ilovepotatos

What's more annoying than listening to Keech yap?
Listening to Keeck say tire. TIRE TIRE!
NOT TYRE!

zoom44

thats pretty damn funny potatohead!! rotflmao:D and drop the chalupa :D

Quick_lude

Wider tires do not increase grip because they create a larger area/contact patch. Wider tires only CHANGE the shape of the contact patch, the actual area stays the same if you compared it to a narrower tire.

ilovepotatos

:D

Sputnik

That is completely incorrect. Wider tires at the same tire pressure do in fact have a larger contact patch.

---jps

jbebernes

I gotta agree with Quick_lude here...assuming equal inflation pressure for the smaller and larger tire.

Roughly 725 lbs to be supported by each tire (1/4 the car's weight) at an inflation pressure of 40 lbs per square inch gives a contact patch of a little over 18 square inches.

A wider tire gives you a wider, but less deep, contact patch. And better cornering grip.

Niebla

Is not just the tire that is wider, the wheel went from a 17x7'1/2 to a 18x10. There is a 2 1/2 inch diference of contact surface before you start.....

Quick_lude

I'm correct only if the overall diameter is set, so when doing plus 1 or plus 2 upgrades. Example, original whee/tire is 195/60/15, upgrade is 205/50/16. In this case "wider" tires don't mean larger contact patch, just different shape.
I'm sure a 305/35/18 tire has a larger patch than a 195/60/15.

MikeW

Shear strength. If the tires were 185's on the rx-8, then when undergoing performance driving the tires would shred away to nothing.

So when the stock 225/45 18 reach their end of life, the replacements will probably be 235/40 18. This is a roughly a 1.5% smaller overall circumference. compared with 225/40 18 3.5% smaller than 225/45 18 (rough average from TireRack numbers).

I see that 235/40 18's 'want' an 8.5 inch width wheel, but if Audi can do it with the new S4 (4000lbs & 60/40 weight distribution) so can the Rx-8.(3000 lbs & 50/50)

P00Man

bottomline, the rx-8 is gunna grip like a tank and speed like a bullet blahAHHA
________
NEW MEXICO MEDICAL MARIJUANA DISPENSARIES

BryanH

How do you figure? A 205 is wider than a 195 (by 10mm), whether or not you're changing the sidewall height and/or wheel size.

That said, it's possible to get tires that are too wide for the car (especially in lighter cars). You spread out the low weight of the car over a large area of the tire, yielding little pressure on each square inch of the contact patch.

wakeech

<<cough>> thatswhatisaid <<cough cough>>

jeez, winter is sucky 'cause it makes me sick. :D

BryanH

Oops. :o That's what I get for skimming some of the posts.

Buger

Hi Niebla,

Sorry I didn't respond to your above sooner but I posted something comparing the 350z and rx-8 0-60 and 1/4 mile times at the following thread: http://rx8forum.com/showthread.php?threadid=1499

Brian

ilovepotatos

Ahh!
I got scared again by buger.

MikeW

Any one see in the Car and Driver article where he (writer/test driver) hit 190kph/118 mph and said the engine was approaching peak power, was that in 4th or 5th gear?

Buger

I am hungry for Potato chips!

ilovepotatos

But I'm just a bunch of plastic and poly fibres!

Buger

With the gearing that I earlier estimated, 118 mph would be at approx 8350 rpm in 4th gear and 6250 rpm in 5th gear.

Buger

Contact patch size is a subject that many people disagree about. The below link does a pretty good job of explaining tire stuff: http://www.autospeed.com.au/A_0996/p...rc=suggestions

An excerpt is below:

"Myth 1: Wider tyres have a larger contact patch than narrow tyres

What actually influences the size of the tyre's contact patch? Is it the width of the tyre, or the profile? The simple answer that it is neither of these; the size of the tyre's contact patch is related to:

the weight on the wheel
the tyre pressure.

For example, say that the weight on the tyre was 900lb, and the tyre pressure was 10 psi. That internal pressure means that each square inch of area can support 10lb, so, in this case, the contact patch will be 90 square inches. If the tyre pressure was 30 psi, the contact area would be 30 square inches, and if the pressure was 90 psi, the contact area would be 10 square inches. This has been found to be almost exactly correct for most tyres (the exceptions being so-called run-flat tyres, or tyres with extremely stiff sidewalls). For most other tyres, carcass structure will have an effect, but by far the major factor is tyre pressure.

So, as you can see, the size of the contact patch of a tyre is not related to the width of the tyre - it is, in fact, proportional to the tyre pressure. What will change with the fitting of a wider tyre is the shape of the contact patch - it will get wider, but shorter longways."

zoom44

buger i think that is what rich said in a similar thread months ago. i'll search later to see if i can find it. unless you beat me to it

zoom44

here is what rich had to say

Not sure if this is what you're looking for, but...

