Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Straight engine vs flat engine on a 1 cyl. motorcycle

  1. Aug 11, 2015 #1
    Hello everybody

    I just got an old underbone bike, which I'm planning to build and finish before the end of this year.
    I found the engine to be quite pathetic (a 4 stroke, carburetted, 100 cc flat engine) so I'm going to do an engine swap with a more powerful but similar engine configuration

    The questions starts here, what advantage would a straight engine have over a flat one ?

    some say that a straight engine will produce more torque as gravity will help it's piston movement, but is it true ? won't gravity also hinder it's piston movement while the piston is going upward ?

    Second, I once read that a straight engine will help make sharper turns as the cylinder movement will contribute to the bike's centripetal force, but I don't see how that could be true as a piston that rocks back and forth like that would also add to the centrifugal force of the bike and both will cancel out each other.

    Not to mention that a flat engine will have a lower center of gravity, thus making it more stable.

    So please help me with my questions here, is there any fact that I miss ?

  2. jcsd
  3. Aug 11, 2015 #2


    User Avatar
    Homework Helper

    The orientation of the cylinder won't have a significant effect on power or handling. A more vertically oriented cylinder will allow a shorter length (front to back) of the motorcycle. A more horizontally oriented cylinder would allow a shorter height (top to bottom). I'm ignoring the case of a horizontal but sideways oriented cylinder, since that would require conversion of the engine's rotation about a roll axis to a rotation about pitch (wheel) axis.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook