red5eight

Hear me out though.

First of all this tire is extremely light, 19.4lbs and combined with Kosei K1-TS at 15.4 lbs that is a combined weight of 34.8lbs vs. about 47 stock. A tremendous savings that might make up some of the ride comfort issues with the extremely small sidewall of a 205/40. Ok so it won't have the ultimate grip of 245 tire but this is a street car not a track monster so my main concern is responsiveness and the super stiff ultra low profile RT615 should deliver in this department. Also the 23.4inch outer diameter vs. the stock 25.9 outer diameter would drop the car 1.25inches with no change in suspension geometry! Finally the smaller outside diameter would effectively give me close to the same ratios as the 2009 edition (see below).

My 2007 has 4.44 rear end, 3.76 first gear and (would have if I do this) 23.4 inch outside tire diameter means a radius of 11.7inches or .975 feet.

So wheel force would be (assuming 160 at the flywheel): (160*4.44*3.76) / (.975) =2740 lbs of force.

On a 2009 with 4.77 rear end and 3.88 first gear and 25.9 inch outside tire diameter means a radius of 12.95inches or 1.079feet.

So wheel force would be: (160*4.77*3.88) / 1.079 = 2744 lbs of force!

Almost exactly the same!

My only concern is: would this affect the ABS system? If anyone can point me to a definitive answer about this I would very much appreciate it. I am considering contacting Tire Rack (I assume mazda would just say don't do it no matter what). Anything else I should be worried about?

nuke0907

iTrader: (2)

yep you're insane.

rx8thunder

Just a tad insane I too am interested in the 4.77 gear ratio and lighter tires/rims.

However, I could never live with the look of such small and skinny tires on my 8, no matter how much performance it delivered. With the big arches on the 8, you need a decent sized tire or your 8 will just look weird.

Here's what I suggest because it's what I'm going to do. Order the 4.77 diff from Mazda and have the dealer replace your 4.44. The dealer I'm working with on this believes it will work but before they open my car up, they will compare it to a 4.44 gear to make sure it will in fact fit. I am supposed to take delivery on or around August 10. Fingers are crossed

In terms of lowering unsprung weight, go with a light 18 inch rim, 8 or 8.5 inches wide. I am going with TE37's. Match it with a light tire, no more than 22 or 23 pounds, putting you below 40 pounds total. Then get some racing brakes and lower the weight another 12 pounds per corner (fronts) and 3 pounds (rears). All of this will cost you some $'s but it will produce a great result I believe and you won't have to sacrifice the looks of your 8.

red5eight

thunder, your plan is sweet except the price difference as you noted. The tires are so much cheaper in the 205/40R17 that they would pay for the wheels ($189/wheel) in about 3-4 sets of tires. I'll consider doing the two piece rotors when these wear out but $1k+ for brakes is too rich for my blood. Also, the tire/wheel look is a personal matter, but I think that 18's look bad with the stock brakes. If the brakes don't fill the wheels then they are too big. What is the point of 18" wheels if you don't use the space?

As for filling the arches, I'm hoping the lower stance will cancel some of that effect out, but overall I agree it will look a little wonky, not a major concern for me.

rx8thunder

^ Take some photos

swoope

iTrader: (5)

the only concern the abs and the dsc has is the outside dia of the tires are the same size..

beers

nuke0907

iTrader: (2)

i think once you put those wheels/tires on you will be greatly disappointed in performance. not to mention it will be very ugly.

swoope

iTrader: (5)

simple stuff here..

the wheels do not fill out the wheel arches. the tires do..

tire dia is the tire dia. the wheel does not know what the tire dia is.

tire dia is what fills out the wheel arches. and lowering the car.

beers

red5eight

I doubt the performance difference will be noticable in street driving, even in fairly aggressive canyon carving I probably never go over about .8-.9g's which these tire should be able to handle. I enjoy the responsiveness of the car more than it's ultimate grip capabilities, so for me it is more important to have short, stiff sidewall than a really wide tire. Basically the weight advantage improves the comfort/responsiveness tradeoff at the expense of ultimate grip. Not a popular angle in the tuning crowd, but since I don't use ultimate grip much...

Fender gaps I can live with, I used to have an '85 fiero gt with 14" wheels and about a foot between the tire and the fender so this will be agressive by comparison :-)

P.s. special thanks to swoope for setting me straight on the abs issue! This upgrade (or should I say sidegrade) is probably about a month away but I'll definitely post pics when the time comes.

swoope

iTrader: (5)

see you lost me on this one..

if you are worried about speed it is tire sidewall height and stiffness and compound..


choose one or the other..

you fail.


street driving if you do anything over 80% you are an ***.. that is what the track is for..

beers

red5eight

That's what this tire is all about, low sidewall height, extremely stiff and agressive compound. I'm not worried about speed in terms of how fast the car is going, but rather in terms of how fast it reacts to steering input. My understanding is that tire width will have little effect on reaction time so 205s should be fine. The tradeoff (as I understand it) is a reduction in ultimate grip, which is fine because as you pointed out you can't/shouldn't get close to the limit in street driving.

