Why Inline Fours?

Discussion in 'Motorcycle Racing' started by ezridernc, Aug 13, 2006.

  1. ezridernc

    ezridernc Guest

    Why do the major manufacturers race, market and sell inline fours?

    I can only think of:

    1. It's The Rules (but AMA has provisions for different engine formats)

    2. It's the Horspower (yet in MotoGP [fastest series] most bikes are
    not inline fours)

    3. It's the Consumer (can't argue this either way but L-twins, V4s and
    Triples sell)

    4. It's Cheapest (doesn't seem logical)

    So, why is it?

    ezridernc, Aug 13, 2006
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  2. ezridernc

    bram stolk Guest

    A tradition that begun in the air-cooled era.
    The only way to properly cool 4 cyls with riding wind is having them
    inline, across the bike.

    Also, a crank shaft that is perpendicular to the length of the bike
    has major benefits: bike remains upright when revving up or down
    (unlike a 2cyl bmw), and it is easy to hook up drive axle with rear
    wheel via a chain.

    bram stolk, Aug 13, 2006
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  3. ezridernc

    Alexey Guest

    It's a combination of several factors, including cost and performance.
    In production based racing (WSBK and AMA), the bikes must retain
    production engine architecture at the minimum. So if you're selling an
    inline-four, that's what it must be in race trim. In terms of
    production, making 1 head per engine instead of 2 is a huge cost
    savings. Ever wonder why twins get to be more expensive? You're
    paying for an extra crank, extra cam chain, extra cam chain tensioner,
    etc. A single head engine also allows for a much easier time shoving
    it as far forward as possible making for a nice long swingarm, which is
    a performance benefit.

    In terms of cylinder count, 4 is a pretty good number as far as primary
    balance in an inline arrangement. I don't recall how well balanced a
    90 degree V-4 is, but I suspect it's not too bad.

    When it comes to pure bred racing machines you see in GP's, the engine
    architecture is dictated by mass centralization, other component
    arrangement, performance-driven cylinder count (4's and 5's get to have
    the same min mass, so 5 is better than 4), and power delivery. That
    last one is where we see engineers' ideas in the most obvious sense.

    Consider V-4 vs I-4. The only thing that really changes from one to
    the other is where the power pulses happen in relation to one another.
    In an I-4, a street firing order is: 1, 2, 4, 3. Each pulse is
    separated by 180 degrees of crankshaft rotation. You can switch that
    to a big bang style firing order with something like this: 1 & 4, 2 &
    3, nothing, nothing. So you get one big pulse, 180 degree rotation,
    another pulse, then 450 degrees before the next cycle.

    I've never had a V-4, so I can't pull up firing orders off the top of
    my head, but the moral of the story is that in a V configuration, some
    of the firing pulses mentioned above can be offset by the angle of the
    V. In a sense, a road going V-4 is closer to a big bang configuration
    right out of the box because it's not gonna have evenly spaced pulses.
    So some people consider it a good compromise between a big bang I-4 and
    a screamer I-4.

    But now the world's changing rapidly because electronics are getting
    better and better. The computer, traction control in particular, is
    taking over more and more of the power delivery problems and solving
    them for the rider no matter what the firing order is. And, while
    still important, I suspect reliability (primary and secondary balance)
    is gonna be more a factor then with the 800's than power delivery on
    its own.
    Alexey, Aug 13, 2006
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