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Old 10-29-2005, 12:45 AM   #1
Endless_Sti
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Question Best Tire Size for 05 sti.

Hello the winter season here in Chicago is not far away, so im planning to prepare early and get the Pirelli Pzero Nero M+S. However, I have one question. Should I get the Stock STI tire size which is 225/45/17 or go with 235/45/17? Or what is the best recommended tire size I can get without any problems. Thank you all in advance.
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Old 10-29-2005, 04:59 PM   #2
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Under no circumstance should you use a 235/45, it's the wrong size completely.

Stock RE070 Specs:

Height: 24.9"
Weight: 27 lbs
Width: 8.9"

235/45/17 PZeroNero Specs:

Height: 25.5"
Weight: 24 lbs
Width: 9.4"

By increasing your height you'll lower your torque at the wheels: BAD
By increasing your width you'll lower your overall traction in snow: BAD
I like the lower weight tho'

The PZeroNero is a fine tire, but you need to get it in the stock size, here are the specs...

Height: 25.2
Weight: 23 lbs
Width: 8.9"

If you want wider summer tires you can get 245/40's and they should fit fine, I like the PZeroNero Rosso's...

Height: 24.8"
Weight: 22 lbs
Width: 9.6"
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Old 10-29-2005, 05:06 PM   #3
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Technically the narrower tire the better in snow. This is because a narrower tire spreads the weight of that corner of the car over a smaller contact patch, consequently there is more weight per square inch of contact patch, which equals better traction on slipery surfaces. Just look at the tires the WRC cars run in snow, VERY narrow. However if you will be driving on merely wet, or even dry roads part of the time you will get better traction with a wider tire because there is more rubber to grip the road with. Even though the weight per square inch of contact patch goes down, the coefficient of friction goes up because the road isn't as slippery as snow. Back to your question, 10mm is less than 1/2 inch so it won't make that much difference. What will make more of a difference is the fact that an all season or winter tire will have a more rounded radius from the side wall to the tread of the tire. I went with Nokian WRs for my winter tire this year. So far my impression is that it's not as good on dry as the RE070 (no duh), and you can feel the car squirm around on the tread and the softer side walls of the WRs. I haven't had it on ice or snow yet though, and I expect that's where it will really shine. I bought a set of Rota Boost wheels to put them on just in case I hit something I couldn't see under the snow. I'd rather replace one of the Rotas than one of the BBSs.





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Old 10-29-2005, 06:23 PM   #4
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Quote:
Originally Posted by STirocket
However if you will be driving on merely wet, or even dry roads part of the time you will get better traction with a wider tire because there is more rubber to grip the road with.
Although I agree with your post as a whole, this part is incorrect. When you change to a wider tire you do not increase the amount of rubber that contacts the road, or add contact patch surface area, you merely change the shape of the rubber that contacts the road. The only ways to increase the amount of rubber on the road or add contact patch surface area are to...

1. Increase the weight over the tire.
2. Increase the weight of the tire.
3. Decrease the amount of air in the tire.
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Old 10-29-2005, 07:06 PM   #5
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Quote:
Originally Posted by UltimateLurker
Under no circumstance should you use a 235/45, it's the wrong size completely.
WRONG-0

There is a great deal of evidince that the 235/45 is a better match to the speedometer than any other size tire for the '05 STi.....it also lengthens the gearing a bit which is real nice too.

Now.....there are WAY-WAY too many people out there that are confusing the NON-STi Impreza with the WRX STi concerning tire size and that is where some people are, obviously, getting confused....they are NOT the same. NO....NOT.
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Old 10-29-2005, 07:17 PM   #6
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Quote:
Under no circumstance should you use a 235/45, it's the wrong size completely.
For a WRX, perhaps. For an STi it's not a bad choice. The STi's short gearing would be helped by a bit taller tire. The 225 P-Zero is taller by a little, and the 235 is taller still. If you keep the stock springs you should have plenty of clearance for the 235.

