NASCAR technical question

[Racefreak24]

OK...this question is racing related but only heard discussed in NASCAR, mabe you can help me understand. On cooler days typically at superspeedways you hear of the crews taping off the grill. Obviously you can only do this in cool enough conditions or for small quantities of time such as qual. They say it creates more front downforce and reduces drag..........OK here we go. How do you INCREASE downforce and REDUCE drag. Seems unusual but mabe I'm missing a formula somewhere.

Thanks

[Conover]

I'm no wind tunnel guru, but from what I gather, you tape off the grill, an opening, that air pushes into and piles up in front of. When you get rid of the opening, and substitute tape for mesh, your providing a slicker surface area that moves the air up along the body instead of piling up in the grill opening. so the air moves more freely which reduces drag, and you give the air something to push down on, the tape provides a surface area for the air to push on, where as before hand it could have even been causing lift, who knows.

Help any? big parachute, small parachute

[Racefreak24]

That's the theory I'v been using all along so hopefully we are just naturally born for this kind of thing. There's just been this question in me that prompted me to ask to see if there was anything i was missing but I think we may be onto something....Interesting though

[BobsAuto]

Exactly. On cooler days, the air has more moisture, therefore, it is heavier. Have you ever watched the air come off the back wing of an FA or F1 on a cool and cloudy day? You can actually see the moisture (heavy air) fall off the back. The heavier the air, the slower the cars go.

[chuck baader]

One other consideration....any air that goes over the car rather than under or through an opening is considerably more aerodynamic...so, you get more downforce with less drag...sorta, kinda, maybe... Chuck

[lateapex911]

Definately. Think of a wing...Lots of lift, little drag. Now, punch a few holes in the leading edge. More drag, less lift.

Or, conversly, fill the damaged front edge, more lift, less drag.

Taping the grill is merely optimizing an imperfect shape. If it wasn't for the darn engine, they could go grilless, LOL.

In summary, the two results are NOT mutually exclusive.

BTW, Nextel Cup cars produce a LOT of downforce.

[Knestis]

Exactly. On cooler days, the air has more moisture, therefore, it is heavier. Have you ever watched the air come off the back wing of an FA or F1 on a cool and cloudy day? You can actually see the moisture (heavy air) fall off the back. The heavier the air, the slower the cars go.
[/b]

Sorry - science misconception alert!

Humid air is LESS dense than dry air, because gaseous water molecules have less mass than gaseous nitrogen and oxygen molecules. Aero functions (lift or drag) are greater in nice, dry air. Liquid water is more dense than air, of course.

Those vortices (vortexes, more than one vortex, whatever) you see off the wingtips of F1 cars or jets are water condensing (going from gas to droplets of liquid) because of the relatively sudden local air pressure changes.

http://www.galleryoffluidmechanics.com/vortex/f18ba10.jpg

K

[BobsAuto]

OK, then why, when it is humid, yet cool, is the air so "heavy" as the meterologists around here say? It is cooler, currently, than over the weekend and more humid. It feels like you have the weight of the world on you. Isn't that heavy air? That's what I was taught, but then again, nuns taught me who had no scientific experience and just used the teachers manual to teach science.
Hmmmm....this may be a huge learning experience.

BTW, the Hood blimp is flying overhead and over the track...the passengers are enjoying the Porsche club event....now where are they for the SCCA events?

PS>>>>LOVED the picture of the F15???? or F10????

[Knestis]

Cooler air = More dense
Warmer air = Less dense

Higher air pressure = More dense
Lower air pressure = Less dense

Lower elevation = More dense (higher air pressure)
Higher elevation = Less dense (lower air pressure)

The problem is that separating cool/warm from humid/dry from low pressure/high pressure might not be very easy out in the real world.

K

[Z3_GoCar]

OK, then why, when it is humid, yet cool, is the air so "heavy" as the meterologists around here say? It is cooler, currently, than over the weekend and more humid. It feels like you have the weight of the world on you. Isn't that heavy air? That's what I was taught, but then again, nuns taught me who had no scientific experience and just used the teachers manual to teach science.
Hmmmm....this may be a huge learning experience.

BTW, the Hood blimp is flying overhead and over the track...the passengers are enjoying the Porsche club event....now where are they for the SCCA events?

PS>>>>LOVED the picture of the F15???? or F10????
[/b]

F/A-18 B, the Blue Angles would fly nothing less. I see them all the time around here. There are two effects going on, temperature and humidity. With a lower temperature, the ability of the air to hold onto the humidity (water vapor) decreeses. Cold dry air is the best, hot humid air is the worst. The main point is that the water vapor displaces the O2 molecules as possible.

