rpilot

I am trying to understand this better and I will ask this question in two parts:

According to Porsche, PTV or PTV Plus vary the torque supplied via braking one wheel when it detects change in direction, yaw rate, etc. PSM apparently does the same thing (ie.. apply brakes to the slipping wheel when it detects understeer or oversteer). Is this simply software which works under different circumstances, ie: proactive via PTV vs reactive via PSM? Or is there a more fundamental difference? Or is it basically the addition of the limited slip diff with the PTV and if so, how does the LSD help in this case that proactive PTV software logic embedded say in PSM or by itself would not?

Also, why is PASM necessary for Porsche to provide PTV? If memory serves correct, Porsche mentions somewhere that PTV uses the braking??? function of the PASM. What does this really mean? Brake subsystem does the braking work. What does PASM do in providing functionality to the braking system?

I hope my question makes sense ...

chuckbdc

As I understand the Porsche literature these are seperate systems made to work together via sensors and software mappings to achieve desired effects by making them work together in preset ways. There are four technologies that can vary physical forces:
- limited slip differential,
- variable rebound shock absorbers,
- automated braking of a slipping wheel,
- automated braking of inside rear wheel in preset cornering conditions

I have seen nothing to suggest that PASM provides functionality TO the braking system per se, just that it works together with it to enhance overall vehicle stability. I don't think it is "necessary" to provide PTV (unless it is because Porsche had decided it is desirable for a vehicle performance parameter (or economic performance for all I know)!

chuckbdc

Hard to see why any of this is complicated. Just mix and match:

PASM, PCCD, PCM, PDCC, PDK, PLDS, POSIPI, PSM, PTV, PTV, Plus, PTM,and PVTS




(Yeah yeah , I know



)

holminator

Haha. He said pee.

Mike in CA

You're on the right track with your description of PTV being proactive and PSM being reactive but there are differences. PTV works by braking only the inside rear wheel in a turn causing more torque to shift to the outside wheel which makes the car to rotate more quickly. This provides sharper steering response and turn in when the car is driven "assertively" (Porsche's description) into a corner.

PSM, on the other hand, reacts when sensors detect that the car is over or understeering and will brake front or rear wheels and modulate power to bring it back into a stable condition.

PASM is an electronic damping control system; basically shock absorbers that adjust to road conditions in real time. It doesn't really have anything to do with the brakes, or traction per se, although all of the alphabet systems are integrated together to some degree. I think Porsche requires PTV and PASM together on the 991 for reasons of packaging and to make you buy both together. They aren't required to make each other work, AFAIK. For example, my '09 Carrera S has PASM, and PTV hadn't even been introduced yet and on the Cayenne PTV is available without ordering PASM.

holminator

It's all explained here in graphic terms.

http://www.porsche.com/microsite/tec...t.aspx?pool=uk

simsgw

Oh, your question is fine. It's the system we're left puzzling over. You have the essence, as Chuck says. And I suspect the essence is all we'll ever manage without access to proprietary documents. I can talk about it a little, but can't be definitive.

First, realize that all those subsystems are just names for the components that control the car's dynamics. That's why they overlap. When we brake heavily, it works better if the nose doesn't dive like an Oldsmobile. PASM gets involved in that, so to in that sense, it aids braking. Ditto lateral maneuvers where the weight transfers too rapidly to one side or the other. PDCC proactively operates to prevent body roll with that weight transfer, but even cars without it get some aid in slowing the roll, smoothing the dynamic transitions.

The distinction between PTV and PTV Plus is between the 7MT and the PDK cars. I think it's only the PDK cars with Sports Chrono that get PTV Plus, but we're talking about marketing material and an aging memory. I'm not sure. I vaguely remember presuming that the 'plus' part is what makes launch control easier, but don't rely on that. The 7MT cars have a traditional limited slip differential. This is one of many analog computers we've been using on cars for nearly a hundred years. The presence of speed difference between the two half-shafts beyond the set-up value causes a clutch (or a brake if you prefer, a controlled friction component anyway) to progressively lock the two axles together. In overrun, the threshold of coupling is different than in acceleration. And limited-slip diffs are tricky in their effects on handling, so that's enough detail. The unit in the PDK cars is electrically controlled, so the on-board computer decides how much braking, or cross-axle coupling, is desirable. That's the 'plus', the computer control.

