GB2548623A

GB2548623A - Truss structure

Info

Publication number: GB2548623A
Application number: GB1605061.9A
Authority: GB
Inventors: Lionel Farmer Benjamin; Hawkins Andrew; Edward Haythornthwaite Merlyn; Henry Ben Robarts-Arnold James
Original assignee: Hieta Technologies Ltd
Current assignee: Hieta Technologies Ltd
Priority date: 2016-03-24
Filing date: 2016-03-24
Publication date: 2017-09-27
Also published as: GB201605061D0

Abstract

A truss structure 2, e.g. a cycle frame, has nodes 4 with at least one replaceable tube 6. Each end of the replaceable tube 6 has a removable fastener 20 engaging plug insert 24/26 to secure the tube 6 to the node 4. The fastener 20 may be removed via access hole 44, which is substantially parallel to the longitudinal axis of the tube 6. The fastener 20 may be a threaded screw or bolt, or may have barbs or hooks to wedge into the fastening hole. The fastener 20 may be hidden. The tube 6 may have crack-stopping indents 34. Internal rib 36 may prevent the tube 6 from rotating and mechanical fretting. The tube 6 may be carbon fibre impregnated with resin (CFRP composite tube), moulded, cast, or made by additive manufacturing. The tube 6 may have a circular or non-circular cross-section (e.g. elliptical, rectangular, polyclonal or other irregular cross-sections). The truss structure may be a bicycle frame, or a three-dimensional structure e.g. vehicle chassis. A tube 6 can thus be replaced, e.g. if damaged, without replacing the whole frame.

Description

TRUSS STRUCTURE

The present technique relates to the field of truss structures.

Truss structures comprising nodes linked by tubular struts can be useful for a range of engineering applications, such as pedal cycle frames, vehicle chassis or frames for bridges, buildings or other engineering applications.

At least some examples provide a truss structure comprising: a plurality of nodes; and a plurality of tubes connecting the nodes; wherein at least one of the tubes is a replaceable tube comprising a plug portion inside each end of the replaceable tube; and each end of the replaceable tube is fastened to a corresponding adjacent node by a removable fastener passing into a fastening hole in the plug portion, wherein the fastening hole is oriented substantially parallel to a longitudinal axis of the tube.

At least some examples provide a truss structure tube comprising: a plug portion inside each end of the tube; each plug portion comprising a fastening hole for receiving a removable fastener; wherein the fastening hole in each plug portion is oriented substantially parallel to a longitudinal axis of the tube.

At least some examples provide a truss structure node comprising: a socket portion having a tubular aperture for receiving a tube; and an access hole on an opposite side of the node from the socket portion, for providing access for fastening or unfastening a fastener inside the node oriented substantially parallel to a longitudinal axis of the tubular aperture.

At least some examples provide a method of manufacturing a composite tube for a truss structure, comprising: fitting plug portions at each end of a tubular mandrel made from soluble material, each plug portion comprising a fastening hole for receiving a fastener, wherein the fastening hole is oriented substantially parallel to a longitudinal axis of the mandrel; laying wall material for forming the composite tube against sides of the mandrel and the plug portions; co-bonding the wall material and the plug portions during curing of the wall material to form a composite tube comprising the plug portions inside each end of the tube; and removing the mandrel with a solvent.

Further aspects, features and advantages of the present technique will be apparent from the following description of examples, which is to be read in conjunction with the accompanying drawings, in which:

Figure 1 schematically illustrates a cycle frame, which is an example of a truss structure comprising nodes linked by tubes;

Figure 2 schematically illustrates a first example of a joint between a replaceable tube and a node using a removable fastener;

Figure 3 shows a second example of a joint between a replaceable tube and a node, in which an outer tubular portion of an insert for receiving the fastener has a tapered thickness;

Figure 4 is a graph schematically illustrating variation in induced stress along the length of an adhesive bond between the insert and the replaceable tube;

Figures 5 to 8 illustrate a method of co-bonding a composite insert to a composite tube;

Figure 9 shows a composite tube comprising the co-bonded insert; and

Figures 10 and 11 illustrate fastening of the tube of Figure 9 to a node.

Example arrangements are below. It will be appreciated that the invention is not limited to these particular examples.

