cupping

What it do everyone, I've been having some issues with my truck's front end and I'm not sure if I'm capable enough to pinpoint exactly what I need to fix. My 93 Ex has been pulling to the right resulting in having to hold the steering wheel to the right about like 10-15 degrees off-center just...

I have just noticed that on the front left hand tyre there is about a 1/2 inch deep groove starting on edge of tyre going 1/3 to atleast 1/2 way round inside tyre edge then it goes back to normal. Its arched shaped. And smooth. I first thought rubbing on something but only goes half way around...