To the best of my knowledge, there’s no “right” answer to many of your questions. I’ll start off with a few cold, hard facts that can’t (I don’t think…) be disputed. I’ll go into a bit of detail on things you say you already understand because I think the groundwork needs to be laid before we get into how things work in a particular application. This first post is basically wheels and tires 101. I’ll try to give some of my thoughts on your question in my next post.
Fact #1: Bigger tires do NOT have an inherently larger contact patch.
Many people think that bigger tires have a bigger contact patch. This is not true. The size of the contact patch is related to the pressure in the tire, the weight that is supported, and the strength of the sidewall. Let’s ignore the sidewall effect for a moment. The pressure in the tire is measured in pounds per square inch (or the metric equivalent, if you live in a civilized country). What does that pounds per square inch measurement really mean? If each tire is supporting ¼ of the 3000 lb. weight of the car, that’s 750 lbs. per tire. If the pressure in the tire is 30 lbs. per square inch, that’s 25 square inches of contact space. This does not change no matter what the size of the tire! If the tire is 6 inches wide, the contact patch will be a hair over 4 inches long. If you stick 12 inch wide tires on, the patch will be about 2 inches long (We’ll get more into this later). Stiff sidewalls can hold up a bit of the weight, so the contact patch will be less than is calculated here, but that is clearly a secondary effect. If that didn’t make sense, try this example. Picture an uninflated balloon. If you set it flat on a table, it will have a fairly large amount of surface area touching the table, because there’s very little pressure in the balloon. Now if you blow it up so that it’s really really full (lots of pressure), only a very small part of the balloon will be in contact with the table. Now, press down with your hand on the balloon. More of the balloon touches the table. You could use this balloon as a scale, if you wanted! All you’d need is the size of the part of the balloon that’s in contact with the table and the pressure in the balloon. Neat!
Fact #2: Unsprung weight is BAAAAD. Unsprung weight is the weight that is not supported by the springs of the suspension. This means everything from the tires, brakes and wheels up to the springs themselves, which are part sprung and part unsprung. It’s much more difficult to explain the physics of why this weight is bad, but I think an example works well. Imagine you’re driving in your car, and a wheel hits a bump. The job of the springs is let the wheel and tire move over the shape of the bump without jolting the car (upsetting its balance) or its occupants (upsetting their coffee). Shocks keep the springs from, well, springing. Left to themselves, the springs would just bounce and bounce and bounce. The shocks dampen this. Ok, what does this have to do with unsprung weight? The more weight the springs have to move, the slower and less effective they are. The more weight that gets moved by the springs, the harder a time the shocks have in settling the springs down. The less weight the springs have to deal with, the more effective they are at doing their job. Now, the sprung weight (everything supported by the springs) factors into this. If you’ve got a very heavy car, you can have beefy springs and shocks which can deal with the heavier unsprung weight better. This is why big wheels and tires have a huge effect on a Miata. Just an extra 7-8 lbs. in each wheel can make a Miata feel like it’s driving through molasses. The RX-8 is going to be about 500-700 lbs. heavier than the Miata, so unsprung weight will be less of an issue, but it’s still an issue. The ratio of unsprung weight to sprung weight is really the important thing.
An interesting aside – Why is it that many Miata drivers use very heavy tires for autocrossing? Simple, autocross courses are usually in a flat parking lot that’s swept before the race. In other words, they aren’t worried about taxing their springs and shocks with up-and-down movements. The stickiest tires are oftentimes heavy, and stick matters more in this context than weight.
Fact #3: Bigger tire circumference has the effect of changing the gearing to hurt acceleration and lowers cruising RPMs. This isn’t inherently bad, it just is. I’m not going to go into the gearing thing again (he hears a collective sigh of relief from the audience), but suffice it to say that if you keep everything the same and just change the wheels and tires, you’ll get more torque to the ground with a smaller rolling circumference than you will with a larger circumference. You will gain acceleration, but your engine will rev higher for any given speed. This isn’t usually a factor, since you screw up your speedometer and odometer if you go with wheels and tires that have a different sized circumference.

also here is the link to the thread if you want to read the whole thing. where is rich anyway?

BryanH

Interesting. I know from personal experience that between identical tires with identical pressures, between 195/55/14 and 205/55/14 there is a noticeable difference -- the 205s have more grip. It's not in my head either, because I ran the car with the 195s up front and 205s in back, and the other way around, and also with 205s all around. Whichever end had the 205s on it had more grip.

I plan to jump to 225mm race tires next season.

Quick_lude

No one is arguing that wider tires don't provide more "grip" This is because the contact patch is now wider and shorter which helps in lateral grip.

wakeech

well, it's not an empirical law that they do. i'm not going to repeat myself with all the luncheon meat i was cutting about how changing the area of contact doesn't affect the gross amount of friction created by a tyre, 'cause i've already said that. i've also already said a few things about tyre tempurature and how that's related to the amount of rubber one has on the road, and how that relates to grip.

just a question Bryan, were the tyres you were using exactly the same make, with the same amount of wear, of the same compound, contruction, and tread design?? if not, it's hard to validate your findings as true... i mean in the impirical sense, i'm not doubting whether those tyres you were using had more grip or not. if you say they did, then i trust they did.

ahhh!! post 666!! i've gotta do another one, quick!!

hmmmm... one thing i've forgotten: how tyre pressure lends affects tyre tempurature...