Aerodynamics

[Bill D]

In the September issue of Hot Rod, they talk about the GT40 and it's aerodynamics. From the magazine, I quote:

[ QUOTE ]
When Ford decided to resurrect the classic '60's GT40 as a limited-production high-end sports car, Ford SVT development engineers borrowed an original from a museum, configured it like the '69 Le Mans winner, and tested the vehicle in Ford's high-tech wind tunnel. Although considered an effective race car in the '60's, the original had, in the words of Ford aerodynamicist Kent Harrison, "an incredible amount of front-end lift."

[/ QUOTE ]

The article went on to say the new GT has a rear diffuser, a front splitter, and horizontal side splitters to keep it stable at speed.

Considering this, how safe are these replicas at 150+ MPH?

Bill D

 

[Ian Clark]

Hi Bill,

Don't you just wish you could get one of these guys to sit down and lay ALL the facts on the table.

Since non of the replicas use the pre '66 nose and all have the rear spoiler, I seriously doubt there's any problem with high speed areodynamic stability in any decent replica. The "standard" GT40 shape we're all familiar with probably has some high speed lift, perhaps 10% of the total weight, nothing like lifting the front wheels off the track which the earliest prototypes reportedly did.

Of course today you can't build a supercar without "downforce", or 15" wheels for that matter. Ad variable assist power steering, ABS, adaptive suspension, traction control and so on, what do you get? a car that gets from A>B looking good but with no soul.

I can say from experience that the CAV is rock stable well over 175mph and the car lets you know whats going on as well has how good you are. These cars require finesse and concentration to drive quickly, try to muscle one around and it will make you look a fool.

Anyone planning to go really fast in a home built GT40 replica should have the whole car gone over buy a competent race car shop first and pay attention to the details.

There's a lot more to be gained in spring/shock rates, ride height, alignment, weight distribution and tire pressures than worrying about the body shape.

Maybe someone should ask Jackie Ickx how unstable 1075 was, he ought to know...

Cheers

 

[Bill D]

Thanks Ian

 

[ChrisL]

That particular comment had been deliberately used extensively in all the Ford GT promotional write ups and you can bet that there is always some commercial license taken whenever promotions are involved. Sure there was lift in the front of that GT40, but what might be "an incredible amount of front-end lift" by 2005 supercar standards can still be incredibly good aerodynamics by any other standard.

My opinion? Given the closeness in appearance between the 69 and 05 cars, I think that Ford were just protecting themselves against the inevitable criticism that Ford had made no gains in 40 years, and it worked. There was no such comment made by any critic.

 

[Bill D]

Chris-

My concern is with the replicas we are all building. Not with the Ford GT. Are our cars safe and stable at speeds above 150 MPH?

 

[Ron Earp]

Why not email Roy Smart and Paul Thompson? Those two are the only ones I know of who have driven their GT40s in confirmed and monitored trials at those speeds on numerous occasions.

 

[Howard Jones]

Now you are asking a question that has two answers. "Above 150 MPH" on the street or for that matter on a empty mile or two of freeway is one question and a long enough racetrack is another.

I have had mine to 5000 revs in 5th a few times and it felt fine. This is aprox 140+++ish. Frankly my Audi A4 is easier to drive that fast. BUT you run out of room VERY quickly at that speed.

There are very few tracks out here west of the rockies that have straights that will allow top speeds near 170+.

So I would say that my GT40 is safe to drive at or near it's limmit given the road condidtions. The last thing to worry about is front end lift. At least at the speeds it can be safely driven. The main concerns are the condition of the road, little bumps at 70mph become JUMPS!!!! at 140, and the QUALITY OF YOUR WORK. Will something important fall off at the wrong time? This is something we all tend to learn the hard way. TIRES!!!!!!!!!!!

Driving cars at 150MPH is very much like flying airplains at very LOW alitudes. Little mistakes have big results.

We all must think very carefully about speeds "above 150mph" Please be careful my friends. I would really hate to read about one of our little familys "big one"

 

[Bill D]

Howard-

I have no intention of driving that fast on a public highway. Willow Springs near Lancaster is only 1 hour from my house and has a long straight. You can get to 150MPH very easily with a strong engine. I would hate to become airborne.

Ron-
Thanks for the tip. I'll forward this thread to them and elicit their comments.

Thanks to all
Bill D

 

[wpeter]

As I recall the numbers for the MK1's were about 250 lbs. of lift at 180 or 200 mph. Considering the front end of a car weighs around 800 lbs it is obviously not enough to pick it up off the ground even at 200MPH. The problem is the dynamics of the suspension and the surface the car is traveling on. Remember a few years ago when the Group C Mercedes flipped over, and they had gobs of down force. It had to do with the car pitching over bumps in the track as well as the track dropping off at the same point. Before 1970 pretty much all race cars had a fair amount of lift. It was just something you got used to. Kind of like power steering.
To get back to the point, replica and original GT40s have pretty much the same flat bottom and the bodies are virtually identical. I would expect the lift numbers to be the same.
Bill

 

[Lynn Larsen]

According to Lee Holman, in addition to a treasure trove of go fast hard parts, one of the most important things Ford passed on to John Weir after they killed factory support for European sports car racing after the FIA effectively outlawed the big block, was a wealth of wind tunnel data. According to Lee, one of the biggies was a set of ride height tips & tricks that allowed the Mk I to be far more civilized in the 175+ mph range, in which it had proven to be somewhat of a handful even after the nose had been changed. I believe the adjustable rear spoiler and the vents to relieve rear wheel well pressures also came out of this data. And, no doubt, the parameters that Ian mentions were addressed as well. Lee did not quantify any of this information for me, but, on the other hand, I didn't ask him to either.

