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Comments on PSM

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There may be nothing new in this article I found on 6speedonline, but it makes an interesting read. (VERY LONG :( )


PSM from the point of view of a pilot

here is a very interesting article about PSM

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 :eek:

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