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grim,

So which is it then, (1) Porsche knows best or (2) Lawyers made them do it? I just want the real story and why wouldn't everyone that cares about performance? I realize there are plenty that will never track their cars, but I certainly will.

I posted looking for maybe something that I don't understand about the setup and dynamics that would make it different than what I understand from tracking my vehicles. Why would you inflate the rear so highly? So far all I see is that we should just accept it because either I'm dumb and Porsche knows best or that it is explained by the liability issue.

Next year I guess I will get a chance to test it, but I strongly surmise that even with reversed pressures (higher in the front), the recommended pressures will make it understeer significantly.

Matt
My "guess"? The lawyers. ITs a SUV. Its NOT a sports car. They cannot have these cars doing 360s because some mother with her kids in the back took a turn too fast and spun the car on loose gravel or any wet surface and the tail smacked a pole. I believe they do NOT want the cars to oversteer. The driver will lift and the tail end will come out. Thats my opinion. Its not a Performance thing, its not a "performance" car. Do you see any Ring times for any Macan? Nope. Its meant for hauling and bad weather. Some here might think it a sports sedan. Fine, their opinion, but its NOT the classic definition of a sports car.
 

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Well it seems much speculation abounds, some think the car has a front weight bias and the tyres are smaller at that end so the pressures should be higher. Others are thinking of the handling while @grim does his best to remind us it's not a 911 or a Sports car!

Handling, I am thankful, in a Porsche world is king. What wins out for company policy is something argued only over the boardroom table by engineers, lawyers and accountants. In our case it would seem that the engineers must have won as it is one of the nicest handling 'SUV's' in the bunch. In our case, the end that has the bigger tyres is the one that's doing a lot of the cornering work. While the front has been tamed and is following the true path, the rear is working extra hard to keep up it's end of the balance equation. Not enough air in the rears would see them folding over on their edges and dragging their (collective) butts around. It's a tallish car with a higher centre of gravity, the weight transfer manifests itself at the rear and to balance that wider wheels and higher pressures have been developed and tested thoroughly to ensure the car handles effectively. Because the GTS and the PP versions are fitted with a slightly stiffer set up and the factory understands the driver will likely push the vehicles harder, they have increased the rear tyre pressure to help keep that balance. But it's not just the vehicle handling that will improve with that higher pressure, when the tyre works hard it heats up, if a higher pressure is not mandated the tyre will overheat and risk catastrophic failure.

That 930 problem of lift off oversteer was caused by the combination of the 'arrival of boost horsepower rush' and what the driver did to compensate, lift off oversteer was born in Porsche road cars with the 356 and continued along with all rear engined models until it became less concerning with the advent of the 993's LSA (live, stable, agile) rear end. The rear axle kinematics of a semi trailing arm do not take kindly to sudden changes resulting from the - 'oh my gosh the powers too much' or off throttle 'oh my gosh I am going in too fast' situation :oops:. The quite large changes in toe angle that resulted, combined with the usual reduction in cornering load, could and did lead to many a scary moment. Thankfully the engineers (again) seemed to have got us over that one. :giggle:
 

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Discussion Starter #23
Gra - thank you for the thoughtful response from Sydney. Much appreciated. I have been to Mascot for work and also vacationed across the country 1x and much enjoyed it.

I question this point: "It's a tallish car with a higher centre of gravity, the weight transfer manifests itself at the rear". Why would this be the case?

We're limited on tracks here in Alaska, but I autocrossed my Jeep Grand Cherokee Trackhawk a half dozen times. It is front-engine and heavier by ~1100lb. I didn't feel like I had it balanced for performance until I had 38 PSI in the front and 30 PSI in the rear. And the rear tires were still not rolling over as much as the fronts, but it was getting close.
 

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I recall watching an early series of Fifth Gear, you know that car testing entertainment program that was considered the thinking man's version of Top Gear - you got some good reviews by folk who knew what they were doing, but with a lot less of the attitude and shenanigans of the latter. Anyway, they tested a Macan Turbo at the Rockingham circuit in the UK. It's a mix of speedbowl and small road course a bit like Daytona in a way. Tiff Needell, Jason Plato and Vicky Butler-Henderson all aboard, one at the wheel and pushing hard. One of the outboard images showed the rear wheel attempting to cope with the load, at one point the drivers were complaining of that outer rear wheel being forced to hop across the track such was the load - the rear tyre was at it's limit and would alternately let go and grip shedding some of the lateral load. Of course the tyre could be seen running up onto it's outer edge.