Quote:
The only ways to increase the amount of rubber on the road or add contact patch surface area are to...
Even assuming you're talking about mounting wider tires on the same given width wheel, I'm having trouble following your logic. Care to elaborate? Perhaps a bit of background on where you came by your information would be of use as well...

[EDIT: not ibU.S., d'oh]
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Old 10-29-2005, 10:39 PM   #7
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thanks for all the great replies. i drive more in the dry and wet then in the snow. i just need tires that will last me in the snow. therefore will u guys still recommend that i get the 225 over the 235? i need to decide quick becuz im ordering my tires next week. also, will the 235 affect nething else other then less traction on the snow, such as rubbing issues(stock suspension and springs), steering, or basically performance wise, and etc...? Thanks again in advance.
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Old 10-30-2005, 10:07 AM   #8
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For dry/track conditions, I'd propose a 17x8.5" wheel - should fit the 05' STi just fine, and you can go up to at least a 245 wide tire. This may give you better cornering (debatable) but definitly will give you more time on any given tire.
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Old 10-30-2005, 11:22 PM   #9
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Scotty: I'm assuming that Subaru knows what they're doing, you're assuming they don't.

Quote:
Originally Posted by gregr01
Even assuming you're talking about mounting wider tires on the same given width wheel, I'm having trouble following your logic. Care to elaborate? Perhaps a bit of background on where you came by your information would be of use as well...
It's simple physics. The amount of rubber in contact with the road is dictated by the amount of air in the tire, the amount of weight over the tire, and the weight of the tire itself.

Imagine you have 2 tires, a 225/45/17, and a 245/40/17. Imagining that the tires are the exact same weight, and the vehicle weight remains constant, it would be natural to assume that changing from tire 1 to tire 2 would give you more contact patch. Unfortunately, it won't because all you've done is change the shape of the contact patch, you've made it wider side to side, yes, but thinner front to back. Still confused?
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Old 10-31-2005, 12:51 AM   #10
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Quote:
Originally Posted by UltimateLurker
Scotty: I'm assuming that Subaru knows what they're doing, you're assuming they don't.



It's simple physics. The amount of rubber in contact with the road is dictated by the amount of air in the tire, the amount of weight over the tire, and the weight of the tire itself.

Imagine you have 2 tires, a 225/45/17, and a 245/40/17. Imagining that the tires are the exact same weight, and the vehicle weight remains constant, it would be natural to assume that changing from tire 1 to tire 2 would give you more contact patch. Unfortunately, it won't because all you've done is change the shape of the contact patch, you've made it wider side to side, yes, but thinner front to back. Still confused?

LOOK....I have researched this a great deal and there is all sorts of evidance that I am correct....the S203 uses 235/45-17's stock and many '05 STi speedometers have been reported to be inaccuarte with a 225/45, but spot on with 235/45's....so...what is your point????

I'm talking about tire size NOT contact shape or size and it you wanna split hairs.....go somewhere else and do it.
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Old 10-31-2005, 12:34 PM   #11
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Quote:
Originally Posted by Uncle Scotty
LOOK....I have researched this a great deal and there is all sorts of evidance that I am correct....the S203 uses 235/45-17's stock and many '05 STi speedometers have been reported to be inaccuarte with a 225/45, but spot on with 235/45's....so...what is your point????

I'm talking about tire size NOT contact shape or size and it you wanna split hairs.....go somewhere else and do it.
1. Reported to be inaccurate? I can believe that, though I've seen no GPS or Radar to prove it, but if you say it's so, that's fine. Subaru can be a little weird that way. I remember looking at the stock WRX wheel/tire combo overall height compared to their Factory Authorized After-Market BBS wheel/tire combo and it was .5" off, which is actually a lot, but still suggested by Subaru.

