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Homework Help: Calculating Spring Rates for Rear Wheel Drive Sports Car

  1. Aug 10, 2013 #1
    1. The problem statement, all variables and given/known data
    A rear wheel Drive Sports car has the following specifications
    The design conditions are that the maximum front ride frequency must be 3.2 Hz and that the rear ride frequency must be at least 10% less than the front.
    Total Mass 850kg
    Weight Distribution 45%Front
    Unsprung Mass each front corner 22kg
    Unsprung Mass each rear corner 25kg
    Front Motion Ratio 1:7:1
    Rear Motion Ratio 1:8:1
    Tyre Stiffness Front 200N/mm
    Tyre Stiffness Rear 230N/mm

    a) Select a suitable spring rate from the information booklet to target a front ride frequency of 2.1 Hz
    b) Select a suitable spring rate from the information booklet to ensure that the rear ride frequency is at least 10% less than the front
    c)Calculate the front roll stiffness as a percentage of the total

    2. Relevant equations

    3. The attempt at a solution
    Ok so I need to be able to complete this question for an upcoming exam and to be honest I'm completely stumped.
    The information booklet for spring rates is unavailable however some pointers in the right direction is all I need telling me what information I need from the booklet.

    In relation to working out the ride frequency
    where k=overall suspension stiffness for relevant corner of car
    m=sprung mass of car

    to work out k
    k spring = spring stiffness (from booklet maybe ?)
    k wheel = 200 N/m (front)
    MR2 = 72 (front) or should it be 1/7 ?
    then use this answer as k in Fn=1/2∏*√k/m and try get as close to 2.1 as possible?

    (Just looking to figure out question a first)
  2. jcsd
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