Vibration at speed?

So I just had my car's wheels changed over from winter tires back to summer ones. All of the tires have their own rims, so the tires don't
have to be taken off and remounted on a single set of rims, just swap the sets around. After I did that this time, I noticed that there is a large vibration at the steering wheel, when the car is travelling from above 100 km/h (60 mph) to 120 km/h (75 mph), and then it dies down again. I'm thinking that perhaps a wheel balance weight might have fallen off, since this vibration didn't happen with the winter set, and I can just have these tires rebalanced. However, I'm worried this could be a more serious problem with the suspension itself. So is it more likely to be a wheel balance issue, or a suspension issue?
    Yousuf Khan
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Every time I've had that problem, it's just been a lost wheel weight. Since it's so easy and inexpensive to get wheels balanced, I would certainly suggest that you try that first. If it works, then problem solved!
Patty
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On 4/24/2017 2:50 PM, Patty Winter wrote:

Thanks, I'm hoping it's just that simple too.
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On 4/24/2017 3:18 PM, Yousuf Khan wrote:

I'm willing to bet everything you own that it's a balance issue.
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On 4/24/2017 4:11 PM, PAS wrote:

So generous of you to bet all of *my* stuff. :)
So it got fixed, it was a balance issue. It was really heavily out of balance.
    Yousuf Khan
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On Tue, 25 Apr 2017 16:01:43 -0400, Yousuf Khan

If it sudedenly went WAY out of ballance keep an eye on it. How much weight is presently on the rim??? You could have a carcass failure starting.
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On 4/25/2017 4:01 PM, Yousuf Khan wrote:

Glad you've got it resolved and that you got to keep all of your money.
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On Mon, 24 Apr 2017 14:19:16 -0400, Yousuf Khan

Given the fact it only happened when you changed tires the probability is very high it is a tire/rim issue.
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On Monday, April 24, 2017 at 11:19:21 AM UTC-7, Yousuf Khan wrote:

If you wanted to, you could start worrying about a potential problem with not only the suspension but also something going wrong with steering, and possibly damaged tires :)))
Many things to worry about in a car.
Since you experienced the problem right after a wheel swap, the most logical thing to worry about would be tire balance, or tire damage.
Basia

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On Monday, April 24, 2017 at 9:21:43 PM UTC-7, snipped-for-privacy@sbcglobal.net wrote:

More logic: you might have damaged the suspension while changing the tires. How about that fo wooring about something. Are your tire changing skill good? Tools in good shape. Did you lift the car appropriately?
Seriously I would check the tires for possible damage, then rebalance.
:)) Basia
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On 4/25/2017 12:21 AM, snipped-for-privacy@sbcglobal.net wrote:

