I bought a 2017 Legacy Limited 2.5 a month ago. I live 125 miles from Van Bortel Subaru in Victor, NY, and I drove home on 9/7 on mostly Interstate highways. A wandering motion was evident at highway speeds from the beginning. From my own experience I figured it was a suspension problem. I found this forum, and my suspicion deepened. Monday i had the dealer order a 20mm sway bar (stock on WRX), and they installed it today. I could have done this myself but I'm getting to be an old guy now and paying the dealer for the requisite half hour of installation time was worth it to me.
We just arrived home a bit ago. What a difference. I was told that I was the first Legacy owner in recent times to swap out the rear sway bar, and their technician who drove it before returning it to me remarked on the substantial difference comparing stock to 20mm. Dealership staff have done this to their personal vehicles, but it seems I was the first customer.
I kept the old one and whenever I trade this car it will go with it. Until then I've solved the wandering issue, maybe ninety percent or more.
Yesterday I drove the car on a back country State road, with a hill that dropped off into a bridge spanning a ravine. That transition would not exist this way if the road had been designed in the past twenty years. My guess is that there is lots of older designed infrastructure in rural areas of New York State, and they won't be fixed any time soon. When I drove over the hill and down onto the bridge that spanned the ravine, the car experienced what I regard as compromised rear stability and the front end moved quickly back and forth maybe three times. I knew enough to hold center and not correct (so as to avoid over-correcting). I'll be on that road again next week, and we'll see how it goes. But I think a 20mm sway bar is a real game-changer.
Thank you sincerely to all who've weighed in on this. I'm brand new to this forum, and I am so thankful for the thoughtful contributions that allowed me to improve this fine car in a genuinely meaningful way.
rhombus