Greyout's guide to spring stiffness & weight transfer

Discussion in 'GT4 Tuning' started by Greyout, Mar 7, 2005.

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  1. Greyout

    Greyout

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    665
    (disclaimer: I'm not an engineer and don't pretend to be one)

    This is not a replacement to M-spec's guide, he's got some great stuff in there.

    In this post, I am looking to answer a few questions that come up, and to try and provide some knowledge so that as you adjust the settings, you know what is happening.

    first up: weight transfer

    It appears that many people are confusing body roll and weight transfer. Although we associate body roll & pitch with the same forces as weight transfer, they shouldn't be considered the same thing. Softening a spring does not increase weight transfer.

    If you were to be riding on a giant sled, with absolutely no suspension, the sled would still exibit weight transfer in a corner, with no body roll at all.

    [​IMG]
    Turning to the left, our little man leaning into the turn.

    In steady state corning, what was 200 lbs on each side might become 150/250. The amount of weight transfer is detirmined by how wide the track is (wider = less weight transfer) and how high the CG is (lower CG = less weight transfer). One can easily see why high performance cars are wide & low to the ground :)

    The reason why weight transfer is important is because of the relationship between how much lateral force a tire can generate and how much downward force is pushing it into the pavement. Obviously, the more downward force, then the more friction, but it does not increase on a 1:1 ratio. X downard force might result in Y friction, but 2X does not result in 2Y - it results in something less then 2Y.

    It is because of this that the greatest traction occurs when there is no weight transfer - i.e. when both tires share the weight equally. Anything other then that, and you are making less then optimal traction. If we could, we would make a car that has a CG at ground level (no weight transfer) but obviously that can't be done.

    next: Body roll

    as we guide our cars around a corner, the suspension allows the body to roll. The axis that the car rolls on, and the amount of roll, depends on the suspension stiffness, the roll center, and the distance of the center of gravity from the roll center.

    The roll center is the instantanous point at which the car rolls on. This is detirmined by suspension geometry. It can be found by extending an imaginary line off of the upper and lower control arms to a point at which they intersect (magenta). A line is then drawn from the center of the contact patch of the tire to that intersection point. The roll center is the point at which the last line, blue, crosses the center line of the car. Its also the point at which the two tire lines cross if you are figuring from the both side, which would be used if the roll center was off to one side or the other (again, remember the roll center is constantly moving as the suspension moves).
    [​IMG]
    The grey box is the tire when viewed from the front. The black lines are the suspension arms. The body has been removed.

    If you connect the front and rear roll center, you have a line. This line is the axis that the car rolls on. Generally, the rear is slightly higher then the front.
    [​IMG]
    Viewed from the right. The axis that the car will roll on.


    As lateral force acts upon the CG, the distance the CG is from the roll center detirmines how much body roll the car will exibit. If the CG was ON the roll center, then there would be NO body roll. the farther the CG is from the roll center, then the more leverage it has on on the body, and the more it will roll.

    It is VERY unlikely that PD had time to compute the roll center of each vehicle, and therefore suspension geometry in general is not simulated. This is unfortunate, but expected.

    What this DOES help you do is understand why adjusting the springs and sway bars does what it does.

    lets take a car that has a perfectly even weight distribution (each number is the weight on each wheel.)

    700 700
    700 700

    in a right corner, we get weight transfer to the left.

    900 500
    900 500

    total weight is still the same.

    When we increase the spring rate, we are not preventing weight transfer. The springs react into the body, reducing body roll, NOT weight transfer Stiff suspension does a few things good for us:

    - reduces suspension travel, to prevent dynamic changes in camber and toe.
    - improves responsiveness
    - allows the car to ride lower without bottoming out

    so, we increase the spring rate by 50% - but we are still at

    900 500
    900 500

    with the stiffer spring, we could lower the car more, which would reduce weight transfer a little, but we won't include that just yet. now lets see what happens when we stiffen up the rear end more.

    850 650
    1000 400

    but wait - if the rear got STIFFER, why is there MORE weight transfer in the rear?

