More Oversteer with Wider Wheels & Tires?
 

I have a 2015 BRZ and was running stock wheels with Michelin PS4S tires. I installed some WedsSport TC105N 17x9 +35 wheels with Continental Extreme Contact Sport tires (245/40/17) and had an alignment done. Suspension is completely stock.

I’ve noticed more oversteer at the limit rather than the mild understeer that the stock setup had. Is that normal? Not sure why I am seeing more oversteer considering the ratio of tire width front to back stayed the same (i.e. tires are wider but are still a square setup).

Any insight is appreciated.

Could be the mould release compound on the new tires.

Agreed, it could be the mold release.
Or it could be the Conti's are a much crappier tire than the PS4S's.

I've had some pretty crappy tires on my car and I think the Continentals were the worst. I might have had a bad batch but the one's I had the sidewalls were so soft you could almost poke your finger through them. The only tires I've ever replaced long before they were due.

The tires I currently have are Forceum Hena's in 235/45/17. They're made in Indonesia. Nowhere near as good as PS4S's but better than POS Extreme Contacts. And a lot cheaper.

Do you have the before/after alignment settings? That could be part of it.

These cars are nose-heavy and overwork the outside front during cornering. Going from 215 to 245 tires could result in a slight change in balance and make the car less understeery due to the wider tire being relatively less overloaded at the outside front. But still, unless you've done a lot of mods the car should still understeer at the limit. Inducing oversteer should require something like a mid-corner lift off the gas or some other driver input.

Get a track alignment. That happened to me with no change in my tires. I figured the alignment from the factory differs from the alignment specs given to dealers and others ( for stock alignment). I got my car aligned by 3 places until someone told me to get a track alignment from a race shop. Since then that is all I get. After a few spins in traffic due to the "stock" alignment I won't be going back to that.

I suspect the tires are a lot worse at a minimum. They slip much easier from a stop than the PS4S did. The Contis only seem to grip well when they’re really warm while the PS4S gripped like crazy regardless of weather or if they’d been warmed up.

Alignment before and after:
Attachment 212790

Before:
Attachment 212791

After:
Attachment 212792

Uploading the pictures from mobile, so hopefully they’re clear enough.

The stock platform is made to take advantage of the 215 tires. When you put big sticky tires (The Conti ECS is a fantastic tire) on with no suspension changes, you overwork the front tires and that little bit of oversteer that was there stock, is exacerbated. My car was very squirrely with only tires. Coilovers fixed all that.

Yup no prob there!

You went from little bit of rear toe-in (not excessive) to pretty much zero rear toe, but IMO like you say it's more on the tires.
FWIW I ran the same 245/40-17 Conti ECS tires on my car, and initially at the track the rear end *was* a bit more dancy-around on left-right transitions (versus serious 200tw tires). But the grip was there. By the time they were ~half-worn they were a lot more well-behaved and were as fast around New Hampshire Motor Speedway road course as Hankook RS-4 "Extreme Perf" tires. Honestly I think they are pretty great *street* tires, just not necessarily as razor-sharp as I'd like. *Especially* at full tread-depth.

No way in hell with a stock car and that alignment that you have oversteer. Im willing to bet my car that it is caused by how you are driving, rather than tires, or whatever other thing people are coming up with. Almost 0 camber up front on mcphersons? No way the front grips more than the rear.

Or rather after installing coilovers you did alignment, which possibly fixed some issues in stock alignment (which has imho too wide "passing" ranges). Overall for most twins owners stock alignment is understeer biased, as these are cars primarily made for daily driving on public roads for drivers with varying (and often low) skill, hence no wonder that big part of "performance/track" alignment is not just to increase negative static camber, but also increase it more in front, to shift stock grip bias to less understeer-ish then stock.
My bet is that your oversteer prior coilover installation due rear toe being out of whack, maybe even toe-out (that shouldn't be there, as even stock rear toe "should" be slight toe-in). Or general mishandling with drivers inputs or choices for almost intentional inducing of rear grip loss for fun sake, when car is loaded in turn or on low grip (eg. winter/in wet/on gravel).

Get a 2013 FRS. Massive oversteer on demand.

And of course check your tire pressures.

I had more than one alignment done on the car when it was stock. On say a 270 degree on or off ramp, at the limit of grip, the back end would have come around before I plowed off into the grass. On 215 tires and more so on 245 tires. At the car's stable limit, an FRS will spin out before it full on understeers.

It's probably the way you're driving the car. More grip and the car breaks away at a higher speed. When they lose grip they lose it faster which compounds the need for smoother inputs with gas and brake. That's one reason the car comes with crummy tires, you'll crash at a lower speed.

You can play around with tire pressures as a first line of changing the setup.