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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?

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    I'm voting to close this question as off-topic because it is not about maintenance or repair. – Solar Mike Jul 12 at 11:01
  • The premise of your question assumed that the tank placement is solely based on gravity feeding the fuel system, but that's not correct. You could easily have a fuel pump with a carbureted engine in a motorcycle with little penalty in weight and cost, the reason they don't move the tank is other factors in design. – GdD Jul 12 at 12:01
  • Where do you suggest it be moved to? Its the only place left to put a fuel tank. – Moab Jul 15 at 15:50
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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.

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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...

  • Yep, surround the fragile fuel tank with a squishy human shaped bumper. In the event of an accident the bumper will protect the fuel tank from major damage. – Mauro Jul 12 at 11:42
  • @Mauro I saw a hospital program not so long ago where the A&E consultant said that the current shape of the tanks was « castrating » motorcyclists very effectively when involved in a head on collision... – Solar Mike Jul 12 at 11:52
  • When I ended up in a head on collision (admittedly at <30 mph) I didnt get castrated just broken armed – Mauro Jul 12 at 12:07
  • @Mauro I get the idea that the consultant sees many more motorcycle collision victims than you, but it was what he said that was surprising... But good to hear you were lucky, hope you are fully recovered. – Solar Mike Jul 12 at 12:09
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    @SolarMike "Castrating bikers in head on collisions" is just watching evolution happening. Only those who can ride without crashing get to breed the next generation of bikers :) – alephzero Jul 12 at 12:52

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