    The reason is because the spring is, again, reacting to the body roll. As the body rolled to the left, the rear spring held up that corner more, increasing the downforce on that side and unloading the inside rear. This results in LESS of a weight transfer up front. The TOTAL weight transfer is still the same, as it should be - 800 lbs difference between left and right.

    the front tires are sharing the weight more evenly, so they will be able to provide more traction.

    In a FWD car, with the same weight, it wouldn't be rare to see this:

    800 600
    1400 0
    [​IMG]

    SWAY BARS:

    a sway bar is simply a semi-flexible bar that connects one side to the other. It adds spring rate to one side in roll, but not in bump. So if you have soft springs and a thick sway bar, then the ride quality will still be soft, and you will still have lots of pitch & dive, but the suspension will resist roll. A sway bar increases the downforce on the outside tire by unloading the inside. Thus, a thick swaybar is not a good idea for the front of a FWD race car. usually, production based race cars will not have any front bar at all, and rely stricly on proper spring rates.

    Discuss :)
     

    Attached Files:

  2. Greyout

    Greyout

    Messages:
    665
    it appears my imagestation links aren't working :( just right click, double click the link, and post in a new window :( sorry

    EDIT: I attached them in the order that they were linked. hope that makes it easier.
     
  3. Kolyana

    Kolyana

    Messages:
    2,353
    I'm not sure I follow this ... sorry.

    I've been studying car handling and performance techniques for just a week or two (I started with IndyCar 2005 and trying to get my ehad around that), and while I know more than I did, there's still something here that eludes me.

    COuld someone explain the above slightly differently so a dumb english chicky can get her head aorund it?
     
  4. Greyout

    Greyout

    Messages:
    665
    imagine the 4 tires of the car are fixed to a imaginary sled with no suspension. As you go around corners, the interia causes weight transfer, even though there is no leaning of the solid sled.

    for a given CG height, track width, and cornering force, you will have the same weight transfer regardless of spring stiffness. its simply centrifigual force at work.

    the body of the car is suspended above the suspension, and is free to flop foward, back, left and right.

    To keep the body off the tires, we require springs stiff enough to support the car. Look at the outside of the car in a turn, with a stiff rear spring. The outside rear spring is going to hold the body off the tires with more force, reducing the amount of body weight that would otherwise be supported by the outside front. You still have the same total weight transfer to the outside, but the rear spring is holding up the body more then the front.

    With pure lateral acceleration, there is no front/rear weight transfer, so that results in more weight being kept on the inside front.

    static:

    1400 lbs---> 700 700

    1400 lbs---> 700 700

    TOTAL:..1400 lbs..1400 lbs

    When at rest, this car has equal weight all the way around (must be nice...). maximum traction is available because each tire is sharing exactly 1/4th of the weight. any bias of weight results in less then optimal Coeffient of friction.


    dynamic (right turn) with equal springs

    1400 lbs ---> 900 500 <--- 200 lbs shift off right

    1400 lbs ---> 900 500 <--- 200 lbs shift off right

    TOTAL....1800 lbs.1000 lbs

    in this right turn, we have 200 lbs shifted off each end, for a total weight transfer of 400 lbs off the right and onto the left (800 lbs difference between right and left). This could be with any spring, or none at all. Each axles traction is reduced by the same amount, as the bias of weight on each end is the same.

    [dynamic (right turn) with stiff rear

    1400 lbs --->750 650 <--- 50 lbs of shift off right

    1400 lbs--->1050 350 <--- 350 lbs of shift off right

    TOTAL..1800 lbs..1000 lbs

    here we stiffened up the rear springs. Note that the total weight of the car, and the total weight transfer from right to left, is the same. The weight on each axle is the same.

    As the body rolled to the left, the rear spring, being stiffer, held up the body more then the outside front. This produces an equal and opposite force downward, forcing the tire into the ground more. This also REDUCES the body's weight over the front left, because the back left is doing some of the work the front left would have been doing. This results in some of the body weight being shifted toward the INSIDE FRONT (this is where corner weighting, or wedge, comes into play). This results in the total weight supported by each axle remaining the same. Because the front tires are closer to an even bias of weight, they are closer to their optimal traction, and can produce more lateral force then the rear.

    Oversteer insues :)
     
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