Truss structures can be weight efficient solutions for structures such as cycle frames and vehicle chassis. In a truss structure comprising a number of nodes connected by tubes, the tubes are typically joined to the nodes by adhesive bonding. However, adhesive bonding is a one-way operation in that it is typically impractical to reverse the bond after it has been made, and so if one tube of the truss is damaged (e.g. by impact), then typically the whole assembly has to be written off and replaced with a new structure.

In the truss structure discussed in this application, at least one of the tubes is a replaceable tube which comprises a plug portion inside each end of the replaceable tube. Each end of the replaceable tube is fastened to a corresponding adjacent node by a removable fastener passing into a fastening hole in the plug portion, where the fastening hole is oriented substantially parallel to a longitudinal axis of the tube. Given the general desire to make the bonds between the nodes and tubes as strong as possible to provide a robust structure, it may seem counterintuitive to provide a joint which can be unfastened, but surprisingly a removable fastening can be provided without significantly impacting on the structural properties of the truss structure. By joining the replaceable tube to the adjacent nodes using removable fasteners which can be undone, then if the tube is damaged it can be replaced with a new one without needing to replace the whole assembly, which reduces maintenance costs.

Using a fastening hole which is oriented substantially parallel to a longitudinal axis of the tube is preferable to a fastener which is aligned to pass transversely through the tube, because the fastener can simply pass into the end of the tube, rather than needing a hole in the wall of the tube (which would have a greater impact on the structural properties of the tube). The fastening hole need not be exactly parallel to the longitudinal access and could be offset a little from the longitudinal axis (e.g. by less than 10 degrees for example). In general, “substantially parallel” therefore means sufficiently parallel that the fastener can enter the open end of the tube and pass into the fastening hole in the plug portion without passing through the wall of the tube. However, manufacture of the node and the plug portion at the end of the tube, and insertion of the fastener, may be simplest if the fastening hole is oriented exactly parallel to the longitudinal axis of the tube.

While the orientation of the fastening hole may be parallel to the longitudinal axis of the tube, it is not essential for the fastening hole to be exactly aligned with the longitudinal axis of the tube - it is possible to offset the fastening hole from the longitudinal axis of the tube. However, manufacture and fastening may be simpler if the hole is also aligned with the longitudinal axis, since this enables the plug portion to be formed by turning for example, and makes it easier to locate the fastening hole when fastening the fastener since its position relative to the node would not depend on the rotational orientation of the tube.

The fastening hole can engage with the fastener in different ways. In one example, the fastening hole may be slightly smaller than the fastener for which it is designed and the fastener may have some barbs or hooks in its shape so that when the fastener is wedged into the fastening hole then it resists removal when the tube is subjected to tension, but the fastener can still be removed with a suitable tool if necessary.

However, in another example the fastening hole may comprise a threaded hole and the fastener may comprise a threaded fastener (e.g. bolt or screw). A threaded engagement between the fastener and the fastening hole can provide better resistance to loads, while still being relatively easy to fasten or unfasten.

Some examples could allow the end of the fastener to protrude beyond the edge of the adjacent node, in which case the fastener may be visible and may be fastened or unfastened using a tool outside the node. However, it may be preferable for the fastener to be hidden within the adjacent node and the replaceable tube, so that the user cannot see the fastener from the outside. This can provide a cleaner appearance, and concealing the fastening inside the node and tube makes it less likely that the fastening becomes dirty or corroded or is accidentally knocked loose. To enable the fastener to be undone when it is hidden inside the node, the adjacent node may comprise an access hole for providing access for fastening or unfastening the fastener inside the adjacent node. For example, the access hole may be in the opposite side of the node from the portion which receives the tube and may be aligned with the fastening hole in the plug portion. The user can insert a tool, such as a screwdriver or Allen key, through the access hole in the node to fasten or unfasten the fastener.

The plug portion may be any portion in the end of the replaceable tube which can receive the fastener. In some cases the plug portion may be flush with the end rim of the tube so that no section of the plug portion protrudes beyond the end of the tube. However, in other examples the part of the plug portion comprising the fastening hole may protrude beyond an end rim of the replaceable tube. This allows a shorter fastener to be used since the fastener does not need to reach as far into the tube. Also, this can allow the end wall of the replacement tube to be separated from a washer or other surface within the adjacent node against which the plug portion is fastened by the fastener, which can be useful for reducing mechanical fretting.

