alexjc4

Who is talking about rear toe out?

If it was just the weight making the difference then a c2 with a full tank would handle the same as a c4 with 1/4 tank. I promise that isn't the case.

ras62

I was making a reference to the inherent stability you mentioned earlier, add toe out at the rear and instability results on either a C2 or C4 i.e stability has nothing to do with the 4wd system. I would also call the C4 tendency to understeer as predictable rather than being inherently stable.

alexjc4

Hmm some semantics here so let's be more explicit.

Look at the characteristics of the c4 in static state cornering at the grip limit. Under these conditions there is no understeer in the c4. When the lateral grip limit is exceeded as corner speed increases, no additional yaw moment is created i.e you get four wheel drift rather than oversteer or understeer and this is a characteristic of the balance of loads (lateral and longitudinal) on the four tires vs the vertical loads. That does indeed have a lot to do with the 4wd system - by reducing the demands on the rear tyres to maintain the forward motion of the car by 30%.

What is notable in the c4 compared to the c2 (which is also surprisingly "stable" in this "grip limit yaw moment" sense) is the margin you have for applying quite severe weight transfers and control inputs even in low grip conditions. You CAN temporarily generate an additional yaw moment but the car reverts quickly to the "stable" yaw demanded by the steering angle once the weight transfer has subsided.

The dynamic throttle understeer (accelerating hard out of the steady state at the grip limit) that I have experienced but some other c4 drivers report not experiencing is "predictable" in the sense that it is repeatable given the same control inputs and cornering conditions. It is also "predictable" in the general sense that to be expected when the steered wheels are also driven - asking a tyre to generate longitudinal grip reduces its capacity to generate lateral grip. Appropriate driving styles and geometry however can greatly mitigate this general characteristic.

All interesting stuff, ras62 have you read Milliken RCVD? It is widely recommended it as a fairly accessible text of vehicle dynamics, and really helped me form a mental model of these things. It is a complex field so though so theory only takes you so far and it's easier to use the theory to understand observed behaviour than predict it.

In this context then the weight distribution and transfer characteristics of a car are the starting point and the static geo is a way to a)manage slip angles at all four corners b)premeditate "unfavourable" dynamic geo changes under given weight transfers and long and lat loads. Along with "favourable" ones like the Weissach axle effect.

ras62

Alex, no I haven't read Millikan. I agree with what you are saying about steady state character, the C2 and C4 are similar in this respect imo. Also when accelerating beyond this steady state the C2 will also understeer at lower speeds, especially true when fitted with a LSD.
The weight transfer and yaw effects you mention is very important and this is where the lower ride height and RS engine mounts really help on both cars.
In extremes I find the C2 is very stable, a LSD further enhances this stability both under braking and when using oversteer. The C4 from memory gets a little ragged in extremes especially if the PDAS is triggered. That is why I asked if you had tested the PDAS on your recent driving course.
It is certainly a complex subject. The older 4wd systems as used on the C4 and the old Ur Quattro I used to have felt quite similar apart from the Audi understeering more. Odd considering the layouts. The later electronic systems as employed by the Mitsubishi Evo are black magic by comparison. Never did quite understand how you could get so much grip and traction while being a hooligan with the throttle. Rob

alexjc4

OK, so I had a chance on Sunday to get a feel for the new alignment. Some laps of small, medium and large roundabouts, and some miles on lovely sweeping and undulating 60mph limit country roads where you can load the car up properly and still stay legal.

Impressions so far:

• With the additional camber the car follows ruts in the road more - which doesn't bother me but might some.
• It does seem more willing to throttle oversteer from a steady state corner.
• Low speed throttle-on understeer still present.
• Still very neutral and not easy to unsettle.
• Lift off oversteer still feels fairly benign and self regulating but I'm not going to push that to the limit on the road.
• Severe straightline braking requires more steering input or rather a firmer grasp of the wheel, partly because it follows ruts and camber in the road - again doesn't bother me too much but might some.
• Turn-in seems better, and with a full tank of fuel the front is more precise than it's ever been.

kos11-12

thanks for sharing your experience,
how was it on motorway speed, what tires pressure did you run, between full and 1/2 tank there is 30/35 kg , do you know your front and rear weight and total weight it would be interesting to compare those numbers,

I will share my numbers sometimes this week, waiting for the weather to improve to measure bump steer, and so on,

jack.pe

Guys, these are the settings that porsche managed to get for me on Sat:



Having now driven the car in anger on a mixed course I can say that understeer is reduced (with PDAS off that is, with it on Carr will always push wide in certain circumstances). Car I pretty neutral and willing to change direction, quite agile really. However the front feels to light for me and lacking in feedback, rather as if car needed much more rake.. Though I don't think that is the case. I am now going to Autofarm next week to see if they can help get some settings that actually work for me. This was with a full tank of petrol..

964TOK

HI Jack,

I would bring the car to Centre Gravity. This is all they do and specialise in Porsche.

http://www.centregravity.co.uk/

Maybe worth a call before going to Autofarm.

jack.pe

Thanks but they're too far away and won't have time before SPA.. Redtek said Autofarm also good so hope they can get some decent settings on it.

kos11-12

GTone is not far,

jack how do you switch off PDAS

freedman

Turn the console swith to enable the low speed AWD mode (hold in your case I think as you dont have the dial) for 10 seconds and it turns everything off

If I recall correctly

alexjc4

Yup, just hold the pdas switch unto the gong goes off, the warning lights come on, cancel the gong with the ! button.

kos11-12

yes mine is a switch so I will need to hold it for 10 sec ? engine running yes...
every time I switch on the car as I imagine it will reset to normal when switched off ,
does it means it will stay on 30/70 all the time,

boxsey911

These are the classic symptoms of not enough castor on a 911. The car will feel easy to turn at low speed and very agile but will then feel quite nervous at high speed. Increasing the castor will make the steering feel heavier at low speed but it will be more direct and have more feedback. At high speed it'll feel very 'planted' and the steering a lot more predictable. Your castor figures show that they were way out before and are still not close to the specified value afterwards.

alexjc4

Yes, yes and yes