Car gets unstable around 100mph

WhiteHeat306 · Apr 27, 2005

Any ideas on why my car seems to wander around on the road when approaching 100? The car is lowered on 17s, with some negative camber up front so the tires tuck.

ram360 · Apr 27, 2005

Wider tires tend to follow imperfections in the road more

tjm73 · Apr 27, 2005

When you say unstable, what do you mean exactly? Is the car wandering back and forth or does the front of the car kinda feel like it's hoovering around? Define what's happening please.

crazypete · Apr 27, 2005

Thats about where the aerodynamics start to kick in and airflow starts to have an effect. Something to think about.

My 87 and 91 both shake and rattle like crazy above 90 mph so nothing unusual there. You need to dial some extra toe-in and it will be much more stable at high speeds. Also maximum camber tends to stabalize it more by causing the wheel to want to remain centered. If you go toe-out, it will wander all over the damn place, frighteningly so at high speed.

Your wheel bearings could be crap. If the front tire "wobbles", it will be more noticable at high speeds.

CrazyCobra · Apr 27, 2005

I have the same problem I have not been able to isolate it yet. however it got worse when I switched to 17's 245 40's. I use H&R red progressive rate race springs( 2" drop I believe) and tokico shocks. The alignment at rest is not the problem so I am going to try a bump steer kit, it may be the alignment changing as the suspention move up and down at the higher rates of speed?

Try this link and see if this sounds reasonable

http://maximummotorsports.com/mmtierod.asp

19865.0 · Apr 27, 2005

sloooooooowwww downnnn

seijirou · Apr 27, 2005

crazypete said:
Thats about where the aerodynamics start to kick in and airflow starts to have an effect. Something to think about.

My 87 and 91 both shake and rattle like crazy above 90 mph so nothing unusual there. You need to dial some extra toe-in and it will be much more stable at high speeds. Also maximum camber tends to stabalize it more by causing the wheel to want to remain centered. If you go toe-out, it will wander all over the damn place, frighteningly so at high speed.

Your wheel bearings could be crap. If the front tire "wobbles", it will be more noticable at high speeds.

If your mustang is shaking and rattling above 90 something isn't happy. Toe in will dampen the effects of steering, which will make the car respond less to your steering _input_ at any speeds. Which if you are shaky at the wheel will stabalize the car at high speeds where small inputs become exaggerated. Camber will help the wheel remain centered because it lifts some of the contact patch of the tire off the road, therefore it will make a wider tire essentially narrower when the car isn't leaning. And as stated before, wider tires will follow imperfections in the road more.

Caster is what truely what stabalizes your wheels at speed. The wheels on the front of a shopping cart are called caster wheels, the caster of the shopping car wheels is what makes them point staight when you push the cart instead of flopping. I'm sure we've all seen a cart with a broken caster wheel that flops around. The caster setting of your front alignment has the same effect.

Bumpsteer only comes into play when the suspension is compressed, or if you lower the car from stock height say 2 inches, you get the same bumpsteer effect a stock mustang would have when the suspension is compressed 2 inches.

The cause of instability can be alignment, wheels out of spec, unevenly worn tires, bad bushings in the control arms, steering rack, shocks, maybe swaybar too but probably not. Bad outer tie rods, bad balljoints, bad steering linkage, bad steering rack, bad bearings, bad spindles.
The list goes on and on.

I do not recommend patching the problem with toe-in (this gives you really really bad ackerman and greatly diminishes turn-in) or heavy amounts of camber (which will decrease already poor braking performance and eats tires)

Find the real problem and get it taken care of, you'll be much happier when you're done.

If you find that the steering response is too twitchy for you at high speeds, you can change to a manual rack, or add a steering dampner.

vristang · Apr 27, 2005

If the problem does not exist at lower speeds, I would guess aerodynamics.
Did you remove that plastic guard that hangs down a couple of inches below the radiator? This was designed to reduce airflow under the car, thereby reducing lift.

crazypete · Apr 28, 2005

Woops, I always mix caster/camber up. Sorry bout that. I wish they made them sound at least marginally different. I found an alignment to greatly reduce these effects for me at least so try that first.

