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Discussion Starter · #1 ·
I will have access to a set of Dunlop SP Sport Maxx RT tires in 235/45-17 size. My question is, will they work on stock wheels and suspension. And yes, I like the stock wheels. TIA
 

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The 235 will be not quite an inch taller and wider and your speedometer will be off by about 2.5% (slower than real speed). They'll fit, maybe not an optimum solution but will work.

Most people go up in size and lower in profile, the stock 215/45R17 would become 235/40R17 and would preserve most of the diameter/height and not impact the speedo.
 

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Discussion Starter · #3 ·
Thanks Ragnar, I am ordering a 2015 WRX soon and itwill be a year round vehicle and the Dunlops are summer only use. I amthinking about Michelin Sport AS 3s for allseason use and then use the take offs on theBRZ as they are summer only and have to be stickier than thePrimacys onthere now. Also the larger size will fill the wheel well somewhat more too. The car would be raised less than a half inch and have almost an inch more onthe ground. I would solve 2 tire issues on 2 cars with one new set of tires.I also have a use for the Primacys, but thats another story, solving another tire issue again. T
 

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235 is a tad wide for the stock 7 in rims, you may get some rollover in hard cornering, i.e. you may be disappointed in the grip at the limits.

I just purchased a set of Michelin Pilot A/S 3 in 225/45 x 17 (just 1% taller than stock) to use for summer, on OZ Alleggeritta 7.5 x 17 in graphite grey.

They won't go on until April though as it is still real winter here. Pirelli Sottozeros work well in our winter, same size 225/45x17 on the stock 7x17 rims and they work fine.
 

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Remember all: The lower the aspect ratio (35 vs 45) the closer the wheel rim is to the road and therefore a greater potential for wheel damage. Alloy wheel repair, I have found, has been a growing industry.
 

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Discussion Starter · #6 ·
The BRZ has 215/45-17 oem and the ones on the WRX will be 235/45-17 ,so the aspect ratio stays the same and the car would have more rubber between the wheel and the road. It would be larger diameter and wider with more rubber on the road.The larger tire would fill the wheelwell gap somewhat too, improving the look.
 

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The BRZ has 215/45-17 oem and the ones on the WRX will be 235/45-17 ,so the aspect ratio stays the same and the car would have more rubber between the wheel and the road. It would be larger diameter and wider with more rubber on the road.The larger tire would fill the wheelwell gap somewhat too, improving the look.
It also effectively changes the gear ratio, making the car slower.
 

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I suggest you stop in and talk with a professional such as at Tire Discount or Tire Rack.

I will have access to a set of Dunlop SP Sport Maxx RT tires in 235/45-17 size. My question is, will they work on stock wheels and suspension. And yes, I like the stock wheels. TIA
 

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Discussion Starter · #9 ·
I probably will talk to Tire Rack, but I seem to have lots of time as it looks like WRX production doesnt start until April. All of this thinking was in finding a good use for the takeoffs from the WRX and knowing they will not have much remaining value after removal, their being used at that point, even with no or few miles. I have seen pics of BRZs and FRSs with some pretty large tire setups and thought this was worth a good look. Just wondering what others have done.
 

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The BRZ has 215/45-17 oem and the ones on the WRX will be 235/45-17 ,so the aspect ratio stays the same and the car would have more rubber between the wheel and the road. It would be larger diameter and wider with more rubber on the road.The larger tire would fill the wheelwell gap somewhat too, improving the look.
This might surprise you:

BND TechSource - Tire Data Calculator

Contact patch area is proportional to vehicle weight and tire pressure. Very little changes with tire profile. The larger tire in this case will be significantly softer unless tire pressure is increased giving poor grip compared to the smaller tire. The larger tire will have lower contact patch pressure unless tire pressure is raised.

In any event, contact patch area isn't very important to grip.

Coefficient of friction goes down with tire load but torso grip goes up, it's a curve not related to contact patch area.

You're correct a larger diameter tire will be closer to the fender lip.
 

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"In any event, contact patch area isn't very important to grip. "

This is not true.


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Tire grip increases with load. Contact patch size isn't relevant. This is why increasing tire pressure (which reduces contact patch size incidentally) increases grip to a point. Increasing tire pressure reduces contact patch size, increasing load per unit area and increasing grip.

It is a very common mistake to think contact patch size is important to traction.
 

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Tire grip increases with load. Contact patch size isn't relevant. This is why increasing tire pressure (which reduces contact patch size incidentally) increases grip to a point. Increasing tire pressure reduces contact patch size, increasing load per unit area and increasing grip.