So you may be getting closer to the limit but still nowhere near it.

Endless Rotaries

iTrader: (6)

That's just not a good thing to do. I can't even emphasize enough.

Check for load ratings on the tires, not too mention speedo will read incorrectly.

Frankly it's a Honda Civic tire diameter going on the rX8. I just don't see how any amount of physics would justify that for street purposes.

It will look CRAZY FUNNY Too. Talk about major wheel gap between tire and fender.

nuke0907

iTrader: (2)

you said the magic words. i was thinking the same. why didn't you just buy a Civic if you are gonna put those crappy fitting wheels/tires red?

red5eight

Why didn't I buy a Civic...ummm...because I didn't buy the rx8 for it's wheel/tire combination obviously. I bought it for the rwd, 50/50 platform and the rotary engine. The load limit on the tire is 1100lbs vs. 1350lbs for the stock tires, so they can handle the load. As I've said the look isn't so important to me, the weight saving is very impressive, the price is very affordable and the gearing change will let me use the transmission a little more all while sharpening up the steering response.

When you take into account the cheaper tires at this size, over the course of my ownership it will be like losing almost 50lbs of rotating and unsprung mass for free!

nuke0907

iTrader: (2)

Endless Rotaries

iTrader: (6)

Just do it then and let us know how it works for you. Sounds like you've already convinced yourself.

Personally i would never ever do it. BOL!!!!!

You could always just buy some Enkei RPF1 17x8" wheels weighing in around 14lbs each and drop a shitload of weight there and keep a stock tire diameter.

red5eight

My only concern was abs so I will be doing this mod in about a month as I said. RPF1s are 15.5lbs in 17x8 according to tire rack, but the main reason for the kosei's is price $189 for 15.4lbs. Which is part of the weight loss. The difference between stock size tires and the 205s is about 5lbs. Thanks for the input and the concern but you are correct that I am convinced this is the right direction for my driving style and my priorities.

MilesJ

With your very small tire choice you may match the much shorter effective gear ratio of the new '09 in first, but in the end you will make all of your gears shorter. The '09 is geared quite differently than the 04-08 cars. This is easily demonstrated with a little gearing calculator I made. At 9K using the OE tire the following MPH's are achieved.

gearing calculator
Redline/ratio/final/MPH
2007 RX8
9000 3.76 4.444 41.2
9000 2.269 4.444 68.2
9000 1.645 4.444 94.1
9000 1.187 4.444 130.4
9000 1.000 4.444 154.8
9000 0.843 4.444 183.6
2009 RX8
9000 3.815 4.777 37.7
9000 2.26 4.777 63.7
9000 1.536 4.777 93.7
9000 1.177 4.777 122.3
9000 1.000 4.777 144.0
9000 0.787 4.777 182.9


You'll notice that the 09 is shorter in 1st and 2nd gear but almost identical in 3rd. It's also shorter in 4th and 5th gear but has a nearly identical 6th.

If you run this 23.4" OD tire your speeds at 9K will be:
37.5
62.1
85.7
118.7
140.9
167.2

What this will mean in your daily life is 70MPH will be at 3750RPM vs 3425RPM with a 25.7" OD tire. Hope you save those extra $$$ for gas.


You mentioned tire load capacity so I figure I'll touch base on that too. BTW- the most important job your tire has is to carry the load of your vehicle.

The OE tire has a Load Index of 91 meaning it can carry 1356lbs at 36psi. The tire you looked at in 205/40R17 must have been an extra load (XL) tire because you mentioned it carrying ~1100 lbs. In 205/40R17 only an XL rated tire carries that much weight. What that means is that a XL tire in that size can carry 1102lbs at 42psi. There is a big difference between standard load tires and extra-load tires.

The specified inflation pressure of the OE tire is 32psi, at that pressure the tires are capable of carrying about 1250lbs. This is the minimum load capacity that Mazda has determined safe for this vehicle.

The XL rated 205/40R17 can make no more than 1102lbs of load capacity period. I would call this an unwise tire choice as it is short almost 150lbs per tire.

Your caveat at this point is the GAWR (gross axle weight rating). On the RX8 it is 1823 F 1986 R. This means that despite being 600lbs short in load capacity they will be safe if inflated to a very high pressure. I would recommend no lower than 40psi F and 42psi R giving you about 2100lbs F and 2200lbs R of load capacity.

At minimum you must maintain at least 36psi F and 40psi rear to make sure the tires load capacity will not dip below the GAWR.

red5eight

Thanks MilesJ, that is some very useful information. Not too worried about fuel economy, hopefully the thinner stiffer tire will offset a little bit the increased revs in 6th but either way I had calculated the rpm in a spreadsheet and found it acceptable. I'll let everyone know what happens to the gas mileage.

The tires were indeed XL rated (Falken RT615s) in that size. Do you have a reference for your 42psi figure required to attain rated load? I've never run across that before.