James

[Racefreak24]

I guess I opened up a topic and a half here cause now you all got me thinkin. Im gonna commit sin yet again here and speak of Drag racing....in drag "racing" they prefer lower elevations....why if higher elevation was less dense air? I hear that lower elevation created more HP......any input here?

And another, if cool air is more dense yet slower.......why are lap times on cool days sometimes FASTER?

Is there a civilian speaking scientist in the house?

BTW

Blue angels F/A 18 Navy/Marines
Thunderbirds F16 Air Force

[Eagle7]

Well, you started discussing aero, now we're moving to lap times. Lower elevation will have denser air. Denser air will give more downforce (good) and more drag (bad). With the right car you can tune for the balance you want.

But the big issue with faster times at lower elevations and cooler days is HP. Denser air can mix with more fuel to give a more powerful mixture - more HP. Yet another reason why qualifying times (morning) are commonly faster than race times (afternoon).

[BobsAuto]

That is also why NASCAR qualifies in the afternoon. They have a more realistic idea of what the car will do at breakneck speed in the race as well. (Going back to that OTHER racing organization.) Also, have you noticed that the big guns in drag racing often race at night? Cooler=more HP.

BTW, topics like this sure make the mind think and work and actually are good learning experiences. Thanks for starting the thread.

[Racefreak24]

Don't thank me....thank you and to everyone who posted to get to the bottom of this.... :happy204:

[Conover]

I saw an article about a month ago, Grassroots maybe, Racecar engineering, I'm not sure where, but they actually improved the aero with racer's tape on a Honda Integra, must've been RE, anyway pretty good article for a club racer, they put the Integra in the wind tunnel and took drag numbers and what not, then they taped up all the seams in the body, including around the hood, along the fenders and so on. Kinda cool, but I don't know who wants to go around with tape all over their car.

[lateapex911]

Not to mention it's not legal.

There have certainly been protests upheld at the Runoffs for exactly that.

[Knestis]

That is also why NASCAR qualifies in the afternoon. ...[/b]

I'll bet that the real reason has more to do with scheduling and attendance. There are a number of Cup venues where they qualify and race at very different times, and most of their races are long enough that conditions just change a lot, as a matter of course...


... Also, have you noticed that the big guns in drag racing often race at night? Cooler=more HP. ...
[/b]

And darker=better show. :)

Seriously - these games are so much more popular and professional than ours (road racing, broadly) because their first commitment is to the fan. We continue to collectively make decisions for the drivers. Or worse, decisions are may BY the drivers - or team owners.

K

[Wreckerboy]

I guess I opened up a topic and a half here cause now you all got me thinkin. Im gonna commit sin yet again here and speak of Drag racing....in drag "racing" they prefer lower elevations....why if higher elevation was less dense air? I hear that lower elevation created more HP......any input here?

And another, if cool air is more dense yet slower.......why are lap times on cool days sometimes FASTER?

Is there a civilian speaking scientist in the house?

BTW

Blue angels F/A 18 Navy/Marines
Thunderbirds F16 Air Force
[/b]

The "squirt racers" prefer lower elevations like the late great LACR which was nearly at sea level for example becasue lower elevations = denser air = more air per a given volume (say a cubic foot) for cramming into the combustion chamber.

[Greg Amy]

How do you INCREASE downforce and REDUCE drag.[/b]

It's not as difficult as you think. First, some basics:

- Lift causes drag. Pilots and aeronerds know that you cannot increase lift with a wing (or any surface, really) without also increasing induced drag (the drag produced not by the resistance to airflow - parasitic drag - but that created by the force of lift itself).

- "Downforce" is a bastardized simpleton NASCAR term; for the most part, very few cars produce "downforce" without wings or special underbody tricks. In fact, NASCAR has done a whole lotta stuff to these things to KEEP them from "producing downforce". In the real world, of which NASCAR is a part, the best you can do for a wingless, undertray-less automobile is MINIMIZE LIFT.

- Airflow beneath a car - without special undertrays - is turbulent, thus creates drag. (That's why you see The Boys faring in exhaust system and crossmembers and such. How many of you are paying attention to such details when building your exhaust...?)

- Airflow beneath a car - absent special undertrays - creates lift due to Bernoulli's Equation.

- Airflow through a cooling system is EXTREMELY draggy, not only because it is extracting copious energy from the cooling system but its passage is horribly turbulent. You'd be amazed at the total drag on most cars simply from the cooling system. ('Tis why manufacturers love small grills.)

Ergo, what is actually happening by taping up the "radiator" for a short while is that airflow through the cooling system is being reduced and airflow beneath the car is being reduced. This is not CREATING DOWNFORCE (thus creating drag) it is actually REDUCING LIFT (reducing drag). Reduced lift = reduced drag; reduced turbulent airflow = reduced drag. Reduced drag = Old Ford Falcons with pretty bodywork and decals go faster.