Each end of the replaceable tube may be inserted into a socket portion of the corresponding adjacent node. The socket portion of the adjacent node may have a rib which extends around the inner circumference of the socket portion, and an end rim of the replaceable tube may abut against one shoulder of the rib, with the fastener fastening the plug portion to a washer abutting against the other shoulder of the rib. By pushing the end wall of the replaceable tube against the rib instead of against the washer, then even if the washer rotates a little while fastening or unfastening the fastener or during general use of the truss structure, the friction between the shoulder of the rib and the end wall of the replaceable tube can prevent the tube itself rotating which reduces mechanical fretting (wear at the contact area between two surfaces under load which move relative to each other). Hence providing the rib between the end wall of the tube and the washer can improve the lifetime of the tube as a whole.

The tubes, including the replaceable tube, can have any cross section and do not need to be circular. For example some tubes could have an elliptical or polygonal cross section, or a more arbitrarily shaped cross section. Hence, the terms “circumference” refers to the perimeter around the cross section of the tube and does not imply a circular cross section.

The plug portion can be formed in a number of ways. In some examples the plug portion may be formed as an integral part of the replaceable tube itself, so that the walls of the tube and the plug portion are formed as one piece. For example, the replaceable tube (including the plug portion at each end) could be moulded, cast or made of consolidated material formed using an additive manufacturing technique for example.

However, in other examples the plug portion may comprise an insert portion which is inserted into an end of the replaceable tube. The insert portion could be of the same material as the tube itself, or made from a different material to the tube. For example, the material used for the replaceable tube may be relatively expensive and chosen for its density or structural properties, but this may not be necessary for the insert portion and a cheaper material could be used for this. For example, if the replaceable tube is made from a composite material such as carbon fibre reinforced polymer (CFRP), a cheaper aluminium insert could be used to save cost.

In some cases the plug portion could be solid all the way through the tube’s cross section (other than the absence of material at the fastening hole). However, to reduce the overall weight of the truss structure, the plug portion can comprises an outer tubular portion for bonding to the inside of the replaceable tube and an inner portion within the outer tubular portion which comprises the fastening hole. The outer tubular portion and the inner portion can be connected by one or more struts (which need not completely fill the space between the outer tubular portion and the inner portion). By reducing the amount of material in the plug portion, the weight of the replaceable tube can be reduced which can be an important factor for some applications.

In one example, the thickness of the outer tubular portion in a direction perpendicular to the longitudinal access of the replaceable tube may be tapered, so that the thickness becomes narrower in a direction moving away from the end of the insert portion which comprises the opening of the fastening hole. This can be useful for reducing the stress on the edge of the bond between the plug and tube which lies furthest down the tube (furthest away from the end comprising the opening of the fastening hole). Tapering the thickness of the outer tubular portion helps to spread the load over a greater portion of the bond, reducing the chance of failure of the bond.

The insert portion can be bonded to the replaceable tube in a number of ways. In one example the insert portion may be secondary bonded to the replaceable tube. For example the secondary bonding may be by adhesive bonding. Hence, an off-the-shelf tube can have the insert portion inserted and adhesively bonded to the tube. This approach may allow the replaceable tube to be manufactured more cheaply.

Another option for forming the plug portion may be to make both the plug portions and the replaceable tube from the same composite material, and to co-bond the plug portions with the replaceable tube during curing of the replaceable tube. Hence, a single curing operation can both form the walls of the tube and bond the walls to the plug portions. This can provide a stronger join as it allows a mechanical interlock between the plug portion and the wall of the tube to be used to strengthen the bond and increase robustness against loads.

In general, at least one of the tubes may be made from a different material to the nodes. For example, it may be desirable to use a material for the nodes which has a relatively high specific strength to provide overall structural robustness for the truss structure, while using a material with relatively high specific stiffness for the tubes in order to provide resistance against deflection. For example, it can be attractive to use a metallic material (either a metal or a metal alloy) for the nodes, while using a composite material for at least one of the tubes, to balance robustness against low weight. For example, the nodes can be made of titanium or steel while the tubes may be made from CFRP. The use of removable fastenings is particularly useful for truss structures formed with metallic nodes linked by composite tubes, as such a truss would otherwise require adhesive bonding of the tubes and nodes and the composite material can be expensive, so enabling replacement of a single tube is attractive.

