Affect of wheel width and offset on stock suspension feel
 

I'm planing on buying lightweight wheels to use with summer tires, and have narrowed down the choice to the following:

1. Enkei RPF1 17x7.5 ET48
2. Enkei RPF1 17x8 ET45
3. 949 Racing 6UL 17x8 ET40
4. Konig Hypergram 17x8 ET40

Weight and price wise they are all in the same ballpark, and I'll be okay with either. The plan is to use these with stock tire size (215/45R17), probably Michelin Pilot 4S or Continental Extremecontact Sport.

My question is how would the wider wheels, and especially the lower offset affect the car's feel on its stock suspension? I don't track or autox, so I'm only looking at street/back roads steering and handling feel, not outright performance. The Enkei's in 17x7.5 would be closest to stock size (of the PP wheels), and are also the lightest, so they seem the best option, right? But they are also more expensive, and I'm okay with the slight increase in weight of the other wheels if it can save me a pretty penny. I also don't care about cosmetics at all, so I don't care at all about looks of the wheels or the flushness of them

A bonus question - I read conflicting opinions about using 215/45R17 tires on 17x8 wheels. This setup is within the manufacturer's specs for the tires, but some people raise concerns about the mild stretch, stating that 7.5 wide tires are better. While others claim that the mild stretch is actually beneficial in reinforcing the sidewall.

P.S. Yes, I'm probably way overthinking this, what with people happily running 9 and 10 inch wide tires with 25mm spacers, but I'd be pretty sad to spend $1000 on wheels that will dull the feel of the car.

It won’t really affect the feel of the suspension much. A wider wheel base will be more stable in high speed turns and less likely to lift the inside wheel/rollover. People typically look at wheel size + offset ratio more to find flush fitment. The 17x8 40 or even 35-38 if you find it would be a great choice for fitment.
A 215 tire will fit fine no issues. however any reason you don’t want to take advantage of the extra wheel width and go with the wider 225s for better grip/handling? It is a pretty good common pairing a lot of people do for NA power levels.

Affect of wheel width and offset on stock suspension feel
 

The tire is going to make a much bigger difference than the wheel width or weight.

If you don’t skimp on the tire you fit (which it sounds like you aren’t), you will notice no detrimental difference in the feel of the car. More grip and stiffer sidewalls of superior tires will make it feel a lot better across the board.

I just put my 17x8 ,45 wheels back on with 225/45/17 SX2s and the car feels immensely better.




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Mainly to keep the car from being "over-tired", as well as weight - 225/45R17 weigh 3 lbs more than 215/45R17. I know that lots of people go up to 225, but many also stick to 215. Since my main concern is the feel of the car, I'm planning on incrementally change it. I might go up to 225 in the future, though.

225 is still far from over “tireing” the car and will still feel amazing. As good or better than 215, and will out perform 215s in every way. 3lbs if it is that much is still minimal and the lighter wheels more than make up for it. 215 is a great size for stock power. If you end up going full bolt ons you’ll really wish you had at least 225. Many just go straight to 17x9 with 245s as 245s are faster around a track even at stock power vs 215/225s and you will future proof yourself for bigger power. I got 17x8 w/ 225s and it was perfect for NA full bolt ons, but i had to replace the rears with 17x9 w/ 255s once i went FI, in hindsight i wish i would have planned more in advance to save money not rebuying. 2 Wheels and 4 tires.

I could always widen the tires later (especially with 8 inch wide wheels), and currently I have no plans for FI (should I really wish more power, I'll keep the BRZ as a slow fun car and get another car that's FI from the factory).
As I said, I don't track, and I don't care about going faster around bends - I want to preserve the feel of the car, which from all I read is very easy to mute with wider tires and wheels. As a first step I still have the stock Primacys anyway, and I find them decent in the dry. The car is extremely fun in back roads, and I want to preserve this.

Ideally your tire sidewalls should be square with the rim width. Not stretched or ballooning. A stretched tire will have little compliance (feel hard), and a ballooning tire will have too much flex (feel boaty).

Though a mild stretch may look cool, too much stretch will make your ride harsh, have higher risk of curb rash, and slide easier. My BRZ with the PP 17x7.5 rim and 215/45R17 Primacy's seems to handle looser than other BRZ's with the standard 17x7 rim with the same tire. So I already don't like a 215/45 tire on a 17x7.5 rim - 225/45 is ideal. I've seen the stock Primacy's put on 17x8 rims and it looks way too stretched. I really don't recommend it.

As for offset, too aggressive (lower) offset without a proportional increase in rim width will increase the scrub radius. While the car is more stable from the wider track, the higher scrub radius has a negative effect on steering effort and nimbleness of the car. Also the car will have a higher tendancy of tramlining. So on stock suspension, I would not mess with the suspension geometry by significantly increasing the offset.

N_Raged: "depends", slight stretch also nets one sharper turn-in/steering response, and i've seen people preferring other way, and adding extra wheel width or going down tire width just to retain that sharpness, with "right size" often feeling too loose/squirming around. Then there is also bit that some tires real width might run a bit wider then "specs". And for example this article, where guy in general suggests mild stretch/preload in most combinations. So imho it's more upto individual driver preferences, just like suspension stiffness, for example in several cases less roll doesn't net more speed not because there is more grip (usually opposite), but because driver may feel more confidence pushing more in such car.

Yea big "depends". I know plenty of performance cars like Porsches come with slightly stretched tires for that sharper response. As long as the suspension was set up for it.

Exactly the conflicting opinions I was referring to :)
I tend to agree with the idea that a mild stretch is beneficial. At the end, 215/45R17 tires are rated from the manufacturer for 7" to 8" wheels widths, so we're not talking about anything extreme or out of specs.

Yes, that was my impression as well. I do wonder how much of a difference would 8mm of offset do, though. This also comes together with a wider wheel (8" vs. 7" or 7.5").

That's an interesting observation, one that I came across several time with regards to the Twins and to other cars as well, and exactly the thing that made me post this. Since I like the way the car handles stock, it would make sense to stick close to the stock specs (although taking this logic too far would mean using just the OEM wheels and Primacy tires). What I would really like to know is how far can I stray from the OEM specs before the handling characteristics of the car are changed?

Ohio Enthusiast: one big "stray from" is already from grippier tires. One immediately looses that tire chirping & rear playfulness fun at legal speeds :(, with loosing traction only at much higher speeds, mostly on track. Many may shun stock primacies, but really, there is something in having fun and not ticket for it :). Then again, when one moves up to trackdays, usually that "street-fun" is one of soon sacrificed things in hunt for more speed.

That's an interesting take. I know Toyota has defended the "Prius tires" decision with similar logic (and it does make sense). In truth there's still some time before I do these wheel and tire changes. Current plan is buy winter tires in the fall and mount them on the stock wheels, then in the spring buy new wheels and mount the stock Primacys on them. This leaves me all of this summer to learn the car with stock setup, and next year to learn the car with stock tires but lighter wheels.

At the end I suppose it also greatly depends on what one considers as fun - I enjoy the wonderful turn in and steering feel of the car, together with the rear wheels pushing in the corners. But this is my first RWD car so I still need to get a feel to what's fun with it :)

You're not going to hurt the feel and the fun of the car with wider and different-offset wheels and tires. Weight isn't that big of a deal either. IMO you can run what you think looks best (within reason!) and don't worry about it. I've gone back and forth between stock 17x7.5 +48 with 225s and 17x9 +35 with 245s at the track and it's just not that big a deal.

Agree with comments above that tire make/model selection is a bigger deal on handling and feel.