Foldable Tandem Bicycle

This invention relates to a collapsible tandem bicycle. In particular, this invention relates to a tandem having separate rigid front, center, and rear frame members joined together by concentric members forming two seat tubes with the frames foldable thereabout.

Foldable tandem bicycle technology is virtually non-existent. As can be appreciated, given the size of the tandem bicycle, the desire to provide a more compact device when stored or being transported is even greater than in the case of a normal bicycle. Two prior art concepts are the take apart, U.S. Pat. No. 579,982 and a collapsible bike, U.S. Pat. No. 3,688.879. Jakobson's U.S. Pat. No. 579,982 was simply unbolted and fell apart. The latest, U.S. Pat. No. 3,688,879 is a collapsible bicycle which can be made into various tandems.

Few however, have attempted to provide a workable tandem that folds into a compact form since the means to date has not been available. With the establishment of the concentric seat tube hinging concept in U.S. Pat. No. 4,448,437, the practical foldable full sized bicycle technology became available. Prior to this patent others had attempted various folding concepts without success see: U.S. Pat. No. 3,015,498. This concentric seat tube technology established that frames could be made absolutely rigid about the horizontal plane through a hinged joint, if that hinged joint remained at the seat tube. The principle has worked for one hinged joint and bicycles using this technology have achieved commercial success. With that knowledge now public, to date its application has not been attempted for a "bicycle built for two".

A variation on U.S. Pat. No. 4,448,437 is a hinge joint just outside the seat tube such as in U.S. Pat. No. 3,015,498. This principle could be used to make up a "Z" fold as a technique of folding. The result however, would still never be structurally as strong and simple as using the concentric seat tube hinges.

Given the virtual non-existent prior art, it is an object of this invention to define a foldable tandem bicycle that is lightweight, has the necessary strength characteristics, and provides riding performance commensurate with contemporary fixed frame tandem designs.

Yet another object of this invention is to define a folding tandem bicycle which may be reduced in size to allow for easy transportation by carrying or rolling alone on a set of wheels for support.

Still further objects of this invention are to define a folding tandem design meeting contemporary safety requirements, that can be marketed economically and served utilizing standard components.

These and other objects of this invention are achieved in a folding tandem bicycle design utilizing rigid frame elements that fold around the seat tubes formed from concentric elements and not at some outside structurally unrelated points to the tandem design. In accordance with the present invention, both the stress of the beam action and the stress of any hinges are resolved within the seat tube itself.

The tandem frame in accordance with the present invention is made up of three interlocking frames, front one formed by the head tube, front top tube, down and seat tubes. The middle frame is formed by the front seat tube, middle top and bottom tubes with a diagonal intermediate brace, and the back seat tube. The back frame is formed of seat and chain stays and the back seat tube. Each of the frame elements is complete except the front frame which is interrupted along the front tube for the introduction of the middle frame diagonal brace. This diagonal brace collar is housed between the securable upper and lower elements of the front frame seat tube. When secured the front frame elements are complete being structurally integral. The seat construction in the back can be made up of two full-length tube and one partial-length tube collared to the full-length tube. A seat post can be sized to slide into the other two tubes.

The present invention defines two preferred embodiments. In the first preferred embodiment, both riding positions, front and back, would be termed non-unisex, with horizontal top tubes. In the second embodiment, the front position would be non-unisex, the back position would be unisex with the lower sloping top tubes. Both embodiments exist presently in non-folding tandems, the first being by the far the most used and strongest construction, the second trading construction strength for ease of mounting and dismounting by the stroker (back rider). In the second embodiment the center frame uses two diagonals forming X-bracing to stiffen its construction. This helps bridge the weight between the two well supports. A single back-sloping tube or dual back-sloping tubes would not be strong enough and/or hit the rear tire stopping the folding action.