Also surely if the axle rating is 1823F and 1986R that would be split between two tires except under cornering. So since I'm not going to be driving aggressively when there is 800lbs in the car (to reach the GAWR) the cornering force would be small and so 900 for each front tire and 1000 for each rear tire gives me a safety margin of 100lbs. Then assuming I only drive aggressively by myself I would have less than 3150lbs total so that would be at most 800lbs per tire static weight which gives me 300lbs for turning. Do you think I should play it safer with 215/40R17s which have a rating of 1200lbs and weigh 1.8lbs more each?

Also, Could you clarify the relationship between tire pressure and maximum load for me?

Thanks again for your post, it was very helpful! I just wanted you to see my reasoning so you could shoot it to pieces if need be :-)

red5eight

One more thing of interest to note: More pressure might be a good idea anyway to get the ratio of width/length of the contact patch closer to stock. Since the width will not change, increasing the pressure will decrease the length of the contact patch, thus I will have decreased both the length and width so the ratio will be closer to stock.

I just did the calculation and 42psi is the exact pressure needed to maintain the same length/width ratio for the contact patch (the 205 width is 7.2inches vs. the 225 which is 8.3in according to Falken's website). Interesting...

MilesJ

Excerpt from Tire Rack's Tire Tech article "Load Range/Ply Rating Identification"

Load Range/Load Pressure
...
Euro-metric
Standard 36(psi)
Reinforced/Extra Load 42(psi)

Load pressure indicates at what psi a tire will attain its MAX load capacity as specified on the sidewall.



As you lower the air pressure in the tire the load capacity decreases as well. Thus a 84W XL tire makes 1102lbs of load capacity at 42psi. That same tire at 40psi can only support a maximum load of about 1040lbs. At lower pressures it makes even less load capacity: 38psi ~ 980lbs, 36psi ~920lbs.

Thus, to make sure you can avoid a possible catastrophic failure you need to run, and maintain at all times, a minimum of 36psi F and 40psi R.

You seem to like equations. If you were to take a 3150lbs car and corner at 0.8G how much extra load do you think is applied to the 2 outside tires accepting that load? I think it'd be 0.4G worth per tire. If gravity makes your car weigh 3150lbs than a load equaling 0.4 times gravity should means 1260lbs of effective weight is being applied in excess of the actual weight of the car.

The question I posed above is surely not 100% correct and someone may pop up to correct me on my engineering/physics principals, and that's fine. Either way I hope it demonstrates to you just how slow you'd have to go to only apply 100-300lbs of extra force.


I think you should run a tire that will make at least 1250lbs of load capacity. That is what Mazda has determined as the minimum safe load capacity and I wouldn't want to test how low I can go.

red5eight

Your assumption that .4G per tire would transfer places the center of gravity at about 30inches above the ground, this is probably a safe upper bound. But remember that load transfer removes weight from the inside wheels and places an equal amount on the outside wheels so it is not like adding 1260lbs to the weight of the car. Rather, since the outside wheels had 1575lbs static they will have 2362lbs in the turn and the inside wheels had 1575lbs static so they will have 788lbs in the turn. Thus there would be 2362/2 = 1181lbs on each outside tire so you are still correct that it would go over the 1100lbs limit.

The question is: Is the CoG height really 30inches?

We can assume that mazda would engineer for at least a 1G turn at full weight rating, which would mean 3800lbs in the car and approximately 1900lbs static on the outside, 1900lbs static on the inside. If .5G of weight transfer occurs during a 1G turn, the outside wheels would have 2850lbs on them, meaning 1425lbs per tire. This exceeds the capacity of the stock tires so there are only a few conclusion I can draw:

-Perhaps the CoG height of the rx8 is well below 30inches, in fact it would need to be below 19inches for the stock tires at 32psi to be safe with 3800lbs in the car. This would mean that in the .8G turn only .24G is transfered to the outside tires. Which would make my setup safe.

-Perhaps the load rating on the tire is a static load and the transient load capabilities are much higher. Ie. for the short time period of a hard turn the tire can handle greater loads. Maybe you can comment on this possibility?

Either way, you have convinced me to go with the 215/40R17 and play it safer with the 1200lbs rated tire. Thanks a lot for your help! If anyone knows the actual CoG height of the Rx8 I would be very interested...otherwise perhaps I will measure it someday and post on it.

MilesJ

You are correct that the load capacity of the tire indicates the maximum weight sustainable by the tire. Placing a load exceeding the rating of the tire instantaneously or for a short duration of time may not cause the tire to fail but a sustained load in excess of the rated load capacity surely would.

Glad to hear you are getting a tire that is closer in load capacity. I would still urge you run your air pressure as high you can stand. The more load capacity the better.

red5eight

Thanks a bunch MilesJ, I will certainly be running above 36, probably more like 38-40 now that you set me straight! In fact just to make sure I'm still up for this I'm going to run my stockies at 40 for a while to see how it is. Hopefully the thinner/stiffer sidewall and the lower unsprung weight will come close to balancing out and it will be a fair prediction of how the 215/40s will be in terms of comfort.

imput1234

iTrader: (6)

It's gonna look/feel like crap do not do it.