The 'Plus' seems to refer to the computer using the limited-slip differential to help with the whole idea of "getting the car to behave the way we want it to behave." That poses the question: "Huh?"

How do we want it to behave, and how does the computer know? Well, the answer to the second question is certainly proprietary, but it is my art, so I'm free to gue... uh, provide a professional inference... about what's going on. Porsche developed the concept of design 'straddle' after killing a few buyers who thought "[the S model] is only for experts" was marketing twaddle. It's an easy concept they've been working to improve for several decades now. Another way of saying it is that proactive vs reactive description, but it's a little more subtle than that.

The digital computers do what the analog computers we call suspension were set up to do unaided in ye olden days. They control the car's dynamics in ways that enhance agility if you approach the car confidently and what was the word? 'Assertive' that's it. If you are assertive, the computer partners are designed to do exactly what you ask and even a little more. If you hesitate, or drive smoothly but quite gently, then they assume any ... let's say over-brisk activity is not what you want. Things are getting out of hand, so the system dampens the car's response.

To skeptics, let me point out that the analog computers used to do this, except in the opposite direction. The one that kills people. I tell my students in early 911 cars that you have to believe in yourself and the car. It will corner faster than you can believe -- if you believe. If you don't believe, it will eat your face. What I mean, without descending into teaching fast driving over the internet, is that you must pick an entry speed into a corner and believe your early 911 can do that speed. If you change your mind in mid-corner, the weight transfer when you hesitate will cause a snap spin. Always stay the course, or even accelerate your way through the corner, no matter what your nerves say. He who hesitates -- in an early 911 -- is looking backward hoping that Cayman reacts fast enough to miss.

The early 911's via their "analog computers" controlling each wheel react well to confident driving. Hesitant driving, they punish. The modern Carreras reverse that unfriendly direction of straddle. They reward good driving by being more agile, quicker to change directions, and willing to assume body attitudes that frighten novices. Very much like the same behavior that killed novices in the bad old days. But if you are a novice, they react more slowly, more conservatively to your input. That is straddle.

PSM is what saves novices and any of us when we get in over our heads. PTV and friends is the same electromechanical components in coalition working on the other side of that straddle: making the car handle so brilliantly.

It sounds like I'm saying it's more marketing speak, that it's the same stuff with different blarney. Really, it would be more fair to say it's a different mode of control that the on-board computers enter when driver reactions make it clear there is a problem. Same car, same parts, different goal suddenly.

I was going to offer gue... uh, inferences about how they do that. This note is long enough already. Let's limit it to this example. When I brake at 1.2g entering the chicane at Fairplex, and roll into 1.2g of lateral left with trail braking, and then reverse the loading to lateral right to make the second apex... when I do all that, the pattern of movements from one control to the next, and the rate of change of car balance, are typical of expert driving. The car is operating at high g levels, but smoothly. If a casual driver on public roads has been tootling along at .1g cornering and suddenly the brakes are hammered and the car jerks left, then any decent computer can be programmed to recognize that as an emergency maneuver. The casual driver never will reach the g loads I consider routine unless the computer helps, so the braking is enhanced, but body rotation is damped while permitting the evasive maneuver that implies. A casual driver would be likely to spin without that help. Now add one set of wheels losing traction and you have a full engagement of PSM.

As to how all the systems interact in real time, I suspect there are as many answers to that as there are scenarios in the Porsche engineering studios.

We're back to the essence. PSM steps in to save our *** when need be. PTV and the rest of that alphabet soup react to assertive control actions to make the car behave like a Porsche should.

And for the record, it works.

Gary

bk_911

I found this post when googleing for PTV on 991 cars. THIS POST IS AWESOME. Thank you for this. I got my answer PTV = Mechanical LSD, and it was entertaining.

Dyim

Gary passed away a while back. Great contributor. Sorely missed.

bk_911

That's sad, I didn't see the "rip" until you said that. Judging by the posts I saw here he was a great guy and a top-notch gearhead.