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    PSM : How it works! ( track suggestions )

    Ok, here is a very interesting article about the PSM on track.

    enjoy the reading

    Porsche Stability Management System: A racer’s perspective
    April 29, 2001 marked the official return to Formula One of electronic
    driver aids, including traction control. In racing as elsewhere,
    technology that enhances (or interferes with, depending on your
    perspective) human performance is controversial. Potential Porsche buyers
    face a similar controversy in deciding whether or not to purchase Porsche
    Stability Management System (PSM) in the new Carrera 2, Boxster, or
    Boxster S. PSM is standard in the Carrera 4 and Turbo and unavailable in
    the new GT2.

    If you never intend to race your new Porsche, the decision to purchase
    PSM is simple. If you can afford it, buy it. It provides a level of
    safety impossible to achieve by driver skill alone. Here’s why. PSM
    monitors the ABS sensors (which measure the speed of each wheel), engine
    speed (RPM), throttle position (via E-Gas), gear selection, lateral
    acceleration (side to side), yaw (the car spinning in a circle), and
    steering wheel position. This enables the PSM to detect oversteer and
    understeer. It basically determines the slip angle of the front and rear
    tires, or more simply, when the car is not going where the steering wheel
    is pointed. Oversteer is minimized by automatically applying the brake on
    the outer front wheel in a bend, slowing the rotation of the car;
    understeer is minimized by applying the brake on the inner rear wheel,
    speeding the car’s rotation. No driver will be able to do that until
    Porsche develops a car with four brake pedals. However, PSM is not only a
    braking system. If you lift off the throttle in a low traction situation
    (wet, snow, etc.) and the back of the car gets loose, PSM will increase
    the engine speed (blip the throttle) to keep the car in line. Also, if
    traction is low, PSM can use engine braking (EDC – engine drag torque
    control) to slow the car. PSM can calculate the amount of available
    traction by comparing wheel speeds at all four corners of the car.

    Recognizing that even street drivers expect excitement from their
    Porsches, PSM allows approximately seven percent slip angle before
    intervening. Five to seven percent is generally agreed to be the limit
    for modern, high performance tires. The biggest difference between PSM
    and the other systems on the market today (Mercedes Benz, BMW, Jaguar,
    etc.) is that PSM is programmed to allow a good deal of slip, as you can
    see. All of these other systems clamp down the moment any slip (i.e., fun
    driving) is detected.

    However, if you require more fun, you can turn the PSM off. When you
    "turn it off," you are taking only the outputs offline. The PSM system is
    still collecting data from the ABS system, the yaw sensor, the lateral
    acceleration sensors and the steering wheel position sensor. If you have
    PSM off, and the levels of slip are exceeded, and you do not touch the
    brakes, the car will continue to slide. If you have not exceeded the
    levels of slip allowed, and apply the brakes (no matter how hard), PSM
    will not active its outputs. However, if you have exceeded the levels,
    AND apply the brakes (no matter how hard), PSM will activate until the
    car has regained control or you get off the brakes, at which point PSM
    stops outputting. PSM assumes that since you hit the brakes that you are
    not comfortable with the level of sliding and that you want it to help.
    This answers the question, posed by Mike Furnish on the PCASD forum, that
    inspired this article, "what happens in a spin when you put both feet
    in?" Presuming that you put in the correct two pedals, PSM will activate.

    So what about PSM and racing? At this point in my career, PSM is an asset
    to my racing. It has allowed me to more confidently explore the limits of
    traction on the first few laps at a new track, particularly in scarier
    corners, e.g., Turn 8 at Willow Springs. I was very happy to have it at
    Phoenix International Raceway, a track with concrete barriers everywhere.
    When PSM activates you can feel it, much like you can feel ABS. It will
    show you where you are losing traction while keeping you on the track if
    the loss was unintentional. When it engages, it may slow you down where
    you might not want it to later, i.e., where you really do want more
    oversteer, but on those first few practice laps, who cares? You can
    actually throttle steer the car quite well with PSM on as long as you are
    smooth, the yaw is not excessive, and the corner is fast enough to allow
    smooth inputs. This in itself is a good training tool. So PSM is good for
    practice, but what about when it matters, during timed laps?

    In a time trial situation, it would depend on the course whether it would
    matter if PSM were on or off. On a tight road course, you would most
    likely want it off. On an autocross track, you want it off for sure. If
    you had sufficient presence of mind on a road course you could turn it on
    and off depending on the corner. You could make sure it’s off for Turn 2
    and 4 at Willow Springs, turns where throttle steering comes into play.
    You could turn it on for Turn 8, the last place on earth you want to see
    your tail catching up with you. I've never done this, but it illustrates
    the point.

    So far, so good. Since you can turn PSM off, why wouldn’t you want to buy
    it, even for a car you intend to race? It seems like the best of both
    worlds. However, remember above where I said that when PSM is off, it is
    still collecting data and if you hit the brakes when the levels of slip
    are exceeded, it will intervene. That could be a negative in one racing
    technique, trail braking, where you are obviously on the brakes and
    turning. There are two reasons to trail brake, one in which PSM is
    neutral or even a positive, and one in which it can interfere with the
    driver’s intention. The first is when you are trail braking to lengthen
    the straight or to maintain a higher speed through the first part of a
    turn. In this case, you want the car to stay on its directed path. If
    things are going as intended, PSM is very unlikely to engage even though
    you are on the brakes. If it does, it is probably because you lost rear
    traction in a pretty big way. By engaging it didn’t cost you time since
    your intention was to slow down anyway and it may have saved you from
    spinning. The second use of trail braking serves a different purpose. If
    you are trail braking to induce some oversteer intentionally to tighten
    the corner, PSM could interfere in the same way as when it is on and you
    lift to oversteer. While I have a lot of experience throttle steering the
    car, with PSM on and off, I don’t brake to loosen the rear of my 996 C2.
    Lifting is normally sufficient. However, I have seen this technique, in
    the form of left-foot braking, used in a friend’s 993 C4 in Turn 4 at
    Willow and Turn 5b at Spring Mountain and presume it would be useful in
    the newer 996 C4. Since the 993 does not have PSM, I cannot tell you to
    what extent it would have interfered. If you are smooth, probably very
    little, if at all. But, this is one possible negative to weigh against
    the aforementioned positives. I think it’s worth it, but let me give the
    last word to Porsche.

    "We wanted the car to perform like a Porsche not a family saloon, so the
    system has been designed for minimal intrusion," explained Thomas Herold,
    the Carrera 4 Project Manager. "Its limits are really high and you can
    reach the same lateral g-force number with the system in or out on a
    steady state cornering circle. Thus, if you are a good driver, you can
    keep the power on in a drift and even adjust the car’s attitude on power
    in a corner without interference. But if you lift off suddenly or brake,
    and the car is in danger of destabilizing, the system will reach out and
    save you."

    "The difference is small around the Nurburgring for a skilled test
    driver," he explained. "Within one second a lap in fact. This is the way
    the car is made. If you are smooth, there is no interference from the
    system. But if you are ragged, the system will be cutting in all the time
    to stabilize the car, so an aggressive driver will be slower with the
    system on."1

    Great artice, thanks Cap'n.


     
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