It is a very common mistake to think contact patch size is important to traction.
So what your saying is that a 225 and a 255 (we can assume the same aspect ratio and wheel size) will have the same amount of grip?


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Tire grip increases with load. Contact patch size isn't relevant. This is why increasing tire pressure (which reduces contact patch size incidentally) increases grip to a point. Increasing tire pressure reduces contact patch size, increasing load per unit area and increasing grip.

It is a very common mistake to think contact patch size is important to traction.
"Grip" does increase with load, but not exponentially. The contact patch is limited to the specifications of the tire. I'm not sure what scientific study you are reading but you can't assume that the air pressure and load or weight on the tire is all that is important. Contact patch is important, as is sidewall stiffness, compound, section width, ect.

Don't worry common mistake.


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Tire grip increases with load. Contact patch size isn't relevant. This is why increasing tire pressure (which reduces contact patch size incidentally) increases grip to a point. Increasing tire pressure reduces contact patch size, increasing load per unit area and increasing grip.

It is a very common mistake to think contact patch size is important to traction.
No, it's a common mistake for you to post about things you know nothing about. Increasing tire pressure stiffens the sidewalls slightly, and decreases grip to the outside of the tires (when cornering, the contact patch shifts from the middle to the outside of the tire). This increases grip how? You wonder why you decrease tire pressure at the drag strip-- to increase grip off the line. At track days, I increase tire pressure slightly (to about 38 psi) not to increase grip, but to prevent chunking the outside of the tires.
 

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I think the most important thing to think about is tire wall stiffness. A 235 on the stock rim is going to leave the sidewalls bulged out and they won't have the support they were designed for. On a drag strip this wouldn't matter, but the BRZ is not really a drag car and I'm sure you'll be using it more for it's handling capabilities. I have read that it helps to actually under size the tires to the rims. So, if you had rims that normally support a 235 tire, put a 225 tire on instead. Now that was something I've read and not experimented with, but it kind of makes sense when you think about how important tire wall stiffness is.
As for contact patch, it's pretty obvious that the bigger the contact patch, the more grip you get. But don't think you need to go out and buy a set of 255's. You need to balance the power and mass of the car to the tire size. Or you wont have enough power to overcome the resistance of the tires.........if that makes sense. So bigger contact patch equals more grip but not more speed!

At track days, I increase tire pressure slightly (to about 38 psi) not to increase grip, but to prevent chunking the outside of the tires.
I'll have to put that to the test when I do my first track day. When I done track days on a motorcycle I always let out 3-4 psi because the operating temp of the tires would go up and so would the psi in the tires. Of course, motorcycle tires are completely different to car tires and don't have an "extreme" load change when cornering.
 

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The extra psi helps roll over for sure, even in auto cross situations. Most tires have a wear indicator on the side that I always strive to reach. Not too much on, but not over the line.

It is worth taking note as you run the tires and they heat up they will increase psi on their own. I usually have to lower them manually after a couple of runs. I believe rule of thumb is 1psi per 10 degrees increase.

Jedi I completely agree choosing tires is important for whatever your purpose. Unsprung weight and rolling resistance is something to keep in mind. Although most competitive road cars tend to try and use the widest tie possible.


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No, it's a common mistake for you to post about things you know nothing about. Increasing tire pressure stiffens the sidewalls slightly, and decreases grip to the outside of the tires (when cornering, the contact patch shifts from the middle to the outside of the tire). This increases grip how? You wonder why you decrease tire pressure at the drag strip-- to increase grip off the line. At track days, I increase tire pressure slightly (to about 38 psi) not to increase grip, but to prevent chunking the outside of the tires.
Plug your numbers in here:

BND TechSource - Tire Data Calculator

Then look at the contact patch size. Make sure you change the correct boxes.

Also, read up a little on friction physics, it will help and the math is very simple.
 

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Bigger contact patch does not produce more grip. Total friction force available varies with load and area, producing a constant mu. For a pneumatic tire mu decreases with load but total grip goes up, to a point. The friction circle available reaches a maximum at a lower mu but higher total force. The physics are fairly simple to comprehend conceptually but the precise effects are devilishly difficult to predict. In fact, the formula commonly used is even called the " magic formula "

http://bsesrv214.bse.vt.edu/Hop/Papers/Tire-Road Friction Coefficient.pdf

http://en.wikipedia.org/wiki/Hans_B._Pacejka
 
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