Don Terrill’s Speed-Talk

Specifically 3rd gen Camaro [which is what I am building] [Ed once said we are all most interested in what applies to our cars and in general he is correct] especially the drag caused by the rear of body shape. What shape or spoiler can help?

Or, since it is 1/8 mile bracket racing, is it a non-issue?

To me, it seems it is important because the last 1/2 of the track is over 80 MPH. Also, if you are racing into a headwind the drag will be increased.

Any feedback?

Don't forget the subtle changes on the underside of the chassis that can make difference too. If you look under a lot of the newer cars, there are little winglets that direct airflow to and from components to not only aid cooling, but also reduce drag.
Just from the top of my head, I had a cheap Skoda from the Volkswagen-Audi Group that was really well thought out aero wise under the chassis. When the beancounters allow this for production of their cheapest, there must be something to it.

I would consider trying out a flat belly pan, covering most of the undergcarriage before adding a rear cone or such drastic aero mods. Closing up the front end and lowering the car as you can within the weight transfer needed, helps.
The bellypan can be made from a sheet of plastic and held up by a beams of thin channel bent aluminum. Just be sure to have it overlap from the front so it doesn't balloon on you.

Automakers are now looking at ways to rid the cars of side mirrors, using cameras and computers instead, because it makes sense for mileage.

Here's a good thread: viewtopic.php?f=1&t=40750

First, in this context I define a rear spoiler as an aero device that actually "spoils' air flow, and that disruption causes a local air pressure increase upstream, acting normally on a flat/slightly sloping horizontal surface, with the main intended result, greater DF. The biggest downside is increased drag. Again IMO a true spoiler has air movement only on one side vs a "wing", with air movement on upper and lower surfaces. A "draggy" wing can also give some "spoiler" type results.

A poorly utilized "spoiler" can reduce drag, by allowing turbulent air downstream of the spoiler to reattach/recombine easier with air stream. An example would be a very laid over spoiler that can almost be flat ( that spoils little and has little DF) that extends past the cars (bluff) body, I call that a "Lip" and one side of a "box Cavity", another feature known to reduce aero drag.

I don't think you'll find any measurable benefit, especially considering that you're running brackets. A few decades ago streamlining was explored by a number of Top Fuel teams. They found that the few extra pounds of bodywork more than cancelled out any potential gain.
https://www.hotrod.com/articles/drag-ra ... dragsters/

Dragsinger wrote: ↑Fri Aug 07, 2020 10:07 am Specifically 3rd gen Camaro [which is what I am building] [Ed once said we are all most interested in what applies to our cars and in general he is correct] especially the drag caused by the rear of body shape. What shape or spoiler can help?

Or, since it is 1/8 mile bracket racing, is it a non-issue?

To me, it seems it is important because the last 1/2 of the track is over 80 MPH. Also, if you are racing into a headwind the drag will be increased.

Any feedback?

A data logger that can capture coast down data, (speed vs time), will permit the calculation of drag forces, and CdA,
as well as rolling resistance.

Knowing the frontal area (A) will permit the determination of Cd. But if there is no change in frontal
area, the CdA number can be used. Remember, it is the total drag force that is important to know.
Also, air density (temp & pressure) will have a measurable effect.

Given: Frontal area = 26 sqft, Cd = 0.45;
at 75 mph, drag power loss = 33 hp
at 100 mph, drag power loss = 78 hp
at 125 mph, drag power loss = 152 hp

MadBill wrote: ↑Thu Aug 27, 2020 10:26 pm I don't think you'll find any measurable benefit, especially considering that you're running brackets. A few decades ago streamlining was explored by a number of Top Fuel teams. They found that the few extra pounds of bodywork more than cancelled out any potential gain.
https://www.hotrod.com/articles/drag-ra ... dragsters/

The Hot Rod magazine article is a very interesting read. The drag racing pioneers certainly had imagination and
creative designs; some even worked. Seems like everyone was in hot pursuit of slick bodies...automotive mostly.

You raise some very interesting issues. A streamline shape that reduces drag is beneficial if power is limited.
But if sufficient power is available, such that we become grip limited, then downforce (even with a drag penalty) becomes more important. Often we find tradeoffs between lift and drag (L/D ratios) forces. And those L/D ratios
can vary considerably.

A Formula One race car can have a drag coefficient (Cd) over 1.2, which is a lot of drag, but has the downforce necessary
to avoid wheel slip. And enough power so that the drag penalty is obviated.

So we are left with several interacting variables: power, weight, tire grip, aero drag, and downforce.

For a given vehicle and purpose, which combination of variables would be appropriate?

