fasttrackfab
fasttrackfab is offline Semi-Pro

Join Date
    Jul 2009
Location
    port huron, michigan
Posts
    126   

Default

    To calculate motion ratio it is a simple formula:

    mr=(d1/d2)^2

    mr= motion ratio

    d1= distance from lower a arm pivot point on frame to centerline of ball joint
    d2= distance from lower a arm pivot point on frame to cneterline of lower shock mount

    a brief explanation of motion ratio:

    (K)
    A-----B-----P

    P is the pivot point, B is the spring mount, and A is the wheel. Here the motion ration (MR) is 0.5... imagine a spring K that is rated at 10 lb/in placed at B. If you want to move A 1 in vertically upward, B would only move 1in*MR = 0.5 in. Since K is 10 lb/in, and B has only moved 0.5 in, there's a force at B of 5 lb. If you balance the moments about P, you get 5(B)=X(A), and we know B = 0.5A, so you get 5(0.5A) = X(A). A's cancel and you get X=5(0.5)=2.5. Which is 10*MR*MR or 10*(MR)^2.


    now the problem you run into when moving the upper shock mount is changing the angle of the shock, ie.. the angle of the shock to the ground, moving the upper mount more outboard will stand take the shock angle closer to 90 versus moving the upper mount inboard lays the shock down and decreases the angle. What this does is the effects the spring rate needed for the same ride.

    To calculate the spring rate needed go to this site and use their calculator. It also explains where to take measurements from. Much easier than me typing it in here.

    http://www.f-o-a.com/calculator.html

    I am being very brief here. if you have more specific questions please ask.

    Last edited by fasttrackfab; 09-11-2010 at 06:39 AM.