NASCAR technical question

Greg, isn't the "downforce" a result of all factors? Not just the removal of internal and underbody drag/lift?[/b]
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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]
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I'd be real surprised if it actually created much vertical force. But again, I don't know that for a fact.
 
***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.
 
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.
 
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.
 
...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.
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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
 
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?
 
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!!
 
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.
 
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!
 
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!
 
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.
 
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...
 
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
 
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]
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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]
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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]
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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]
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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]
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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
 
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.
 
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
 
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
 
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?
 
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
 
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
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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
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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.
 
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