In accordance with the present invention, the folding process is achieved expeditiously without the requirements of having to utilize a large number of tools or to materially disassemble the tandem. Rather such traditional steps of removal of the front wheel, seat post, the front one carrying the strokers' handlebar, all with the throwing of standard quick releases. The folding process is continued with the rear set post safety catch being then depressed, the three binding quick releases are undone and the rear frame section is folded around over the middle section. Similarly, the front section safety catch and quick releases are undone and it too is folded around over the middle section but on the opposite side. In a folded position, the approximate size of the tandem is determined by the wheel size in the height direction and by the wheel size plus the chain wheel projection in the length direction.

What started out as an unwieldily, unstorable object over 8 feet long has been reduced to a 3 feet by 3 feet compact package. In such a folded position, the tandem bicycle can easily fit into the trunk space of a small car, conforms to airplane travel requirements, may be carried aboard elevators or rolled along on wheels as a suitcase on wheels.

Further, in accordance with the present invention, standard derailleur mechanisms, standard brakes including dual rear tandem brakes, standard tandem drive mechanisms including eccentric crank chain tighteners can all be used. Other than the addition of concentric seat tubes with their accompanying quick releases and safety catches, the components follow the norm of all standard tandems.

FIG. 1 is in side view of a 2 tandem bicycle in accordance with a first embodiment of this invention;

FIGS. 2A and 2B are exploded views of the two embodiments of the frames of this invention; and

FIG. 2C is a view seat tubes of this invention.

Referring now to FIGS. 1 and 2A, the first preferred embodiment of this invention is shown. FIGS. 2A, 2B show the fundamental alternative rigid frame configuration used in accordance with the present invention. The important characteristic of all the embodiments is the concentric nature of the seat tubes used to interlock the three frame members. In FIGS. 1, 2A, 2C, the front frame members 5 comprise a head tube 22, a top tube 32, a down tube 34, and seat tube component 50. These elements are constructed utilizing conventional bicycle technology by welding tubular elements to form a rigid construction. In FIGS. 2B and 2C components 50 and 44 are collared into complete continuous components. The collar elements 45 are stops for the partial frame elements locking their vertical position on the continuous frame. The middle assembly 7 comprises seat tube 44 and 76 front and back, bottom tube 70, diagonal tube 68, pedal housing 60, and a top tube 66 in FIGS. 2A and 2B or a X-bracket members 68 and 69 in FIG. 2C. The rear assembly number 9 comprises a pair of seat stays 94, 96, a pair of chain stays 100, 102, a seat tube 80 with a pedal housing 86. In FIG. 2B the (lower part of) seat tube (80) could be cut and collared onto the continuous seat tube member 76 but the front derailleur has to be set on member 80 half way up, so there is nothing to be gained. Finally, the tandem bicycle is configured to have the front section folded in only one direction and the rear section only in the opposite direction from the straight position. Metal blocks 58 and 92 ensure this folding mode and they have alignment pins to adjust for the return to the exact alignment configuration.

FIG. 1 illustrates the complete tandem assembly which apart from the double frame members utilizes completely standard components. The embodiment of FIG. 1 is constructed in a manner consistent with the frame assembly shown in FIG. 2A. That is, the same front frame member 5 is shown comprising a head tube 22, a top tube 32, a down tube 34, and a seat tube 50. As in standard conventional and tandem bicycles the head tube 22 houses a concentric internal handle bar stem 24. The handle bars 28 have attached brake handles 26, derilleur controls 30. It is understood, but not shown, that the brake and derailleur controls have cables disposed to the front and rear wheels, front and rear derailleurs. Shown, however, is the first part of the cable having the brake quick release lever 20 which opens the front brake 18 far enough so the front wheel 2 can be removed. The front brakes are located on the front fork 16. The front wheel quick release must be backed off to unsecure the safety pin 12, then the lever 14 releases before the front wheel can be removed. The front seat tube is made up of the two concentric tube members, the outside member 50, part of the front frame 5 and the inside member 44, part of the middle frame 7. The outside tube 50 is drawn tightly around the inside tube 44 by the three quick releases 51, 52, and 53 each working a section of the tubes' length to make both tubes act as one. The front seat tube 44 is capped with the seat 36, on the seat post 38, attached to which is the conventional stroker's handlebar 42, handlebar stem 40. The seat post can be removed by undoing the quick release 46 below the latch 48. At the bottom is the front rider's chain wheel 62, and conventional elliptical pedal housing 60 used to tighten the intermediary chain 90. Element 58 is the frame alignment stop, and 64 the retractable coaster wheel to roll the tandem on when it is folded.

