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Discussion Starter · #1 ·
Had a new sensation this Fall when coming up the Esses at Watkins Glen. New engine combo has the car much faster. When arcing out onto the back straight, the car had some instability coming from the back at roughly 135MPH full throttle. I do have a front spoiler and small "winglets" on the front corners, and a small diffuser under the car in back.

I have checked the rear suspension carefully, all mounts, brackets, components are good, nothing broken or loose. Even had just corner-weighted the car. All seems mechanically good. Never had this before, though also never carried this much speed up the Esses.

Could I have an imbalance, so that the front is pushing down too much?
 
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Remember when Audi added the little pop up spoiler to the TT because over a certain speed the back end got light and caused a couple of high speed accidents? Similarly some drag cars have what looks like a flat extension of the trunk lid to reduce rear lift at speed. Perhaps you are experiencing a similar thing. Maybe a small lip spoiler across the trunk lid or something similar might help? Or perhaps your diffuser is 'stalling' at the higher speeds? Might be tough to diagnose without access to a wind tunnel.
 

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I think Shelby had Cobras way above 135 w/o problems.......
For what its worth I had the same problem with my 427 1968 Vet at 140. .I solved it by keeping it below 140.
Thinking its ALL about aerodynamics .
 

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I had the problem with the front of the car at 140 down the main straight at Road America. I have the chin spoiler, hood vents and the side ones but couldn't avoid the front lifting - I decided not to be "that guy" and kept it under 140. No issues with the back but I am running the 5-link set-up, no sure if that matters. Please check your shock tops for tightness (that will make your rear go squirrely - been there done that). BTW, the Glen is a great track but unfortunately my FFR was in upgrade mode when the track date came up, ended running my BRZ - I have a lot of admiration of those 2.0L guys that through their cars around to keep speed.
 
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They all get weird in the 135/140MPH range. You might try to get a look at one of Shelby's real race cars to see what was done to them to keep them on the ground. Cobra #198 driven by Dick Smith was allegedly clocked at 198mph at Daytona. Centrifugal force could possibly by itself hold the car down in the turns but not on the straight sections.

Seems like some of the Cobra racers are running wings.
Wheel Tire Car Land vehicle Vehicle
 

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Discussion Starter · #9 ·
Read the article, though they give minimal details about the undercar spoiler. I'm wondering how similar it is to my diffuser?

No Wings! I hate the look on a Cobra, even though they do work. Only aero device up in the Windstream to work.

Yes, I do think the place where I feel it can also be attributed to centrifugal force. I feel this rotation as I am coming out of the Esses under full power, so the car does have some rotation going on as I am turning right. Could also possibly give some credit to short wheelbase, so replicating a short pendulum of force? Obviously with a longer wheelbase, this would not be as easily felt.
 
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A Cobra (back in the mid 60s) was responsible for England having a "posted speed limit".Read 1/2 down this page.Car was clocked at 185.
Maybe someone has a picture of this Cobra to see IF it had wings............
No wing just a whole different body. The tail extension was a type of aero at that time

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Jack Sears 185MPH AC GT Was not a Shelby car but a competitor. Don't believe the car was actually clocked but the math was run base on a claimed 6500RPM in top gear.
 

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Discussion Starter · #12 ·
Well.....

Since I have a full windshield, the only easy aero add is a wing, though not my style. But it would push the rear of the car down.

But, I have another item in my bag of tricks, side skirts underneath. I raised them up for access, maybe if I again make them longer and lower to help create more of a low pressure area under the car? I could fabricate some sort of external jack bar that can be slipped into a tube attached to the chassis for jacking up the car from the side with a floor hack.
 

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Discussion Starter · #16 · (Edited)
OK, big diffuser. Mine is half that size. That will definitely help keep the back down.

As well, mine is higher up in the body. I'm thinking the diffuser needs to have some specific distance between the roadway and the vanes?

Maybe I can Have one to mount like the one in the pic for high-speed track days? Or extend my vertical elements closer to the ground? Doing some research on diffusers, I am thinking mine needs some "tweaking".

Some threads with my design:
 
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Chepsk8,
Thanks for posting this thread. I really like the idea of adding aero and a rear diffuser but I've not yet taken the plunge. I even purchased a whole sheet of aluminum to cover the underside of the chassis, which I have yet to do. I have experienced what you describe in the kink at RA, although not on the straight where I have pushed past one-fifty. I now think it's attributable to the balance of the car left to right. With me in the driver seat and no passenger the car is biased to the left. The thought of adding weight to balance the car isn't my favorite choice, so I plan to move some weight around and minimize what is added. to the right front. How well your car balanced may affect your performance in the turns, especially at high speed.
 

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Looking at your pics and the Superformance pics you need to have your diffuser as an independent unit. If you look at the pics their diffuser actually looks like an inverted wing. There is a space between the diffuser and the underbelly of the car. Yours is the underbelly of the car. If you fabricate a frame for your diffuser and mount it so the front section opening could adjust from closed against your underbelly to say a 3in opening and have the rear mount spaced off under belly about 2in +/- as shown in their pics you would have a functioning rear wing. This way you would have an adjustable wing under the rear of your car out of sight. Rear view Side view
 

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Had a new sensation this Fall when coming up the Esses at Watkins Glen. New engine combo has the car much faster. When arcing out onto the back straight, the car had some instability coming from the back at roughly 135MPH full throttle. I do have a front spoiler and small "winglets" on the front corners, and a small diffuser under the car in back.

I have checked the rear suspension carefully, all mounts, brackets, components are good, nothing broken or loose. Even had just corner-weighted the car. All seems mechanically good. Never had this before, though also never carried this much speed up the Esses.

Could I have an imbalance, so that the front is pushing down too much?
It will be interesting to find out how this works out for you :) Please keep us advised .
 
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