Why does a longer swing arm give better traction?

Discussion in 'The Perfect Line and Other Riding Myths' started by boatpuller, Aug 12, 2013.

  1. boatpuller

    boatpuller Long timer

    Jan 4, 2012
    Central fly-over land.
    By 2 inches.
  2. Jan from Finland

    Jan from Finland Been here awhile

    May 2, 2005
    That's a good question. Even if this thread went down since page 2.

    I think that there are two reasons. Longer swingarm moves less front-to-back for given suspension movement. Additional horizontal movement at tires's contact patch reduces traction. The other reason is that longer swingarm resist wheeling so you can accelerate harder. That works like added traction.

    The BMW's rear suspension is not a simple swingarm. It's a form of paralleogram suspension. If it were true parallelogram, where the facing sides are of equal length, effective swingarm length would be infinite and tire would only move only up and down (like in equal length double wishbone suspension in a car). In that case adding 2 inches to length of swingarm would do nothing.



    However, in BMW's paralever all opposing sides are not equal length. A to B is not the same as C to D. B to C is different than A to D. That's because there is build in anti-squat in the design. It uses shaft effect to prevent squat during acceleration. Therefore BMW's swingarm length is finite and adding 2 inches actually lengthens the swingarm. At least in theory. I am not sure how much adding 2 inches to about 55 inches effective swingarm length actually helps. At least it gives the marketing department some bragging rights...
  3. JDK111

    JDK111 Been here awhile

    Jun 22, 2010
    W. Canada
    Agreed with both points .... AND ... There's a u-joint (or cv joint) at the swing arm pivot point. For a given length of suspension vertical travel, the longer the swing arm, the less the angle the driveline joint will see. The less this angle is - the better.