To PDCC or not PDCC? That is the question.
Discussion
This post relates specifically to the Porsche 991, but I am aware that PDCC (Porsche Dynamic Chassis Control) is available on other Porsche models, so it would be interesting to hear from people running other cars with this technology.
To anyone simply interested in getting into a car and driving what follows will seem like extremely dry ponderings, but I am aware that there are a few people here contemplating whether to add PDCC to their 991 order so I thought I would share what I have found out. I must make it clear that I am neither technically savvy nor the Stig, so please excuse me if my opinions stray into factual inaccuracy.
I have now driven four Porsche 991s, two with PDCC and two without, both on track and off. I have also spent the last two afternoons jumping in and out PDCC and non-PDCC cars and hooning about on sunny, bendy B roads in the Vale of Belvoir. Life can be tough at times.
Firstly, at the risk of revisiting a well discussed topic, I think PDCC might have a lot to do with why some journalists have described the 991 as numb. They have blamed the new electric steering, but as anyone who has tried it will testify the steering is brilliant and, whilst the 997 twitchiness is gone, it is communicative and of a quality befitting the 911. Driving a car fitted with PDCC, however, is an immediate challenge to the senses of anyone who knows the 911 models up to and including the 997.
To try to put sensations into words, the non-PDCC 991 feels like a bit more like an old school 911 in as much that the body rolls side to side, front to back and diagonally with the profile of the road. This roll is progressive with speed and the cornering forces applied. It is quite a comfortable ride and when pushed hard the car will behave with familiar 911 traits, that is to say it bobs and rolls about (up to a point). The PDCC car, on the other hand, doesn’t do the bobbing or the rolling, it stays fairly flat no matter what. The car darts about from one corner to the next and changes direction like a kart, but whilst retaining a comfortable ride.
The reason I have had so much opportunity to test both cars back to back in the past couple of days has been because my car was seemingly suffering with harsh rear suspension. It was fine when driven hard but around town it was tiring and hard. My local OPC took it in swiftly, gave me another 991 and set about finding the problem. Which, as it turned out, was a simple one that was having greater than expected consequences: The rear tyres were inflated to the maximum, whereas the minimum is better suited to the car.
Lowering the rear tyre pressure by 0.5 bar lowered the side wall by 10mm and the difference it made to low-speed driving was very noticeable. All 991S cars run 20” wheels and are consequently firm, and that half a bar is what makes the difference between acceptable and unacceptable ride comfort. I had previously tried loading the car (with children) and it made no difference; the sensation was that the car was in Sport Chassis mode at all times.
Tyre pressures, then, seem to be quite critical where the 991 is concerned and I believe especially so with PDCC fitted. The reason for this (and this is where my technical knowledge is sketchy) is that as PDCC works to reduce roll the tyres are forming a greater part of the movement in the running gear. A change in tyre pressure, therefore, will change more noticeably the small degree of roll present in the car.
Something else that came out of the workshop’s investigations is that PDCC, contrary to what I have read, appears to be active even with the car stationary and with the car in a straight line. Also interesting is that there is no variability in PDCC settings, whereas there is in other systems in the car. I am not sure it is required, but I am surprised nonetheless that it isn’t available.
So what does any of this mean really? Well, I took a punt on PDCC when I ordered my car. In earlier test drives I thought the real difference was fairly subtle and I flip-flopped regarding whether it was necessary or not. Furthermore, having been reintroduced to the non-PDCC car yesterday I was charmed by its old school 911-ness. But today, having got my PDCC car back with the tyres running lower pressures, I went out and did the same B roads as I did yesterday. Today the roads were busier and I found that to pass traffic I sometimes needed to carry greater speed into bends. I was startling how the car took it in its stride. There were a couple bends in particular where I would not have dared corner at the same speed in the non-PDCC car. For fast driving, then, I think that PDCC is a must-have. It allows the car to be quite civilised and comfortable but as flat as a racing car a millisecond later without any button pushing.
A word, too, about comments from people who have said that PDCC does not (and I think this was the term used) “telegraph the transition from grip to slip”. In my experience, getting one end of a 911 or the other to slide is usually the result of either inadvisable weight transfer, excessive speed or a slippery surface. To push the car hard enough that it is about to let go means that any sensation is likely to be coming from the tyres, which applies whether the car has PDCC or not. To be pushing the car that hard, to my mind, means that the suspension is pretty much used up and it’s all about tyres. I stand to be corrected of course.
