Joint for a bicycle frame

An improved joint for a bicycle frame has a first sleeve for receiving a head tube and a second sleeve projecting laterally from the first sleeve, for receiving a top tube or a downtube, wherein the second sleeve includes an inner tube and an outer tube, which coaxially project from the first sleeve and which are spaced from each other with a circumferential clearance which is slightly larger than the thickness of the top tube or the downtube so that the top tube or downtube may be snap fit into this clearance.

BACKGROUND OF THE INVENTION 
The invention relates to a bicycle frame, particularly to an improved joint 
for a bicycle frame. 
Generally, a conventional bicycle frame, as shown in FIG. 1, includes a 
head tube 1, a seat tube 2 for supporting a seat (not shown), two seat 
stays 3 connected to the upper portion of the seat tube 2, two chain stays 
3' connecting the lower portion of the seat tube 2 to the rear portion of 
the seat stays 3, a top tube 4 connecting the head tube 1 to the upper 
portion of the seat tube 2, and a downtube 4' connecting the head tube 1 
to the lower portion of the seat tube 2. In the conventional frame, the 
head tube 1 is connected to the top tube 4 and to the downtube 4' by two 
respective joints 5, as shown in FIG. 2. Each joint 5 includes a first 
sleeve 5A for sleeving a head tube 1 and a second sleeve 5B projecting 
laterally from the first sleeve 5A, for sleeving a top tube 4 or a 
downtube 4'. After the head tube 1 is inserted into the first sleeve 5A, 
and after the top tube 4 or the downtube 4' is inserted into the second 
sleeve 5B, the engaging area between the tube 1, 4, 4' and the sleeve 5A, 
5B must be welded, using a copper solder or a spelter solder. This 
requisite welding connection causes several drawbacks. Firstly, it is 
necessary for the person skilled in welding technique to ensure the proper 
positioning 4 the components of the frame after welding. Secondly, welding 
increases manufacturing costs. Also, since it is desired that bicycles be 
as lightweight as possible, a light solid material such as aluminium alloy 
is used in fabrication. This material, however, may be difficult to apply 
by a welder. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a firm joint for a bicycle frame 
which not only can be made of a light, solid material, but which is also 
quickly manufactured. convenient and stable. 
According to the invention, an improved joint for a bicycle frame includes 
a first sleeve for recieving a head tube and a second sleeve projecting 
laterally from said first sleeve, for recieving a top tube or a downtube, 
characterized in that said second sleeve includes an outer tube and an 
inner tube, which coaxially project from said first sleeve and which are 
spaced from each other with a circumferential clearance which is slightly 
larger than the thickness of said top tube or said downtube so that said 
top tube or said downtube may be snap fit in said clearance. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 3 and 4, an improved joint according to the invention 
includes a first sleeve 10 including an inward flange 11 for restricting 
passage of a top of a head tube 1, and a lower opening 12 indented in an 
irregular curved shape for to reducing the amount of sleeve material 
required. A second sleeve 20 is provided. This second sleeve 20 includes 
an outer tube 21 an an inner tube 22, which coaxially project laterally 
from the first sleeve 10. The outer tube 21 is shorter than the inner tube 
22, and the two tubes 21, 22 are connected by a radial wall 24 at a base 
portion thereof and are spaced from each other by a circumferential 
clearance 25. This clearance 25 is slightly larger than a thickness of a 
top tube or a downtube 4 so that the top tube or downtube 4 may be snap 
fit in the clearance 25. The outer tube 21 includes an irregular shaped 
opening 211 and the inner tube 22 includes three axially extending 
elongated protusions 221 on the outer surface of the inner tube 22. This 
arrangement enhances snap fitting of the top tube or the downtube 4 with 
the second sleeve 20. Optionally, the engaging area of the first sleeve 10 
and the head tube 1 as well as the engaging area of the second sleeve 20 
and the top tube or the downtube 4 may be applied with an adhesive to 
enhance the security of the snap fitting. The thickness of the adhesive 
layer on the engaging area is decided by the thickness of the protrusions 
221 for the purpose of ensuring the stability of the joint of the second 
sleeve 20 and the top tube or the downtube 4. Thus, the security of the 
joint according to the invention is the same as other prior art joints 
(such as that of FIG. 2). However, the joint of the instant invention is 
quicker and easier to assemble as it is easier to position the elements 
thereof. Furthermore, no welding is required by the joint of the present 
invention. 
Referring to FIG. 5, the structure of another improved joint according to 
the invention is disclosed. This joint is similar to the joint of FIGS. 3 
and 4 except for a plurality of bumps 222 on the outer surface of the 
inner tube 22. The bumps 222 are adapted for frictional engagement of a 
top tube or a downtube. 
Aternatively, the lower opening 12 may be in any suitable form, such as a 
circular shape parallel to the upper opening of the first sleeve 10 so as 
to increase the engagement area of the head tube 1 with the first sleeve 
10. The openings of the outer tube 21 and the inner tube 22 may also be in 
any suitable form. For example, the outer tube 21 may be elongated 
relative to the inner tube 22, and the protrusions 221 and the bumps 222 
may be provided on the inner surface of the outer tube 21. 
With the foregoing, the splitting of usual single-tube sleeves applied by 
an adhesive in other fields can be avoided, when an external force is 
applied thereon. 
With the invention thus explained, it is apparent that various 
modifications and variations can be made without departing from the scope 
of the invention, it is therefore intended that the invention be limited 
as indicated in he appended claims.