kev

I have a set of front K-28's in 8"x18" ET50 (8"x10" ET65 rears) that will be upsized to 8.5". From reading I assume that a 8.5" can fit a 265mm tire in front. I understand that contingencies such as suspension, tire brand, wheel brand are in play here, but speaking generally, does anyone have advise on offsets that work best? Is it simply a matter of adding back the .5" to the offset (ET50 to ET38)?

Greg Fishman

Kevin,
What tire are you wanting to run? I have run both Hoosier in a 275 and the Michelin 265's in front and was not happy with the set up for numerous reasons. Mchelin should have a 235/18 available soon. If they are Hoosiers look at the 245/18.

When I upsized my Kinesis wheels from 8.5" to 9" we just replaced the inner wheel half.

kary993

Kevin, I do not know of any 265's that would fit on a 8.5" rim, at least very well and perform as they should. I would have to check but I think you would need at least a 9" or even a 9.5" wheel at a minimum to put a 265 in the front of a 993. Anyone else concur with that thought? I seem to recall that Bill W has put 265 on the front of his 993 track car, might check with him.

I have some BBS wheels that are 10.5 and 8.5 (rear/front) that I think are a 45/40 (or maybe 40/45) offsets respectively, I cannot remember exactly. They require a fair amount of camber to fit under the fender even with rolling the rear fender lips The front only requires some minor rolling. If I take out the camber, say down to 2 degrees from 3.5 degrees, they will rub the fender lip on the rears.

There are so many factors here. When you put larger wheels in front you have to worry about what springs you have. I never was able to fit anything above and 8.5" wheel in front with a stock suspension. Required smaller diameter springs like PSS9's. You will get rubbing on the inside with larger wheels and a tire that big for sure if you turn the wheel near lock. Rears you need to worry about your offset on inside and outside if you want to fit anything like a 11" wheel with 305's for example. Very tricky business for sure if you want it right!

kev

Greg,

Gripwise, I was thinking that bigger is always better so I was entertaining the notion of trying to fit 265/295 MPSC on 8.5"x18" and 10"x18" wheels. Competitve track driving is a possibility in my future, so I am trying to keep this option open for down the road.

Could you elaborate on why the 265s MPSC and 275s Hoosiers didn't work out? Are 235/285 MPSC or 245/285 Hoosiers a better race setup, even with less rubber?

kev

Kary,

You may be right about the 265 on a 8.5", as I was mainly extrapolating from the fact that I've gotten a 225 tire on the 7" Cup wheel. Another motivation to get the 8.5" to work is that I only want to pay for rebuilding the front wheels, since as I understand, either a 1.5" or 2" differential from F to B is optimal. I am flexiable when it comes to compromising for performance (fender rolling, smaller springs, additional camber) so I don't anticipate these factors being issues for me.
I plan to talk to Kinesis about this but appreciate all the advise from experience. Thanks.

kary993

I agree with Greg that a 245/275 Hoosier set up work well for 993's. You can do this on 8.5 and 10 wheels. This is what I used in the past as far as tire and wheel sizes. I have not used Hoosiers in a year or more but if you try to go to 285's you get 30 series tires instead of 35's. This gives you different diameters and the rake of the car gets off due to the lower diameter in the rear as compared to the front. The 245/35/18 and 275/35/18 work best. With this set up you should be fine from all the modifications I mentioned above given the Kinesis wheels are not pushing the limits of offset.

I currently use a 245/546/18 and 285/645/18 Pirelli slicks and like the set up very much. Have been able to dial in the suspension, alignment, and adjustable sway bars to set the car anyway I need it for any track (understeer, oversteer, or neutral) depending upon preference. It makes driving this car a real pleasure when it is so adjustable!

Glen

Kev in answer to your question. 55 mm offset or so in a 9 is about right. I of course loved the 265 in front...lol

Glen

Stock boxster 9" rears will work as fronts without spacers btw as far as a cheap date for wide fronts in an 18....

Greg Fishman

Kev,
The 265 MPSC need at least a 9" rim. The 295's should be on 11's. This was according to the Michelin engineer I spoke with at Road America. I didn't like the 265's on the front because it took so long for them to heat up and once they did it really messed up the balance of my car. Severe understeer and the beginning of a session, severe oversteer at the end of it. The 225's worked better IMO.

I used the 275 Hoosier on the front end of my car for a long time and then tried a different set up with the 245's. The extra grip I got from the 275 was great but with the 245 I could lower the car another 10+mm and then got a better weight distribution and that made up for the extra rubber.
I use a Hoosier 305 on the rear wheels. I would go with the 245/285 tire for the size rims you are looking at.
Just my opinion and as Glen mentioned it worked well for him and probably others.

Martin S.

The bigger the tire the better...BUT, be certain the rolling radius front to rear has no greater delta than 5%, and maybe a stretch to 6%. Anything greater than that and your anti-lock and ABS lights will come on, and your ABS will turn itself off. This is a great way to flat spot some tires...ask me, I know from experience. See my earlier post for details, of shoot me a private e-mail with your e-mail address and I will send you the calculations I did based on comparing front to rear.

DJF1

Will the 235 size fit good on an 8 inch rim?

JimB

I ran 265 MPSCs on the front of my 996 with the same results as Greg. They turned like wagon wheels when cold then stuck like crazy when they warmed up. Not a great combo.

joey bagadonuts

Kevin,

You don't necessarily need to change the offsets. If you use a 50mm offset on a 7, 8 or 9-inch wheel, the distance from the centerline of the wheel relative to the hub will remain unchanged. The extra width will be added equally to both the inside and outside edges.

It's kind of tough to determine the right offset since differences in suspension, camber, ride height and fender clearance factor into the equation. Also, if you plan to modify your suspension and alignment specs in preparation for Club Racing, you might want to put off the wheel update until then. What works well for your current setup might not be optimal for your race car.
0.02

Martin S.

DJF1,

One tire manufacturer recommends on a 235/45ZR17 tire: _7.5" minimum wheel, 8.0" acceptable wheel width, and 9.0" ideal width. I can see why it would be any different for wheel size whether it be 17" or 18" since the 235mm is the width of the tire, approximately 9.3" wide.
_

Greg Fishman

I would think it would be on the small end of what was acceptable. I thought you were switching to Hoosiers next year?

A bigger tire is not always better, a larger tire takes more time to heat up and takes more hp to push it as it increasing the rolling resistance.
Another issue is that a tire with a larger diameter (rolling radius) will cause an effective change in your gear ratios.