Darebren

I'm an ex Ford/Mazda employee, I know that some of the Mazda's are more "Ford" than Mazda, but this one is an another exception like the Milennia and Miata. I saw one or two of these RX-8's around the Ford Headquarters as far as a year back, prototypes, but I knew it was coming and I knew I needed one! 18 inch tires, and they even throw in a back seat! excuse me, may I move to the front of the line please? when can i order....

Alienate

Hailz!

If there is a good thread explaining what I am about to ask.. just point me there.

I have been thinking about tires and tire sizes in general.

Why do ppl want larger rims?

First off.. all else being equal, it raises the car's CG.

But, if you use a lower profile tire to offset that issue, you lose the tire sidewall height, which results in less AIR, and therefore, a stiffer.. read BUMPIER ride.

Wouldn't going to a 16" rim versus an 18" rim.. with the same profile tire... give you a lower CG and quicker accelleration?

Or use a 17" rim with a higher profile tire to give you the overall height of an 18" rim with the low profile tires?

And! Can't you put a WIDER tire in a wheel-well if the rim is smaller.

I love wide tires. :p

Thanks in advance for your thoughts.

N8

PatrickB

Larger rims in the same diameter tire result in less sidewall on the tire. You are of course right that this means a bumpier ride, since there's less air and rubber sidewall between the road and the car. Unfortunatley, taller sidewalls also mean the tires flex more on turns, resulting in softer, mushier handling, as I understand things. All of this is balanced by the hardness of the rubber used in the tire, too. Softer rubbers are considered grippier on cornering, but would of course result in a softer sidewall, so tire sizing and design is a tradeoff between grippiness of the tires, stiffness of the sidewall, and treadwear.

Performance cars want the grippiness of soft rubber tires, but the stiffness of short sidewalls. Passenger cars (and runflat tires) use harder rubber and taller sidewall for tires, giving better treadwear and softer ride and handling, but at the cost of some of the ability to grip the road.

-Patrick

Quick_lude

If you don't like the ride of a sports car with low profile tires, this car is not for you. Get a cushy sedean or an suv..

said7

Aside from that, 18 inch and up rims look awesome with lowering springs.

Id sacrifice ride quality for handling any day.

BlueAdept

How bad are your roads anyway?

I notice about 50% more road noise and slightly harsher ride on low profile tires, but this is almost gone at highway speeds on a good surface.

Alienate

Hails!
Thanks Patrick. Your reply helped me clear up some of my ignorance on the subject.


Quick_Lube, on the other hand.. made a smartarse reply that shows why newbies have a hard time feeling at home on this board.

Luckily, I have been on many boards over the years and tend to overlook cheap shot artists.

I don't need some ******, still wet behind the ears, telling me what car I should or should not buy. :p

N8

zoom44

it depends on what part of this continent you are driving on.
northeast u.s. is bad but getting better. southwest u.s. is pretty good, northwest is ok but getting worse. i seem to remember florida roads being sort of bad but that was a long time ago.

BlueAdept

There are some truely bad ones here, but mostly it's very good... The biggest problem is those damn humps they keep putting everywhere to stop you finding a short cut! If you're ever lost, in a town in England... just find a road with humps, it's SURE to go somewhere useful!

Amusing fact,

Some of the original Roman roads in England have only needed resurfacing 3 or 4 times in the last 2000 years... I can't remember where I heard that, but I'll ask. I guess they never had 35 tonne trucks back then!

Also, while quick_lude's remark was a bit short and perhaps unwise... there is an element of truth in it, there are tradeoffs in every car.... and this one will sacrafice ride and silence for handling on ANY tires, although especially on the 18" low profiles. Equally, your point would be just as well made without a "cheap shot" of your own.... I imagine it's all too easy to forget that people from all walks of life are here, and people tend to assume a bit much somtimes... no insult intended I'm sure.

Let's not all fall out BEFORE the car arrives, we've got so much to do!!!!

Finally, incase it wasn't clear... the OVERALL diameter of the wheel&tire will be the same on 16's as 18's .. .it's all a matter of profile.

The measurement on a tire I think will be on the RX-8 is 225/45R18 where:-

225 = Width of tread in Millimeters.
45 = Profile height as a percentage of tread width.
18 = Wheel diameter in inches.

So, that wheel is 18 inches or 457.2mm in diameter, the sidewall is 45% of 225*2 = 202.5mm so the overall diameter of the wheel is 659.7mm

My guess is that the 16" rim will be reduced to at most 205, the rim being 406.4mm. This means that the profile has to make up 253.3/2 = 126.7mm

127.6/205 =0.618, so the ideal profile would be 62, but 60 is the closest.... Thus the tyres on the 16" rims would be 205/60R16 giving an overall diameter of 123mm*2 + 406.4mm = 652.4 mm.

The final, overall diameter is as close as it can be! I hope that's interesting/useful.

As for wider tires/higher profile... I hope somone will correct me, because I'm winging this... but the weight of the car is supported by the air pressure in the tyre, so there shouldn't be any need to increase the profile as you make the tire wider... just the opposite, you'll have to lower the profile as the tyre gets wider just to keep the sidewall height the same (remember that it's a percentage of the width!)