Quite simple, really...



...in drag "racing" they prefer lower elevations....why if higher elevation was less dense air?[/b]

Lower elevation = more dense air; more dense air = more oxygen molecules per cubic inch of air. More of that means bigger "go boom".

The power increase from the increased oxygen in denser air far exceeds the increased drag from it; to a point. For example, if I am williing to let the engine in my light airplane rev to whatever it wants, I can exceed the redline at sea level and make some serious speed from the inreased power. However, if I limit my flying to redline my best speed at attained at around 8,000 feet with full throttle (less intake restriction) due to the reduced drag. Any lower and I have to pull back on the throttle to stay within redline; any higher and I cannot make enough power to reach redline.

A turbocharged or turbine airplane, on the other hand, can fly much higher and will make better speed up high because it will hav ethe decrease drag from less denser air PLUS increased power from turbocharged manifold pressure. That's why commercial aircraft, almost all powered by turbine engines, fly as high as possible.


...why are lap times on cool days sometimes FASTER?[/b]

More oxygen = more power. More power = faster lap times.

[lateapex911]

Ergo, what is actually happening by taping up the "radiator" for a short while is that airflow through the cooling system is being reduced and airflow beneath the car is being reduced. This is not CREATING DOWNFORCE (thus creating drag) it is actually REDUCING LIFT (reducing drag). Reduced lift = reduced drag; reduced turbulent airflow = reduced drag. Reduced drag = Old Ford Falcons with pretty bodywork and decals go faster.

[/b]

Greg, isn't the "downforce" a result of all factors? Not just the removal of internal and underbody drag/lift?

My thoughts would be that the resultant increasse in downforce was due to the removal of the lift quotient under the car AND the increase in surface area of the carefully angled grill...which now acts like a diveplane.

Also, and i wish I could find the souce, I read some figures about downforce and I was amazed at the positive numbers these cars produce. (As long as they aren't going backwards)

(It makes what the guys like Lorenzen and Petty did back in the 60s more impressive)

[Greg Amy]

Greg, isn't the "downforce" a result of all factors? Not just the removal of internal and underbody drag/lift?[/b]

Without detailed information, of course it's impossible to do anything but "educatedly" guess. However, I suggest reduced lift from taping up of the cooling duct is primarily due to reduced underbody airflow...


...carefully angled grill...which now acts like a diveplane.[/b]

I'd be real surprised if it actually created much vertical force. But again, I don't know that for a fact.

[ddewhurst]

***Ergo, what is actually happening by taping up the "radiator" for a short while is that airflow through the cooling system is being reduced and airflow beneath the car is being reduced. This is not CREATING DOWNFORCE (thus creating drag) it is actually REDUCING LIFT (reducing drag). Reduced lift = reduced drag; reduced turbulent airflow = reduced drag. Reduced drag = Old Ford Falcons with pretty bodywork and decals go faster.***

Greg, if I may. Because you are IMHJ saying that reduced air flow through the radiator is reducing the lift & I don't buy that thought for a given radiator in a given race car. (Air speed through any given radiator is approx 40 mph.) IMHU, as described below taping is creating creating more down force to overcome bottom side car lift.

Air passes through any radiator at approx 40 mph (wind tunnel proven) independent of car speed & because of lack of flow through the radiator creating a turbulant ball of drag air in front of the radiator/car.

Turbulent airflow through a given radiator can be controled with core rows/size, fin density/shape & the radiator rear shroud.

Pressure drop front to back of the radiator is important & can be impacted by during a race widening the front fenders. Also helps down force to overcome bottom side car lift.

In my humble understanding when the car radiator intake is taped the size of the turbulant ball of drag air is reduced creating less drag at front car opening & the air flow over the taped area is allowing more non-turbalant air flow over the hood creating more down force to overcome bottom side car lift.

Continue the Fun ;)
David

ps: C&R radiators estimates a 2 inchs of tape across the opening will up the down force by 50 to 70 pounds.

[Greg Amy]

Could very well be, David. As I said, I'm offering simply educated guesses.

I'm confident that the main reason for taping the radiator is to reduce drag. While the airflow may only be 40 mph the car has to absorb all of the energy to get it from 200 mph to 40; in addition while it may only be 40 through the rad it's a very large volume of air.

The thing is, though, as you speed up the air over the car, it will reduce the pressure; any reduction in pressure above the car results in lift. If it is true that lift is reduced when the radiator is taped up, then the only immeditely logical cause for that would be due to a reduction in airflow underneath the car, thus a reduction in static pressure below the car. That reduced underbody airflow - absent any other changes - can only seem to come from the airflow through the cooling system...

