y-

shaped configuration. Tubes which have the latter shapes, but which include internal reinforcing structures that have the spar-like configuration shown in Figs. 5 and 6, may also be built using the method of the invention, but are not shown herein because of their similarity to tubes which are shown herein. It will be understood that the tubes shown in Figs. 7-9 have the same common features discussed previously in connection with the tubes of Figs. 1-4.

Referring to Figs. 10, 11 and 12, there are shown tubes which include an internal reinforcing structure having at least one internal passage with a longitudinal axis that is aligned with the longitudinal axis of the tube. Fig. 10, for example, shows a tube 10-10 having an outer wall 12-10 which is internally reinforced by an off-center reinforcing structure that has three round internal passages 22-10, 24-10 and 26-10 formed within a thick monohthic reinforcing body 40-10. Fig. 11 shows a tube 10-11 which is internally reinforced by a thick symmetrically distributed reinforcing body 40-11 that has three round internal passages 22-11, 24-11 and 26-11 formed therein. Fig. 12 shows a tube 10-12 having an outer wall 12-12 which is internally reinforced by a reinforcing structure that includes a concentrically disposed tube 22-12 supported by three radially disposed inner wall segments 14-12, 16-12 and 18-12. It will be understood that the tubes shown in Figs. 10-12 have the same common features discussed earlier in connection with the tubes of Figs. 1-4.

The steps by which the tubes shown in Figs. 1-12 are made will now be described with reference to Figs. 13-16. Referring first to Figs. 13A through 13F, there is shown the sequence of steps used in making a tube having the

internal reinforcing structure shown in Fig. 1. Step 1, shown in Fig. 13A, comprises providing a plurality of heat expandable mandrels 50-1 and 50-2 which have semi-circular cross-sections that are similar to the semicircular cross-sections of the elongated chambers 13-1 and 13-2 which separate inner wall 14-1 and outer wall 12-1 of the tube of Fig. 1. More particularly, mandrels 50-1 and 50-2 each have a rounded outward facing surface and a generally planar inward facing surface. In order to strengthen the intersections between the tube wall and the inner walls of the reinforcing structure, the edges of mandrels 50-1 and 50-2 are rounded or bevelled so that their use produces the fillets that are shown at the upper and lower ends of inner wall 14-1 of Fig. 1.

In the preferred embodiment, mandrels 50-1 and 50-2 are composed of a silicone rubber material which expands when heated. One example of a commercially available material of this type is sold under the name "Tooling Elastomer SG Base" by Dow Corning. These silicone mandrels are formed by curing the silicone base material in a mold having a molding cavity with a size and shape dependent upon the size and shape of the elongated chambers to be formed in the tube with which they will be used. Because the molding process used to produce silicone mandrels is known to those skilled in the art, it will not be described in detail herein.

Once heat expandable mandrels of the desired size and shape are available, they are ready to be used in performing the second and third steps of the method of the invention. In a first of these steps, mandrels 50-1 and 50- 2 are individually wrapped with layers 52-1 and 52-2, respectively, of a suitable

composite material, i.e., a material that includes an array of filaments that has been pre-impregnated with a suitable curable resin. Thin planar pieces of this material, commonly known as "prepregs", may include filaments of any of a number of different materials such as graphite, fiberglass, etc.. These filaments are usually formed either into a linear array, commonly known as a unidirectional tape, or into a woven array, commonly known as an isotropic cloth. Both of these types of prepregs are commercially available in a form in which they have been pre-impregnated with a suitable, curable resin, usually an epoxy resin, which allows them to be handled as discrete self-adherent units. One example of a commercially available prepreg materials which is suitable for use in the method of the invention is that sold under the trade designation "Prepreg Fiberite 7714". The latter prepregs are impregnated with a curable epoxy resin that cures at a temperature of approximately 250°.

In individually wrapping mandrels 50-1 and 50-2 with layers 52-1 and 52-2 of prepregs, unidirectional and isotropic prepregs may be applied interchangeably, or in combination, and with any desired orientation, depending upon the strength properties which the finished tube is to have. The mandrels may, for example, be wrapped by applying partially overlapping layers of unidirectional tape and/or by applying fully overlapping layers of isotropic cloth, either or both of which may have any desired orientation with respect to longitudinal axis of the mandrel. It will be understood that all such wrapping configurations are within the contemplation of the method of the

invention.

Once the heat expandable mandrels have been individually wrapped, they are ready for processing in accordance with the next step of the invention. This step comprises the forming of the individually wrapped mandrels into an assembled multi-part body 53-1, as shown in Fig. 13B, which body has a shape generally similar to that of the desired finished tube. Assembled body 53-1 may at this time be handled as a unit because the stickiness of the resin with which the prepregs are impregnated tends to hold together both the layers with which the mandrels are wrapped and the wrapped mandrels.

