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Thread: Jacking force

  1. #1
    I've gone through many posts, calculations regarding Jacking Forces on the forum, but could not get a clear answer of what exactly is the effect of jacking force on the net Tire load(both Steady state and Transient).

    From my understanding of the concept, the Jacking force depends upon the RC height and the RC Lateral Position(or IC as per some) and also lateral forces. So if during a cornering maneuver the lateral force on the outside wheel is more(assuming RC to be static for simplicity, RC Height above ground), the net jacking force on the chassis would be in the upward direction, thus causing a vertical acc. and lifting the chassis.

    Thus net force on 4 wheel = Mass*(gravity-vertical acceleration) ; vertical acc: due to jacking
    so the net effect should be a reduction of the net normal load.

    What I dont understand is why some of the calculations on the forum:

    a) First add up the jacking force on each side of the suspension to the net tire normal load on that side "(Lateral force is perpendicular to normal force(in tire coordinates), so why does it DIRECTLY add to some tire normal force ???)" and then decrease the same force from the spring and keeping the net tire load same ??

    or to say:
    (No effect of cg movement or roll etc etc.)
    Lateral Forces ---> Jacking Forces ---> Vertical CG movement ---> Spring being longer ---> Reduction of net Normal Load. (Did i miss something somewhere?)

    If this is true:
    Then during constant cornering maneuver, if the lateral force on the outside wheel is always more than on the inside, the net vertical force due to jacking will act to lift the sprung mass and this in effect will decrease the net tire load due to the spring being longer, and this condition is maintained till the exit of the turn when the forces on both side is same. (Which has a large effect on my Dynamic Model

    Any help, advice is appreciated.


    PS: I hope to get Unscented Kalman Load observer working one day :/

  2. #2
    You seem to be getting crossed up between which forces are internal to the vehicle (i.e. between wheel and chassis) and which are external (normal load at the tyre CP). You are right that 'jacking forces' can act to lift the sprung mass, but what is reacting this? answer - the tyres! You haven't magically reduced the mass of the vehicle, or load transfer, just the way it is transferred.

    The easiest way to get your head round this, is consider how the forces are fed into the chassis, and what effect this has on the body's motion, taking moments about the CG of said body. Just remember, all the tyre forces feed into the chassis at the IC for that wheel... With ICs on the ground plane (in side and end view), all CP forces (Fx and Fy) are fed into the chassis at the ground, which is the CG height (Hcg) from the CG. This produces a roll (and/or pitch) moment on the body which will account for 100% of the load transfer; all of it goes through the springs/bars (no jacking).

    Now imagine on of your ICs is above the ground (Hic). The Fx and Fy forces for this corner act at this point, so your moment on the chassis is now less because the forces act at a distance of Hcg-Hic, resulting in less roll/pitch. This also introduces a vertical component between the chassis and wheel though due to the angle of the swing arm. This acts upwards on the chassis at the IC (so creating its own roll/pitch moment on the body), but also downward onto the tyre. Hence the net vertical load at the tyre is maintained, but your chassis will have rolled/pitched less, transferring less through the springs, but some has been transferred instantaneously through the links. This is what some call the geometric or inelastic load transfer - effectively the 'jacking force'.

    Of course in reality each IC can be anywhere along the instant-axis, depending on the proportion of Fx/Fy force at the CP, making it a 3D problem. But to help evaluate the jacking forces, you can take a side view and do the longitudinal forces only, then an end view for the lateral forces. Repeat this at all 4 corners with their individual ICs and you'll be able to resolve the actual roll/pitch moment on the body.

    Hope this helps!

    Lee Stretch
    UHRacing Alumni

  3. #3
    Another way to thinking about jacking force, is to compare it to antidive and antisquat.

    They are really all very similar related effects created by the interaction of cg and instant centre heights.

    Jacking does remove some of the vertical load off the springs, but it replaces it with an equal extra vertical load component on the suspension links.

    The vehicle rises on it's springs, but total vehicle mass obviously has not changed.