It's not uncommon for a car's optimum qualifying setup to be high downforce, but needing to be 'trimmed out' for the race, since the former's high drag leaves the car a sitting duck in traffic on a long straight.

MadBill wrote: ↑Wed Nov 25, 2020 12:23 am It's not uncommon for a car's optimum qualifying setup to be high downforce, but needing to be 'trimmed out' for the race, since the former's high drag leaves the car a sitting duck in traffic on a long straight.

Quite right. But then we try hard to increase downforce while also reducing drag. They are not always mutual opposites
as indicated by wide L/D ratios often seen. Well, not seen but measured.

You can't compare Formula cars to enclosed body drag cars. There is a world of difference.

Lap times on a road or oval circuit are not the same as a 1/4 mile ET. On a road circuit, balance and down force are primary. A example is, when I raced in Formula Atlantic, we ran in the same race as Formula 5000 (1000 lb / 200 HP vs 1500 lb / 490 HP). The 490 hp cars went 180 mph, the 200 hp went 150. Because of the wing angle the F5000 had to use and their wider/taller rear tires, to corner, it knocked their max speed down. The FA cars cornered faster with less wing, because less wt is less lateral force to control. Drag slowed the F5000 so that they only had 20% more speed, with 2.5 times the HP but, they had more acceleration with the higher hp/wt ratio (3/1 vs 5/1). At Riverside there are 2 slow corners, 6 & 7 both ~60 mph, the rest of the track, was accelerating to top speed or 100+ mph but, for the top lap times, there was but 7 sec difference, between them, in 2.7 mi.

It takes about 4 times the HP to double your max speed, with the same drag and in 1/4 mi racing, unless you trap speed is above 120, I don't think there is much to aero for enclosed body cars and a spoiler, isn't going to help your ET as your speed is way to slow while hooking the tires. If it does anything, it will help keep the front up, at speed, not good for drag! Most, 1/4 mi drag cars, don't reach their max aero speed, in that distance, if they did, lots of aero things would apply. In a 1 mi drag race they would apply, too.

That said, attention to detail is always good, if it is in the right direction and a front spoiler could be of slight benefit if, it doesn't create to much drag.

But, in drag racing, it is hp to wt, hp to wt, hp to wt and getting the wt to the drive wheels, with enough traction. At higher speeds, aero can increase your trap speed but, won't do much for ET. It's advantage, comes to late. It means much, much more, in circuit racing and can be the winning edge.

frnkeore wrote: ↑Tue Jan 26, 2021 4:32 am It takes about 4 times the HP to double your max speed, with the same drag

4 times the tractive effort, 8 times the HP?

I'm talking about aerodynamic drag.

i.e. If you can attain 100 mph, with 150 hp, with the same amount of drag, it will take about 600 hp, to attain 200 mph.

VW published a SAE paper about passenger car aerodynamics a ways back.

Their charts and tables instantly chopped my confidence in the "more downforce = more drag" bromide.

Not altogether surprising since driving down the road is heavily into "bluff body" aerodynamics and not so much airfoils and etc.

frnkeore wrote: ↑Wed Jan 27, 2021 8:10 pm I'm talking about aerodynamic drag.

i.e. If you can attain 100 mph, with 150 hp, with the same amount of drag, it will take about 600 hp, to attain 200 mph.

The aero package listed above will require 1200 hp to attain 200 mph; 600 hp will only attain 158.8 mph.

As I learned t Chaffey College, a rough constant for cars is 2/3 of total drag of car, is nose to base of windshield. Consider if you clean-up your aero package, will gain some high speed stability,( traction not withstanding).

Dragsinger wrote: ↑Fri Aug 07, 2020 10:07 am Specifically 3rd gen Camaro [which is what I am building] [Ed once said we are all most interested in what applies to our cars and in general he is correct] especially the drag caused by the rear of body shape. What shape or spoiler can help?

Or, since it is 1/8 mile bracket racing, is it a non-issue?

To me, it seems it is important because the last 1/2 of the track is over 80 MPH. Also, if you are racing into a headwind the drag will be increased.

Any feedback?

Larry, look at a PS car→ no grille openings, low ride height, and a rear deck extension.
The radiator and air wandering around under the hood is drag. The pressure gradient at the rear of the car due to the abrupt end of the body creates a huge "wake" which is drag. Lowering the body reduces frontal area a little and keeps some air out of the underside which is another source of drag. Address all three and you'll have incremental changes that will work but be hard to measure without real good baseline data. For driving into the wind, you need to keep good records and record what headwind speed costs over .006-.01 ET so you can dial for it. My brother's car is about hundredth slower into a 7mph headwind in the 1/8 mile at 140-141mph.