Tire cupping in the middle

[four.8is]

My rear tires are almost gone but they have been pretty decent in terms of noise(Michelin Diamaris )
Well lately, I've been noticing some kind of tire noise that increased with speed...don't know exactly how to describe it but sounded like a really worn tire that has a huge metal screw in it...at speed, you can get an idea of what it sounded like...
I narrowed it down to the left rear tire, went and checked it and sure thing it cupped in the middle...put a level on the tire and I noticed a gap of about 1/4" ...checked the right rear one and it was perfect...
I guess my question is : what causes this?
Worn suspension components?
Too low/too much air in the tire?
The noise bugs me to no end so any opinions are welcome.

[motordavid]

Low tire pressure, rear shock gonzo at that wheel location = 'bouncing' of tire at speed, and a few other guesses. Just guessing from here...
GL, mD

[knucklebuster]

You have to much air in the tire .As the tire heats up it expands in the middle causing the center to wear more .Run lower psi .

[TwinsPoppa]

If it's evenly worn in the middle it's probably over inflated. If it's actually cupped/scalloped (uneven) in the center it might be bad shocks.

Compare tire pressures to start then start looking at suspension wear. When was the last time you had tires balanced and suspension aligned?

Good luck!

[four.8is]

Ok checked pressure today...rear 30psi front 30psi
Inflated to 35psi rear and 32psi front

Thing is, I had the rear bags and sensors replaced along with te compressor this spring..after bringing it home, I noticed the left side sits lower than right and I thought it might be cuz they didn't put the right values in when they adjusted all the corners...called them up, the mechanic said everything was within spec...but now it might just be the shock that's bad..

Twins, I got the car in February and I've never balanced or aligned since mainly because all the tires wore evenly and I haven't had any noise or shudder in the steering wheel pointing to unbalanced wheels...well now all of that changed...

I was thinking to get an alignment sometimes in spring when I should overhaul the suspension but maybe I should at least road force balance the wheels by then...

[Ricky Bobby]

How many miles four?

You're around 100k if I recall? If so there's a good chance your lower ball joints in the rear and the upper control arms have some wear, you will find out when you go for an alignment and they aren't able to get it in spec.

An alignment would be helpful just be sure the toe is to .01 to wear the tires even. I always run 33psi on the rear and 32 on the front, you have the right idea with 35

[four.8is]

RB, yes about 108k miles
Will try to get an alignment done to see where I am right now with the suspension before replacing the tires..otherwise I might be wasting my money on tires just to have them end up like the ones on the car...

[Ricky Bobby]

True, at your mileage you will most likely need some susp parts addressing, have any been replaced by you, or the PO in recent years?

I'm at 77k and my whole front has been done, my rear I just have done subframe bushings and sway bar bushings/links, but my ball joints and control arms are due as I'm only able to get to -2.9 of camber on the RR.

My shop actually let me bring the X back to them for the alignment after I replaced the tie rods because initially the tie rods were original but they ended up being seized, I didn't want to pay the shop 4x the price to replace them so I did it myself and brought it back the next week.

[four.8is]

Nope, I have not changed any suspension parts yet and neither did the PO(got the records)
So I'm def in need of suspension work...
Quick question: are the lower control arm bushings suppose to be as soft as mine are?if I yank it side to side the arm moves a little...and when i did so, I noticed some fine cracking on the bushing...
Both arms are doing the same...not to mention when I hit a bump or a pothole from time to time I get that metal on metal sound that points to bad bushings or...?i'm a total newbie to suspension components..

[Ricky Bobby]

You're talking about the thrust arms?

If those are original I guarantee they are shot, can't believe you don't get the shudder when braking. the arms should be tight, and the bushings should show no cracks. I'd start with the tension struts/thrust arms, ball joints, and wishbone control arms in front to begin with, and for the Rear, I'd do the lower ball joint, and upper rear control arms.