I'm sure there are much more definitive answers to this somewhere out there.

[dickita15]

I remember reading an article a few years ago is a magazine (probably C&D or R&T) about testing a top stock car at Road Atlanta. The driver was not a stock car guy. I do not remember who wrote it or which top team the car was from. The driver took a few laps and complained that car understeered in turn 6 and 7. The crew chief took a pair of channelocks and gently bent the leading edge of the front fender out a little. The driver was sure this would have no effect and was amazed when the front end stuck in those turns next time out.

Another interesting tid bit I heard is that before qualifying at an Impound race they push the brake pads all the way back. Testing on a dyno said that this was good for 11 horsepower in drag savings. When it is not an impound race they put in thin pads and gain even more.

[Knestis]

...Another interesting tid bit I heard is that before qualifying at an Impound race they push the brake pads all the way back. Testing on a dyno said that this was good for 11 horsepower in drag savings. When it is not an impound race they put in thin pads and gain even more.
[/b]

Topic Drift Warning:

I've thought of using drum rears on the Golf for this very reason. If they weren't such a PITA to work on, I would already have done it.

K

[JeffYoung]

Ditto on PITA working on those things.

Kirk, I typically have to adjust my shoes outward enough so that they are "touching" the drums a bit anyway just to get decent pedal feel. So, I wonder if any low drag advantage drums have over discs is lost in the interests of getting a firm pedal?

[Racefreak24]

OK so I'm showing how NEW I am to racing.....not being a FAN but DOING it. What does pushing the brake pads to the back mean and whats the Pros and cons of having rear brake DRUMS?

PS Yall are crazy....I can't believe how long this topic has morphed and stayed alive. Awesome!!

[dickita15]

Disc brake pads are always lightly touching on the rotors causing drag and using power. In normal operation they are only pushed back by the rotor in that the rotors are slightly out of round. In the Nascar qualifying situation they push the pads back so they do not touch and the driver does not touch the brake pedal until his run is over. By the way with that much pad movement it takes 2 pumps on the pedal to get brakes.
Drum brakes have springs that pull the shoes back so there is less drag. One of the down side of drums is it take more pedal movement to engage them so discs give a firmer faster pedal.

[joeg]

We might as well get into putting baby powder over the car for qualifying--better than wax-- 0-weight motor oil, ceramic wheel bearings, etc...

Keep the thread going!

[Colin Harmer]

Back to Aero....

It's always been my understanding that the tape was purely to keep the air moving up and over the car, and not get stalled or turbulent at the radiator opening. The teams tend to get very aggressive with whis at some tracks, as the engines can handle temperatures that would make us club guys wince! There's a few other contributing factors as well.

If you get more air over the car, you'll get more air on the rear spoiler which will in turn push the rear end of the car down. This will change the centre of pressure on the car (actually shift it forward a bit) and change the pitch angel of the car. This may give them the aero balance that they need in a qualifying lap, and also has a lot to do with why some cars run so well in traffic and not so well out front, as that aerp balance and center of pressure is critical.

Most teams are trying to run the right front springs in coil bind around the center of the corner, as this seals the valance off with the track and as such gets as much air as possible moving up and over the car, again improving aero.

Now here's where it get interesting....

I've had the oppertunity to attend a couple of private Nextel Cup tests this year, and the change in rake of the car makes a significant change in the cowl pressure for the intake. So much so that when I was in Kentucky, the engine team would reject the carbs if there was a sustained change of more that 1 degree in the cars rake angle. They could actulally see the change in the AFR on the Lambda data. Which of course is also affected by water temperature, which is affected by the amount of air going through the radiator....A circular statement...my favorite.

Every time I have had the oppertunity to be around the top teams in Nextel, I was absolutely amazed at the level of engineering that they bring to what many view as a "hay seed" style of racing.

More aero bits...keep them coming!

[Dyno]

At the risk of contributing to topic drift, I submit the following...

One of my acquaintances holds literally dozens of land speed records. OK -- it's on motorcycles, but aero is important there, too. Once, when visiting his shop, I asked if he could share a couple of his secrets.

The first thing he showed me was a photo of himself at 200+ at Bonneville. He pointed out that his leathers were a little baggy that day and appeared to be flapping in the breeze. He said he returned to the pits, wrapped duct tape around his chest and calves (that duct tape thing again!), and found the few mph he needed for another record.

At that point we turned to his bike, which was on a stand next to us. He took one finger and started the front wheel spinning very slowly -- about two revs per minute or so. I swear, that wheel did about 20 revolutions like that before it finally stopped. It was like watching the second hand on a Rolex. I don't what he did to acheive that, nor would he tell me.

