No, not all motorcycles have fuel tanks mounted on the frame. While it is true that many motorcycles have their fuel tanks positioned on the frame, there are exceptions to this design.
Some motorcycles, especially sport bikes and some cruisers, have fuel tanks located under the seat or in a more centralized position. This design is often referred to as an underseat or underbody fuel tank. Placing the fuel tank in this location helps lower the bike's center of gravity, which can improve stability and handling characteristics. It also allows for a more aerodynamic body design.
On the other hand, motorcycles with a more traditional design, such as standard, touring, and off-road bikes, typically have fuel tanks mounted on the frame between the handlebars and the seat. This location provides easier access for refueling and maintenance purposes, as well as allows for a larger fuel capacity in some cases.
The positioning of the fuel tank can vary based on the motorcycle's intended purpose, design, and engineering considerations. Ultimately, the choice of fuel tank placement depends on factors such as the bike's style, performance, ergonomics, and desired aesthetics.