Dynamics - Suspension Components

Owned by Evan Dodd
Last updated: Feb 02, 2022
7 min read

  • Road unevenness is characterized in terms of a sinusoidal wave function

  • For time function:

  • This pattern is then transferred into non sinusoidal, but periodic motion

  • Road unevenness as a function of time isn't that important, but rather the statistical mean road unevenness is

  • Components of Suspension System

    • Tires

      • The wheel load is transferred to the tire by the rum and pressure is applies to this contact area

        The following chart shows the proportions of load carrying components of a tire as a function of compression

      • The respective operating point of the tire also influences the spring rate, with increasing driving speed the influence of the centrifugal force at the tire circumference causes an increase in the spring rigidity, the side forces generated at the tire due to the side slip angles lead to a lateral displacement of the tire-road contact area, which results in an uneven load on the tire shoulders and hence the reduction in spring rate

      • Tire damping

        • Tire damping is a form of material damping, which leads to a rise in the temperature of the tire and increases with the number of plies and the material mass in the tire.

    • Body Springs

      • Elements of wheel suspension systems, which produce a restoring force under elastic deformation

    • Leaf springs

  • Classical design of springs

    These springs can also be used as structural elements for the coupling of the body and axle

  • Multilayer leaf springs inherently possess damping characteristics due to frictional forces between the leaves

  • Commonly used nowadays in combination with rigid axles in only a few car designs

    Leaf spring has a linear characteristic curve shown below

     

  • Usually large spring lengths are necessary in practice

    • May not be that feasible for our solar car

  • Because of the linear characteristics there are two main disadvantages

    • Spring Travel

      • When a load is applied on the spring, the available spring travel can often reduce to insufficient values

    • Natural frequency is not constant

      • Since the spring stiffness does not change with the load, the body natural frequency in the unloaded condition is larger than the natural frequency in the loaded condition because the mass loaded is greater than the mass unloaded

    • These disadvantages can be reduced through the use of additional auxiliary springs

    • Another disadvantage of leaf springs involves dry friction which occurs both in the suspension as well as between the leaves of the multi layer leaf springs

    • These disadvantages can be handled by the use of shackles instead of sliding shoes in the suspension or by introducing plastic layers between spring leaves

  • Torsion Bars

    • Used more frequently in passenger cars and light commercial vehicles

    • Torsion bars are rods made of spring steel - they are stressed by torsion

    • They are clamped at one end and free to twist at the other end

    • There is a crank at the end that is free to move

    • The strut of the axle or the suspension forms the crank

    • Torsion bars are normally arranged along the body-side bearing axis of the strut, at the opposite end of which the vertical wheel force acts as an external load

    • There is a twisting angle and a moment

 

  • The shear stress of a torsion bar with a circular cross section is

  • The diameter of the bar cannot be "random" it must be a function of the maximum twisting moment and the admissible shear stress

  • Typically, to achieve suitable spring stiffness, large torsion bar lengths are required

    For front wheel suspensions torsion bars are used often in combination with wishbones and arranged parallel to the wheels

  • Coil Springs

  • The most common spring in a passenger car

    The equivalent of a wound torsion bar

    Loaded mainly y torsion

  • R is considered as the lever arm and F acts as the spring force and D/2 as half the diameter of the spring coil spring stiffness is defined by:

 

  • The normally linear characteristic by the variation of the overall diameter, the wire thickness and the gradient

  • This effect is based on some coil windings coming into contact with one another

    • Which means the effective length of the wound torsion bar shortens

  • The combination of all three measures creates a "mini block spring" shown below

  • The low overall height of the mini-block is an advantage

  • Since coil springs can essentially withstand higher forces along their longitudinal axis in comparison with their transverse axis, they are used similar to torsion bars in combinations with the struts of the wheel suspension, which support those components of forces which cannot be withstood by the springs

  • Gas Springs

    • Previously the springs absorbed energy as a result of deformation with gas springs the energy is absorbed through a variation of volume

    • Principle structure of an air spring

 

  • Hydro pneumatic Springs

  • All in all Steel springs have more linear Force distribution then Gas/Air Springs

  • Shock Absorbers

    • Shock absorbers serve both to guarantee driving safety of a vehicle as well as optimize driving comfort

    • Driving safety is highly influenced by the road grip of the wheels

    • Oscillations of the wheel masses have to be minimized by damping, since they are inly suspended on the tire spring but not on the vehicle body suspension

    • Vibration dampers or shock absorbers differ in the type of friction, which causes dissipation into heat energy

    • Different damper designs:

    • Mechanical friction has since been replaced by fluid friction since the resulting exponential relationship between damping force and the spring rate allows for a better adaptation of the absorption characteristics to the vibratory vehicle system

    • Fluid dampers also show a much better response

    • Lever dampers (fluid friction)

      • Oil is forced through a valve by a piston which is displaced by a lever

      • The lever is often realized in the form of a transverse arm

      • This type of damper is rarely used nowadays

    • Telescopic shock absorbers

      • are used almost exclusively today

      • Apart from wearing of parts such as levers, bearings and operating cams, these dampers are advantageous because of the possibility of more precise adjustments then lever types

      • This is due to the fact that the soring travel and plunger lift mostly correspond

      • Due to the large plunger lift, the liquid recirculation is large

      • The direct damper can hence operate with a much lower pressure compared to lever dampers

        • This has a favourable effect on lifespan

      • There are two types if telescopic dampers

        • Twintube

        • Monotube

 

  • The mechanical friction is absorbed and transferred into heat and travels over an external tube of the damper