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As i understand it, fuel tanks for carburettor bikes were mounted on top of the engine since then you wouldn't need a fuel pump. But modern bikes have one anyway, so why didn't the tank move somewhere lower to lower the center of mass and make some room up top (e.g. for cables, a bit of stroage)?
Is it just optics or are there technical reasons?
There have been carbed road bikes with the fuel tank under the engine. Various late 1980s 125cc 2 stroke Gileras for example, where in some cases it released space to allow a storage area for a crash helmet where the fuel would normally be.
Problem generally attributed to this design is a more noticeable difference to hanling from the centre of gravity moving as fuel is used.
Many carbed bikes did use a fuel pump, allowing the engine to lean forward, with downdraft carbs to provide a straight path to the head, while avoiding needing the fuel tank to sit above the high mounted carbs. Yamaha FZ750 was possibly the first designed like this in the mid 1980s.
On a more modern injected bike with a large 4 stroke engine I suspect an under engine fuel tank would provide packaging problems for the exhaust (trying to wrap a plastic tank safely around a hot exhaust and catalytic converter). While the exhaust could be run over the engine to avoid this, this would result in more complicated exhaust routing, and interferring with the fairly critical airbox.
What would you move?
Put the engine on top and the tank below? Would that complicate the drive to the wheel?
Make the tank as a "surround" to the front wheel? Except for the hazards or collisions...
The design has been "fairly" standard for approximately 100 years or so, and seems to be the "best" all-round solution...
