Rear Driver side tire cupping wear - bad shocks already?

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SF Bay Area
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'17 CX-9 Signature
My car is a 2017 Signature with about 36k miles on the clock. Seems to ride pretty well.
I replaced the rear brake pads two weeks ago (yes, worn out at 36k miles) and in the process, i rotated the tires.

As soon as i put the rear tires on the front, i started to feel a very slight, but noticeable, vibration in the steering when driving slowly on parking lots.... and also, i can hear a weird thum-thum-thum sound from the front.
I looked at the front driver side tire (it was on the rear for the last 8-9k miles) and i noticed that the INNER edge of the thread has patchy, uneven wear. The thread goes up and down in wear every couple of inches. This is what they call 'tire cupping' in the tire business.
The tire is the original Falken OEM. It still has about 40% left. With this cupping wear, it's garbage now, since it makes the ride extra noisy and the slight vibration at very low speeds is not fun. I will replace them with some Michelin Defenders very soon.

The big question is WHY did my tire get cupped on the rear? I asked a couple of tire shops, and without even seeing the car, they both said - "bad shock", "blown strut". One of the tire guys claimed that they had a 2018 Ford Escape with only 20k miles and 2 visibly blown shocks on the back.

I still find it hard to believe. My car rides well. The rear shocks are bone dry. No leaks. No bouncing. No strange noises. The car was never overloaded. ... and it has only 36k miles. Yet, the tire shows real cupping, only on this side.


Anyone had similar experience? Any ideas why this may be happening?


I may take it to Mazda dealer, but i am 90% sure they will blame it on a bad tire. Not sure if other than visual inspection, they can do much about 'testing' the rear shocks.


Thanks!
 
Possibly bad shock in the rear, but it could also be due to lateral runout due to suspension misalignment. Cupping could also be caused by improperly balanced wheels. I'd suggest a wheel alignment. You could do it now to rule out a misalignment and reduce the risk of cupping to your remaining tires, but I would definitely suggest an alignment when you get new tires mounted.

How bad is the cupping on the tire? Does the lowest point go lower than 2/32nds? The reason I ask is because you may be able to just get the cupped front tire shaved (and the other front tire shaved to match), but I don't know if that's a feasible repair since you'd essentially be taking life off of the good tire (and possibly the rear tires if you get them shaved down to match the diameters of the front tires).
 
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You can un-rotate your tires. Leave the cupped one on the back unless it is too uncomfortable for you. Rotation is to extend the life of the tire tread. If you're OK leaving the cupped one on the back, and there is sufficient tread depth remaining for safety, there's no need to get new tires yet.

Get the 4-wheel alignment checked. Require that the shop put all settings right on the middle of the spec, not just anywhere "in spec." Get that tire balance checked. A shop can examine the shock.
 
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My car is a 2017 Signature with about 36k miles on the clock. Seems to ride pretty well.
I replaced the rear brake pads two weeks ago (yes, worn out at 36k miles) and in the process, i rotated the tires.

As soon as i put the rear tires on the front, i started to feel a very slight, but noticeable, vibration in the steering when driving slowly on parking lots.... and also, i can hear a weird thum-thum-thum sound from the front.
I looked at the front driver side tire (it was on the rear for the last 8-9k miles) and i noticed that the INNER edge of the thread has patchy, uneven wear. The thread goes up and down in wear every couple of inches. This is what they call 'tire cupping' in the tire business.
The tire is the original Falken OEM. It still has about 40% left. With this cupping wear, it's garbage now, since it makes the ride extra noisy and the slight vibration at very low speeds is not fun. I will replace them with some Michelin Defenders very soon.

The big question is WHY did my tire get cupped on the rear? I asked a couple of tire shops, and without even seeing the car, they both said - "bad shock", "blown strut". One of the tire guys claimed that they had a 2018 Ford Escape with only 20k miles and 2 visibly blown shocks on the back.

I still find it hard to believe. My car rides well. The rear shocks are bone dry. No leaks. No bouncing. No strange noises. The car was never overloaded. ... and it has only 36k miles. Yet, the tire shows real cupping, only on this side.


Anyone had similar experience? Any ideas why this may be happening?


I may take it to Mazda dealer, but i am 90% sure they will blame it on a bad tire. Not sure if other than visual inspection, they can do much about 'testing' the rear shocks.


Thanks!

Off the top of my head, there are several possibilities beyond a worn strut:
1. Bad alignment. Cupping on one side is sometimes due to the rear toe angle being out of spec. I'm not sure if this is even adjustable on these cars. Have you hit anything that may have bent something?
2. Loose upper shock mount bolts.
3. Bad/worn suspension bushing or wheel bearing.
4. Bad tire. It's possible that a belt in the tire is messed up.
5. Bent rim. Have the runout checked before putting a new tire on the rim in question.
 
I just went through the same with our 2016 Signature with 30k miles, tire cupped. I was pissed at the dealer that sent me an e-mail about that but did not bother to tell it to my wife who was waiting for the car to be serviced at the shop and they even proceeded with the useless (at that point) rotation. I hated the Falken Ziex and they were pretty worn out on top of the cupping (at 30k I know, not great), so I went shopping for new tires and an alignment. The shop told me that the alignment was seriously out of spec, hard to say what caused it but from your story and mine it looks like the CX9 is prone to go out of alignment around that time, not sure if it is a design flaw or an external factor like a pothole...
 
I have had same problem. 16 GT.
I changed OEM tires with 34k miles and I think I will take a close look.
 

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