Rotating mass. We're trying to reduce it. So any wheel (rim) that is lighter (and hopefully stronger) than the OEM rims will be a good choice for upgrades.
I got a sweet deal on a mint set of JDM STI wheels which are 6-7lbs lighter. They are "flow formed" using Enkei's MAT process. OEM+, light, strong and a good price? K'mon! on they went.
You know the drill, good tires means grip. A decently priced and Summer tire is worlds apart from the stock OE Spec Yokahomas. Since Imprezas is an AWD car, grip is the game. Treat yo'self.
A common modification for aesthetics is adding wheel spacers to get a more "flush" look. However there are some performance benefits for adding wheel spacers, mainly widening track width. I've added 20mm spacers all around which increases the track width of the Impreza to 61"/61.5" which is wider than a stock WRX. Adding track width increases lateral grip when cornering giving a more predictable slip when cornering at the limit. Adding wheel spacers softens the suspension by effectively lengthening the suspension arm "lever".
Adding wheel spacers does not come without some detractors. There are some things that the tuner must consider. Adding spacers increases rotating mass, which reduces acceleration, and braking. Since the spacers push wheels out further, it also acts as a longer lever which in certain situations can amplify bushing flex in control arms. There is also the effect of slower initial turn in, however this corrects itself quickly on more steering input. There will be more "scrub", which can cause tire edges to wear quicker. Finally there is also slightly more wheel bearing wear.
The use of spacers is similar to that of using smaller offset wheels, so both would have similar effects stated above. Consider the reasons for adding wheel spacers. More often than not, the OEM offset (+48 or +53 for most Subarus) is often the best compromise for handling. Another way to achieve a more "flush" look with the fender is to run wider wheels, but those also have their disadvantages mentioned below.
Update I have since, removed the wheel spacers in order to regain the original steering feel of the OEM setup. I noticed increase wear on the inner edges of my front and rear tires. After running spacers for over a year, I will say that while it is safe to run spacers, however the extra wear on tires makes them something to reconsider. If using spacers, a small increment of 5-10mm would be agreeable. Anything more, will show up in tire wear and noticable handling differences.
While I am not here to dictate how you build your car and platform, I have noticed one place where many people fall victim to style-tuning vs performance-tuning. Often times I will see people slap on heavy and wide wheels purely to achieve a stanced look. Often times this results in poor acceleration and braking. While cornering load is technically better with wider wheels, a balance must be struck in order to attain all-round performance. A tire no wider than 235 has been quoted as the upper limit of this platforms tractive needs. Any wider will have significant scrub. Heavy and wide wheels may look good, but they will also hurt performance. Take note.
As a tuning reference, the STI optional wheels for this car are 17x7, Taking it one step further in the OEM+ direction, more performance oriented Subaru platforms BRZ/Forester utilize a 17x7.5 wheel. If we were to apply the +1 rule, 18x8.5 would be the ideal upper limit for wheel sizing.
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