So, Bill, from everything I have learned, the Mk I, in stock form is OK in the 150-175mph range. Could it be better? Undoubtedly, and chin splitters like Mark LaVea makes and as seen on the RF in recent posts by Hersh and Robert would contribute to a better handling car at the upper boundaries of its capabilities; as would an effective diffuser. In addition to other mods, brakes being the most obvious, I think it would be prudent to improve the car aerodynamically if one truly intends to attempt speeds beyond 175mph. Further, I think the improvements should be undertaken in, at the very least, consultation with people who know and understand how cars behave at these speeds. And, the effectiveness of the improvements should be tested and verified by equally knowledgabe people. If a given GT40 owner has experience driving at the limits, well and good; otherwise, be honest with yourself about your capabilities and let someone do the testing who understands the cause & effect relationships of the modifications being undertaken.

Regards,
Lynn

 

[Sandy]

Bill -

I have been on the big track at Willo a few times with the 65 mustang and you can definitly wind it up where it get a bit scary towards the end where it is a game of 'chicken' to see how far you can stay in it. The back of the fglass hood lifts off the car an inch or so which adds to the scare factor. I can't imagine a GT40 having worse aero then the Mustang (open front, no front spoiler, nice tapered back for lift).

Hey, nothing a 90lb bag of cement couldn't fix /ubbthreads/images/graemlins/smile.gif

Don't forget the GT40 is a car from the early 60's and was probabally pretty good at that time.

Bill - Don't know if you have seen some of the threads on single vs. double snorkle hoods, that that is a factor too (May have been you that started the thread, can't remember)

 

[Bill D]

Thanks Lynn and Sandy. I'm leaning towards adding an adjustable rear spolier extension as seen on some of the original cars. I will talk with Mark about a chin spoiler.

I also noticed that some of the original cars had canards on the front fenders. I assume they were added to improve downforce on the car. They look funny, but probably were effective.

Regards
Bill D

 

[Bill D]

I got this picture from the restoration website for 1076. As you can see, they added a rear spoiler and some holes to aid evacuation of air from the rear. I will look into the feasibility of doing the same on my car.

Bill D

 

[Ron Earp]

Bill, I think you'll find this discussed on this site here:

http://www.gt40s.com/ubbthreads/showflat.php/Cat/0/Number/64359/Main/63260/#Post64359

Ron

 

[Sandy]

The rear spoiler is one thing I keep kicking around in my mind, but I like the way the car looks without it. It is something that is easy to add a bit later if needed. I think another thread has some discussion of the vented rears as well. Not going to worry that much about it until I get the car and see how she goes.

Sandy

 

[Bill D]

Thanks Ron. All this is very helpful

 

[Mitch D]

I like the fact that the aluminum spoiler is adjustable - in the above picture of restored 1076 it looks like it can be lowered a good one-inch from its current position. However, then it looks like you cover half of the pressure relieving vent holes. Seems odd that they didn't make the vent holes oversized to accommodate the full range of adjustment.

I requested my RF has the vent holes, so someday after she arrives perhaps I'll make such a spoiler as a DIY project too.

Mitch D

 

[John W]

According to my books they would adjust the spoiler to suit different tracks, just the same as they would adjust ride height, spring rates, anti-roll bar settings, etc.

I would guess that if they could afford to drop the spoiler that the effect from air in the rear arches would be less, so not so much of a problem if the holes were covered up a bit...

 

[Paul Thompson 'Hooligan']

Hi Bill

Roy and I have run his Mk1 GTD to 175+ on a few occasions and in general it felt fairly stable. This was done both with and without a front splitter and both in the dry and when raining fairly hard. (that was interesting!)

The car was always fine to 150Mph, approaching 170 the car was a little 'light' of feedback at the front but was marginally better with the spiltter than without. I suspect at faster speeds, the spiltter would have made a more notable difference.

One thing that was a little unnerving, was the tendancy to 'veer' in strong cross winds. You'd correct to allow for the wind from one side (load the steering slightly) and once it stopped, the tendancy was to veer the other way...
A bit strange at first but the trick was to ensure you don't over-correct, being as gentle with the corrections as possible.

We run larger diameter rear road tyres than the slicks, so there is a tendancy for the car to have a 'nose-down' atitude when on the 'road tyres'. This seems safer and presumably reduces the possibility of the wind getting under the main chassis flat area. I would think that at 170+, getting much 'nose-up' would be bad news indeed, generating enough lift to really spoil your day! - just my opinion as I have not done any maths on it....

One thing though, Roy and Ray Christopher did some experimenting with a 10th scale model and some DIY spoilers etc. Roy made a windtunnel comprising a 3-phase fan of some 10KW or more and fitted it with electronic weighing scales, drawing air across the model and making measurements from the scales. OK till the model took off and entered the fan blades..... One 'shredded' 40! - doh!

 

[Lynn Larsen]

Bill,

From what I have been told, the dive planes on the front of the car didn't really do much at all, but were better than nothing.

If someone knows otherwise, please us know.

Regards,
Lynn