Plan on making an aluminum air difuser for my 00' hatch. I've tried to research this on the boards, but havn't been able to find much other than photos of JohnG's EG hatch. Does anyone know if one design is any better than another. Bassically id like any info or a link to where i can read up on this subject. I'd rather not just go out, slap something onto my bumper and hope it works with no incline of what i'm doing.

* also, where can i get a pop rivet tool to fasten the aluminum onto the bumper and about how much do they cost?

thanks for any info or help you can give me.


[Modified by JDM Style EK, 6:57 AM 3/8/2003]

 

Unless you have wind tunnel testing this is exactly what you will be doing

Series rules will also limit what you can do. Check with John at http://www.roadracegear.com

 

I dont think you can go wrong with using an air dam though. Blocking any amount of air from flowing underneath the car will help in reducing lift.

 

Yes, reduce lift, how do you decrease lift? Ah yes, I remember increase DRAG. If you go to big on the front air dam/spoiler/spliter you can and will in fact slow your car down. This will be very noticable on long straights.

Good Luck

 

Not quite true. Drag and lift are not interdependant. For example, you could stick a big sheet of plyboard on the side of your car, and it's going to increase your drag, but it will not seriously effect your lift. Obviously a gross example, but its point is valid. The point of air dams are to divert air that would normally go underneath the car faster than the air on top (the very rough method of causing lift) around the side of the car. Air dams do not increase drag. There is also a difference between wings and spoilers (heh, my gliding instructor always annoyed me with this, but there are difference between air brakes and spoilers as well). Wings produce downforce by inverting a wing. It produces lift, but the lift is down, so it's called downforce. Spoilers interrupt the flow of air causing turbulance.

Sorry that was off topic, and reaaaaly basic, but I thought the point needed to be made. --edit damn my grammar sux0red


[Modified by bork, 8:19 PM 3/8/2003]

 

what about splitters...will they slow you down?

 

out of my area of expertise. The few pictures i could find of them were bad angles of old BMW m3s... but they look essentially like an air dam for cars that suck their air in higher up than our hondas.

that's purely a guess though...

 

Wouldn't this cause some sort of drag and slow the car down...or not?

 

All of my engineering books are in boxes in the basement, so I apologize if I don't use all of the terms precisely as I would have 20 years ago but...

Vertical forces on the car are generated by a pressure difference between its top and its bottom, over an area. If the area is 6x12 feet, that is 72 square feet, or over 10,000 square inches. An "air pressure differential" - between the top and bottom of this surface - of only 1 pound per square inch (psi) would realize a force of 10,000 pounds!

That is NOT going to happen in the real world of production-based cars but it gives a sense of the power of aerodynamics. In this world, "lift" is when this force is upward and "negative lift" or "downforce" is when the arrow points at the ground - a good thing, generally. We also have to be VERY sensitive to the way lift is distributed between the front and rear wheels. If a car carries 1500# of static (sitting still) weight on the front wheels but generates 1600# of lift up front when moving at 100mph, what happens? It is sometimes useful to think of a car as several surfaces - front, middle, and rear - as you play aero games, but understand that the actual air pressure on each infinitely tiny spot on the car will be different than its neighbors.

There are lots of kinds of DRAG, which can be thought of as "negative horsepower" that tends to slow the car down - it's a force pointing back at you from the direction you are trying to go. It is true that drag is induced simply by creating lift but we do not "decrease lift" by "increasing drag." The drag is a byproduct of efforts to stick the car on the ground.

Whether the car is "slowed down" or not is largely dependent on what you are doing with it. If you are running at Bonneville for maximum top speed, you only want enough negative lift to keep you from flying. If you on a road course, you will likely lower your lap times by generating downforce, even if you give up speed on the straights, because cornering (and therefore average) speeds increase.

Aero tuning can also be used to control the over- and understeer balance of a car at different speeds. An oversteering car can be great fun at low speeds (to get it to change direction quickly) but you can tune out some of that characteristic at high speeds - where oversteer would send you backwards into the woods to talk to God - by tweaking air dams, spoilers, and wings.