2. The second part of the post was not intended for you, but for gregr01 .

3. Don't tell me to go somewhere else, you have zero authority to do such .
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Old 10-31-2005, 12:55 PM   #12
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Going back to the original question: For snow tires, narrow is usually better. I'd go with the 225s.

Wider is usually better for dry traction, but since you're talking snow tires, you won't really appreciate the slight benefit in running 235s. The tire compound isn't going to be track oriented, it's going to be of a softer snow/ice makeup. So, there's not much sense in going with 235s, as you won't really notice much of a dry road grip benefit. (Hard cornering will just wear away the nice grippy blocks and sipes on your tire's edge anyway, and those are pretty handy in the snow.)

As UL mentioned, the contact area is pretty much the same with different tire widths. However, it's the shape of that contact area that is a factor. Think of the frontal length. A narrow tire will create a narrow channel in the snow, while a wide tire has a wide channel. The wider the tire, the more snow that has to be pushed or crushed out of the way.

Lastly, as others mentioned, different tire ratios do affect your gearing. Again, there isn't much of an impact with a 225/45 vs. 235/45. You're input with the clutch and throttle will have more of an effect on things than a small change to your gearing.

KR
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Old 11-01-2005, 12:07 AM   #13
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My speedo is way off with 225/45's, slightly less off with 235/45's. But it still reads fast (as indicated by those "your speed is" roadside radars.)

I went with 235's for winter just because I think it is a better match for the 8" rim. The amount of actual deep-snow driving one does in winter is typically pretty limited, so while an extra 10mm of width may not be that noticeable in the dry, conversely, I am not sure how noticeable 10mm less would be in the snow.

Basically I wanted a little extra rubber for rim protection in the event of sliding into curbs and such.

Quote:
Originally Posted by Endless_Sti
also, will the 235 affect nething else other then less traction on the snow, such as rubbing issues(stock suspension and springs)
apparently people are stuffing 275's onto stock '05s for autoX, so you are not going to have a problem with 235's
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Old 11-01-2005, 12:43 AM   #14
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Quote:
Originally Posted by UltimateLurker
Although I agree with your post as a whole, this part is incorrect. When you change to a wider tire you do not increase the amount of rubber that contacts the road, ...
I'm still having trouble with this... My thinking is that if all other factors are the same (car weight, tire weight, air pressure, tire diameter) a wider tire WILL give you a bigger contact patch. Are you suggesting that a 6" wide tire will have the same contact patch as an 8" tire? If that's the case why do they make wider tires? This isn't physics, it's simple math. A wider tire puts more rubber on the road. If the air pressure and the diameter stays the same the width of the contact patch front to rear will not change regardless of the width of the tire side to side. Consequently a 6" wide tire on an 8" wheel will have a smaller contact patch than an 8" wide tire on an 8" wheel. Am I missing something here? Please clarify...
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Old 11-01-2005, 02:23 PM   #15
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Yeah, you're missing something, but it's common, just think about it a little harder.

If the vehicle weight, tire weight (make, brand, etc.), and tire psi remain the same, a 2" wide tire and a 15" wide tire will have the exact same amout of contact patch surface area, it'll just be a different shape, the 2" tire's will be longer front to back, and the 15" tire's will be wider side to side. When you add contact patch to the side by making the tire wider, you take it from the front/back.

Again, the size of the contact patch is dictated by 3 things:

1. The weight of the vehicle.
2. The weight of the tire.
3. The amount of air in the tire.

If you compared a 15" solid steel tire to a 2" solid steel tire you'd of course have 13" more contact patch surface area on the 15", but if the tire is made out of rubber, and filled with air, you have a little thing called distribution.
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Old 11-01-2005, 05:44 PM   #16
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....so why the hell does putting bigger tires on a car make it handle better?????

More g in lateral excelleration???

Try autoxing with a 2" wide tire....
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Old 11-01-2005, 06:22 PM   #17
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I think its worth mentioning that the 225 on a 8" rim is on the narrow range, and will leave your rims very vulnerable to curb rashes that a 245 would likely avoid.