Actually the steering was the next thing I was worrying about. :)
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I had that due to a tire with a bias separation. It had been ran flat for 20 miles starting from home. It felt a bit squirrely so I pulled over and the wheel had been riding on the tread. Ouch. Reinflated and replaced a leaky valve stem when I got back home. Still drove on it for awhile but it caused vibration only at about 62 mph (not there at 60, or below, or at 65, and above). Was going to get a new set but totalled once of my Subies so grabbed the mags and tire set off of it (the insurer only deducted $30 but required a set of 4 wheels+tires so I found some spares for really cheap - they just needed it to roll).
Another time a balancing weight got thrown off so, yeah, it vibrated. Had cheap steelies on at the time with no wheel cover and slapped into a hard (iced) snow bank. When digging out the packed ice-snow stuck in the wheel (because that really threw off the balance), I could see the shinier (less dirty) spot where the weight had been that was now gone. Fixed with a tire rebalance. Maybe you bent the wheel while offroading but the summer tires went into storage before you noticed the vibration. Balancing won't work. Need a new wheel or someone to bang out the old one (but I wouldn't trust the old wheel).
Got any deal with the tires that includes free rotation? They might throw in a balance check (but doing the rebalance would cost). I also keep getting $25 off cards from a local car shop that's done a lot of work for me and that covers a chunk of a balance (of all 4). I've seen where off-car balancing was insufficient compared to on-car balancing.
I had a wheel that banged into a curve when I spun around on ice in a turn. Had to replace the bent wheel. Then I noticed a vibration. Turned out it was an out-round-tire that didn't exhibit noticeable vibration on the old wheel but did on the new wheel. Kept using it (on the back axle, though) until all tires got replaced.
Some places will do a free brake check. Have them check if the pistons in the calipers are sticking and pushing the pad at an angle to the disc. Not sure how changing the tire set would suddenly expose a sticking brake pad. After driving for awhile, like from work to home, press your hand against the wheels (no covers) to see if one is a lot hotter than the others (the fronts will be warmer than the rears but one on the same axle shouldn't be a lot hotter than the other). The brake check should include steering check, including them trying to wobble the wheels when lifted to check for wearing wear, spinning on the car to check for bearing noise, and checking the connects to the wheels (e.g., look for slop in the steering dampeners).
I don't recall a misalignment causing vibration, just pull or drift. I usually wait until just after pot-hole season to do a yearly alignment check unless something makes me suspicious. Worn tie rod ends can cause vibration but I would think it would've beent here with your winter set unless the winter set were so fat and knobby that their own increased road noise masked the vibration. Do you just feel or hear the vibration or can you see it by having 1 finger resting atop the steering wheel?
There might be a stablizer bar shock (aka anti-vibration shock aka steering dampener) you might to have checked. I've not looked into Subies if they have one. I've had off-road vehicles with huge knobby tires and they had a steering shock. Without it, the tires would start to wobble and it would quickly accelerate to make driving very dangerous and the car would start bouncing left and right. Scared the crap out of me. When I go underneath, I could see where it used to be mounting. They forget to put it back on after some engine or tranny work. See:
http://perrinperformance.com/i-22342620-steering-dampener-lockdown-for-subarus.html
That mentions a steering dampener that might be worn or shrunk as it dried out over time. However, I can't see how steering slop would suddenly show up by changing a tire set but maybe it's possible slop was less noticeable with your winter set.
Can you feel any play in the steering (lag before response)? That could indicate (other than steering system problems) a worn wheel bearing. Any griding noise or increased friction noise when you turn left or right? However, you said the vibration showed up immediately after switching to the summer tire set. The bearing would've been worn before with your winter set but I suppose a different (wrong) offset in the wheels could mask that (and probably have caused the wear).
Vibration can be caused by a flat spot on a tire. That's usually from leaving the car parked for a long time without moving it (to get a different spot on each tire to take the static load). Hopefully the summer tire set weren't on another car sitting unused all during the winter. I had a 24 year old Subie that got used little so it had flat spots on the tires. Took a couple miles to roll it out. Does the vibration go away after driving for awhile?
I'm presuming you checked all tires have the same and proper pressures. Did you check for any feathering at one side of the tread? Maybe when you put them on again, the feathered edge is one the wrong side - but then you need an alignment job or new wheel bearing. My aunt's Subie had some severe feathering on one side of the tire and wasn't noticed until she complained about a change in handling after a tire rotate (I would've thought the shop have told her about the wear problem but it was a freebie coupon and she got what she [didn't] paid for - just the rotate).
I've seen vibration just from lug nuts. One guy only replaced his old ones because he wanted new shiny ones. Vibration gone. I suspect he had a mismatched set of lug nuts. I have to wonder if those locking lug nuts might not be different weight and profile than the standard ones.
Did you check for mud packed into the wheel? Don't know how you use your summer tire set. I've had both packed mud (summer) and packed snow (winter) throw off the balance.
Does this Subie have automatic tire pressure sensors? Those have batteries inside the stems that last 10 years or 100K miles. If someone worked on your summer tire set soon before storing them, maybe they put in a standard valve stem but the wheels had previously been balanced with the sensored valve stems. I'm sure the battery has more weight than the air space inside a standard valve stem. Looked at the blob on the end of this one (hidden inside the tire so you won't see it unless the tire is unmounted from the wheel):
https://images-na.ssl-images-amazon.com/images/I/81YswZvo7BL._SL1500_.jpg

https://www.youtube.com/watch?v=ETFlaRLGsw8

I didn't bother to hunt around to see if anyone reporting the sensor body flying off the stem and rolling around inside the tire. Someone putting in a standard valve stem and not rebalancing the tire afterward would probably make it off balanced by just a little bit which would show up at high speed.
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