frankvan

CH,
first, it's good to see yet another canadian on the forum!
second, I agree with the comment about Kumho's - with tires you really do get what you pay for (with very few exceptions). I had a set of Kimho's on my Alfa and they were alright under ideal circumstances but throw any adversity at them (a little rain, some dust on the corner, etc...) and they lost their composure very quickly. They were also very hard to balance - Had them into the shop I use all the time for wheel balancing several times but couldn't get rid of vibrations between 50 and 70 mph (80-120 kmh).

Finally, I looked at all the options for a good performance tire (on this forum and through google searches) and found very few suitable to our unique northern climate. The Michelin Pilot all season seemed quite good, but very pricey (over CDN$2000 for a set of 17" tires). Then I looked at my snow tires which are phenomenal (sp?) and they say Nokian on the side. Nokian (based out of Finland?) makes summer performance tires as well as winter tires.
So I got the Nokian NRZ-i for one primary reason - their summer tire rubber compound is designed to perform down to zero C. I couldn't find that claim from any other manufacturer! Living in the frozen Prairies (we even get ice storms in July in Edmonton...) that seemed farily important.
Although I haven't tracked the car, the tires seem more than capable for everyday street driving.

One caveat - the NRZ-i's are classified as a summer performance tire. They got me through one surprise snowfall in October, but they weren't happy. There is no substitute for good snow tires, even on an all wheel drive vehicle.

Oh, and price for a set of 4 - CDN $1300 incl mounting, "high speed" balancing (with the glued on weights hidden on the inside of th rim) and assoc taxes and environmental fees.

C H

The Kuhmo's I have have a ton of glued on weights, inside and out. I will only drive the car April or May till October, so cold temperatures are not an issue for me. I want maximum dry grip, and the AWD should take care of wet traction, as I won't be taking corners at 100 MPH in the rain.

I drove the car home in December 1600 miles and had no traction issues. I floored it in light snow and the rear end fished slightly, but straightened out right away.

I am more concerned about the noise, but since one tire is almost bald on the inside that is probably the cause. At least I hope.

C H

Update:

Alignment done and changed bad tire. The noise is almost completely gone. Slight whoop whoop sound at lower speeds. I may need to change the other tire, or perhaps something else is worn. The alignment took 3 hours and was a modest $195 CDN. The camber was 7 degrees out on the bad tire and the others were all out of spec!

Indycam

Mismatched tire rolling diameters on a C4 is a no no . If you have
one new tire and three worn down tires , they have different rolling diameters .
At $195 CDN did they corner balance your car ?

C H

The other 3 had low mileage and tread depth difference was minimal. No alarms on the dash either. I think I am going to change the other rear tire to see if the noise goes away completely, but for now the valve adjustment is taking precidence. It is fairly loud when warmed up, I was quoted $130 for parts and about $200 for labour. Sound reasonable? I could probably do it, but I really don't want to at this point. I just want to enjoy it for know, and that means not laying on my back for half the day and spending the other half hunched over. I don't believe the car was corner balanced, the P-car mechanic has a C2 himself so I trust he knows what he is doing. I will ask him when I get the valves done.

How exactly does is corner balancing adjusted? My read up indicates that it is balancing the weigth evenly, ideally each wheel has the same weight but it is acceptable to have more at the front or back if evenly distributed.

This is my first Porsche so I have no reference point, but I tested its cornering abilities and with the wide 18's, the 30mm lowering and now that it is aligned properly it is outstanding and couldn't be happier with it. I am waiting to bring it to the track to really push it so I can feel the understeer that the car is suppose to have. I have not felt it at all yet, and even though this is likely the most expensive thing/hobby I have ever bought it is well worth it.

This concludes my novel

Indycam

Hiya CH
"How exactly does is corner balancing adjusted?"
If you look under your fenders , at your struts / shocks , you will see the spring perches , the things under the springs , they are adjustable . Adjusting them is the way to set up corner balance .
Weight scales are put under each wheel to see how much each wheel is holding up . The thing is spun up or down on each corner until each is set "right" .



"The other 3 had low mileage and tread depth difference was minimal."
How minimal ? If the difference is out of spec , you are indangering you diffs .

C H

I thought the aluminum perches were for ride height? Anyways, decided to order the other rear tire and get it put on the same time as the valve adjustment.

Indycam

"I thought the aluminum perches were for ride height?"
Yes they are , they are also 1st used to set the C.B.

"Anyways, decided to order the other rear tire"
All four tires gotta have same same rolling dia , or very very close to the same same R,D.

adwriter

The Porsche specifications for rolling diameter are not too bad. The tires have to be within 4% diameter front to rear, and within 4% of the original Porsche tire spec.

The Porsche Club of America has a good explanation for a 964 C4 with 17" inch wheels:

"Width-wise you can run 235 front and 275 rear with a fairly tight fit, sometimes a 235 is too wide and you need a 225. But your wheel offsets will have to be perfect. The factory race wheels for your car, 10 years ago, were 17" one piece 8x17 front ET 52 if memory serves, and rear 9.5 x 17 ET 68.5, and then you had to run a 21 mm spacer to get the rears to clear the RR oil line and the trailing arms. 68.5 minus 21 is 47 mm, that is your ideal rear wheel offset for a 9.5" wheel with a 275 tire. You can run a 10" wheel if you are really good at this stuff.

"The shortest tire you can then run is a 225/40-17 at 24.1" diam, -3.7% from the reference, and 275/35-17 at 24.6" diam and -1.7% from reference. The more conventional 275/40-17 is too tall for a 24" front tire unfortunately, so for the 275/40-17 which is what most people run, at 25.7" and +2.6% over reference, you need more like a 225/45 up front, 25.0" and -0.1%. All that is so you can stay within 4% of spec and within 4% front to rear on the diameters."

Hope this helps!

Chris

Indycam

Hiya Adwriter
How much , % , does a tire change from new to old ?
How much , % , are the specs , same axle , side to side ?

adwriter

No idea what the specs are for same axle, side to side. I'd assume it's 4% too, but I might be wrong.

As for how much a tire changes from new to old, a tire will typically have 8/32" of tread depth when new, of which you can only use about 5/ or 6/32".

There are two surfaces included in the diameter, so you can use up 12/32", which is 3/8".

The diameter of a 245/45-17 tire is 25.7" and 3/8" is also 0.375". So from new to old you could go from 25.7 to 25.325. That's about 1.5% and well within specs.

Indycam

The side to side spec is said to be 0 , if the tires are not same same the dif has to work all the time ,
if the front to rear is not same same , somethings got to give . So best of all worlds , all tires have the same rd . How much wear and tear mismatched rd has on the drive train is hard to say but if you want the car to last as much as possable .....

C H

So you could blow your diff if the tire pressure is different on the same axle, as this would change your rolling diam as much as a slightly used tire.

Indycam

Rd does not really change with inflation . You can change the size and shape of the contact patch with inflation but not rd . The belting in modern tires control the rd , you can not blow up a tire to have a larger rd , the belts will not stretch .

"So you could blow your diff if the tire pressure is different on the same axle"
Its not a matter of blowing a diff , its that it will be used up sooner . The greater the rd difference the greater the speed of rotation of the diff . If you are driving in a straight line the diff should be nearly stopped . It should spin when going round a turn , not all the time . The parts cost for this car are far more than a tire or two , better to spend some on tires than using up the diffs .

C H

Thanks Indy, I will get everything measured to assure all is within tolerence and take it from there.