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Car Unstable at Interstate Speeds

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I have a 15 GT that I recently bought with 2K miles on it and have been experiencing some issues after installing a slightly wider tire on the rear (285). I did not notice any issues driving the car the first few weeks until I installed these wider rears but now at speeds above 80 on the interstate the car is very floaty and unstable feeling, changing lanes and slight curves at speed is sketchy to say the least, Just driving around town you dont notice it at all... The car is lowered on Eibach Sportlines from first owner.
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Alignment settings? Bumpstops trimmed enough/at all?
 

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I had this probem. You need to make sure the lower control arm on the rear is really tight with a big breaker bar. There can be just the slightest gap that causes the toe to change. Alot of guys including myself try to put the springs on by removing the lower control arm and arent able to torque it back up properly leaving just a slight amount of slack in it which is enough to notice at freeway speeds especially when you are going 70 and you go to accell the car will sway side to side
 

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I had this probem. You need to make sure the lower control arm on the rear is really tight with a big breaker bar. There can be just the slightest gap that causes the toe to change. Alot of guys including myself try to put the springs on by removing the lower control arm and arent able to torque it back up properly leaving just a slight amount of slack in it which is enough to notice at freeway speeds especially when you are going 70 and you go to accell the car will sway side to side
lmao I'm gonna check this out.. sometimes I get a similar feeling. ford touched my car last so we'll see.
 

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20x9 OEM wheels... Here is my recently alignment..
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Check tire pressures and whether or not your bumpstops are trimmed. There have been others with the same problem who did not have trimmed rear bumpstops. The rear camber is a bit high (too negative) but not enough to cause that problem.
 
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Tire pressure is good, how can I check rear stops? I know I've never felt the car hit them.
 

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Tire pressure is good, how can I check rear stops? I know I've never felt the car hit them.
On sportlines, you'd be on them 100% of the time if they're not trimmed.

Jack the car up, remove the shock (much, much easier with wheel off but possible with it on still) and pull the shock mount and dust boot off. The bumpstop should be inside the dust boot.
 

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At first I thought the title said "Car Unstable at interstellar speeds" had to see what kind of space ship you were driving.
 

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On sportlines, you'd be on them 100% of the time if they're not trimmed.

Jack the car up, remove the shock (much, much easier with wheel off but possible with it on still) and pull the shock mount and dust boot off. The bumpstop should be inside the dust boot.
Yea this things not riding on the stops man hell even on huge bumps I've never felt them.

PO had Ford install the springs so I would hope everything is well with that...
 
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Of course, Nitto 555 G2.
 

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Of course, Nitto 555 G2.
You say 'of course' but sometimes folks put on used/like new tires and there may be some wandering on the highway...so just had to ask to help eliminate. Those used/like new tires may have some issues.
 

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Yea this things not riding on the stops man hell even on huge bumps I've never felt them.

PO had Ford install the springs so I would hope everything is well with that...
I would not assume they're right, ask [MENTION=15379]MikeyPee[/MENTION]
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