It is not necessary for all the tubes of the truss structure to be made from the same material. As well as reducing cost when repairing damage to a tube, another advantage of using a removable fastener is that this allows a user to mix and match which type of tube is fitted in a given position of the truss structure. For example, while generally a composite tube may be desired to reduce weight, if the truss structure is going to be used in a situation in which particularly high loads are expected, then the composite tube could be replaced with a metallic tube (e.g. steel) which is more robust than the composite material, to reduce the likelihood of damage. Later, in a situation where high impact is less likely, the composite tube could be refitted to reduce weight once more.

It is not essential for all of the tubes of the truss structure to be replaceable tubes joined using removable fastenings. In addition to the replaceable tube, there may be at least one other tube which is joined by a non-removable join such as using adhesive bonding. Using a removable fastening may slightly increase the weight of the truss structure, for example due to the additional weight of the insert as discussed above. For tubes likely to become damaged the additional weight may be acceptable given the trade off of reduced cost of repair. However, for other tubes which are less likely to be subjected to damage, it may be preferable to make the tube non-replaceable and simply use an adhesive to bond it to the adjacent nodes, so that no plug portion is required. Also, the particular physical geometry of the truss structure for some applications may make it relatively difficult to undo the fastening for a tube at a certain position in the truss structure, so such tubes could be made non-replaceable to save weight. A truss structure as discussed above can be used for a range of applications, including for example a pedal cycle frame, a chassis for a motor vehicle, or part of a bridge. The pedal cycle frame could be a frame for a bicycle or a tandem for example. The present technique can be particularly useful for mountain bike frames due to the increased impact (and likelihood of damage) for mountain bikes compared to road cycles. Composite mountain bike frames are attractive for their low weight, but the relatively expensive nature of the composite materials may discourage many mountain bikers from using such composite frames since the likelihood of damage is high and if the entire frame needs to be replaced when one tube is damaged, the cost is expensive. By providing the ability to replace an individual tube without needing to replace the whole frame, the removable fastenings allow composite-based mountain bike frames to become more attractive as the ongoing maintenance cost following damage is reduced. For similar reasons, for a chassis for a motor vehicle, the cost of the chassis as a whole is generally relatively high and so it can be attractive to allow a single tube of the chassis to be replaced when damaged.

As well as providing an overall truss structure including tubes and nodes, the tubes and nodes can also be provided separately and then subsequently assembled downstream. Hence, a truss structure tube can be provided with a plug portion inside each end of the tube with each plug portion comprising a fastening hole for receiving a removable fastener, with the fastening hole oriented substantially parallel to a longitudinal axis of the tube.

Also, a truss structure node may be provided comprising a socket portion which has a tubular aperture for receiving a tube, and an access hole on an opposite side of the node from the socket portion for providing access for fastening or unfastening a fastener inside the node which is oriented substantially parallel to a longitudinal access of the tubular aperture. For example the access hole may be aligned with the end of the tubular aperture when viewed along the longitudinal axis of the aperture.

Figure 1 schematically illustrates a frame 2 for a pedal cycle, which is an example of a truss structure. In this example the frame 2 is a mountain bike frame, but the technique could also be used for road bikes. Also while Figure 1 shows a bicycle frame the technique could also be used for tandems or other types of cycle. The frame 2 comprises a number of nodes 4 made from the metallic material such as titanium or steel, which are connected by tubes 6-16 made from composite materials such as carbon fibre reinforced polymer. For example, the tubes include a top tube 6, down tube 8, seat tube 10, 12, seat stays 14 and chain stays 16. It will be appreciated that the cycle frame may have other components than those shown in Figure 1, for example suspension may be provided. In this example the seat tube 10, 12 is formed of two portions as there is a intervening node for connecting to suspension, but a one-piece seat tube 12 could be used for other frame designs. A truss structure can be a weight efficient solution for a relatively lightly loaded structure such as a bicycle frame or vehicle chassis. By placing materials with relatively high specific strength (force per unit area at failure divided by density) and largely isotropic materials at the nodes 4 in the form of lugs, and materials with relatively high specific stiffness (elastic modulus divided by density) in the tubes to connect the nodes, this can provide a relatively lightweight frame which is structurally robust and resistant to damage. The nodes can be made of cast metal or can be formed by additive (layered) manufacturing (AM or ALM) techniques such as selective laser melting. However, metallic nodes would typically be bonded to composite tubular struts using adhesive bonding, which has the drawback that the bonding is a one way operation and cannot be reversed. This means that if a tube is damaged by impact then the whole frame has to be written off and replaced, which is expensive.

