Steering light at high speeds 110+ on dry road

...Obviously not public road ofcourse! As the subject mentions, this is the only symptom that I'm experiencing. There no shudder/vibrations or "steering pulling" to the left/right but the car feels very "light" when manoeuvring corners ect. I recall there was a post earlier about the same being experienced by some when taken to trackdays and think the culprit may be in the tyre pressure. The car is rarely driven on a full load, with two persons at most 80% of the time. The tyre dimensions are 225/30/18 and rear at 285/30/18 with the psi set in accordance with the manual at Front - 36 psi and Back - 44 psi. Unless it seems to point to something else, can someone recc at what psi is used for general driving to combat this without causing unnecessary wear on the tyres. Cheers!

Those pressures sound high to me. I fun factory on the road of 32 front 37rear cold. This however didn't help me with my floating at speed. Mine was resolved when I had new suspension installed and the ride heights reset to factory heights. My ride height had sagged an inch at the rear giving the car a slightly nose up attitude. This won't have help aerodynamics at all.... It's now glued to the road... If you think about it like this. Ferrari don't put spoilers onto the rear of their cars as they use ground effects. They use the slowing of air under the vehicle to provide downforce. This is done by rake and the transition at the rear. As pressurised air enters the front, it slows down as the height between the car and the road changes (the rake), it creates a vacuum compared to the air pressure above the car. A clean transition back into the main stream with a diffuser at the rear finishes the job. Unfortunately this is the bit Porsche can't do as easily with a rear mounted engine.... This is the reason the underside of the Porsche has so many trays to keep it flat and keep the air moving cleanly.....

The air will accelerate under the car creating a lower air pressure, Bernoulli's theory. My front tyres are 225 40 18 but I dont know if that would make any difference to the feel, I run the pressures 34 & 42psi {2lbs lower} TB :wink:[/b]

Well the overall car air above and below conform to Bernoulli's theory with the top being low pressure, the bottom being high pressure, hence lift, but then rake then creates a vacuum affect as the same volume of air that went under the front then having to take up more space.... But then we transition it nicely with a diffuser to help suck it through reducing the high pressure under the car as you state to produce less lift... It's this interaction of rake (theoretical slowing) with diffusing (speeding up) that hurts my head. I can say that I know it works Best leave it all for CAD

I'm :? now. Isn't TB correct. The car in profile is a wing. Accelerate the air beneath the car (by making the underside smoother) creates more downforce by lowering pressure. Slowing the air over the top (spoiling it by breaking up its flow), has the same effect by increasing the pressure differential but also drag. By concentrating on ground effect and also feeding air from lower to upper surfaces, increases downforce but does not increase drag to the same degree. Isn't that what has been so clever on F1 and recent road going Ferrari? And wasn't this the sea change in Porsche thinking in the transition from pre-964 911's to the 964 and subsequent cars?

Yes TB is 1000000% correct. I made a mistake in my thinking. As the air has to travel further over the top of the car, it must speed up to match the air underneath. Faster air = lower pressure hence lift. So the rear diffuser is there to transition the exiting rear air as smoothly and efficiently as it can and in a sense speed it up to reduce the high pressure region under the car. You then have rake which provides a vacuum under the car with the theory that the same amount of air that went under the front must now expand to take up a larger area. My mistake in thinking is that I figured this would slow down the air......... A good albeit crappy drawing website which explains it all is here a bit ... http://www.up22.com/Aerodynamics.htm Better one.... http://www.mulsannescorner.com/diffuser.htm

I should know the answer to this off the top of my head considering i read for Mech Eng degree for my Under Grad....oh well, that was money well spent! Hmmm... FYI I run 34 front, 40 back on 18"s too. Was at 36/42 until last Wednesday, but feels much better now. Taran :?

:hijack: Well I supposed we had gone off the point; check the tyre pressures first but :hijack: I know I'm going to get this wrong soon but this is about pressure differentials, not vacuum, which I'm told doesn't exist. You can create lift with a flat piece of paper set at an angle of attack to the airflow; increase the chord depth and you increase lift because the air passes faster over the top (lift is increased at slowe speeds) ; increase the speed below, the lift is reduced; make the air flow quicker demonstrably quicker beneath ( as in a car with a flat floor plan) than over the top, then it will cause a downward pressure. Wings on top of a car can create downforce acting as an inverted wing but they also create huge drag, so power needs to be increased. But make the bodywork act as the wing, then drag can be reduced as can power, engine size weight etc etc. The Audi TT was a good example of lift; reach a certain speed the car took off (as did the early 911); add a spoiler to disrupt the air flow cures the problem but adds drag. Another example is the good old 930. I am told at 100 mph plus, one could feel the effect of the front spoiler and rear wing reducing the lift to such an extent that the car felt more settled at these higher speeds. What I would be very interested in would be getting a better understanding of how passing air from the bottom of a car to the top works .... which I believe happens on a recent Ferrari but I'm afriad I loose interest too soon when I see the badge ....

oooh actually I think I know the answer ... if one can create a duct from thelower section to the upper part, then it might be possible to suck air from below in the same way as the old Brabham F1 twin fan car did, creating a higher presure differential and effectively sucking the car down on to the road ... The problem then must be at low speeds, when the air flow would not be fast enough to create the differential and then one is totally reliant on mechanical grip ... Does that sound right ?????? :? :?

and now I've seen the section on spoilers ...... :coat:

don't forget about Gurney flaps!!!! :twisted: TB

Mk1 had one, mk2 gt3 didn't..... I wonder why? I'd have been inclined to put the mk1 gurney onto a mk2 lid, but then I'm messing with science....

A diffuser does a very simple job. It takes the air that is running at ground level under the car and encourages it to raise upwards (ie follow the diffuser upwards) so that it exits at a higher point from the rear of the car. This higher point is in a part of the airstream that has a much lower air pressure than ground level behind a car so in effect sucks air out from beneath the car increasing the effect of the car being sucked to the ground. Ian.

hijacked - most definetly! Interesting information however! Will reduce the tyre psi -2 to see how it goes while i try to digest some of the information here! Thanks!

Hi The advice on whether your car is now sitting level, ie has the back end dropped is a good one to check out. When on trackdays how much fuel load are you carrying? I have heard testers comment that the front end can seem light when the fuel level is low which is understanable given the position of the tank. You might want to consider lowering the car, since I agree with the view that the more air you exclude from the underside the lower the pressure under the car, and therefore the pressure on the top of the car holds it down more. Best Regards Big Bob