Question on amount of vehicle pitch during braking

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MSI
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Question on amount of vehicle pitch during braking

Post by MSI »

Q: Is there any literature study indicating how much the bonnet of a car would dive on hard braking? What sort of range of values to expect as the vehicle weight shifts forward?

A: What i believe you are asking is are there any tests with measurements of the vehicle pitch as a result of braking. Unfortunately i am away from the office and can't do a search for possible test sources (for example if any research or conference tests have quantified pitch response during braking tests). Most braking tests are meant to measure the effectiveness of the braking system, (effective deceleration as a function of available friction) whereas you wish to determine the amount of vehicle pitch or dive that occurs.
Just thinking in general, the amount of vehicle pitch or dive of a vehicle during braking is a function of several things:
  • The center of gravity (CG) height
    • A higher CG would tend to pitch more than lower CG with all other things being equal
  • The suspension stiffness
    • A stiffer suspension would resist pitching more
  • The presence of any anti-pitch or anti-dive suspension features
    • Possibly a design feature of the OEM vehicle.
      • Some vehicles include in their design a consideration for the angle of the front upper control arm to produce lift of the front end during braking and thereby reduce the pitch response. There is also a patent which some vehicles may include in their design for attaching a bar to the back of the brake backing plate which performs a similar 'anti-dive' function.
      There are also some after market add-on component for anti-dive which claim to produce anti-dive, however we have not evaluated the effectiveness of any such device
  • The vehicle loading
    • The more occupant weight and/or cargo in the vehicle (particularly as a function of the percentage of overall vehicle weight) could produce greater weight shift forward and therefore more possible pitch
  • The type of braking system and distribution of braking torque
    • What percentage is applied to front/rear of the vehicle
There are ways to quantify, in general, for your application.
  • For example from wikipedia
      • To determine the percentage of front suspension braking anti-dive for outboard brakes, it is first necessary to determine the tangent of the angle between a line drawn, in side view, through the front tire patch and the front suspension instant center, and the horizontal. In addition, the percentage of braking effort at the front wheels must be known. Then, multiply the tangent by the front wheel braking effort percentage and divide by the ratio of the center of gravity height to the wheelbase. A value of 50% would mean that half of the weight transfer to the front wheels, during braking, is being transmitted through the front suspension linkage and half is being transmitted through the front suspension springs.
        For inboard brakes, the same procedure is followed but using the wheel center instead of contact patch center.
A quick check on SAE produced a couple related papers (available from SAE for around $20 each)
  • Quantifying the Change in Ride Height Due to Braking: Brake Dive Test Data, SAE 2006-01-1563
    • ABSTRACT The concept of brake dive is not new and most drivers are aware of this phenomenon. However, no known data is available to quantify the amount of change in vehicle height during braking. ...The study will also review the factors that affect brake dive and the parameters under which these factors can be determined or be reasonably estimated. ...However, no known data is available to quantify the amount of change in vehicle height during braking. Often investigators involved in under-ride/over-ride collisions or relatively low speed collisions are faced with questions such as, “How much does the front bumper dip during braking?”
    Elimination of Roll, Squat, and Dive Through Biased Suspension Response,2000-05-01
    • ABSTRACT: Conventionally sprung vehicles are subject to rolling and pitching of the sprung load as the vehicle corners and accelerates, respectively. Designs which incorporate active elements seek to control these movements, frequently resulting in compromised performance or reduced fuel economy. This paper considers the possible replacement of conventional springs, shocks, and anti-sway bars with specified fluid spring components. The fluid spring components offer biased response to dynamic load variations in the following manner: provide support of the sprung load equal to the magnitude of the load at each moment, and either compressing readily to absorb upward forces originating in the wheel assembly which exceed the magnitude of the load at that moment, or extending rapidly to provide support equal to the magnitude of the load at that moment, in the event the wheel assembly tracks through a hole.
Let me know if you find any additional information and i will add it to this thread. Also once back in office i will check for any specific full scale tests.