Aerodynamic Data for GT40s

Bob Childress · 02-01-07, 01:48 PM

In another thread “Frontal Protection in a '40” the discussion moved around to aerodynamics. With a tip of the hat to Ron who recently suggested we avoid mixing the topic of threads, I though I would ask for additional comments about GT40 aerodynamics here.

In that other thread I think Lynn makes a good observation about practical vs. theoretical aerodynamic GT40 issues. Likewise another thread named “Rear Spoiler” (imagine that) also reveals that despite its functionality, everyone thinks a rear spoiler is a very cool styling detail (and it is relatively easy to fabricate). Now heaven knows it is very difficult to actually measure performance differences even in highly controlled environments but I cannot tell if we really know enough to make any data based decisions. The data Lynn referenced was:

Lift/Drag for the MK II
100 lbs. @ 150 mph, with 525 lbs. of drag
144 lbs. @ 180 mph, with 756 lbs. of drag
177 lbs. @ 200 mph, with 933 lbs. of drag

That data is also quoted elsewhere (for example: Products - Aerodynamic Devices) but I have not been able to find more detailed information. Does anyone know the configuration of the car being tested. Were the measurements for the MKII as raced in 1966 with a 2” rear spoiler, no front winglets, and 3” of ground clearance? How are the lift numbers for the car divided front to rear?

DRAG
Most of our cars are MKI configuration with no spoiler, no snorkels, and only 1 air scoop per side. I would guess that the extra drag created by the MKII snorkels and scoops would be very significant. It seems like they are in the optimal position to create drag. And, because the frontal area of the scoops and snorkels is about equal to that of the exposed lip on the spoiler, one could argue that the drag created is greater than or equal to that of the spoiler itself. If drag on a MKII at 200 mph is 933 lbs, what might be the drag on a MKI at the same speed with no spoiler, no snorkels, and no extra scoops?

LIFT
Presumably the intent of the rear spoiler is to maintain the front to rear weight distribution of the car as the speed increases. Up to a point as speed increases does the air pressure on the front of the car press down as lower pressure at the rear creates lift? Is it generally believed that the spoiler cancels lift on the rear of the car so front to rear proportion stays constant?

Mike Drew · 02-03-07, 04:26 AM

Thought you might be interested in the graphs depicted here. They are taken from a 1970 edition of a book/magazine called Style Auto, which was an inside-the-industry publication in hardcover, exclusively for those in the auto manufacturing industry. This data is from an article they published on the then-new Pantera, and compares the wind-tunnel data from the Pantera, Boss 302 and GT40 (type unknown--presumably a Mk 1???)

J. Salmon · 02-03-07, 08:39 AM

It does say "racing GT40" and also references other issues.

Looks like a little forward tilt (lower front ride height than rear) would be a good rule-of-thumb set-up. Without wind tunnel data, I don't know of any other way to help balance the front and rear.

Or maybe...I wonder if a front splitter might actually work. I think if you are using a rear diffuser, you can start having problems by directing flow away from the diffuser. I would think with a 40, any possible increase on the front would be helpful, and there is no rear diffuser to confuse the issue.

Gary Gibbs · 02-03-07, 12:41 PM

Buy book by Joseph Katz. Great read.

Lynn · 02-03-07, 01:29 PM

Bob,

I think Graham's point was an argument against more spoiler in the rear for aesthetic reasons without considering its effect on handling. As I understood it, as more down force is added to the rear with more spoiler, the induced squat (which can also come from acceleration and be augmented by the rear suspension setup) will cancel out the "rake" (lower ride height in front, which was one of the aerodynamic tricks that John Weir received from Ford based on wind tunnel testing of the MK II, along with all the goodies (his words) that he received when Ford discontinued their racing efforts) mentioned by J.Salmon. So, while the adjustable spoiler may look cool, if the rear suspension isn't configured to handle the increased down force over the rear axle, the end result may be a poorer handling car.