[Greg Amy]

Simon McBeath has published a book compiling a lot of his aero articles from Race Car Engineering magazine. I'm 1/3 of the way through it (I keep getting distracted) but it's a very nice compromise of technical and everyday language (leans towards the technical.)

http://tinyurl.com/ek6a3

Don't know if this specific issue is addressed, but if it is I'll report back...

[88YB1]

Let me begin by saying I am not an engineer. Just a guy who holds a private pilots licence. As a student pilot you are taught why airplanes fly. Or in some cases don't fly. Airplanes fly by moving an airfoil (designed by an engineer) through an air mass. If you looked at the cross section of an airfoil you would see that the bottom is realative flat while the upper surface is curved. As this airfoil passes through the air some of the air goes under the wing and some goes over the top. Since the air going over the top of the wing must travel further to reach the trailing edge it slows down. This slower moving air creates a low pressure area. As long as the angle remains the same, you have lift. There is no way you can increase the air pressure therefore there is no upforce or downforce created. (hovercraft excepted) Only a low pressure area.
Now to apply this to a race car. Air that strikes the front of the car. Some goes over some under. The air that goes over the top will move slower than the air that goes under the car creating a low pressure area. Wings are used to break un the air flow going over the car and prevent the low pressure from developing. Secondly due to the proximity of the bottom of the car you actually build a pressure zone under the car. Air is being forced (compressed) under the car. That's why we use fairings and splitters on the front of our cars. To prevent some of that air from goining under the car. Now that we have done a pretty good job of minimizing air being forced under the car what do we do with the air being forced through the radiator. Since exhaust ducting (engine compartment vents) is not allowed we run it through the engine compartment then force it under the car. Now we have pressurized air under the car and with air traveling the longer distance over the car, we have a poorly designed airfoil. But, it still generates lift. How can we prevent this lift? We could put fences on the top of the car to disrupt air flow and kill the low pressure, but this would create drag and slow the car. This gets us back to removing the pressure (lift) under the car. Since we've already got this puppy as low to the ground as the rules allow and an air dam built to the limit, there is only one area left. The air coming through the radiator. Reduce this airflow to the max. I'm sure that the NASCAR team engineers have developed the air inlet to the very minimum to provide the necessary cooling for a four or five hundred lap event. During qualifing though they only run two laps at full speed. It is therefore possible to get by with less air flow through the radiator and still have the engine survive. So they reduce the size of the radiator opening which allows less air to pass throught the radiator and under the car. Net result is less lift producing what NASCAR calls downforce.

Chuck
ITA Pontiac Fiero with waaay too much lift under the car

[Greg Amy]

Some corrections, Chuck.


Since the air going over the top of the wing must travel further to reach the trailing edge it slows down. This slower moving air creates a low pressure area.[/b]

Since the air must go farther, it SPEEDS UP. When you increase the speed of the air, the pressure drops.

(Side note: that's also the basis for a carburetor: speed the air up through a venturi, pressure drops, it suck in gasoline from the jets.)


There is no way you can increase the air pressure therefore there is no upforce or downforce created.[/b]

That's not quite correct. In addition to the Bernoulli effect of the speeded-up air, there's also the force from angle of attack. AoA diverts the air downward, and Mr Newton assures us there's a corresponding upwards force (as well as induced drag backwards).

You'll find some CFIs that swear by Bernoulli, some that swear by AoA; the truth is that it is a combination of both.


(hovercraft excepted)[/b]

No need to except hovercraft; it's the same idea: delta P. Higher pressure underneath versus lower pressure underneath = a rise from the ground, up to the point where the forces are equal and the rise stabilizes.


Now to apply this to a race car.[/b]

That's a mixed-up jumble. Airflow over a car increases speed, and the pressure drops; thus, lift is created.

Airflow under a car slows down, thus pressure increases, and lift is created.

Airflow under a car also "packs up", further creating higher pressure underneath the car.

Airflow under a car is also subject to a myriad of turbulent device, causing significant drag.

Airdams, splitters, and trays are devices used to divert the airflow from going underneath the car, thus reducing static pressure under the car.

Wings are aerodynamic devices used to create lift/downforce through a mount.


We could put fences on the top of the car to disrupt air flow and kill the low pressure...[/b]

Longitudinal fences on the tops of cars are there to smooth the flow of air and attempt to keep it as laminar as possible and not spilling over the sides, thus reducing drag. Vortex generators (e.g., Mitsu Evo) are devices intended to create localized vortices to also allow the air to remain as laminar as possible without flow attachment.

Reverse spoilers on NASCARs are devices intended to detach the airflow and dump lift when the cars get bass-ackwards.