So what did it demonstrate? Firstly, race drivers (especially in small groups) always like to take it to the limit - a well known trait. Secondly, it was that outer rear that was doing all the work. The load was transferring off the inner rear due to the height of the cars centre of gravity (which of course had been increased by the three bodies inside). Further, the front anti roll bar was doing it's job of transferring the load diagonally off the front of the car to the rear, this helped the front keep it's balance. If you care to look under the front of your Macan you might be able to recognise the large front stabiliser (anti roll bar). A well renowned race engineer once suggested that any bar greater than one inch in diameter should be considered as a 'solid axle conversion kit!'. Of course he was talking race cars, they were usually lighter vehicles and lower with relatively stiff springing, but it's a reasonable argument. In our case, we are looking at vehicles that weigh around 2 tonne and carry that weight a lot higher, so the stabiliser bars are going to be bigger if we are to limit roll. The job of the engineer is to balance the handling by limiting roll, controlling the bump, rebound and wheel (spring) rates etc. Important details are keeping the weight down and low - in our case they lowered the roof and seat mounting heights in comparison to the Q5 and they made the chassis stiffer too. Our cars are in the majority rear wheel driven, so that rear outer tyre is working at providing drive and carrying the lateral load transfer. Controlling the way the weight transfers is often an overlooked area, good drivers learn about this, early 911 drivers especially... These days, they have made a lot of progress by using a lot of electronics to aid in the process e.g. using torque vectoring to make a car 'rotate' into a corner better.

I have no experience of a Trackhawk and I am not sure they sell many or any over here (my nephew has a Cherokee), but I bet it carries it's weight relatively high and as is the flavour, no doubt has a very large front stabiliser bar in an effort to flatten front roll and put the balance more to understeer - given it's obvious copious power. The engineer at Chrysler would favour front 'push' as it's called over there instead of a 'loose' rear as it's safer for the average 'Joe'. By increasing your front tyre pressure, you were trying to address the understeer which I bet could, at times, probably become massive and plough like if you rushed it in and or called up the horses too soon. On a car like that, I would be looking to make the rear looser by decreasing the rear tyre pressure (looks like you done that), increase the front tyre pressure and by increasing the rear spring/stabiliser rate. But too much of the latter can lead to power down (wants to spin and not grip) issues. As @grim has cautioned us before, these large SUV vehicles are not good track cars, that doesn't stop the owners from having some fun at autocross or track days - but that's about all it is. However, it's not impossible, I did see some great footage of an old diesel truck doing some dramatic hill climbing up Pikes Peak recently ... ;)

Glad you enjoyed your visit here, it's a truly great place ... with or without the shrimps and the barbie
 

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Discussion Starter #26
Gra,

Thank you for your thoughtful and detailed comments. I much enjoyed reading them.

Your explanation on weight transfer to the rear helps me visualize the case you were describing. Certainly there are any factors involved and now I wish I knew what pressures they were running at that track as another data point.

The Trackhawk was crazy fast, but the chassis was weak in my opinion. I think it likely was doing a very poor job in dynamics and is a result of them stuffing insane power in a chassis that wasn’t designed for it. Hence my move to the Macan GTS. I’m gladly going to roughly half the horses to get better handling and better build quality.

Again, thanks for the detail you shared.


Sent from my iPad using Tapatalk
 

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Interesting topic. I have a couple other data points to throw in, along with some opinions.
First, the Macan's platform mate is the Audi Q5. I had a B8.5 SQ5 alongside our Macan S for a while and it made for a very interesting comparison.
The major difference between the two, beyond basic suspension tuning, is the rear biased viscous-coupled center diff on our Macans compared to the 40/60 Torsen center diff in the SQ5. This is the only difference that I could imagine creating additional rear tire load on the Macan. If you ask a tire to do more -- e.g. Not only handle cornering load but also apply power, you're either going to need more tire on that end of the car, or you're going to have some interesting handling balance.
IMHO Porsche was already conservative in this regard (biased towards understeer) when they decided to put wider tire on the rear. I ran mine with 295mm square tires for a while, and drove it hard, and it was amazing. Handled super well. Where it suffered was steering feel. With a narrower section width tire in the front, you're going to have better feel. It was pretty muddy with 295s on the front.

