There are a lot of things you can do to your car that will affect the aerodynamics, but there is rarely any information about how. And it matters. A small change in front end lift can have a big, noticeable impact on stability (for instance, the difference between a 996.2 C2 and GT3 is only about 12kg at 200 kmph). And reducing front lift without a similar change at the rear Ė which Porsche has always been very careful not to do Ė is often said to make the car quite unstable at high speeds.
Iíve read many many threads on bumpers, lip spoilers, ducts, wheel well vents, ducktails, wings, and diffusers, and with very few exceptions, there is zero data. Iíve called top German tuners and other highly regarded manufacturers of aero add ons, and again, zero data. (Manthey wings are a notable exception.) If you want to know the impact of a change, you are going to have to figure it out yourself. Or perhaps, other members and I will use one of the below methods and find out for you (or at least share our findings online).
The good news is that you can measure changes in lift and drag without a wind tunnel. The bad news is that I still canít tell you exactly how. Indeed, I started this thread to share what I know, and ask members who know some of the missing bits to help out.
Iíll save drag for another post, because thatís the easy part. Iíll also save ďwhy not just go out on track and check the lap times?Ē. As for lift and downforce, if you Google, youíll find a number of ways, but Iíll focus on three that strike me as most promising for our cars. And if anyone knows of any others, please let us know!
You get sensors called linear potentiometers/shock pots, install them to your suspension, and attach them to a very high quality data logger (if thatís the right word), e.g. from Pi or Stack. The gear will cost something like $6k. And to give credit where itís due, I learned this from another Rennlister: stvsxm.
I wonít be making that sort of investment into this. One might however see if one can find a racing team or race prep shop who can lend you the equipment and/or do this for you. Alternately, the sensors are the least expensive part, so although I understand they typically canít be coupled to an amature data logger, Iím wondering if they might nonetheless work with the sort of high end data amature data loggers I can probably borrow from friends, e.g. one of Race Logicís devices that can acccept numerous external sensors.
The second idea is to somehow tap into the data coming from the ride height sensors. (996s with Xenon headlights have ride height sensors. Presumably 986s and all later Porsches with Xenons do to. Donít know about earlier cars.) And at least on some cars, it is possible to tap into such data. Hereís a link where someone did it with a Lexus: http://www.autospeed.com/cms/article.html?&A=113225
There is a simple way to translate changes in ride height into kg of lift or downforce. Put some weight between the wheels on one axle (e.g. in the frunk or behind the rear seats on a 911) and measure how much the height changes. This tells you how many kg are needed to move the height by X mm, so if you know the car rises Y mm at speed, you can calculate how much lift you are generating. Note though that for the car to sink enough to measure a meaningul change, youíll need a lot a weight, especially if you have stiffer than stock springs. 10 kg surely wonít do it unless you have a very precise measuring device and method.
(Or if you already know your effective spring rates front and rear, then you could calculate the lift or downforce without even measuring the impact of adding weight over one axle.)
Unfortunately, this is where my knowledge of how to do this starts to run out. How to get the data? I may call around to people who do chip tuning. How to save or display the data? On a PC? Or perhaps one could again use a data logger, good ones that accept external sensors. I may call data logger manufacturers like Race Logic.
Iím hoping someone else on the forum will have more of a clue how to do this, and Iím quite certain many (most/all?) will know more about our cars electronics and how to plug into them than I do. So any thoughts/advice/insight about this would be greatly appreciated!
This is probably simplest and cheapest, but also the least useful. Indeed, it probably wonít work, because itís just my idea, not tried and tested by someone else so far as I know, but anyway, here it is: The idea is to use a spririt level or bubble level Ė but a precise digital one, not one where you look at a bubble, so that it tells you precise angles in numerical terms.
First, you mount this in the car. Maybe you use a GoPro mount or a radar detector mount. Or maybe you tape it onto the center console or the handbrake with lots of masking tape to immobilize it. Though Iíd be surprised if wiser readers donít have better mounting ideas than mine. Then you find the angle when the car is standing, then at low speed, and then at several different, high speeds. That tells you how much more the nose is rising than the tail at each speed (and if your car is a 911, the nose will almost certainly rise more than the tail). Then you do your aero mod to one end, and test again, and see how the angles change.
If you changed the aero at the front, then an change in angle is probably (almost) all due to a chance in height at the front. Do a bit of trigonometry (or add weight at one axle as above, but measuring both the change in height and the change in angle) and you can calculate the change in height at the front, and the change in lift.
But if you changed the aero at the rear, then if the angle changes in a way that suggests that the rear is lower, itís fairly likely that it indeed is lower, but also that the front is higher. How much? Apparently increasing the rear downforce by 25 kg on a 996 GT3RS versus the GT3 caused front lift to increase by 9 kg at 200 kmph. And thatís a car with pretty stiff springs. On a normal Carrera, the difference would be much greater.
Still, this method can help with both front and rear. If the car has roughly the same angle as it did stock, then you are not far from having a balanced setup. And if someone posts the angle of their GT3RS (or Turbo) at speed X, and your Carrera has the same angle, then you probably have an even better setup for track (or road).
A FEW MORE DETAILS:
With all these methods, you probably want to:
- Have roughly the same amount of gas when you measure. Iíd aim for half a tank, since thatís probably the average amount.
- Make sure you start out with stock ride height if you want to see how the car behaves stock, or at least the same relative height front to rear.
- Make sure your test road is level, which you can do by driving too slow for aerodynamics to make any meaningful difference (Iíd think 60 mph/100 kmph is quite safe unless you have a monster rear wing) and verifying that you have the same heights and/or angles as when standing.
- Drive fast enough. At least 90 mph/144 kmph, I would think. Ideally 124/200+.
- Donít accelerate, brake, or turn while measuring.
- Use a GPS device to know your precise speed.
That's what I know so far. Additions and corrections are welcome!!! (Questions too - I hope someone can answer them.)