Figures 2 and 3 show examples of joining a tube 6 to a node 4 using a removable fastener 20. The tube 6 slides into a tubular socket portion 22 of the node 4. The tube 6 is a replaceable tube having inserted into the end an insert 24 which forms a plug portion plugging the end of the tube 6. It is not essential for the insert 24 to completely cover the end of the tube. In this example the insert 24 is made from aluminium and is manufactured by CNC (computer numerical control) turning, but other materials could also be used for the insert 24. The insert 24 has an outer tubular portion 26 which is bonded to the inside of the wall of the tube 6 by secondary (adhesive) bonding, and an inner portion 28 which includes a threaded fastening hole 30 which is oriented substantially parallel to a longitudinal access 32 of the tube. Struts 29 connect the inner portion 28 to the outer tubular portion 26. While in Figures 2 and 3 the struts are positioned at the end of the tube, they could also be placed further down the tube. The outer tubular portion 26 includes indents 34 which are crackstopping features, to improve the strength of the bond between the insert 24 and the tube 6.

The socket portion 22 of the node 4 includes an internal rib 36 which passes circumferentially around the inside of the socket portion so that there is a temporary narrowing of the socket portion. The end wall of the tube 6 abuts against a first shoulder 38 of the rib and a washer 40 abuts against a second shoulder 42 of the rib. A threaded fastener 20 is inserted through the washer into the threaded hole 30 in the insert 24 to fasten the plug portion 24 of the tube against the washer 40 with the end rim of the tube 6 forced up against the first shoulder 38 of the rib 36. This fastens the tube 6 onto the node 4 in a manner which can resist tension or compression loads on the tube and node structure. By providing the rib 36, the tube is less likely to rotate and be worn due to mechanical fretting.

The node 4 has an access hole 44 in its casing which is positioned on the opposite side of the node from the tubular socket 22 for receiving the tube, with the access hole 44 aligned with the threaded hole 30 in the plug 24, to permit access for undoing or doing up the fastener. For example, a screwdriver or Allen key can be inserted into the access hole 44 to fasten and unfasten the fastener 20.

In Figure 2 the outer tubular portion 26 of the plug has a constant thickness in the direction perpendicular to the longitudinal access 32 of the tube (other than at the location of the struts 29). However, as shown in Figure 3 it is also possible to provide a plug insert 24 in which the outer tubular portion 26 has a tapered thickness which narrows in the direction 48 moving away from the end having the opening of the fastening hole 30. This is useful for reducing the stress induced at the leading edge of the adhesive bond between the plug 24 and the tube 6 at the edge furthest from the end of the tube. As shown in Figure 4, with a single lap adhesive bond between two surfaces loaded under tension, the induced stress F varies as a function of the distance x from the edge of the bond. When the walls being bonded have a constant thickness then the stress is concentrated strongly at the leading edge of the bond and then reduces rapidly so that subsequent parts of the bond bear less load. This makes the leading edge of the bond much more likely to fail and increases the likelihood that the two layers being bonded peel apart at the leading edge. On the other hand, as shown in the dotted line of Figure 4, tapering the wall thickness so that it is narrower at the leading edge of the bond and then gets wider (i.e. the thickness gets narrower in the direction 48 moving down the tube), this smooths out the curve in Figure 4 so that the induced stress is spread more evenly along the length of the adhesive bond, which reduces the chance of the bond failing.