The middle frame 7, as shown in FIGS. 2 and 2B, can be constructed as in first embodiment with a top tube 66, diagonal bracing 68, bottom tube 70, and seat tubes 44 and 76 or it can be constructed with X-bracing members 68 and 69, bottom tube 70, seat tubes 44, 76 as in the second embodiment. Diagonal braces 66 and 69 must be oval tubing set vertically so that the rear tire does not hit them restricting the folding process.

The rear seat tube as in the case of the front is constructed of the inside member 76 and the outside member 80 is drawn tightly around it by the three quick releases 82, 83, and 84. The rear pedal housing 86 is cut around and attached the seat tube 80. This housing holds the main chain ring 90 and on the opposite side the rear connecting chain ring 88. The pedal housing and the chain ring are set high enough above the bottom tube 70 and away from the diagonal tube 68 so that when the rear frame is folded this chain ring will clear both of these members. In between the two chain rings is the alignment stop 92, and below another retractable coaster wheel 64. The rear drive train mechanism of the tandem in accordance with this invention utilizes standard components found on non-folding tandems. That is the larger drive sprocket assembly 90 is controlled by front derailleur mechanism 98 which moves the chain 104 to different sprockets. The chain connects back to the rear set of sprockets 108 controlled by the rear derailleur 112. It is also noted that during the folding of the tandem, none of the gear mechanisms are in any way affected by that operation. The front chain 90 which is the front rider's drive mechanism becomes disengaged and must be removed during the folding operation.

The rear frame 9 which is made up of the conventional seat stays 94, 96 holding the rear brake 92, chain stays 100, 102 all connected to the seat tube 80. An optional drum brake 106 used on tandems can be installed. The rear wheel 10 can be removed with the standard quick release 110. Not shown on either crank housing are the pedals, or on the frame members the brake and derailleur cables.

Referring now to the phantom line portion of FIG. 1, the folding operation in accordance with the present invention will be discussed. The first step is to remove the two seats utilizing the quick releases 46 and 78. Next, the front wheel is removed by utilizing the quick release 14. The tandem is then set down to rest on the front forks 16 and the rider moves to the rear, positioning himself over the rear seat tube. Quick releases 82. 83, and 84 are undone, the back latch 48 is depressed, then the middle frame is lifted to take the weight off the rear tire. Then, the rear frame section is folded around until it is tight against the diagonal 68. It can be secured in the folded position by tightening any one of the quick releases 82, 83 or 84. The chain 90 is picked up and stored. The rider then positions himself over the front seat tube and lifts the front end of the tandem up while extending the front coaster wheel 64. Quick releases 51, 52 and 53 are undone, the front latch 48 is depressed, then the front frame is lifted to take the weight off the front forks, then the front section is folded around until the handle bars are tight against the rear tube 80. Again, one quick release 51, 52 or 53 can be tightened to secure it in the folded position. Then the rear coaster wheel 64 can be lowered to below the bottom of the folded front forks so the folded tandem can be rolled along. The handle bar stem can be equipped with a quick release 31 so that the handle bar and stem can be pulled out of the headset, clipped onto the top tube 66 to achieve a tighter folded package.

Finally, the tandem can be outfitted with conventional accessories, some of the embossments for such illustrated. Elements 54 are for water bottles, 109 for rear luggage carrier. While the first and second embodiments have been shown, there exists in non-folding tandems, unisex frames for both front and back riders. They are weak, heavy tandems but they could be designed to fold. However, the folding frame concept could be applied to such a configuration.

Further modifications can be made to this invention without departing from the essential scope thereof.