As far as tire pressure, I'm not an expert, but a few things I've picked up over the years. Obviously if you go too low, you're going to run the risk of the tire rolling off of the rim. Bad. Go too high and you sacrifice the contact patch and sacrificing grip. But, higher pressure typically will give you better load handling. At the expense of ride. Also, you're going to have crisper "feel" at higher pressure because there's less deflection of the sidewall. So, it's a balancing act. I always end up going with Porsche's recommended pressures fwiw.
 

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Further, the front anti roll bar was doing it's job of transferring the load diagonally off the front of the car to the rear,

I have to question that. Isn't it correct to say that a front roll bar transfers load from the rear to the front? Early 356 series were a classic example. The oversteer was [not quite] managed by fitting softer springs to the back of the car, to allow roll, with the addition of a rear "camber compensator" spring which was a totally weird device, pivoting in the middle of the car to increase the rear spring rate, while still allowing a conventional roll bar on the front of the car to transfer some of the load from the rear to the front. Not that it helped much with the swing arm suspension!

At the opposite end of the scale to cars with the engine behind the rear axle, would be a car like the Subaru, where the engine is in front of the front axle. Their natural tendency is heavy understeer, though interestingly many owners of these cars use heavy front sway bars with lighter rear sway bars. This comes down to the preference for wash-out understeer to OMG oversteer.

Jeep Trackhawke ... as is the flavour, no doubt has a very large front stabiliser bar in an effort to flatten front roll and put the balance more to understeer
Quite, a fairly heavy front engined car so it has a tendancy to understeer. I would guess that it has large anti-sway bars front and rear to limit roll but still favouring understeer wash-out at the front.

The natural tendency of a rear engined car, particularly one where the engine hangs out behind the axle, is of course, oversteer. the natural tendency of a front engined car like the Macan is understeer. Because the centre of gravity of Macans is quite high, the sway bars play a big part in limiting body roll as well as transferring load from one end of the car to the other.

at one point the drivers were complaining of that outer rear wheel being forced to hop across the track such was the load - the rear tyre was at it's limit and would alternately let go and grip shedding some of the lateral load. Of course the tyre could be seen running up onto it's outer edge
Yes, the question of getting power to the ground while maintaining traction does rather complicate things :). If the tyres are running up onto their outer edges I wonder if Macans could benefit from more negative rear wheel camber? It's a classic mod. with the trade off being wear on the inside of the tyre when driving doesn't involve lots of cornering. What sort of camber angle changes happen with Macans during cornering?
 

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Ah the 356, an excellent example of an early suspension design system that worked to a point but quickly founds it's limit as tyre technology improved and we graduated from cross ply to radial tyres.

The 356 trailing arm front suspension was great in a straight line, but as soon as you turned a corner the body roll would increase the positive camber (top of the wheel tilting outwards) of the loaded wheel and this would lead to greater slip (or understeer) - 1 degree of roll = 1 degree more positive camber! But it did provide a degree of balance for the equally (and some may say more sinister) difficult swing axle rear suspension. At the back the swing axle had a very high roll centre and this could lead to a term called 'jacking', where the rear end could raise itself up if it had to deal with bumps in a bend - the rear wheels would tuck in more and more with each successive up and down, this increased the positive camber and would lead to greater slip (or oversteer). To some extent, the inherent understeer of the front end design was used to compensate for the inherent oversteer of the rear.

In this design, limiting front roll is important to increase front grip - hence the stabiliser bar which apart from increasing the spring rate on the outer loaded front wheel also reduced the amount of body roll (helping reduce positive camber change and slightly helping keep the rear more level too). At the rear, while a soft suspension was needed to enhance grip and compliance, it was not good at limiting the 'jacking' effect. So either a compensator spring (a single leaf spring pivoting on the gearbox with the ends connected to each wheel) or a Z bar (a Z shaped bar with the centre line of of the Z bolted to the gearbox area and the outer bent ends to each wheel) were often employed to increase (stiffen) the vertical wheel spring rates as both wheels moved into bump (bending the extra spring) and limit the 'jacking' effect. Unlike a normal stabiliser bar, the Z bar had no effect in roll.

Driving a 356 was not unlike driving an early Beetle, the driver had to develop a style of driving where balancing the car required input from both throttle and steering, a bend was taken rapidly as a series of steering input, throttle modulate, steering correction, throttle modulate etc. This style developed it's own moniker, it was called 'Wischening' - or Wiping in English, much like a wiper moving back and forth across the screen, the driver moved the steering wheel in small movements left and right to balance the car. Driving these cars well required skill and it was therefore more rewarding, this was the time when cars had character and driving well was character building.
 
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