Referring to Fig. 13C there is shown the next step of the method of the invention. This step comprises the overwrapping of assembled body 53-1 with a plurality of layers 54-1 of prepregs of the same type used in underlying layers 52-1 and 52-2 to form the overwrapped assembled body 55-1 shown in Fig. 13C. As in the case of the prepregs of underlying layers 52-1 and 52-2, the prepregs of overwrap layers 54-1 may be of either the unidirectional or isotropic types, or any desired combination thereof, and may have any desired orientation depending upon the strength properties which the finished tube is to have. Once completed, overwrapped assembled body 55-1 is relatively self-adherent and may be handled as a unit.

Referring to Fig. 16, there is shown an enlarged end view of overwrapped assembled body 55-1 of Fig. 13C. As shown in Fig. 16, body 55-1 includes mandrels 50-1 and 50-2 which have been individually wrapped with two layers of prepregs 52-1 and 52-2, respectively, making a total of four layers available for curing into inner wall 14-1. In Fig. 16 the individually wrapped

and assembled mandrels (body 53-1) are collectively overwrapped by four layers of prepregs 54-1, making a total of six layers for curing into outer wall 12-1. These relative values for the numbers of layers in tube 12-1 and in internal reinforcing structure 14-1 are a matter of design choice which is dependent on the desired strength and stiffness properties of the finished tube. Thus, the thickness of the walls of the internal reinforcing structure may be approximately equal to that of the tube wall, as shown in Figs. 2, 4, and 7-9, or may thinner than that of the tube wall, as shown in Figs. 3, 5 and 6, simply by selecting appropriate relative values for the numbers of layers in each of these parts of the tube. It will be understood that all combinations of relative thicknesses are within the contemplation of the method of the invention.

Referring to Fig. 13D there is shown the mold 60 that is used in the performance of the next steps of the method of the invention. As shown in Fig. 13D, mold 60 includes mold segments 60-1 and 60-2 which together define an internal molding cavity 62 within which overwrapped assembled body 55-1 is positioned. This molding cavity preferably has a shape and a size which, taking into account the thermal expansion and contraction which it will undergo during curing, will yield a finished tube having the desired size and shape. Once the parts of the mold 60 are clamped together with body 55-1 positioned within molding cavity 62, the entire assembly is raised to the proper curing temperature and held at that temperature until the resin in the prepregs of all parts of body 55-1 are cured. As this curing proceeds, mandrels 50-1 and 50-2 expand isotropically. Part of this expansion is directed

outwardly and serves to compress the layers of curing prepregs at the periphery of body 55-1 against the unyielding surface of molding cavity 62 and thereby form the wall of the tube. Part of this expansion is directed also inwardly and serves to compress the layers of curing prepregs in the interior of body 55-1 against one another and thereby form the inner walls of the tube. As this occurs, curing proceeds simultaneously in all parts of body 55-1, i.e., all parts of body 55-1 are co-cured.

During the early stages of the curing process, the impregnating resin in all layers and prepregs assumes a relatively fluid state which allows it to coalesce or congeal into a single continuous monohthic mass which exhibits few if any traces of the layer and prepreg boundaries which existed before curing. As the curing process runs, to completion this continuous structure is frozen in place and thereby preserved in the finished tube. Thus, as previously stated, a tube constructed in accordance with the present invention is free of all separately cured joints.

After the curing process is completed, the cured body is allowed to cool somewhat and then removed from mold 60. The highest temperature at which the cured body may be removed from the mold depends on the type of curable resin used, but will in general not require that the cured body be retained within the mold until the temperature of both the body and the mold have fallen to room temperature. It will be understood that all combinations of cooling time and temperature which produce a finished product having the

desired structure and properties are within the contemplation of the method of the invention.

Referring to Fig. 13E, there is shown the next step of the method of the invention. This step comprises allowing the now consolidated, cured tube and the mandrels located therein to cool to a temperature low enough that the mandrels contract sufficiently to be individually pulled out of the elongated chambers formed thereby. Once all of the mandrels have been removed, the newly cured tube is ready for final processing. This final processing will typically include the trimming off of any flash formed during the curing process, the grinding and polishing of the exterior of the tube in a centerless grinder or the like, and such additional surfaces finishing steps as are necessary to prepare the tube for incorporation into a bicycle frame. An oblique view of a finally finished tube is shown in Fig. 13F.

Referring to Figs. 14A through 14F there are shown the steps used in producing an internally reinforced tube with the relatively more complex internal reinforcing structure of the tube shown in Fig. 6. As in the case of the method illustrated in Figs. 13A through 13F, the method illustrated in Fig. 14 begins with the step of providing a plurality of preformed mandrels, each mandrel having a cross-sectional shape which is similar to that of a respective one of the elongated chambers that are to appear within the tube. In the embodiment of Fig. 14 these mandrels include a centrally disposed generally triangular mandrel 70-1 and three peripherally disposed arcuate mandrels

72-1, 72-2 and 72-3. Each of these mandrels may have the composition and be made in the same way as the mandrels described in connection with Fig. 13.

