Possible rear suspension issue - unstable at higher speeds
 

Note: bought the car used with 19k miles and the handling issue was not that noticeable since it took a while to notice it.

For a while now I thought I was having issues with the front suspension settings like perhaps the toe was off and it was causing the car to fly into the next lane with just slight input but I learned this was not the case. I was also running a 265 tire up front and I was wondering if this was the issue but I am inclined to think this is also not the issue.
Everyone kept telling me alignment is the only way but this too was crossed off the list.

Work done: SPL Rear camber arms and toe bolts, Swift springs, power mods not relevant, 275 front and 285 rear tires.

Well, I got new tires installed with the above mentioned sizes and I decided to go to my alignment shop, which takes care of customers with drift cars, and have them check things out. The alignment was corrected. Note I used to think the issue was in the front suspension but now the car feels really precise up front with the new settings but I still have the issue!!!

So this is the issue now. With any steering wheel correction made...Front: very precise control and car will go exactly where I want it no complaints...Rear: it almost feels like the rear is loose? Like it wags it's tale a bit back and forth when I go from one lane to another at highway speeds. Frankly it feels unstable. Now imagine the fact that I am not driving aggressive at this point. If I do get aggressive it gets worse. If I was on a track I would have to fight this thing with any sudden course corrections.

So my question is this normal? What have you all done to combat this issue?

Do you have the same exact tire brand and model on both front and rear? If not, this is your prob.

The car's pretty stable in stock form. I'd go after your list of changes (to suspension setup, tire setup, etc) when trying to diagnose a cause. When you say the rear is "loose", do you mean it's bouncy? Do you mean it loses traction too easy when you give it gas? Does it act like a solid axle instead of IRS? (hoppy accelerating over bumpy pavement?).

My thoughts:

1: post the sheet from your recent alignment
2: check torque on suspension components
3: most likely, you are tramlining with 275s up front

Same brand tires front and rear. Continental Extreme Contact DW

I actually noticed the issue in stock form which is why I am doubting my changes has caused any issues.

I know why it's bouncy but that is not what I mean. It does lose traction to easy but this is not the issue I'm inquiring about either as I would see the traction control light go off when the issue happens. I've never had a solid axle car at highway speeds before but I admit when the issue happens its the first thing that comes to mind. Almost like the mounting points for the solid axle car was loose and the axle is moving side to side.

Above speeds of let's say 50 mph is where it becomes apparent. During lane changes on perfectly smooth surface too. I navigate the car into the next lane the rear just feels loose to the point that when the lane change is over with the tale is still in motion for a split second. Like a split second of oversteer. If I were to put this car on an autocross and go through esses I have no doubt it would be a battle to keep the rear in check.

1) I need to look for it
2) I had these checked but maybe I will have them take a closer look at the rear
3) I was worried about this but many of the racers have this setup without issue. I used to run square with the Rx8 and that was perfect.

What is your toe setting in the rear?

Same question, here.
Sounds like rear toe out.

You had a "drifting" guy do your car's alignment, sounds like a disaster to me

If rear toe is toe-out, get it reset to factory spec's - a bit of rear toe-in.

If still a problem, then something has to be loose .... you have rear camber arms and toe bolts, so check that the toe bolts are not loose - they might be tight on the spanner, but there may still be a degree of freedom under lateral suspension loads so look carefuilly for shiny metal indicating that even though the bolts are tight, something is moving.

You are decribing steering from the rear which is really unsettling to ay driver - is usually caused by toe-out at rear, but could be loose suspension as well.

If the traction link has a bush that is flogged out, that may provide enought movement to give you some dynamic toe-out at the rear and that will always give a driver a does of the squirts ...

Put the car up in the air (axle stands or hoist) and go over it closely with the Mark I Eyeball and a proper spanner (wrench) check - that answer is in the car somewhere.

RB

you should check the bushing at the rear differential cover, see if its leaking silcon fluid,
this could be giving you the loose feeling.

Hey man I had the same issue with my car!....I couldn't go faster then 110 because the rear end of the car felt like it was loose...I solve by installing the camber kits all around and alignment to factory specs ...and problem solve for me....good luck and keep us updated !!

my nismo does this all the time since ive bought the car..., the whole bouncing n constantly having to correct the steering because it just goes side to side on the hwy but speeds over 85 it settles in and drives fine.... weird? Its definitely unsettling as my previous car wouldnt do it at all. i changed my nismo suspension to kw v3 coilovers and it has been reduced but still does it and my tires are stock sized 245 up front 285 in the rear.

Stock/neutral toe.

The client base is dealerships and high end luxury/sports cars but they have a few drift cars as well as autocross racers that frequent the shop.

Good info thank you!!

This reminds me...a while back the rear diiferential mount appeared to have been leaking but when the dealer cleaned up the fluid I never noticed a leak after that. Could this be the issue??

You don't happen to be driving on crowned roads? Does the movement happen on the same roads in the same spot? My car has always handled fine, but there are a few roads around here that cause my car to wander due to heavy crowning of the road. Very noticable in my Z but hardly notice it in my other cars.

Just a thought-