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I have a 2015 Mustang Roush Phase 2 supercharged. Vehicle has 8,8xx miles on it. For some reason at higher speeds above 65, the vehicle feels like it rocks side to side when driving. If there is a slight rut or bump in the road the whole vehicle drifts that way. I have no idea where to even start looking. Anyone else hear of something like this? Thanks in advanced.
I bought the vehiclr used a couple years ago, and don’t believe it has anything done to the suspension besides what Ford or Roush added. Everything seems to be oem.
 

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Have someone push/pull the rear of the car side to side, while you look at the underside and see where the problem is. You need to take it to a shop so they can investigate.

It could even be in the front-end, and it just feels like the rear end. I suspect you have worn anti-sway bar bushings somewhere, or something related.
 
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Yeah I knew that the S550 has IRS rather than a solid axle, so what does prevent the differential (rather than axle) from moving laterally?
 

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I have a 2015 Mustang Roush Phase 2 supercharged. Vehicle has 8,8xx miles on it. For some reason at higher speeds above 65, the vehicle feels like it rocks side to side when driving. If there is a slight rut or bump in the road the whole vehicle drifts that way. I have no idea where to even start looking. Anyone else hear of something like this? Thanks in advanced.
I bought the vehiclr used a couple years ago, and don’t believe it has anything done to the suspension besides what Ford or Roush added. Everything seems to be oem.
It's almost certainly an alignment issue. Some tires are also more sensitive than others. Too much toe in will make it actively want to wander side to side in a disconcerting way, and tend to hold that turn. Too little will resist turning, and feel very vague and floaty. Incorrect toe will also often result in following ruts and lines. Too little caster will result in serious instability that gets worse with speed. There's no such thing as 'too much' caster, except that the car gets hard to turn. And lastly, camber usually doesn't 'feel' any particular way, but causes a lot of tire wear.

(alignment issues with the rear will cause different weirdness, but the idea still applies!)
 

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Yeah I knew that the S550 has IRS rather than a solid axle, so what does prevent the differential (rather than axle) from moving laterally?
Missed this question. Thankfully a spammer made me re-look at this thread! lol

Anyway, Dino, on IRS setups, the pumpkin (and center of the diff) are bolted directly to the frame. They can't wiggle around at all. Only the outer suspension parts and half shafts (axles with CV joints or U-joints) can move.
 

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Are there grooves or seams on the road when it happens, it almost sounds like tramlining. But I would still make sure something is not loose or broken.
Tramlining is the tendency of a vehicle's wheels to follow the contours in the surface upon which it runs. The term comes from the tendency of a car's wheels to follow the normally recessed rails of street trams, without driver input in the same way that the train does. The same effect is sometimes called Nibbling.
 

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Good call. It could indeed be tramlining and that usually indicates excessive toe out of the front wheels. Obviously that would require correction with a wheel alignment but while the car's on a lift, the control arm bushings and inner/outer tie rods would need to be checked for excessive play.
 
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