    The net effect is that it raises the effective CG, and increases the roll couple
    Cheers, Tony

  4. #4
    Senior Member
    Join Date
    Mar 2005
    Tsk, tsk, tsk.... Such a simple subject, and so much misunderstanding....

    "IC's", "Instant Axes", and other childish 2-D gibberish used when discussing a 3-D problem?


    Use "n-lines". They are simple, and work in 2-D and 3-D. Here the relevant n-lines are all the straight lines that pass through the wheelprint and are perpendicular to its path-of-motion, as constrained by the suspension linkage (which may include the driveshaft).

    "Forces from the wheelprint act on the chassis at the IC"?


    Forces act along their "lines-of-action". ANY point on the line is just as good as any other point (not considering structural problems). This "forces at the IC" is possibly the biggest source of misunderstanding in this subject. Especially so when only the horizontal force component (Fx or Fy) is shown at the IC. Very common, and very wrong!

    BTW, the forces carried by the suspension linkage (ie. everything other than the spring-dampers) act along the abovementioned n-lines.

    Jacking forces?

    Since the Olympics were just on, think about the pole-vaulters. They run fast, stick the pole into the ground, and what happens???


  5. #5
    I'd argue that 2-D childish gibberish is just fine when the problem or specific answer you're trying to address is frequently, almost entirely two dimensional.

    I always go with the most simple method of analysis for any problem so long as it meets required accuracy.

    Same reason ADAMS simulations aren't the best choice for everything even if MBD is the most complete way of doing things.

  6. #6
    Originally posted by exFSAE:
    I'd argue that 2-D childish gibberish is just fine when the problem or specific answer you're trying to address is frequently, almost entirely two dimensional.
    Absolutely !
    Cheers, Tony

  7. #7
    Senior Member
    Join Date
    Mar 2005

    "I'd argue that 2-D childish gibberish is just fine when the problem or specific answer you're trying to address is frequently, almost entirely two dimensional."

    I have yet to see an FSAE suspension that is anywhere close to being "entirely two dimensional". Some suspension types are sort of 2-D-ish, or planar, such as trailing-arms, but yet to see such in FSAE...

    "I always go with the most simple method of analysis for any problem so long as it meets required accuracy."

    To calculate, say, lateral jacking forces, only one kinematic number per wheel is required, namely the lateral n-line slope. The "IC" approach requires two numbers per wheel, the IC's Y and Z positions. (Incidentally, n-line slope is (with qualifications) the ratio ICz/ICy). To calculate lateral and longitudinal jacking requires only two numbers per wheel (the lat and long n-line slopes), whereas the "Instant Axis" concept demands four numbers (ie. 2 x IC positions).

    I figure one is simpler than two, two simpler than four, etc...

    But, more importantly, the childish 2-D gibberish of ICs and Instant Axes frequently gives the wrong results. Almost nobody is aware of this. So nobody knows what accuracy they have, so they have no idea whether it meets requirements.

    So the more complicated method is also the less accurate.

    I smell fear of change....


  8. #8
    Well I can make the argument then that this is all effectively meaningless if you're not using the actual tire forces and moments, and many chassis engineers wouldn't know if they have a good tire model to work off of or not. What are your jacking coefficients really telling you without it?

    Not that I really get down to this level of nitty gritty that often anymore anyway.

  9. #9
    After years of careful observation (and participation), I submit the following hypothesis:

    Of every topic discussed on this board, roll centers have the highest ratio of words typed in this forum to actual importance to the laptime of a vehicle.


    To the OP: have you quantified your jacking forces? I'll bet you're likely to add more force to your sprung mass while trying to pass tech!
    "Gute Fahrer haben die Fliegenreste auf den Seitenscheiben."
    --Walter Röhrl

  10. #10
    Discussion is always worthwhile, if it helps to clarify the thinking.
    And sometimes you think you understand something, until you try to explain it to someone else, then begin to realise the complexities go a bit deeper.

    Some of the more abstract ideas can be difficult to get ones head around, and I don't see discussion as being a waste of time.

    You could argue that any one single aspect of vehicle dynamics (taken in isolation) as not being that significant in itself to lap times.
    But the final combination of compromises sure are.
    Cheers, Tony

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