This invention relates to bicycle frame design and manufacture.

Contemporary bicycle frames are well known structures, usually "diamond" or sometimes "lady back". This invention seeks to improve such frames by increasing frame stiffness and reducing frame cost.

With the introduction of bicycle suspension systems the requirement for frames to be flexible and resilient has changed to a requirement for frames to provide as rigid a chassis as possible, to maximise the suspension systems efficiency to absorb shock.

There are currently several frame construction techniques available to achieve high stiffness, but they are limited by unit production cost and in some cases high weight.

There are six principle objectives of this invention.

The first is to provide a lightweight frame structure.

The second is achieve this with low production cost in both set up costs and unit production cost.

The third is to achieve in plane and torsional stiffness in excess of contemporary frames.

The fourth is to permit production variability to provide frame design freedom.

Tne fifth is to ensure the frame will not suffer high fatigue.

The sixth is to ensure this is achieved whilst maintain low aerodynamic frame drag.

This invention is a new method of constructing the mainframe of a bicycle.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 shows in side elevation a contemporary bicycle frame and wheels.

Figure 2 illustrates in perspective the proposed construction technique.

Figure 3 illustrates in perspective a reinforcement gusset.

Figure 4 illustrates in perspective a fiat sided bottom bracket shell.

Figure 5 illustrates in perspective an bottom bracket and rear frame pivot housing combination unit.

l here is illustrated a bicycle mainframe 1 which includes a top tube 2, down tube 3, seat tube 4, bottom bracket shelf 5.

Also illustrated is a bicycle rear frame 6 comprising a pair of seat stays 7 and chain stays 8 respectively and a rear wheel 11.

Also illustrated are other well known bicycle features such as handlebars 9.

The three members of the main frame 1 , the down tube 3, the top tube 2, and the seat tube 4 all have a hollow, partially flat sided section. The width of the distance between each respective member's flat side's inner wall are different. The top tube's 2 inner wall width is the same as the seat tube's 4 external width. This enables the top tube 2 to be cut to overlap the seat tube 4.

The down tube 3 can have the same internal side wall width as the top tube 2 enabling it also to be cut to overlap the seat tube 4. Or the down tube 3 can be the external top tube's 2 width enabling the down tube 3 to be cut to overlap the top tube 2, with a spacer being used to ensure the down tube 3 closely fits when it is cut to overlap the seat tube 4.

Or the walls of these members may have moderately reduced or increased inner wall widths to achieve snug overlaps at joints to avoid the need for spacers.

The area of the top tube 2 which continues past the seat tube may be cut to accept; a pair of seat stays 7, a suspension assembly, a uni-stay, or even a cantilever type pannier 10 as shown in figure 2. These joints feature an overlap area 12 which can be affixed by physical or chemical means. The assembly can be further enhanced both structurally and aesthetically by the addition of gussets 13 as illustrated in figure 2. The gusset 13 is hollow and has four faces, two side walls 14, a back wall 15 and a mating wall 16 which provides a complete matching surface to that the surface it mates to on the frame tube. The hollow area is necessary to facilitate manufacture and reduce weight, but its edges 17 also fit closely to the tube that it mates to. These gussets 13 can be fixed for example by bonding or welding.

The mainframe can be affixed to a conventional head tube 18, either plain, lugged or otherwise or the head tube could form part of the assembly prior to this stage.

Figure 4 illustrates a bottom bracket shell 5 which features flat sides 19 which if passed through a hole in the area of the seat tube and down tube junction it will not twist because the flat areas will provide a lock. Furthermore if the seat tube 4 features flat portions of it's front and rear sides its lower end then there will be an overlap area between bottom bracket and seat tube to facilitate fixing.

Figure 5 illustrates a combined bottom bracket shell and rear frame suspension pivot 20.

This construction technique can be applied to the bicycle frames, tandem bicycle frames, recumbent bicycle frames, tricycle frames or any structural frame similar in principle.