The ornamental design for a bicycletoptube, as shown and described.
FIG. 1 is a perspective view of the bicycle top tube of the present invention showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front end view thereof;
FIG. 4 is a side elevational view thereof;
FIG. 5 is a rear end view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing the bicycle are provided for the purpose of illustrating environmental structure and forms no part of the claimed design. Furthermore, the broken lines showing portions of the bicycle top tube illustrate portions of the article that forms no part of the claimed design.