- GA, PPSEL/Instrument

[88YB1]

Thanks for the corrections. It's been decades since I've flown, and the memory is a bit rusty. The physics still work just stated wrong.

Chuck
No wonder my car is slow. I must be going backwards.

[Dyno]

Look... All you need to do is go to your EA Sports Formula One computer racing game, use the options menus to reduce the radiator size on the vehicle of your choice, and watch your lap times tumble!

What more could you possibly want to know?? :D

[Knestis]

I was afraid that Greg was going to do it and he did - beat me to the conversation about Bernoulli vs. Newton.

At some point we all get the "air goes faster over the top of the wing" thing but, as Greg suggests, there are a lot of aero guys out there who adhere to the idea that all you have to do is push the air down, to make the plane go up. (Or push the air UP to make the car stick DOWN.) After repeated conversations with a couple of these guys, who will tell you that the trick to getting good lift is controling laminar airflow (along surfaces) well enough to "turn it down" as much as possible, I've resolved the question in my head by thinking about the wing on a Fokker DrI differently than that on a F35 Lightning II, for example. Thick wing chord, low-speed situation goes to the Italian; the flat plate jammed through the air with jets, point for the Englishman.

Back in the IMSA GT days, you could tell which 935s or Prototypes were making the most downforce by who shot their roostertail highest when they ran in the wet. That is NOT about little nitrogen and oxygen molecules hurrying along the diffuser on the bottom of a car, trying to catch up with their dance partners from whom they were separated by the splitter!

Food for thought.

K

[Racefreak24]

What's the deal with the SCCA not allowing certain downforce enhancements? Well, beyond the front air dam if I'm not mistaken. Downforce seems to be fairly cheap (in comparison to other mods we make) with small wings, fender flare, ride height etc. Seems like it would be a big help at the low speed technical tracks such as NHIS or even a faster track such as Mid-Ohio where there are elevation changes and high brake points. I'm sure theres a good reason for it....right?

[Knestis]

If they were allowed - and proved to be advantageous - everyone would need them. Once everyone had them, the balance of power would essentially stabilize and all that would have been accomplished is that everyone went a little faster. That's not really the priority that the category was designed to pursue.

More subtley, different cars would react differently to more open aero rules. For example, any given airfoil mounted above the rear window on my Golf (a la the old F2 rally rules) would return a different benefit than one to the same rules mounted on the trunk of a Civic sedan.

Too much hassle. No real value for the category.

K

[Z3_GoCar]

I was afraid that Greg was going to do it and he did - beat me to the conversation about Bernoulli vs. Newton.

....
Food for thought.

K
[/b]

If I remember right Bernoulli was Swiss, and his equation only works for invicid (non turbulent) incompressible flow of a non-reacting, themodynamically ideal (non changing specific heat) fluid. In reality Newton and Bernoulli describe the same thing. Actually the governing equations are coverd by Navier-Stokes from which Bernoulli can be recovered after some simplifying assumptions. Civil engineers use Bernoulli almost exclusivly along with the Moody friction factor, Mechanical engineers only use Bernoulli as a very rough first assumption, but often embedded are much better solvers with different forms of Navier-Stokes simplifyed, as it's a non-linear differential equation. As far as non-linear differential equations go Navier-Stokes is a particularly tough nut to crack, so much so that there are graduate level Mathmatics courses taught on this one equation alone B) . Now, do we want to start on Eulerian referance frames verses LaGrangian?





If they were allowed - and proved to be advantageous - everyone would need them. Once everyone had them, the balance of power would essentially stabilize and all that would have been accomplished is that everyone went a little faster. That's not really the priority that the category was designed to pursue.

More subtley, different cars would react differently to more open aero rules. For example, any given airfoil mounted above the rear window on my Golf (a la the old F2 rally rules) would return a different benefit than one to the same rules mounted on the trunk of a Civic sedan.

Too much hassle. No real value for the category.

K
[/b]

What about is there any concern when the aero advantage turns into a liability, and cars start to go off course like large sheets of paper in a breeze? Ala, the IMSA prototype race at Road Atlanta back in the early 90's.

[Knestis]

...Mechanical engineers only use Bernoulli as a very rough first assumption, but often embedded are much better solvers with different forms of Navier-Stokes simplifyed, as it's a non-linear differential equation. ...[/b]

But that approach completely neglects the Areola-Spanakopita index of turgidity, well recognized as having an deleterious influence on the magnanimity of non-linear Navier-Stokes salutations. The only proxy-delamination opticient to the pentile clarendon, is the twixt-inductee caroming fork of the manic prong method. We see this appendication in prescient turd-swirlers - but you knew that. (See Nottybitz, 2004)

K

[Black91n/aRX-7]

Ok, this week's assignment is to read all of the articles found here:

http://www.advantage-cfd.co.uk/Newsletter/...r-Articles.html (http://www.advantage-cfd.co.uk/Newsletter/Newsletter-Articles.html)

You can thank me later :happy204:

[Racefreak24]

Well keeping to the AERO topic, and since I have such a knowledgeable group here can someone explain undertrays and other under car aero and it's effects. Is this equipmant you can buy or is it individual car adjustments such as muffler, brake pads. I know open wheel have an extensive underbody aero system.

