Downforce, Traction, and Aerodynamics

[thinkmonkey]

I suppose this should be in a different forum, but it applies to the body of the car. I'm building my car from the unibody up as a weekened racer and so I've been addressing certain issues as they come at me. Lately the issue of high speed aerodynamics has come to my attention. The smooth sweeping shape of the roofline into the hatch of the Z is very aerodynamic and creates a low pressure zone because the air flows faster over it. (Bernoulli) On the other hand, underneath the car the air is very turbulent and creates a high pressure zone. All this adds up to lift at the rear, and I've heard at 70mph anywhere from 120-180lbs of the rear tires!

This concerns me because I live in seattle, where it rains like the ****ens, and we have very wet roads. AT 70mph my car oversteers which is not a very good situation. I can come up with a few solutions to this, and I want your guys opinion on effectiveness.

Front Air Dam: I'm going to get the MSA II one, no question. It is stylish and is supposed to force air underneath the car in order to create a low pressure zone by moving the air faster. Skirts are to keep the air trapped underneath. Has anyone noticed a significant handling change as a result of this airdam, or is it more cosmetic.

Flat Underbody: This is less practical because it would require a lot of work and have many problems, but would be very effective. Smooth out the airflow, create low pressure, less lift maybe downforce. Mclaren pioneered this in the 60's/70's in F1. AIr diffusers could be built into the underbody. Diffusers look like channels pressed into the underbody that widen towards the back of the car. High pressure air goes in, expands in the channel, creating a low pressure zone. Thats the basic theory.

Visually and Aerodynamically distracting spoiler: I have to say, I'm not a fan of the wing. Supra's, F40/50's, Rice, Countach's; all very tacky with the spoiler. I've come to realize though that this may be my only option. Cosmetically, I tolerate the molded lip spolier the most, although I doubt it's effectiveness. What I'm looking for is a removable and adjustable wing to go over the hatch so that I could keep my pride during the week, and save my neck on the weekends. http://www.geocities.com/spized1969/ has a wing that I may look into. I H A T E how it looks, but it looks very functional.

When it does come to traction and aerodynamics, let me clear up a few things. On a three box car design (like a sedan) a spoiler actually does improve drag. The trunk area creates a lot of turbulence, even with a swept window. The wing helps to smooth out the air and guide it away from the body. On the Infiniti G35 it improves the drag coefficent .01 from .31 to .30, which is somewhat significant. But this is why the fatback was invented, to help improve drag. On a Z it will only decrease top speed, but will increase top speed handling.

With fatter tires you do not get more grip. Plain and simple. The force of friction, which equates to how much force you can apply to the road from the engine, is dependent of two factors. The normal force (weight of the car) and mu (the coefficient of friction). Mu changes according to the to the two surfaces in contact, and temperature. Hot Pirellis are sticky, while cold firestones are not. Fat tires do however improve handling and can take more stress because there is less weight per square inch.

[byunique]

Front Airdam from MSA: yes, it works very well. My car used to feel very floaty at 70mph and faster. I installed the fiberglass airdam, and the front end started hugging the road. Yes, a definite performance advantage, not just for looks. Your theory holds true. They have fibreglass and urethane dams. The difference is the fibreglass ones are deeper and would go with that one if you are careful about not hitting parking lot stops. Next best thing are the urethane ones, which are not as deep but will not break. I also have a rear spoiler, but installed that merely for looks, so I wasn't paying attention if any performance was added.

One more thing... A Z is very much like a airplane wing very prone to take off. From what I heard from Z racers, no matter what you do to the car.....at 150mph, it is very scary and unstable.

[hmsports]

From everything I've read in my performance books over the last eight year, the rear wing/spoiler usually only does any good at over 150mph. Most of the aftermarket wings you see are just to make the cars look cool. I'd find it hard to believe that the wing on the site from your link has had much wind tunnel testing - it was probably designed by an artist, not an engineer.

What kind of racing are you doing? I know from personal experience that lowering the car and having a good front air-damn to keep the air from lifting the front of the car will keep it quite stable.

My vote is for you to save the money on the wing and see how the air-damn and side skirts work out first.

FWIW, Rick

[EScanlon]

A magazine in the 70's did a comparison on the 240Z stock and the MGB GT, and the Opel GT. Their results were published in Sports Car Graphic; Mar 1970, and reprinted in the book "Datsun 240z & 260Z Gold Portfolio" which is a reprint of many articles.

In that article they showed that the 240Z Stock was the only one of the three to have downforce on both the front and rear axles. The other two showed significant UPforce. All of these results were published as being at 100mph.

Now, at higher speeds from what others have posted and purportedly other magazines (haven't seen or heard), there is a feeling of "lightening" or lack of control through the steering mechanism. Haven't myself experienced it, since I've been doing some major rework on my car and haven't had it out on a long stretch of highway where I could let it get past 100.

However, a couple doubts as to your original post occurred to me.

Quote:

"Front Air Dam: I'm going to get the MSA II one, no question. It is stylish and is supposed to force air underneath the car in order to create a low pressure zone by moving the air faster. Skirts are to keep the air trapped underneath. Has anyone noticed a significant handling change as a result of this airdam, or is it more cosmetic. "

Maybe I'm completely wrong here, but I had always been told by other enthusiasts and racers as well as my engineering classes that the purpose of the front air dam was to force the air in front of the car UP AND OVER the car, and what could not be forced up (due to the plowing effect) would in fact be pushed to the side, PREVENTING the air from entering the area underneath the car. THIS effectively reduces the volume and pressure of air beneath the car which causes it to "suck" down to the pavement. Skirts on the other hand, were to maintain that "plow" and NOT allow the air to rush back in under the car just past the front wheels, and thereby TRAP the LACK of air pressure beneath the car for as long as possible.

Now the design of the air dam is what's important in whether it diverts the air to the side or forms a rolling turbulence that builds in front (and this causes some lifting) which then causes the bulk volume of the air to go UP and over the car.

Now the rear air dam or spoiler, it's shape was an attempt at reversing the front air dam. By forcing the air sharply upwards, it would cause it to tumble and fill the area behind the car quicker while at the same time creating a vortex that would cause the bulk of the air stream above to ride over and above. Yes drag was increased (made worse; something you didn't explain) but the offset in drag was explained in that the entrance of fumes in to the car were diminished.

Now, I studied engineering, and this is from what I recall from 20 + years ago, I may be all hogwash, or it could be that studies since then have demonstrated that what I recall to be erroneous.

Looking forward to your reply.

[hmsports]

EScanlon is right on with the purpose of the air-dam. You want to keep as much of the air from getting under the car as possible. A vertical air-dam with a horizontal plate, of at least an inch, going forward works best. It will force all the air around the car so none spills under. It is also important the the contour of the air-dam is extended far enough out that it directs air around the front tires. It is amazing how much flat frontal area a tire has that can cause a lot of air flow disturbance.