I will be interested to hear the verdict of people better qualified than me to comment, but I think the answer to whether to go for PDCC or not, which car is better, will come down to who the driver is and what he or she (OK probably he) wants from the car. If it is someone who wants a 911 that is just like previous 911s but better, the car without PDCC is the way to go. But for a more advanced and, dare I say it, exciting experience, PDCC is a must-have.
Would I order PDCC again? Yes, in a heartbeat for my current car, because I wanted a more sports orientated car and specced it that way. If I wanted a weekend toy that I never planned to drive fast I might not bother. But I don’t think there is a right or wrong answer either way.
So, PDCC, just when you thought Porsche gave you too many options already.
SFO said:
beautifully written and very well thought out.
thank you.
with reduced tyre pressure, are you still within Porsche recommended pressures?
Yes, at the bottom of the range at room temperature. thank you.
with reduced tyre pressure, are you still within Porsche recommended pressures?
I am usually the sort who considers tyre pressure to be right if the tyres are round and the rim isn't touching the ground, but I think I need to reevaluate my approach. In fact in hindsight ordering the tyre pressure monitoring on my car might have been a good call.
Had PDCC on a Cayenne GTS and it was great. Improved the handling dramatically, no roll and cornered like a 911. That was a 2 ton car though!
Not sure what it adds to the 991 as that should already handle well enough and surely takes away some of the driver involvement.
So I would be a NO
Not sure what it adds to the 991 as that should already handle well enough and surely takes away some of the driver involvement.
So I would be a NO
Great write-up, Manks! I think you've hit the nail on the head! You're quite right that as you load up the suspension, eventually you are reliant on the performance of the tyre. In a non-PDCC car, this will generate a slip angle where the direction the wheels are turning and the direction of the tyre are slightly different - in a 997, you will sense this limit of grip as an oscillation in the steering wheel (it'll start to writhe). With PDCC, it's theoretically possible (I'm actually guessing here) that the slip angle between wheel direction and tyre is much smaller (maybe even non-existent), so that car will corner much, much faster but when the limit of grip is reached there's nothing to tell you that you're there? The oscillation is either far less or actually non-existant, so when the tyre does lose grip, you'd be far more reliant on your own reactions and of course PSM to bail you out! Perhaps a technical expert can come on and let us know if this theory holds any water!
J-P said:
Great write-up, Manks! I think you've hit the nail on the head! You're quite right that as you load up the suspension, eventually you are reliant on the performance of the tyre. In a non-PDCC car, this will generate a slip angle where the direction the wheels are turning and the direction of the tyre are slightly different - in a 997, you will sense this limit of grip as an oscillation in the steering wheel (it'll start to writhe). With PDCC, it's theoretically possible (I'm actually guessing here) that the slip angle between wheel direction and tyre is much smaller (maybe even non-existent), so that car will corner much, much faster but when the limit of grip is reached there's nothing to tell you that you're there? The oscillation is either far less or actually non-existant, so when the tyre does lose grip, you'd be far more reliant on your own reactions and of course PSM to bail you out! Perhaps a technical expert can come on and let us know if this theory holds any water!
We are getting into something here that I am really unsure about. But why would a PDCC car not generate slip angle? Weight transfer is less with PDCC cars but the tyre sidewalls still deform as the outer wheels loads up. So slip angle may be less, but not absent.Additionally, how is PDCC different (with regard to predicatbility) from having suspension like that on a GT3 for example? PDCC simply means that the suspension stiffens up only when cornering, but you've effectively got a situation like on a GT car - i.e. stiff suspension and low profile tyres. I seem to recall you said that the GTRS was very predictable with regard to the limits of grip.
I am not suggesting that you are wrong, by the way. Simply trying to understand the logic of what you say.
Manks said:
We are getting into something here that I am really unsure about. But why would a PDCC car not generate slip angle? Weight transfer is less with PDCC cars but the tyre sidewalls still deform as the outer wheels loads up. So slip angle may be less, but not absent.
Additionally, how is PDCC different (with regard to predicatbility) from having suspension like that on a GT3 for example? PDCC simply means that the suspension stiffens up only when cornering, but you've effectively got a situation like on a GT car - i.e. stiff suspension and low profile tyres. I seem to recall you said that the GTRS was very predictable with regard to the limits of grip.
I am not suggesting that you are wrong, by the way. Simply trying to understand the logic of what you say.