A front splitter would, no doubt, help maintain the initial rake setting and the increased down force over the front axle that is the reason for the rake to begin with. (While the data shows a net down force for the whole car at speed, there is lift over the front axle albeit less than the down force over the rear axle.) Even with a splitter, I would be inclined to think that there is enough air coming back under the car to make contolling that air with a diffuser a productive exercise. I would be very interested in data about diffusers as compared to spoilers; like the down force/drag ratio, the extraction of air from the engine compartment, the amount of "draft" provided to a following car, etc. Intuitively, if feels like a diffuser might have more upside, but I have no evidence to base that on. Unfortunately, there is apparently precious little aerodynamic data on the MK I at all, let alone, for different configurations.

Bob Childress · 02-09-07, 05:19 PM

The Quest for Automotive Holy Grail
GT40 Aerodynamic Information

It would be hard to imagine what technical information would be more interesting and maybe more useful to all the current GT40 fans than real scientific aerodynamic test result of the GT40. Don’t you think it is strange that GT40 aerodynamic information is not available somewhere either from the original cast of GT40 drivers, owners, designers, crews, etc. or from at least from the personal collection and library of some passionate GT40 car buff. Wouldn't it be interesting to know all the details!

To help find this information we need help from every forum member and from your personal network of other car experts and historians. We are looking for a hardback trade publication called “Style Auto,” specifically English language volumes 20, 22, and 23.

A one-page flyer { Search for GT40 Aerodynamics ( GT40 Aerodynamics in Style Auto.JPG )} has been uploaded to the Gallery section to help track down Style Auto Volumes 20, 22, and 23 which may have the real controlled test data about GT40 aerodynamics. Please copy the file and forward the flyer to anyone who you think might help in this search. As the search continues we may find other references and add those to the list of search publications.

What do you think? Can we make this a worldwide group project?

Bob

Canuk40 · 02-09-07, 05:59 PM

Hi J.

I've always recommended a .250 wedge in the car (nose down) when setting ride height. The last CAV we had on the scales for setup had 1172 lbs on the front wheels and 1527 lbs on the rear, full up - driver passenger and gas.

So 120kg or 265lbs would lift the front end by less than a quarter inch. However if you started out nose up or got light over a crest and caught air under the nose at those speeds, you'd get a ton of lift.

Anyways, the more information out there the better. Seems like the later single outlet nose (68'69) worked well enough at Le Mans that the side chin spoilers weren't used (too much drag) where close to 330kph was acheived.

Cheers

Canuk40 · 02-09-07, 06:16 PM

Hi Bob,

Just had a quick look at you drag figures for the Mk11. Looks like 525lbs vs 390 lbs for a (presumably) Mk1 at about 150mph. Only a casual observation, seemslike the Mk1 has 30% less drag than the Mk11. They needed those 7 litres!

The Mk1bs with Gulf wide fenders and 14" wide rear tires (extra frontal area) still managed over 210 mph on 425hp. That wasn't the aerodynamic brick wall either, just no more strait to accelerate on

On the topic of drag, I read somwhere a current F-1 car will generate 1g in braking at 200mph just by lifting off the gas. You could say then that the drag has to equal the weight of the car, what's that 1200 > 1300 lbs with driver? Wow

I know from going faster than I'll admit in the CAV GT40, it is directionally stable at speed. If front lift was eliminated without moving the areodynamic pressure center forward then it would be even better.

Cheers

ChrisL · 02-09-07, 08:20 PM

The figures on the graph surprise me. I know it’s been said before on this forum that the rear end of he GT40 actually produces aerodynamic down forces but I have to admit that I have been a little skeptical. But I would be hard pressed to argue against this evidence.

However, I would still expect that the body shape on its own can only develop lift (the rear spoiler reducing the amount of lift) and therefore the only principle that can produce a down force in the case of these cars would be the ground effects created be the wedge stance. If I'm right about this (opinions please) then the undertray shape/length is critical when attempting to reproduce these figures on our replicas.

