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Discussion Starter · #1 ·
The Mazdaspeed6 suspension/tires and the Michigan weather beaten/truck abused roads do not mix. The OEM Bridgestone 050A and snow DO NOT mix. I'm looking to improve the ride. I was considering 225/50R17s on 17x7 w/ 50mm offset. The 225s might offset the reduced traction in summer.

Options I have considered are:
1) Change out the Bridgestones for all season tires (e.g. Pirelli PZero Nero M&S). I suspect ride improvement would be small.
2) Change out the OEM wheels and tires for lightweight 18" wheels (18.5#) and all season tires. I suspect the ride would improve more.
3) Change out the OEM wheels and tires for lightweight 17" wheels (17.2#) and all season tires 215/50R17 or 225/50R17. Will the 225s on 17x7 50mm offset work on the speed6, w/o modifications? Can I run tire pressure less than the OEM 38 psi?

Keith_MS6
'06 Speed6, Liq Plat, GT

PS - thanks socsndaisy and poohster for your feedback.
 

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heavier wheels absorb more road imperfections than lightweight. if you go to light wheels your ride will suffer slightly. If you went to a 225/50 the heavier tire and slightly taller sidewall will help improve ride, especially on an all season tire. the 225/50 will have a larger contact patch due to both the increased tread width and diameter of the tire and will give you slightly better straight line grip.
there's a lot of "slightly" in there. if you want to improve your ride, just read the reviews of the tires in the size you want on tirerack.com and look for what you think will be the best compromise.
of course I traded my oem touring tires for extremely stiff summer tires and barely notice a difference. Our roads aren't the best either.
 

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Discussion Starter · #5 ·
postosh: How is "the 225/50 will have a larger contact patch due to both the increased tread width and diameter of the tire " a misconception? The OD is about the same so length of the contact patch does not change much while the width is increased.

Keith_MS6
 

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The Mazdaspeed6 suspension/tires and the Michigan weather beaten/truck abused roads do not mix. The OEM Bridgestone 050A and snow DO NOT mix. I'm looking to improve the ride.

3) Change out the OEM wheels and tires for lightweight 17" wheels (17.2#) and all season tires 215/50R17 or 225/50R17. Will the 225s on 17x7 50mm offset work on the speed6, w/o modifications? Can I run tire pressure less than the OEM 38 psi? [/b]
.........Bingo, light (not heavy) 17x7" wheels, and 225/50's all seasons will come in SL load ratings, vs oem XL, and will take less pressure to hit target load capacty, like about 32 psi. Step down to V speed rating and will ride much better over those rough roads. Search for fit issues and offset. Handling has much to do with whose hands are on the wheel.
 

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How is "the 225/50 will have a larger contact patch due to both the increased tread width and diameter of the tire " a misconception? The OD is about the same so length of the contact patch does not change much while the width is increased.

[/b]
See the very lively discussion here.

A bit less technical, a bit less back-and-forth: (A very long page. Use Text Search to find: Fat or thin?)

Summary: as a practical matter, the size of the contact patch (as opposed to its shape) is determined by the mass the tire bears and the pressure to which the tire is inflated.
 

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See the very lively discussion here.

A bit less technical, a bit less back-and-forth: (A very long page. Use Text Search to find: Fat or thin?)

Summary: as a practical matter, the size of the contact patch (as opposed to its shape) is determined by the mass the tire bears and the pressure to which the tire is inflated.
[/b]
*************************
Fat or thin? The question of contact patches and grip.
If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm².
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm²
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm² / 22.5, which is 7.8cm.

And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less the same. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.

The area of the contact patch does not affect the actual grip of the tyre. The things that do affect grip are the coefficient of friction and the load on the tyre - tyre load sensitivity. Get out your geek-wear because this is going to get even more nauseatingly complicated now.

http://www.carbibles.com/tyre_bible.html
 

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Discussion Starter · #10 ·
I gather that there may be little traction to be gained between the 215 and 225 widths. Ok. My priorities are: all season traction (to include 2-3 inches of snow) and improved. Maybe VR speed rated 215/50/17s are the way to go. Chances are pretty good that I will not see 150 mph. I can install the OEM wheel/tire should I choose to auto-x.

I thought the 225s would being .2 inches higher and would fillup the wheel well better while smoothing the road than the 215.

Still have not heard an affirmative yes or no on the fitment of a 225/50/17.

www.carbibles.com was a wealth of information; well written with bits of personality.

The KZ-Vs (tirerack) and the 5Zigen Fn01r-c (edgeracing) look nice and light. The Falken ZIEX ZE-512 look ... inexpensive (edgeracing).
 

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Discussion Starter · #12 ·
You can find the answer in the stickied topic at the top of the Wheels/Tires section:

Choosing the Right Size Tire
[/b]
The information provided in the "stickied topic" is excellent. But don't the tire sizes lisyt4ed depend on the wheel application, specifically offset?

Keith_MS6
 
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