rotarenvy

some have you may have spotted my wheel thread .
I got the first run with them this afternoon as I was waiting to get some hub-centric rings. I got to say the car is transformed! the smaller 235/40/18 with 2kg less per wheel is a winner in terms of acceleration. it feels much faster and my partner commented on how smooth it was. I thought all the light-weight wheel performance-hype was placebo. i.e. after one pays $3000+ for a set of wheels you want to feel an effect lol. but there is one :D

I got these from www.j-plus.com.au he is in Sydney and gave excellent service. I will be dealing with him again. he doesn't have much in stock at the moment that would suit the rx-8 but I'm sure when the next load arrives there will be some to choose from.

like any modification there are tradeoffs.

there is a big difference in overall diameter between 225/45/18 and 235/40/18. you notice it when you change from 235 back to 225 and have to wind up the jack to fit the 225. the 235 was spinning in the air and the 225 can't fit as it hits the ground.
a smaller tyre diameter might mean scraping up the driveway, higher fuel consumption, Speedo/odometer error. I'm not worried about these I can compensate by changing my driving style and being aware of differences. I also only do 10K a year so fuel and extra Km's are not an issue.

larger size 9" and 39+. these effect handling. the car has lost some of the sharp turn in could be due to the cheep tyres and the stretched fit of 235 on 9". oversteer on corner exit has increased but I thought my car understeered anyway. overall it has made me feel more confident to enter corners faster and my cornering speeds seem higher. the car seems stable on rough surfaces but it was a little floaty at speed which I'll put down to green tyres for now. I chose 9" as it is a good size for the 245/40/18 which is popular but after the acelleration gains from 235 I would now choose a 8.5" rim to suit with a 40+ and I reckon it would rock (i still might make the change )

hotpot

Thanks for sharing.
I've been wondering about the effect of smaller tyres on the acceleration of the car. Glad to hear that you can actually feel a difference. Some 'before & after' acceleration times would have been interesting.
I guess the speedo will overstate the actual speed even more now.

takahashi

I am still digesting you experience in a 235/40.

Very interesting experience, on one hand you understeer and easier for you to powerslide.
I can only draw one conclusion from that.

Hymee

iTrader: (1)

Understeer - perhaps less grip up front?

Powerslide - perhaps less grip up back? Plus the smaller diameter is allowing for more torque at the tread.

Cheers,
Hymee.

rotarenvy

probably both I would guess the tyres are worse than the std re-40's and they are covered in mold release for the first 2-5k.
but I still went 20km/h + more through a corner than I could on the std wheels? I also haven't had the dsc kick in once yet but I'm driving very soft on the green tyres.

Hymee

iTrader: (1)

I asked the dealer about my yoko's being in the green state. He reckoned on 30 to 50 km would have sorted that out. Somehow, I still reckon they felt green about 150km later on a track day.

Cheers,
Hymee.

rotarenvy

I have heard that it's like 1000km or more. I reckon it would depend on how fast a tyre wears. 1-2mm of surface rubber is probably contaminated.

I was thinking; the car may have less grip on the front than rear, explaining the understeer oversteer. but this dosn't mean the car has less grip than std.

It just means the suspention balance has changed. higher cornering speeds would suggest an improvment ie lower centre of gravity, wider stance, and maybe grip. but deciding if the setup has less grip than std can only be tested by running std wheels on one end and the new set on the other and swap for each combination.

Hymee

iTrader: (1)

Yeah - I didn't say I necessarily agreed with the tyre dude though

I reckon they are a lot better now. I must say I was initially slightly dissapointed with them when they were "green".

Cheers,
Hymee.

rotarenvy

it makes those 24hr tyre 'guarantees' a bit of a joke as you can't tell how loud they are going to get or how much grip they realy have. I'm a bit anoyed with the info you get from tyre companies to decide what to buy. like kumho reckons every tyre is a ultra-high perfomance tyre? you can only go by recomendations and just because someone says they're great dosn't mean they are.

labrat

I'm not sure what people mean when they say "green" or "contaminated" tyres. Polymer science has come a huge distance from the old days, and with the knowledge about polymerization catalysts and the like, you can be sure that the tyre compounds are polymerized to the max when they come out of the mould. You have to compare the mould temperature to the temperature that the tyres see on the road to appreciate that they aren't going to do too much more in the way of curing after they get out of the mould. Chemists use a rough rule of thumb which says that reaction rates double for each 10 degrees rise in temperature, so since road contact temperatures are unlikely to be much above say 80C (unless you're doing burnouts), they don't compare to mould temperatures which are likely to be over 200C. This means that any improvement in polymerization is going to be incrementally small.

Tyre moulders use a mould release compound (probably a silicone) to get the tyre quickly and cleanly out of the mould, and it is probably wearing off this slippery stuff and taking off the shiny surface which contributes to improved handling. I wouldn't imagine that penetration of the mould release stuff would be very deep, perhaps only tens of micrometres, but it might take a few km to wear down. It's a bit like scuffing in a new pair of leather soled shoes.

Obviously at the other end of the scale, tyre running temperatures do matter. When temperatures get too high (say due to vehicle overloading and speed), the rubber decomposes and delaminates from the reinforcing. You see that in truck treads by the side of highways. I remember that at the bauxite mine at Weipa, the haul roads from the pits to the crushing station are kept sopping wet to keep the tyre temperatures down - Bridgestone use Weipa as an open lab for researching new compounds for heavy haulage tyres.

The chemistry of rubber is now so well known that they can control polymer chain length (which contributes to elasticity) and cross-linking (which contributes to stiffness), not too mention the amount and type of filler (carbon black, silica, etc) for strength and wear resistance. What's left to the driver is to choose the right tyre for their car and driving style. Unfortunately, there's little objective evidence out there upon which to make a decision. Sure, there's lots of subjective comment (along the lines of what is discussed in the forum), but I'd like to see some real data. Believe me it exists - in the test labs of the tyre companies, and they all use standard test gear. However, I don't see much chance of seeing it all on the same spreadsheet unless some consumer organization gets interested.

rotarenvy

well I was just refering to mould release compound, and speaking form what I have heard.
how porus is tyre rubber? how much release agent do they use and how much gets mixed with the rubber as it's injected? all good questions as to how long the release agent hangs round for.

anyway my comments were just to point out it takes more than 1 day of driving to get tyres up to full grip levels. The rest is just hear say 50km, 100km or 1000km not a lot of proof out there. my impressions are based on driving one the tyres with no time to scrub them in.