Does anyone have picture of the undertray of an original GT40, specifically the rear?

ChrisL · 02-09-07, 08:30 PM

Quote:

Originally Posted by Canuk40

... it is directionally stable at speed.

I'll second that. I had my car to 230kmh last weekend and it was very stable. Even under hard braking down hill at those speeds, the back wiggled only a little until I got down to about 160mkh (a guess) where it then felt planted again.

BenL · 02-09-07, 11:41 PM

Quote:

Originally Posted by chrisl

Does anyone have picture of the undertray of an original GT40, specifically the rear?

Take a look at the picture of the underside of an original GT40 at: http://www.racingicons.com/gt/challenge.htm

Canuk40 · 02-09-07, 11:46 PM

Hi Chris,

I won't admit to 300kph but it felt great

Ian

Bob Childress · 02-10-07, 12:22 AM

Addendum - Volume 22 has been found and is on order. Yea!!

A one-page high resolution flyer is in the Picture Gallery section to download and send to your contacts. Help track down Style Auto Volumes 20 and 23 which may have the real controlled test data about GT40 aerodynamics. See Gallery { Search for GT40 Aerodynamics ( GT40 Aerodynamics in Style Auto.JPG )}

Russ Noble · 02-10-07, 01:14 AM

Quote:

Originally Posted by bchildress

A one-page high resolution flyer is in the Picture Gallery section to download and send to your contacts. Help track down Style Auto Volumes 20 and 23 which may have the real controlled test data about GT40 aerodynamics. See Gallery { Search for GT40 Aerodynamics ( GT40 Aerodynamics in Style Auto.JPG )}

Well I would if I could!

Anyone want to instruct me on downloading from the gallery? Or point me to an existing thread that covers this? I tried a search on "download" but couldn't find the info.

Cheers

Steve Briscoe · 02-10-07, 02:03 PM

Quote:

Originally Posted by bchildress

What do you think? Can we make this a worldwide group project?

Bob

I hope so. Current data on the downforce with various options will be very useful. Especially above 150MPH. In that event, the data can be trusted and the driver can think about braking, shifting, acceleration and the other cars around them versus whether or not the front end is going to lift. There was a related article in the March issue of Hot Rod. It's called "Wind Camp". Bob Eaker has a business in Mooresville, North Carolina that will wind tunnel test cars. Bob is a former senior project engineer from GM's Advanced Aero Group and was later the aerodynamacist for Hendrick Motorsports. The wind tunnel is powered by four fans that suck the air past the car versus blowing. It costs $345 an hour for the first two hours and $490 an hour after that. Various downforce measurements are taken at points on the car. Wings, dams, and other aerodynamic devices can be added and subtracted for different readings. After testing, a data sheet is provided showing downforce with the car's different configurations. If I was in North Carolina, I would be scheduling my car. I don't want to speculate on what happens at speed. Maybe one of our forum members has an interest in this and can schedule an appointment to see the tunnel or even test their car and share it with us. Bob's website is A2 Wind Tunnel. The article in Hot Rod is very impressive. If you have it, I would recommend reading it.
Steve

Bill Hara · 02-10-07, 04:57 PM

Bob

Have a look at this:

eBay: 6 1960's STYLE AUTO BOOK 1969 HC automobile design (item 140083055338 end time Feb-10-07 19:41:42 PST)

I think it contains number 20 and 23.

Ron Earp · 02-11-07, 07:05 AM

Quote:

Originally Posted by 60-61

There was a related article in the March issue of Hot Rod. It's called "Wind Camp". Bob Eaker has a business in Mooresville, North Carolina that will wind tunnel test cars.

I heard about this from a friend last week but had not seen the issue. The prices seem reasonable and I think it might be worth the money for sure before the Lola goes out for racing. It'd certainly ease my mind a bit and give me some "aero" confidence. If someone decides to take their 40 down I'd like to come along as well. I'm in NC, as are Lynn and a few other fellows nearby in SC/TN, and it'd be a good gathering.