labrat

Following discussion about reduced steering response after putting 245/40 18's on RX-8 wheels, I wrote to Fulda/Goddyear's customer relations centre in Europe and have just received this reply (in part):

"I seems that a possible reason for the loss of steering response on a wider tyre, can be the rim width. In fact the 225/45R18 original equipment tyre is mounted on a 8x18" rim. This rim width is on the higher end of the optimum rim width for the OE tyre. In fact for this size the optimum rim width is 7.0 - 8,5.
For the 245/40R18 the optimum rim width is 8,00" - 9.00". So your OE rim on the RX-8 is quite narrow for the 245/40R18.

Another reason is the fact that the initial steering input you give on the steering wheel is transmitted to the road mainly through the center part of the tyre.
Because a wider tyre has a different pressure distribution, the initial steering input is not transmitted as precise to the road as with the narrower tyre.

Try to increase your tyre pressure somewhat (0,2 -0,3 bar) to change the shape of the mounted tyre. (rounder shape) . This should normally help to improve the steering response."


When I checked my tyre pressures this morning in the garage before driving off, they were all around 230kPa (2.3Bar, 33.8psi). I increased the pressure to 250kPa (2.5Bar, 36.8psi), and immediately noted the improvement in steering response. I would now rate it as close to the stock Potenza's. I'm thinking of experimenting with another 10kPa to take it to 260kPa or 38.2psi.

The Fulda/Goodyear's guy's comment on the OE tyre size compared to the rim size is instructive. It would appear that Mazda has deliberately chosen a tyre that is somewhat too narrow for the rim, probably in order to influence steering response.

NickG

Interesting post Labrat. I read somewhere recently that Mazda had literally tested hundreds of tread compounds before settling on the OEM tyre for the new MX5. It's hard to imagine they could have put that amount of work into the RX-8 only to decide on a mundane tyre like the RE040. But perhaps there was method in their madness as far as the tyre/rim size combination is concerned, after all.

BTW I think you did a great deal on those Fulda's. That's a heck of a good price for a German tyre, compared to what you pay for Continental Sport Contacts for example.

RXP33D

Labrat, I'd have to agree. On the stock tyres the steering response was better compared to the 245. Is it because the Goodyear GS-D3's on mine have softer sidewalls compared to OE?

The car seems more floaty at high speeds. Regardless of the tyre, is it ok to set them at
40PSi(280Kpa)?

How are your tyres coming along labrat?

NickG

It's OK for most tyres to run at 40psi and steering response will of course definitely improve. I ran the RE040's at 38psi for most of their lives. Aside from ride harshness there is another penalty to be paid however. At 38psi my tyres were severely worn in the centre, with thousands of k's left on the edges.

labrat

[QUOTE=NickG
BTW I think you did a great deal on those Fulda's. That's a heck of a good price for a German tyre, compared to what you pay for Continental Sport Contacts for example.[/QUOTE]

I can't claim any credit. That was the asking price via Mazda Grand Prix Aspley.

The tyres are fine. I can hear myself think, and the car stereo sounds better. The car seems well glued to the road, only the car wasn't as twitchy. Now it seems much better at the higher pressure.

I think that regarding tyre pressure, it is possible that Mazda's recommended pressure of 220kPa on the OEM Potenza might be too low for a wider tyre, and it could result in wear at the edges. I ran the OEM's at 230kPa throughout their life, and had very even wear across the tread. I suspect that for 245/40's that the Fulda guy might be right at running 30kPa higher. I think ideally, I should pump them to 260kPa, and they will naturally decrease to 250kPa over a couple of weeks.

Revolver

Thanks for the update labrat.

I wonder if anybody out there has fitted 245/40s to 18x9 rims? Applying the rationale in the response you received, that should eradicate squirm.

I also wonder what the optimum tyre size is for 19x9 rims??

takahashi

It will be a 255 Dave.

I agree the 8" is on the small side for the 245 but any increase in width will reduce steering response. The steering will feel heavier - since you have more "meat" on the road

Side note: Why is that sponsor link all of a sudden. When u type Mazda or something - it will turn green

timbo

I think way back in the early tyre threads we talked about 8.5" rims being optimal for 245s. But, faced with virtually no 225/45s of any value, I had to act. I run mine at 36psi and can notice very little difference relative to the OEMs. Certainly, noise is lower, and the ride less harsh (which surprised me).

Ultimately, any tyre choice is going to be a compromise of some sort.

takahashi

Funny... I have look at a Japanese magazine and 225/40 18 is a common size there and it is listed with other selected common 18" sizes like the 245 and 265s.

Australian distributor is out of touch on size choice?

I know there are not many cars with 225/45 -18 in Australia = only RX-8 and 350Z's front tyre is this size. But I know there are many Japanese cars are having this size stock. hmm.... it is tough being a minority I guess (*don't I have this exp recently)

Revolver

So does all the above mean that the best matches between rims and tyres are as follows:

245/40 = 18x8.5

225/45 = 18x8

And that in 225/45 we are pretty much limited to the OEM tyres and more expensive stuff like Michelins and Pirellis whereas there is a greater choice in 245/40s (e.g. Yokohamas, Fuldas, Trampios, etc).

???

(Forgive me, I'm a bit slow with all these numbers and I'm just trying to pull together what I've picked up in various tyre threads into a format I can follow). :o

timbo

spot on, I reckon

rotarenvy

most tyre manufacturers will give an optimum rim width for the tyre size. 245 to 8.5" is the usual recommendation. I did come across one tyre recommending a 245 on 9" but this far from common.

Revolver

Thanks for that Rotarenvy.

I'm starting to think I should buy 8.5" aftermarket rims for street driving so I can get replacement tyres easier and use my OEM wheels and tyres for track work and replace the OEM tyres with semi-slicks when they wear out.

If this isn't getting off-topic too much, looks aside do you guys reckon there is any performance advantage to be gained from going to 19" or even 20" wheels? Seems to me the extra weight is a factor.

Gibbo, Ezzy, you guys have been running 20" wheels. Pros, cons?

timbo

Actually, for pure performance, you might want to consider 17" -- lighter -- wider range of track-useful tyres....but mate, this sickness...it will pass

Revolver

Like it did for you after you bought the Hymee grille kit, the Hymee catback, the shortshifter, the scanalyser...(need I go on?) :D

timbo

That's supporting local industry, mate -- very different :o

Revolver

Hey, I'm happy to buy Australian. If anyone knows a locally made wheel that can compete with the SSR's and Volk's for looks and weight, please let me know.

Hang on, isn't it a yank that makes and sells the shortshifters (Richard Paul)?

timbo

Yes... and Mazda is made...?? I am just having a lend....:p

Revolver

:D