While Figures 2 and 3 show only one end of the replaceable tube 6, it will be appreciated that the other end has a similar plug portion 24 which is fastened to another adjacent node 4 using a removable fastener 20 in the same way as discussed above. Hence, if the tube is damaged, both ends of the tube can be unfastened from the respective adjacent nodes and the tube can be replaced with a new one.

In the example of Figures 2 and 3 the plug portion 24 is an aluminium insert which is inserted into a carbon composite tube and adhesively bonded in. This has the advantage of reduced manufacturing cost since standard off-the-shelf carbon tubes can be used and no special composite manufacturing technique is required.

However, another approach is shown in Figures 5 to 11 in which an insert made from a composite material is co-bonded with the composite carbon tube, which can provide a stronger bond due to mechanical interlock between the insert and the tube, to reduce the likelihood of failure of the bond and increase robustness against loads.

Figures 5 to 8 illustrate steps of a method of manufacturing the tube comprising cobonded plugs. Two plug portions 50 made from composite material are provided, each comprising an outer tubular portion 52 with struts 58 connecting the tubular portion 52 to an inner portion 54 which includes a threaded fastening hole 56. In this example the inner portion 54 protrudes out from the outer tubular portion 52 and is connected via diagonal struts 58. In other examples the inner portion 54 could be aligned with the tubular portion 52. The outer tubular portion 52 of each plug portion 50 includes a number of barbs or hooks 60 which protrude out from the outer tubular portion 52. The barbs are for interlocking with the subsequently formed walls of the composite tube to increase the bond strength.

As shown in Figure 5, the two plug portions 50 (or “inserts”) are fitted at opposite ends of a tubular mandrel 70 made from a soluble material using temporary fasteners 72. For example, the bottom end of the threaded fastening hole 56 may include a further hole for inserting the temporary fastener 72. Also, each of the plug portions 50 is fastened to a tooling jig 74 so that the mandrel and the two insert portions 50 are raised up. The same threaded fastening hole 56 (which would eventually receive the fastener for fastening the tube to the nodes in the truss structure) can be reused during the manufacturing process to fasten the assembly onto the jig 74.

As shown in Figure 6, wall material 80 for forming the composite tube is then laid against the side of the mandrel 70 and the plug portions 50. For example the wall material 80 may be carbon fibre impregnated with resin. The wall material is hand laid, bagged, cured, debagged and post-cured to form a CFRP composite tube. During curing of the wall material, the wall material 80 is co-bonded to the plug portions 50 (the resin in the wall material bonds with the plug portions 50 as it cures). The barbs 60 in each plug portion 50 mesh with the CFRP walls 80 to provide mechanical interlock. As shown in Figure 7, the tooling jig 74 is then removed so that the carbon composite tube 6 now comprises the plug portions 50 co-bonded at each end. The mandrel 70 remains between the plug portions. The temporary fasteners 72 are removed, and as shown in Figure 8, the mandrel 70 can be removed by washing solvent through the tube using hoses connected at each end of the tube, to dissolve the mandrel 70.

As shown in Figure 9, this leaves only the CFRP tube 6 with the plug portions 50 remaining. Using this method of manufacturing a composite tube for a truss structure, a strong bond can be achieved which is less susceptible to failure. As shown in Figure 10, the CFRP tube 2 with the pin tangled inserts co-cured with the tube at each end can then be inserted into the lugs forming the socket portion 22 of the node 4 and a fastener 20 can be inserted into the threaded fastening hole 56 in the insert, to fasten the tube onto the node 4 as shown in the assembled view in Figure 11.

While the example shown uses circular tubes, it will be appreciated that the same technique could be used for tubes with non-circular profiles, such as elliptical, rectangular, polygonal, or other irregular cross sections.

While Figure 1 shows an example of a bicycle frame, it will be appreciated that this technique of joining tubes to nodes can be used for any truss structure, such as a vehicle chassis or structure for a bridge or building construction. The truss structure may be a largely two-dimensional truss structure, such as a bicycle frame, or a three-dimensional structure or space frame, such as a vehicle chassis.

Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.

Claims

1. A truss structure comprising: a plurality of nodes; and a plurality of tubes connecting the nodes; wherein at least one of the tubes is a replaceable tube comprising a plug portion inside each end of the replaceable tube; and each end of the replaceable tube is fastened to a corresponding adjacent node by a removable fastener passing into a fastening hole in the plug portion, wherein the fastening hole is oriented substantially parallel to a longitudinal axis of the tube.

