Rocking in the rear under acceleration or quick lane changes... Car feels unstable

[CrazedAntelope]

I have an Ecoboost PP car with a manual transmission. It feels like the rear of the car can skip or wag 1 foot to the left when I go into 2nd gear under full acceleration (when I shift very harshly). Everything on my suspension is stock.

Is this the type of rocking you are referring to?

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[5ABI VT]

My guess is oem bushing deflection. I just finished changing out the toe, camber and vertical links to bmr/steeda and a bmr cradle lockout. I haven't driven it since but the reason I did it was pretty much what you are describing. under full power in 3rd or 4th I was really feeling the bushing deflection causing the rear to wiggle under power and feel ike it was slightly changing directions (which it was im sure) The toe arms in particular have such soft bushings that I can wiggle them with my hand which Im sure was one of the culprits.

 

[DanDglassman]

On my previously stock GT non-PP i had a odd sensation while cornering that could best be described as a "rubber-band" feeling. I believe it was compliance of the subframe bushings. It was unnerving. Felt like the start of the tires breaking loose - but it wasn't.

I installed the Steeda rear IRS bushing support kit and the IRS braces and the sensation went away. Much improved cornering afterward. I was inspired by the improvement so I added the Steeda springs and swaybars. Tracks like a go kart now.

 

[David@Steeda]

On my previously stock GT non-PP i had a odd sensation while cornering that could best be described as a "rubber-band" feeling. I believe it was compliance of the subframe bushings. It was unnerving. Felt like the start of the tires breaking loose - but it wasn't.

I installed the Steeda rear IRS bushing support kit and the IRS braces and the sensation went away. Much improved cornering afterward. I was inspired by the improvement so I added the Steeda springs and swaybars. Tracks like a go kart now.

Thanks for providing your feedback! Glad to hear the car's handling as it should

 

[markmurfie]

I have the same unstable feeling in my car as well. It also seems that the more suspension parts to stiffen things up I put on my car the worst things get. I can put my foot on the top of a tire and rock the entire car side to side and the car moves in the same motion as I feel while driving. I would describe the motion as the last few moments before a spun coin comes to a stop. A rocking circular type motion. It appears to me to be all side wall movement. I have the base 18x8 rims with 235 50 18 tires so they do have quite a lot of sidewall. My first mod should have been rims and tires, it will be my next.

 

[Brady01]

Did you ever get this fixed mine is doing the same thing

 

[Edyuly]

Old post but did anyone figure this out?

 

[Guard5.0]

Not sure on the 17 and up cars but when the 15’s first released I added lowering springs and kept the pp dampers. An initially average IRS turned to junk. I called it oscillation in earlier threads and others had the same experience. Adding power (FI) made it even worse. I threw the BMR and Steeda catalog at it and it helped but the biggest benefit came via the coil overs. These cars needed a lot of help in the handling department but matching appropriate springs with dampers really helped.

 

[fightermatt]

Just in case others are searching for solutions, here is what I have found. Make sure the axle-spindle nut, or whatever it is actually called, hasn't started backing off and let the wheel bearing begin to separate; also make sure all the bolted joints connecting the rear knuckle to the car are torqued to spec. Especially the toe rod, mine was loose where it bolted to the knuckle and my rear end was all over the place.

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