BlackGT89 · Apr 28, 2005

Yeah, sounds like "lift" is rearing it's ugly head. Wide tires can also hurt aerodynamics (take a look at the skinny tires they use for record runs on the Bonneville Salt Flats). It's not always a popular belief, but for those kind of speeds you are beginning to need front air dams and rear spoilers to keep it pinned to the road.

tjm73 · Apr 28, 2005

vristang said:
If the problem does not exist at lower speeds, I would guess aerodynamics.
Did you remove that plastic guard that hangs down a couple of inches below the radiator? This was designed to reduce airflow under the car, thereby reducing lift.

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

WhiteHeat306 · Apr 28, 2005

tjm73 said:
When you say unstable, what do you mean exactly? Is the car wandering back and forth or does the front of the car kinda feel like it's hoovering around? Define what's happening please.

Sorry man I should have clarified better. It's like the car wants to steer itself, the steering wheel moves back and forth just a little bit, and I feel like I don't have good control of the car. It's not like I whoop ass in the car every day, but the few times before when I was going that fast I really didn't want to stay at that speed for long because I felt like I didn't have control. Plus 100 isn't even that fast.

tjm73 · Apr 28, 2005

WhiteHeat306 said:
Sorry man I should have clarified better. It's like the car wants to steer itself, the steering wheel moves back and forth just a little bit, and I feel like I don't have good control of the car. It's not like I whoop ass in the car every day, but the few times before when I was going that fast I really didn't want to stay at that speed for long because I felt like I didn't have control. Plus 100 isn't even that fast.

What everyone said about aerodynamics is true/correct. What you further explain here makes me think sounds like it's more than just that. AS far as a cause goes...man I don't haev any idea. Maybe the alignment gets out of whack as the front of the car rises from the aerodynamic lift.

bhuff30 · Apr 28, 2005

WhiteHeat306 said:
Sorry man I should have clarified better. It's like the car wants to steer itself, the steering wheel moves back and forth just a little bit, and I feel like I don't have good control of the car. It's not like I whoop ass in the car every day, but the few times before when I was going that fast I really didn't want to stay at that speed for long because I felt like I didn't have control. Plus 100 isn't even that fast.

Check your inner and outer tie rods. Its really easy... grab the sides of the tire and try to rotate it back and forth like you are steering. It shouldn't move at all. If it does, you'll have to look around and figure out where it's loose at. Both inner and outer tie rod ends tend to go bad, espeically with the age and milage of these cars.

WhiteHeat306 · Apr 28, 2005

Thanks for the help guys, do you think it could also just be the Essenza tires that came with the rims? They are pretty much brand new and in great shape, but maybe they are not high-speed rated.

poneypower89 · Apr 28, 2005

19865.0 said:
sloooooooowwww downnnn

Probably the best advice given so far

My car rides like a dream at 130+ speeds... No shaking, no uncontrollable actions of any kind... Completely stock GT (suspension wise) with a set BFGW Traction T/A's. So i wouldn't blame it on aerodynamics.

Daggar · Apr 28, 2005

This is how I fixed that VERY problem:

http://forums.stangnet.com/showthread.php?t=539559

BlackGT89 · Apr 28, 2005

White Heat, I clicked on your "Check Out My Ride" link and I'm going to take back what I said. I notice you have a Saleen type rear wing and a front air dam / spoiler already, so maybe you do need a few new front end parts instead. I'd check the speed rating on those tires as well. Nice ride, BTW !

WhiteHeat306 · Apr 28, 2005

bhuff30 said:
Check your inner and outer tie rods. Its really easy... grab the sides of the tire and try to rotate it back and forth like you are steering. It shouldn't move at all. If it does, you'll have to look around and figure out where it's loose at. Both inner and outer tie rod ends tend to go bad, espeically with the age and milage of these cars.

I just did that, and the wheels stay put, nothing loose at all.

WhiteHeat306 · Apr 28, 2005

BlackGT89 said:
Nice ride, BTW !

thanks man

Car gets unstable around 100mph

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