As shown in Fig. 14B, the next steps in the making of tube 10-6 include the individual wrapping of mandrels 70-1 and 72-1 through 72-3 with a plurality of layers of prepregs 74-1 through 74-4, respectively, and the assembly of the wrapped mandrels into the assembled multi-part body 53-6. Once so wrapped, assembled body 53-6 is then overwrapped with a plurality of layers 54-6 of prepregs, of the same type used to wrap the individual mandrels, to form an overwrapped assembled body 55-6. Overwrapped body 55-6 may then be placed in curing mold 60, as shown in Fig. 14D, and maintained at the previously mentioned curing temperature until the curing process is completed. Thereafter, once the cured tube is cooled and removed from the mold, the mandrels may be removed therefrom as shown in Fig. 14E to produce the tube 10-6 shown in Fig. 14F.

Because the reinforcing structure shown in Fig. 14 includes a mandrel that is surrounded on all sides by inner walls, it lends itself to fabrication by a variant of the method of the invention which uses a mandrel that is not of the heat expandable type. More particularly, the reinforced tube shown in Figs. 6 and 14 may be made with a non-expandable, incompressible mandrel in place of expandable mandrel 70-1. If such an incompressible mandrel is used in practicing the method of the invention, it is inserted between individually wrapped mandrels 72-1, 72-2 and 72-3 at the time of their assembly into assembled body 53-6. This substitution has no significant effect on the other

steps of the process or the results produced thereby, because the presence of an incompressible, centrally disposed mandrel interacts with the surrounding expandable mandrels in much the same way as a centrally disposed expandable mandrel. Thus, the use of an incompressible mandrel to form an internally disposed polygonal chamber is equivalent to the use of a similarly shaped expandable mandrel for the same purpose.

Because, except as specifically noted, the method illustrated in Fig. 14 is so similar to that illustrated in Fig. 13, the method illustrated in Fig. 14 will not be further discussed herein.

Referring to Fig. 15 there are shown the steps used in making a tube having the internal reinforcing structure shown in Fig. 10. The steps used in the method illustrated in Fig. 15 are generally similar to those described in connection with Figs. 13 and 14, except in two respects. The first of these is that the tube shown in Fig. 10 includes an internal reinforcing structure having a plurality of relatively small passages 22-10 through 26-10 therethrough. The second of these is that these passages are located in a reinforcing structure 40-10 which is relatively massive or thick and which is offset from the longitudinal axis of the tube. In spite of these differences, the tube shown in Fig. 10 may be constructed by the method of the invention by including therein an additional step which makes provision for the relatively massive nature of the internal reinforcing structure.

The making of the tube shown in Fig. 10 begins in the same manner as making of the tubes shown in Figs. 1 and 6: by the provision of a plurality of

heat expandable mandrels 80-1 through 80-4 having cross sectional shapes which are similar to those of the elongated chambers to be formed thereby, as shown in Fig. 15A. These mandrels are then individually wrapped and formed into an assembled multi-part body 53-10, as shown in Fig. 15B.

Because of the small size of the mandrels used form the passages 22-10 through 26-10 shown in Fig. 10, the fitting of the individually wrapped mandrels into an assembled body causes relatively large open spaces to appear at the boundaries between the wrapped mandrels. In order to prevent these relatively large open spaces from giving a rise to voids within the finished tube, the method of the invention shown in Fig. 15 includes the additional step of filhng these open spaces individually with sets of build-out strips 82-1 through 82-6 of prepreg, as shown in Fig. 15B. Once the resulting assembled body 53- 10 is overwrapped with a plurality of layers of prepreg 54-10, as shown in Fig. 15C, and processed in the previously described manner as shown in Fig. 15D, E and F, strips build-out 82-1 through 82-6 become integrated into the structure of the tube body and lose their separate identities in much the same manner as the other pieces of prepreg used therein. Because the steps used in the variant of the method of the invention illustrated in Fig. 15 are similar to those described in connection with Figs. 13 and 14, the method of Fig. 15 will not be further described herein.

While the step of including build-out strips in the assembled multi-part body has been described in connection with the filling of open spaces, it may also be used to create additional structures or thickness gradients along or

across any desired part of the tube or its internal reinforcing structure. One example of such additional structures are locally thickened outer wall regions, such as offset region 40-10 of Fig. 10. Two such regions 42-10 and 44-10 are shown in dotted fines in Fig. 10. Thinner versions of the latter region may be constructed at any desired places on the inner surface of the tube by placing a suitable number of build-out strips over the part of the assembled multi-part body that is adjacent to the region to be thickened. Because of the compressibility of the expandable mandrels, and the incompressibility of the external mold, these thickened regions will extend inwardly from any surface to which they are applied.

While the method of the invention has been described with reference to a number of specific embodiments, it will be understood that the true spirit and scope thereof should be determined with reference to the appended claims.