[JeffYoung]

And the corollary to Kirk's post:

Beavis say: "The angle in the dangle is inversely proportional to the heat in the meat."

So there.

[Z3_GoCar]

.... We see this appendication in prescient turd-swirlers - but you knew that. (See Nottybitz, 2004)

K
[/b]

What is that like a temporally displaced toillet?? Kirk, I'm calling B.S. on that :P



Well keeping to the AERO topic, and since I have such a knowledgeable group here can someone explain undertrays and other under car aero and it's effects. Is this equipmant you can buy or is it individual car adjustments such as muffler, brake pads. I know open wheel have an extensive underbody aero system.
[/b]

An undertray is one large cover that keeps all the moving drag inducing elements out of the air flow on the bottom. Some of the first users of undertrays were the dry lakes racers. They'd reached the point that all the little details on top were taken care of, and were looking for places to cut drag even more. A later incarnation of the undertray used the channeled air flow to increase down-force, or at least to locate a low pressure zone so that the race-car is properly balanced. The problem with depending on an under-tray for down force is that one little bump and all of a sudden the down force you were depending on goes away. Also, with production based cars, that's a rather large area to cover, and it's often not allowed to cover it anyway per the rules. Formula cars can get away with an undertray as there's not much down there, or the rules alow it. My friends FC Van Deimon has a nice little carbon tray under the power plant that opens up into a diffuser. He's running VARA since there's no place for his car in SCCA anymore :unsure:

[lateapex911]

The IT ruleset kinda prohibits aero devices with the exception of airdams and factory equipped spoilers.



However, the airdam is free essentially, and as long as it fits within the prescribed limits, all is good. Splitters can be incorporated, and undertrays could be integral as well, but they need to terminate where prescribed by the airdam rule limit.



Some creative types might weld a large flat plate covering much of the underside to the muffler, and call it part of the exhaust system, which is free. That would result in some chuckles, some rolled eyes, and maybe a protest based on the "No device can perform a prohibited function".



Might make it by the stewards, might not...;)

[Greg Amy]

Sometime in the last year or so there was a topic on airdam spoilers and splitters. Started by Bettencourt, as I recall. In it I uploaded a PDF of Race Car Engineering articles on that very topic of spoiler, splitters, and undertrays. Use the search function and see fi you can find it...

[ddewhurst]

***But that approach completely neglects the Areola-Spanakopita index of turgidity, well recognized as having an deleterious influence on the magnanimity of non-linear Navier-Stokes salutations. The only proxy-delamination opticient to the pentile clarendon, is the twixt-inductee caroming fork of the manic prong method. We see this appendication in prescient turd-swirlers - but you knew that. (See Nottybitz, 2004)***

Forget this ^ BS, it's not approved Man Law. :rolleyes:

[evanwebb]

Just to throw in my $.02, (trying to remember my fluid dynamics from grad school...) I think that it is the case that airflow over real cars isn't laminar past the top of the windshield, if it gets that far. At that point (I think Greg? alluded to this mentioning the vortex generators on EVOs or WRXs or something?) the flow can either become turbulent or separated. These are two rather different states, and turbulent airflow is still considered "attached" and is much less draggy than separated flow. Vortex generators actually create turbulent flow by adding energy to the airstream. The combination of how much energy is required to create turbulent flow vs separate the flow, and how much bodywork is in contact with the turbulent or separated flow, results in how much drag is created. Reducing the amount of bodywork in contact with these flows reduces the amount of drag on the car, and the short chopped-off tail of the cars known as Kamm tails use this strategy (see the Cobra Daytona coupes of Peter Brock for example). Also, this is why golf balls are dimpled rather than round, the airflow over "real" surfaces in the real world doesn't stay laminar very long, and then it goes through a very quick transition to turbulent and then separated flow. The dimples actually act to cause the flow over the entire golf ball to be turbulent for as long as possible before transitioning to separated flow, hence the ball is less draggy and you can hit it further. Unless you are me, in which case you mostly don't hit it at all, or you hit a "worm burner" along the ground...