To be honest, I am also unsure if this is correct or even in any way accurate - as I said it's a guess but the logic behind it is this. The car leans less, so in turn puts less stress on the tyres. This could mean that the tyre contact with the road is far, far greater, so the tyre deforms far less - maybe enough for it significantly less noticeable? GT3RS doesn't have an "anti-lean" suspension, so the tyre will eventually deform much in the same way that a normal car will but I'm not sure how PDCC works, so it may be holding the car level to such an extent that the car goes grip, grip, grip, let-go without much warning. Of course I could be talking completely out of my arse (I most probably am) - I've no idea, I'm not technical enough to know for sure and I'm a long way short of a racing driver - It's just a thought.Additionally, how is PDCC different (with regard to predicatbility) from having suspension like that on a GT3 for example? PDCC simply means that the suspension stiffens up only when cornering, but you've effectively got a situation like on a GT car - i.e. stiff suspension and low profile tyres. I seem to recall you said that the GTRS was very predictable with regard to the limits of grip.
I am not suggesting that you are wrong, by the way. Simply trying to understand the logic of what you say.
garyhun said:
Why would less lean put less stress on the tyres? Surely the lateral forces woud be the same, it's just the body being kept flat by the firmer suspension?
Honest Q as I'm not technical either.
True - I've no idea. My thinking was that there would be less weight on the tyre as the suspension is actively pushing the car up, so whilst the lateral force will be the same the deformation of the tyre would be less because there's less weight being pushed onto the tyre - As I said I've no idea if this is true and I should probably stop posting as I'm sure there'll be somebody on here soon, that will actually know and prove me to be a complete moron!Honest Q as I'm not technical either.
J-P said:
garyhun said:
Why would less lean put less stress on the tyres? Surely the lateral forces woud be the same, it's just the body being kept flat by the firmer suspension?
Honest Q as I'm not technical either.
True - I've no idea. My thinking was that there would be less weight on the tyre as the suspension is actively pushing the car up, so whilst the lateral force will be the same the deformation of the tyre would be less because there's less weight being pushed onto the tyre - As I said I've no idea if this is true and I should probably stop posting as I'm sure there'll be somebody on here soon, that will actually know and prove me to be a complete moron!Honest Q as I'm not technical either.
It's a good discussion though and highlights the fact that many/some of us contemplating this technology have no real understanding of it. garyhun said:
Why would less lean put less stress on the tyres? Surely the lateral forces woud be the same, it's just the body being kept flat by the firmer suspension?
Isn't this simple? Surely more lean on say for example a sharp right hand bend - the passenger side 2 wheels would push into the ground harder and heavier than the drivers side = equals imbalance in weight distribution and more stress on the passenger side rubber. If the car leans less, tyre contact on all 4 wheels would be more equal = 4x strong grip vs 2x strong grip and 2x light grip. So, less lean = more equal balance and grip on all 4 cornersAm i talking b
ks? 
Phooey said:
garyhun said:
Why would less lean put less stress on the tyres? Surely the lateral forces woud be the same, it's just the body being kept flat by the firmer suspension?
Isn't this simple? Surely more lean on say for example a sharp right hand bend - the passenger side 2 wheels would push into the ground harder and heavier than the drivers side = equals imbalance in weight distribution and more stress on the passenger side rubber. If the car leans less, tyre contact on all 4 wheels would be more equal = 4x strong grip vs 2x strong grip and 2x light grip. So, less lean = more equal balance and grip on all 4 cornersAm i talking b
ks? J-P said:
To be honest, I am also unsure if this is correct or even in any way accurate - as I said it's a guess but the logic behind it is this. The car leans less, so in turn puts less stress on the tyres. This could mean that the tyre contact with the road is far, far greater, so the tyre deforms far less - maybe enough for it significantly less noticeable? GT3RS doesn't have an "anti-lean" suspension, so the tyre will eventually deform much in the same way that a normal car will but I'm not sure how PDCC works, so it may be holding the car level to such an extent that the car goes grip, grip, grip, let-go without much warning. Of course I could be talking completely out of my arse (I most probably am) - I've no idea, I'm not technical enough to know for sure and I'm a long way short of a racing driver - It's just a thought.
Hmmmmmm, surely all that less load means is that the same deflection will take place but at greater cornering speed?Gassing Station | Porsche General | Top of Page | What's New | My Stuff