Porsche Stability Management System: A racer’s perspective By Jack Miller

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

References

1. http://212.53.73.128/roadtest/porsche1.html

Thank you to Jeff Southall of Porsche Cars of North America for the
technical information in this article.