Last winter I ran my 225/45/17 Hankook W300s on my 17x8" rim, and the tire sidwall was angled pretty sharply away from the rim. Why doesn't the stock STi tire do this? I dunno, but some people have speculated that the stock 225's are very wide for a 225.

Unless you know you'll be doing lots of deep snow driving where a narrow tire will benefit, go for the 235/45/17 or if you're nervous about overall tire height, a 245/40/17.
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Old 11-01-2005, 08:36 PM   #18
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Quote:
Originally Posted by fragment
I think its worth mentioning that the 225 on a 8" rim is on the narrow range, and will leave your rims very vulnerable to curb rashes that a 245 would likely avoid.
Hmm, that is a good point to consider. You do want to avoid the dreaded curb rash, especially if you are using the stock BBS rims. However, I'm not sure I would run 245s as my winter tires (just my opinion). Likewise, I'm not sure which, if any, tire manufacturers have included sidewall rim protection in their tire design. (I know some used to use a different sidewall radius on their snow tires, so they would help protect the edge of the rim.)

Quote:
Originally Posted by Scotty, he's our Uncle!
....so why the hell does putting bigger tires on a car make it handle better?????
Ah, the 'why is wider better' question. I'm sure this has been discussed in detail elsewhere, and I'm no expert, so in a nutshell:
1. The shape of the contact patch has an effect on cornering power. A nice, uniform shape is good. Narrow tires can have an oval shape, while wide tires have a nice rectangular shape. Still, too wide a tire will have a concave shape, and might not grip quite as well.

2. Don't forget about your sidewall. A wide tire generally has a stiffer, and shorter, sidewall. That plays a role on your cornering power.

3. Lateral forces and gee-whiz formulas about friction co-efficients and abrasion factors, etc, etc, all play a role as well. While being forced sideways, your tire is also being distorted, and the forces acting upon it are not simply being applied in a linear way on one axis. Your poor contact area is now taking localized loads, and the leading edge can get the brunt of it. (The leading edge varies as your slip angle changes as well).
3a. Basically, it comes down to the shape of your contact patch again. A uniform shape can take, and distribute, the variance in forces better than a non-uniform shape.
3b. Take a really wide tire, and a really narrow tire. Stand them both up, and then try and tip them over. Which is easier to tip? What just happened to your contact patch?

4. The crown radius of the tire also is a factor. Wide tires generally have a bigger crown radius, and allow for a nice uniform contact patch.

There's other minor factors as well, stuff happening on a very small scale but still having an effect on things. There's also the fun technicality that the contact area does change slightly if the tire width changes (due to the change in amount of sidewall taking the load, and that the load across the contact patch is not uniform, even when the tire is at rest.)

KR
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Old 01-09-2009, 10:57 AM   #19
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I know this is OLD, but reading it gave me a headache. Why doesn't a wider tire increase the contact area? Let's say a 10 inch wide tire has a 2 inch front to back contact area and an 8 inch side to side contact area for 16 sq inches. Wouldn't a 20 inch wide tire still have 2 inches front to back, but say 18 inches side to side, for a 36 sq inch contact patch? I don't see how that can be wrong, but many people in this thread said width doesn't matter. Don't know if the original posters are still around, but maybe someone can explain this.
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Old 01-09-2009, 03:44 PM   #20
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Wow this is old.
Your missing the whole load part (weight over the wheel) of the contact patch. Keeping the weight the same in the example you explained above makes your math look like this:

10 in wide wheel:
2x8=16 sq inches of contact patch

20" wide wheel:
1x16=16 sq inches of contact patch

Yea not the best example but i tried to use what u had to better relate. There is no way to get a 36 sq inch contact patch unless u increase the weight of the car or weight over that wheel. You are basically taking the same load and distributing it over a greater area so the shape will change but not the overall area hence the 1x16 verse the 2x8.

To lazy but look up pressure loading equations and that will help.
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