Car gets unstable around 100mph

[CrazyCobra]

I ment to mention your ride, sweet , very clean, love the white cobra's, i'm runnin on boring old chromies .

I'm convinced my problem (same as yours) is bumpsteer, how much lower is your car than stock? Check the angle that the tie rods are at, they should be nearly staight, more angle=more pron to toe in with any suspension compression, as when moving down an uneven road at a high rate of speed, more toe change=unpredictable, unstable, unwanted no input steering of its own. Thats where my next change is going to be.

Good luck, I continue to whatch this post for any other suggestions

 

[seijirou]

Aerodynamics can play a factor yes. But 100mph aerodynamics on this particular car is not going to push it around the lane unless we're talking about 100mph crosswinds.

The speed rating of a tire has nothing to do with the car's stability at speed. It's a guide to the structural integrety of the tire.

Play in any of the front suspension components as little as a millimeter can cause your problem. Locking the steering wheel will help to expose the sloppy bits. If you are not practiced at checking suspension components, get a friend who is and have them help you. It's hard to find what you can't identify, there's nothing wrong with getting help, it's how we learn. Especially in this area.

Alignment only matters after you are sure everything else is happy. You can set the alignment on a car with worn balljoints, but that alignment won't stay true when the car is rolling again.

Also, do any noises or vibrations come in at the same time the car beings to become unstable? And secondly, does it occurr simililarly on different roads? Some roads, particularly trucking routes, can have a groove worn in that you can't necessarily see. In particular the ones you can't see are the ones that aren't going to come into play at normal highway speeds.

 

[ninety15.0]

I have noticed this same problem in my stang...but it only happens on this one highway near my house. The ruts or grooves in the highways from the big trucks can cause huge problems, making the can hunt and dive around in the grooves. Get your car out on a newly paved highway that you are sure doesnt have any grooves in it and see what happenes....i bet you will noticed the problem greatly decreased maybe even gone away. the suspension geometry of these cars isnt great....especially when slammed 2 inches. give it a try and tell us how it goes. good luck

 

[WhiteHeat306]

Aerodynamics can play a factor yes. But 100mph aerodynamics on this particular car is not going to push it around the lane unless we're talking about 100mph crosswinds.

The speed rating of a tire has nothing to do with the car's stability at speed. It's a guide to the structural integrety of the tire.

Play in any of the front suspension components as little as a millimeter can cause your problem. Locking the steering wheel will help to expose the sloppy bits. If you are not practiced at checking suspension components, get a friend who is and have them help you. It's hard to find what you can't identify, there's nothing wrong with getting help, it's how we learn. Especially in this area.

Alignment only matters after you are sure everything else is happy. You can set the alignment on a car with worn balljoints, but that alignment won't stay true when the car is rolling again.

Also, do any noises or vibrations come in at the same time the car beings to become unstable? And secondly, does it occurr simililarly on different roads? Some roads, particularly trucking routes, can have a groove worn in that you can't necessarily see. In particular the ones you can't see are the ones that aren't going to come into play at normal highway speeds.

Thanks for all the helpful info man. Honestly, I can't hear a damn thing over my exhaust even at idle, so I don't know if there are any noises coming from the suspension. I don't really notice any vibrations on the car other than when I come to a hard stop, the steering wheel vibrates. Also, I have to have the steering wheel just a BIT, I'd say no more than 2-3mm, to the right to get the car to track straight.

 

[vristang]

No it wasn't. It is their to direct air up and through the radiator. Particularly on GT's with have no grill.

I am looking at a great diagram right now that shows how the front air dam reduces flow under the car (I would post a pic of it but the scanner is not working). The flow under a vehicle is highly turbulent due to the rough/uneven surfaces of the undercarriage. High tubulence equals high pressure (lift when acting upon the underside of the car) and aerodynamic drag. Both of these reasons are why the air dam is in place on the Mustang. If the GT bumper were that bad, then the LX would not have an air dam. My LX does. If you look head on, from a low height when comparing the LX and GT bumpers you will notice that the GT has a more direct air path to the radiator than the LX. Maybe the air dam under the Mustang does help to direct air toward the radiator, but this is not its primary function, nor its secondary function. If you are interested the book is Competition Car Downforce by Simon McBeath pg 41.

After reading through the posts again, I am still unsure how bad the problem is. There is some bouncing around that will occur when running wider tires. My GT will handle great on a good road, but on certain sections of I-5 I swerve like a drunk.
19865.0 has the best solution so far. There is no need to go that fast on a public road. Or are you actually racing this thing?

 

[mpswirl]

I have that same problem but it happened after i changed my tires, i went from F1's to dunlop and the car handles like *****. I do alignments at my job and its not that but it could be your problem. You have to ask yourself when did it start happening, after you changed tires, lowered it, etc. Then check the alignment if none of those apply.

 

[NUTCASE]

try just going through uyour entire front and rear sus to make sure everything is in good shape. espesially balljoints, tie rod ends, and and wheele bearings as mentioned above.

if all is tight try adding sfc's and a g-trac bar. that should definatly help.

I have the stock tire size and sfc's and so far I have been stable up pretty high

 

[WhiteHeat306]

I have that same problem but it happened after i changed my tires, i went from F1's to dunlop and the car handles like *****. I do alignments at my job and its not that but it could be your problem. You have to ask yourself when did it start happening, after you changed tires, lowered it, etc. Then check the alignment if none of those apply.

the suspension was like it is when i bought the car

 

[WhiteHeat306]

try just going through uyour entire front and rear sus to make sure everything is in good shape. espesially balljoints, tie rod ends, and and wheele bearings as mentioned above.

if all is tight try adding sfc's and a g-trac bar. that should definatly help.

I have the stock tire size and sfc's and so far I have been stable up pretty high

sounds like a good idea