Just to throw in my $.02, (trying to remember my fluid dynamics from grad school...) I think that it is the case that airflow over real cars isn't laminar past the top of the windshield, if it gets that far. At that point (I think Greg? alluded to this mentioning the vortex generators on EVOs or WRXs or something?) the flow can either become turbulent or separated. These are two rather different states, and turbulent airflow is still considered "attached" and is much less draggy than separated flow. Vortex generators actually create turbulent flow by adding energy to the airstream. The combination of how much energy is required to create turbulent flow vs separate the flow, and how much bodywork is in contact with the turbulent or separated flow, results in how much drag is created. Reducing the amount of bodywork in contact with these flows reduces the amount of drag on the car, and the short chopped-off tail of the cars known as Kamm tails use this strategy (see the Cobra Daytona coupes of Peter Brock for example). Also, this is why golf balls are dimpled rather than round, the airflow over "real" surfaces in the real world doesn't stay laminar very long, and then it goes through a very quick transition to turbulent and then separated flow. The dimples actually act to cause the flow over the entire golf ball to be turbulent for as long as possible before transitioning to separated flow, hence the ball is less draggy and you can hit it further. Unless you are me, in which case you mostly don't hit it at all, or you hit a "worm burner" along the ground...

[nswanberg]

Any of you computational fluid dynamics experts ever seen any of the studies dealing with random surface imperfections and reduced drag on water vessels? I have wondered if there might be any high speed automotive applications. Only problem is our cars would not look all shinny and pretty.:(

NS

[lateapex911]

..... Only problem is our cars would not look all shinny and pretty.:(

NS
[/b]

I guess you've never been to an SCCA Prod car grid!

Kidding, kidding..

(just a little rejoinder to the Prod car guys who criticise IT for being a category of junkers...;) )

[Eagle7]

... Also, this is why golf balls are dimpled rather than round, the airflow over "real" surfaces in the real world doesn't stay laminar very long, and then it goes through a very quick transition to turbulent and then separated flow. The dimples actually act to cause the flow over the entire golf ball to be turbulent for as long as possible before transitioning to separated flow, hence the ball is less draggy and you can hit it further....
[/b]
So my orange peel paint job makes me faster! Cool. :happy204:

[Dyno]

So my orange peel paint job makes me faster! Cool. :happy204:
[/b]
My college fluid mechanics book had two interesting photos... The first was a side view of a bowling ball dropped into a glass tank of water from some unspecified height. The second photo was that same bowling ball dropped into the same tank after it had it's leading hemisphere coated with glued-on sand.

The wake created by the sand-coated ball was much smaller than that created by the smooth ball.

Orange peel is good!

[ddewhurst]

A gillion years ago Petty Enterprises put a layer on their race car roofs similar to the surface of a golf ball.

[Dyno]

A gillion years ago Petty Enterprises put a layer on their race car roofs similar to the surface of a golf ball.
[/b]
New product idea: Produce a helmet that looks like a golf ball. Sell to formula guys. Wouldn't they look cool?

[lateapex911]

Yea! AND whats cooler would be all the Callaway Big Bertha marks from confused duffers all over the helmet!

[Conover]

If you look underneath a lot of New cars you'll see dimples all over the bottom. The most recent one that I've seen is the MKV Jetta. Apparently they've got the airflow under that car figured out pretty well, because there is an automatic transmission cooler mounted horizontally level with the rest of the underside of the car.

And I didn't see any Turd Swirlers anywhere near there.

[sstecker]

If you look underneath a lot of New cars you'll see dimples all over the bottom. The most recent one that I've seen is the MKV Jetta. Apparently they've got the airflow under that car figured out pretty well, because there is an automatic transmission cooler mounted horizontally level with the rest of the underside of the car.

And I didn't see any Turd Swirlers anywhere near there.
[/b]

a bodyboard manufacturer came out with boards years ago with a dimpled bottom surface.

[Racefreak24]

OK here we go again.....why does nascar have the roof spoiler at Daytona and talladega...I know it slowes them down and keeps them in a bunch but what aero effects does it have? And also they have a vertical fin down the drivers side of the rear window at these tracks as well. Any input?

[lateapex911]

As far as I recall, the roof dam is all about drag and slowing the cars down.

The longitudinal fins on the rear window are part of the 'spin and lift abatement program'. I seem to remember they came into effect when all the cars started floating up when they got sideways to the wind, and that fin spoils the airflow and kills the lifting effect. The roof flaps that pop open when the roof sees the low pressure during a spin are there to create the same result.

[nswanberg]

New product idea: Produce a helmet that looks like a golf ball. Sell to formula guys. Wouldn't they look cool?
[/b]


I got to git me one of these helmets!!!!

I never heard of the Petty golf ball roof???? I will have to look that one up but it does sound like something Lee and Richard would have tried!!!

Go fast and trun left!