OK, Jad and Dan, I understand the sentiment that you guys are trying to express--that learning (and teaching someone) to drive one of the newer Porsches at the limit is problematic, due to the advanced PSM systems with which they are equipped. I agree with that idea, and think that anyone trying to learn basic car control skills should start in a vehicle without them, or lacking that opportunity, should begin their training in a controlled environment with PSM turned off. There is nothing quite like losing control of a car to induce the proper perspective on performance driving risks, and the necessity of developing the correct reactions to prevent such occurrences.
That said, I think you are both mistakenly perpetuating some myths and misinformation about the design and actions of the PSM systems that are not helpful in understanding the nature of the problem, or in developing solutions to the difficulties they present in driver training. The actual situation is quite complex, due to the differences in PSM operation in differing driving modes as well as in differing models of Porsches. Without a complete understanding of these complexities, progress will not be made in overcoming these difficulties. My comments are interspersed in your discussion below.
Jad wrote:The new systems are no longer good learning tools.
Agreed. When active, they mask mistakes in a way that minimize consequences. This is a good thing in terms of preventing damage to the vehicle or injury to the occupants, but is a bad thing in terms of informing the driver of the error they made, thus preventing development of proper inputs and actions in a given situation. I understand your dilemma as instructors in attempting to explain a mistake to a student whose car corrected it instantly. They don't believe you because there was no serious consequence. That is a problem.
They intervene all the time with no warning or feedback to the driver.
This is not true. Every intervention by PSM is signaled to the driver by the PSM warning light on the dash, if the system is functioning. This is explicitly stated in the technical documentation, and Porsche would be negligent to not inform the driver, as PSM intervention is a warning that the driver's actions are not appropriate in the current driving conditions and should be altered. They state in several sections of the driver's manual that
"Your Porsche sports car features a complex integrated system made up of all control systems acting in power transmission and in the chassis...Despite the advantages of transmission and chassis control systems, it is still the driver's responsibility to adapt his driving style and maneuvers in line with road and weather conditions, as well as the traffic situation. The increased safety that is provided should not induce you to take greater risks with your safety. The limits set by the physics of driving cannot be overcome. These systems cannot reduce the risk of accidents due to inappropriate speed. Adapt your driving style, maneuvers and speed to the road and weather conditions, as well as the traffic situation."and
"In spite of the advantages of PSM, it is still the driver's responsibility to adapt his driving style and maneuvers in line with road and weather conditions, as well as the traffic situation. The increased safety that is provided should not induce you to take greater risks with your safety. The limits set by the physics of driving cannot be overcome, even with PSM. Risk of accidents due to inappropriate speed cannot be reduced by PSM." If Porsche made the operation of these systems "invisible" to the driver, they would incur considerable liability, and they know this. They absolutely want the driver to know when it is happening.
Unfortunately, in a performance driving situation, a dash light flashing on may not be noticed by either the driver or the instructor, due to attention being focused on the track. I happen to think that there should be a distinctive audible "chime" (or maybe a "buzzer") associated with any PSM intervention as a more effective warning, but that is another subject, one I have actually communicated to Porsche engineers separately. However, there are other effects of PSM intervention that are definitely noticeable, by both driver and instructor, and if we do not become more familiar with them. progress in dealing with this dilemma will be impossible. I have mentioned them before in this thread, but maybe reading them in the actual manual for the 2012-2016 Carrera model would be useful:
http://www.porscheownersmanuals.com/2012-911-carrera-manual/6/161/ABD-automatic-brake-differentialThe events below inform the driver of PSM control operations and warn him to adapt his driving style to the road conditions:
– PSM warning light on the instrument panel flashes.
– Hydraulic noises can be heard.
– The vehicle decelerates and steering-wheel forces are altered as PSM controls the brakes.
– Reduced engine power.
– The brake pedal pulsates and its position is changed during braking.All the various Porsche electronic control systems are networked with the aim of combining the best possible driving performance with maximum safety. Many of the "performance" systems truly are "invisible," like PASM, PDCC, PADM, and rear-wheel steering, etc., but the safety portions of PSM to ensure vehicle stability in extreme situations are very much apparent, especially in "Sport" driving mode when pushed to the limits of stability. Without being aware of them, and having enough seat time in such cars to detect their effects, both students and instructors will be at a loss to deal with the issues raised here.
Dan Chambers wrote:It appears that a good portion of the stability control feature relies on the rear brakes keeping the car going in the desired line based on steering input, yaw, wheel rotation speeds related to each other, throttle input, etc. One of the computer's responses is to apply braking to the rear wheels ... independently of the use of the brake pedal ... and individually from rear wheel to rear wheel ... to help steer the car in the desired direction. This application of brakes to the rear wheels for stability purposes can occur at over 60-times a second. Yes; 60 times a second! AND, the braking pressure is calibrated to be "unnoticeable" to the driver.
These algorythms were written in to the stability control system to make the driving of the car in a spirited fashion seem easy and smooth. So, when a driver says something like "No, I don't actuate PSM/PASM. I never feel it." My reply is "That's right... and you never will. That's how it was intended and designed."