2. The truss structure according to claim 1, wherein the fastening hole comprises a threaded hole.

3. The truss structure according to any of claims 1 and 2, wherein the fastener is hidden within the adjacent node and the replaceable tube.

4. The truss structure according to any preceding claim, wherein the adjacent node comprises an access hole for providing access for fastening or unfastening the fastener inside the adjacent node.

5. The truss structure according to any preceding claim, wherein at least part of the plug portion comprising the fastening hole protrudes beyond an end rim of the replaceable tube.

6. The truss structure according to any preceding claim, wherein each end of the replaceable tube is inserted into a socket portion of the corresponding adjacent node.

7. The truss structure according to claim 6, wherein the socket portion of the adjacent node comprises a rib extending around an inner circumference of the socket portion, wherein an end rim of the replaceable tube abuts against one shoulder of the rib, and the fastener fastens the plug portion to a washer abutting against the other shoulder of the rib.

8. The truss structure according to any preceding claim, wherein the plug portion comprises an outer tubular portion for bonding to the inside of the replaceable tube, and an inner portion within the outer tubular portion, the inner portion comprising the fastening hole.

9. The truss structure according to claim 8, wherein a thickness of the outer tubular portion in a direction perpendicular to the longitudinal axis of the replaceable tube is tapered to become narrower in a direction moving away from the end of the plug portion comprising an opening of the fastening hole.

10. The truss structure according to any preceding claim, wherein the plug portion comprises an insert portion inserted into an end of the replaceable tube.

11. The truss structure according to claim 10, wherein the insert portion is secondary bonded to the replaceable tube.

12. The truss structure according to any of claims 10 and 11, wherein the insert portion and the replaceable tube are made from different materials.

13. The truss structure according to any of claims 1 to 11, wherein the plug portion and the replaceable tube are made from the same material.

14. The truss structure according to any of claims 1 to 10, wherein the plug portion and the replaceable tube are made from a composite material and the plug portion is co-bonded with the replaceable tube.

15. The truss structure according to any preceding claim, wherein at least one of the tubes is made from a different material to the nodes.

16. The truss structure according to any preceding claim, wherein at least one of the tubes is made from a composite material.

17. The truss structure according to any preceding claim, wherein the nodes are made from a metallic material.

18. The truss structure according to any preceding claim, wherein the plurality of tubes comprise at least one tube adhesively bonded to adjacent nodes.

19. The truss structure according to any preceding claim, wherein the truss structure comprises a frame for a pedal cycle.

20. The truss structure according to any of claims 1 to 18, wherein the truss structure comprises a chassis for a motor vehicle.

21. The truss structure according to any of claims 1 to 18, wherein the truss structure comprises part of a bridge.

22. A truss structure tube comprising: a plug portion inside each end of the tube; each plug portion comprising a fastening hole for receiving a removable fastener; wherein the fastening hole in each plug portion is oriented substantially parallel to a longitudinal axis of the tube.

23. A truss structure node comprising: a socket portion having a tubular aperture for receiving a tube; and an access hole on an opposite side of the node from the socket portion, for providing access for fastening or unfastening a fastener inside the node oriented substantially parallel to a longitudinal axis of the tubular aperture.

24. A method of manufacturing a composite tube for a truss structure, comprising: fitting plug portions at each end of a tubular mandrel made from soluble material, each plug portion comprising a fastening hole for receiving a fastener, wherein the fastening hole is oriented substantially parallel to a longitudinal axis of the mandrel; laying wall material for forming the composite tube against sides of the mandrel and the plug portions; co-bonding the wall material and the plug portions during curing of the wall material to form a composite tube comprising the plug portions inside each end of the tube; and removing the mandrel with a solvent.

25. A truss structure substantially as herein described with reference to the accompanying drawings.

26. A truss structure tube substantially as herein described with reference to the accompanying drawings.

27. A truss structure node substantially as herein described with reference to the accompanying drawings.

28. A method of manufacturing a tube for a truss structure substantially as herein described with reference to the accompanying drawings.