Sputnik

As a general rule, but there is a point of diminishing returns. Regardless of whether you want a softer ride or not, some give in the tire is necessary so that the tire can flex to keep an optimum contact patch on the surface (not only on imperfect surfaces, but when the wheel happens to tip left or right in cornering).

---jps

Grimace

Didn't R&T run a test on a BMW outfitted with several different tires a year or two ago? There were 60-series, 16", 50-series 17", 40 series 18" or something along those lines. I think they concluded that an aspect ratio of 45 or 50 is juuuuust right, as Goldilocks would say.

Rich

Just right in that test doesn't really mean much for any other car in another situation. For instance, in a large and heavy car with an extremely capable suspension (I'm thinking maybe a Diablo), a few pounds of extra unsprung weight probably won't make much difference, so a larger wheel (heavy) might give you the benefits of a shorter sidewall without a big drawback. Also, the wider tires mean that for a given aspect ratio, you get more sidewall height than you would in a narrower tire. In the end, on that car maybe a 35 series (just guessing here) tire would be just right. Lots of people think that for the Miata the ideal aspect ratio is around 50 or 55 because of the lower unsprung weight, which is extremely important in a lightweight car.
Also, as the aspect ratio for a given changes, so does the construction. That means that when you're going to a smaller aspect ratio, you're also changing other parts of the tire, and one tire's changes may not match anothers. In other words, even if you found a sweet spot for one tire, on another tire the sweet spot may be different.

Long story short, even if 45 or 50 was just right for that BMW, it doesn't say much about what will be right for the RX-8.

Quick_lude

Learn how spell my name or get more original jokes. If I had a dollar for everytime I heard "quick_lube"

As far as I understand Mazda's mission for this car is for it to be a sports car with 4 usable seats, ergo low profile tires. So if that is the case the tires should be in the 40-50 profile range. The ride will be bumpy compared to most cars, it's something we'll have to accept.

Good enough answer granpa? :p

Alienate

Nice calcs Blue.. thank you... and Grimace makes my point... Given all the above "banter".. I believe a 50 series on 17" rims will be the best ride FOR ME.

N8

Quick_lude

But since the aspect ratio is also affected by tire width/rim size you need to take that into effect right? So if you had a 255/50/17 tire and a 215/50/17 tire, the sidewall height and comfort/handling characteristics will be much different.
Also are we absolutely sure it will be offered with 18" rims only? Mazda could still offer 16 or 17" 'base' rims and perhaps recalibrate the speedo and spring/shock settings.

Grimace

True, I should have been more clear. The BMW example is valid though, in my humble opinion. Similar weight, wheelbase, suspension tuning (a guess here), rear wheel drive, etc. But like you said, it all comes down to the details.
You wouldn't want to put 35's on a Escalade, for example... although everyone does around here... :p

BryanH

I don't mean to single you out but I think this isn't always the case. Performance high-profile tires, while they may have a softer compound, are constructed with stiff, thick sidewalls to keep the tread stable. On my MR2 I have a set of DOT-approved race tires (Kumho Victoracer V700s) which are 195/55/14. Their ride is no better (perhaps even worse) than the 195/50/15 street tires I use.

The tires that roll over the most under hard cornering seem to be cheap, long-lasting touring tires. Performance rubber is usually pretty good, no matter what size it is.

Hercules

I got Kuhmo Supra Esctas on my car.. can't complain.. they are plenty grippin in the wet and dry.

BryanH

Those are my street tires too. For the price, I can't complain either. But I've had better street tires. Just got a big hole in one last week so I just got some Yokohama AVS ES100s to try. I hear the Miata guys like these better than the Kumho ECSTA Supra 712s.

Hercules

*nod*

I just needed cheap tires for a replacement, and those did it well... My car is going to my brother after I get the RX-8, so I really don't care after that

BlueAdept

I don't think we can be totally sure of anything, but the option sheet we saw from Germany showed that the car comes with 18" rims, or 16" in the 5speed (auto?) version. That version has smaller brakes thou as the "uprated" oned won't fit inside anything less than 17" and only 192 PS.

There shouldn't need to be any more than trivial speedo calibration, if any due to the overall size being as close to the same as makes no odds, but somone who knows might like to tell me what suspention adjustment of any might be wise between say 18" and 16" rims.

I'm guessing, with the justification from the earlier post that the car will have 205/60R16's or 225/45R18's I could work out the size necessary for 17" rims and a given width if anyone likes! but I think that if somone wanted 17" rims then they would either have to make a deal with their dealer or find somwhere to sell the 18's.

In truth, I don't know how much difference it would really make, but my guess would be "not much", if any.

Spoonie

16" Rims? Baby Shoes! 17's are okay but if you want the look get 18" rims. They look waaaaaay better. In this day and age 16" rims look super tiny. Hey, why not get 14" or 13" wheels. Now that would look sweet on a high performance car.

Quick_lude

Yeah but 17" tires are still relatively cheap. 18" high performance tires are $230US/$450 Cnd and they don't last long either.