How do I know that drivers in cars with advanced PSM/PASM are consistantly over-driving their cars? Because their rear brake pads are wearing as fast, or faster, than their front brake pads. In cars without PSM/PASM the front brake pads wear faster than the rear pads by a factor of about 1.5 to 1. Because 60% or more of the braking force is distributed to the front of the car in non-PSM/PASM cars, the brake-pad wear is consistant with the braking force delivered to the braking system. With newer cars equipped with stability control, the people who consistantly over-drive have different brake-pad wear.
This is incorrect for a couple of reasons, Dan. First of all, there are two different systems involved in this situation which you are conflating, PTV (Porsche Torque Vectoring) and ABD (Automatic Brake Differential). PASM (Porsche Active Suspension Management) is not involved with PSM or brake activation at all, it is simply an active electronic adjustment of the dampening characteristics of the shock absorbers, so why you mention PASM at all in this context is a mystery to me. All new Porsches are delivered with PSM (calibrated in different ways in different models) but only a portion of them are even optioned with PASM, so stating "PSM/PASM" makes no sense to me.
It is true that PTV uses the rear brakes alone, in conjunction with the rear differential lock, to distribute a greater amount of drive force to the outside rear wheel in a turn, which has more traction from being loaded, and prevents wheelspin on the inside wheel, which is unweighted. This also imparts a greater axial force on turn-in, making it feel more crisp and "sporty." However, this is a gentle application of the brakes, which can indeed be "invisible" to the driver, and happens even without "overdriving" the car. It engages as a performance enhancement even in moderately "spirited" cornering, when vehicle instability is not yet an issue. It is not a component of PSM at all (although PTV Plus, which is only available on PDK cars, can work in conjunction with PSM in a minor way on road surfaces with varying grip as well as in the wet and snow, due to its fully variable torque distribution).
ABD, on the other hand, is the "stability management" portion of the electronic braking system, and it intervenes for safety, not performance, in hard cornering situations (although it does have some minor performance application along with ASR in controlling wheel spin when accelerating in a straight line as well). When it intervenes, it is an abrupt and strong influence on the car's motion, and definitely noticeable, always slowing the car considerably and making weird hydraulic noises. It operates on all four wheels independently, not just the rears. If you suffer extreme understeer in a turn, the rear wheel on the inside of the bend is braked as necessary to bring the car onto the desired arc. If you suffer extreme oversteer, the front wheel on the outside of the bend is braked. It is conceivable that someone who constantly tries to kick out the rear to rotate the car and corners in an oversteering slide all the time (without ever entering corners too hot and suffering understeer) could actually wear the front brakes faster than the rears due to ABD intervention. I tend to believe that the reports of accelerated brake wear in the rear are coming from people driving cars hard that have no limited slip differentials at all, and ABD is kept really busy limiting wheel spin in the rear. This is especially true of many of the newer mid-engine cars that are often not even optioned with PTV or any LSD and don't have the rear weight distribution of the 911. After working tech inspection for almost 20 years now, I certainly wouldn't assume that differential F/R brake wear is necessarily a sure sign of someone overdriving the car. It can easily be that someone has replaced the front pads without doing the rears, since the fronts wear so much faster. Unless the owner verified that pads have never been changed, or all 4 wheels were done at the same time, I'm not sure you could make an automatic assumption that PSM caused the differential wear.
Finally, the differences between PSM in different modes and models must be understood for effective instruction to take place. In the lower tier models, activating Sport or Sport Plus mode widens the allowable range of deviation. This is what the manual says:
"PSM interventions occur later than in Normal mode. The vehicle can be maneuvered with greater agility at its performance limits, without having to dispense with the assistance of PSM in emergency situations. This helps to achieve optimal lap times, particularly on race circuits and on a dry road surface." (This is a tacit admission by the folks who make these cars that PSM does not help you go faster than a good driver could with it off, but while it may slow you down, at least you won't crash before you can make it around the track!) After you turn PSM off, when you then apply brakes in the ABS control range, the vehicle is stabilized even when PSM is switched off. One-sided spinning of the wheels is prevented, even with PSM switched off. The vehicle retains its enhanced braking readiness through prefilling of the brake system even when PSM is switched off. Wheel-specific brake interventions and the anti-slip control system (ASR) are also switched off. The automatic brake differential (ABD) remains on. By contrast, in the GT models, there is no Sport or Sport Plus mode. PSM is permanently calibrated in a sportier manner, allowing more latitude than even Sport Plus in the other cars. PSM can be switched off in 2 stages:
– Stage 1 ESC OFF:
Switches off Electronic Stability Control (ESC).
– Stage 2 ESC+TC OFF:
Also switches off Traction Control (TC).
There is no stabilizing brake control in either of the switch-off stages (even when the brakes are used in the ABS range). Note that Traction Control cannot be disabled without first turning off Stability Control.
I hope this information better informs our discussions of these issues.
(BTW, after 3 years of AX events, my GT3 brake pads are showing no abnormal wear differential, they are at about 70% F and 85% R)
TT