Rear Tire Inboard Cupping

[Sand and Dust]

Hope you can help me out on this. My ride is a 2007 Yaris Sedan. I am experiencing rear tire cupping on the inboard sides (chassis side of the tire ) both left and right. My tires are 195/60 R 14 mounted on 6.0 alloy wheels. Have no idea about the off-set but the wheels are flush with the fenders and does not extend out. Maintained all tires at 32 psi. The tires have ample clearance inside the wheel wells. This problem has already happened twice. My previous tires were Hankook Optimo 195/60 R 14 and had the same tread wear problem. Both rear tires (Hankook) actually failed from belt edge / tread separation (one tire actually bulged so badly that the belt wire stuck out of the tire tread). Both failures happening at the inboard side. I noted that the tread shoulders of the Hankook tires were square and were a bit thick which I suspected was building up heat. It gets quite hot here in Saudi Arabia (ambient around 120 deg F) and I drive long highway stretches at more than 120 kph. I switched to a Chinese brand because of the tread profile (rounded tread shoulders) and they did fare better - only noted cupping but no separation.

It seems my rear wheels have a camber problem but the service manuals state they are fixed and cannot be adjusted. I also noted that the rear tires have the same tread depth at the outboard and inboard side and only the cupping at the inboard side is obvious. Any other thing I have to look into as I am totally mystified with this problem. BTW the rear tires are very noisy at 100 to 120 kph.