It is not necessary to have a windtunnel to test aerodynamics. The stopwatch and driver feedback will tell you if downforce (total or distribution) is changed on a skidpad or race track. You can also do coast-down tests to see if drag is changed - start at a set speed on a big chunk of level ground, shift the car into neutral and coast until it stops. Repeat this a few times to get some baseline data, then put on the lip/spoiler/whatever, and run the tests again at the same speeds, in the same place. If the car stops in less distance, drag has increased and vice-versa. Remember that you want to maintain all of the other variables to the greatest extent possible - weight of the car+driver, weather, temperature, etc.

Most generally, the idea of a "splitter" (a horizontal surface poking out of the front of an airdam) is to trap the air and keep it from spilling under the airdam. This increases the pressure differential over the nose of the car. They can be an aero "good deal" because they have a good "net effect" - favorable returns in downforce with minimal additions to total drag. Extending the lip back to a tray under the front of the car is generally thought to increase this effect.

Have fun!

Kirk

EDIT - if you are one of those folks who think that aero forces are only an issue at high speeds, I have a demo that I can do in a classroom that will illustrate otherwise.





[Modified by Knestis, 2:41 PM 3/9/2003]

 

hmm...I'd be interested in seeing this.

 

I'd venture to say he's talking about a paper airplane

 

Heh, i love engineers :D very well put.

 

Disclaimer: I'm NOT an engineer - I just play one on the Internet.

I bailed on the ME program and got a BFA in industrial design - it was way more fun - but ended up teaching junior high school science and engineering classes. We can learn a LOT from simple toys - like LEGO.

I used to do a unit where I provided teams of kids identical packages of LEGO parts including four wheels. We had a Pinewood Derby kind of inclined plane race track that we set up in the hall, and the races were an integral part of the activities introducing Newtonian physics.

As an example of the learning that took place, one team was kicking everyone's butts, the contest being to see which car would go the farthest down the hall from a dead stop at the top of the ramp. I had my suspicions but let things play out. Their car kept going farther, and faarther, and faaarrrther down the hall, until on one run it smacked into a door jam and busted open - to reveal a secret compartment filled with pennies. (Damn JH kids cheat like IT drivers.)

It was a hoot - everyone yelling about cheating - so we went back to the room and I had the protestors present their case in scientific terms: How was it an advantage to have more mass in the car? They managed to explain that more mass at the same height made for more potential energy, which turned into additional kinetic energy down the track and into the hall. Case closed - cheaters DQ'd. (Rules NERDism supported.)

At the end of this unit, I did a couple of runs for them with a car that I made, that had a flat LEGO plate as the top of its "bodywork". I made three runs and recorded the distance achieved (no pennies), and then took the flat plate (maybe 3x7"?) and stuck it up at 90* to the airflow, like a sail. Tests were repeated and the car went dramatically less far. My question to them was, since they knew that I hadn't changed the mass of the car and understood the mechanics by now, "why?" The next unit was aerodynamics (fluids, states of matter, pressure vs. force).

The magnitude of the dynamics involved are a LOT lower but so were the speeds although in fairness, I probably quadrupled the frontal area of my racer. The little cars might have reached a velocity of 10fps (maybe 7mph?) but it was enough for aero drag to become a significant issue.

Two Germans - Kamm (of Kamm-back fame) and Paul Jaray - did some groundbreaking low-speed automotive aerodynamic testing before WWII. Jaray got significant gas mileage and performance improvements at speeds as low as 20mph, with clever bodywork and dinky engines (working with Auto Union, I think?) - his cars had Cds (coefficients of drag) substantially lower than most modern production cars (which ALL come in at about .40 [+/-.02] regardless), and they were practical to boot. Cd is a derived value (which can be determined using coast-down testing, BTW) and is dependent on frontal area.

If you are in the mood for some math, "drag HP" can be determined using the formula (if my notes are correct):

Drag (D, in pounds of force) = 1/391 x Cd x A x V^2

That's the coefficient of drag, times the frontal area, times the velocity squared, divided by 391. Drag HP is then calculated:

Drag HP = D x V x .0027

That's drag, times velocity (again, drag HP increases with the cube of the velocity), times a constant to convert to ponies.

A Civic with a Cd of .39 and a frontal area of 10 sq. ft. sees about 5.5 drag HP at 50 mph. For what it's worth, downforce increases with the cube of the velocity too, as I recall...

As always, someone should check my work 'cause I am rusty as hell.

K