Joe Weinstein

Hi.
I am considering lighter wheels, and for what sort of tire I want,
I can see a good tire combo if I could run a 9"x18" in the front.
Has anyone already done this? What was the wheel and back-spacing?
(Distance from inner face of hub to the plane of the inner rim)
thanks
Joe

Trojan Man

9" will definitely fit, but it will depend on the offset.

Joe Weinstein

Ok... What would the offset be?

A stock 8" wheel fits but it depends on the offset

NJ-GT

9" x 18" ET 52 (factory rear Boxster size) fits the front of any 996.

If you are planning to use 255+ size front tire then go with it.

If you're custom ordering wheels and want to use 245/235/225 front tires then go with 9" x 18" ET 45. Increasing front track is a good thing on a car that understeers so bad.

Joe Weinstein

Thank you very much. I will see if I can pick up some used boxster
rears... Am I right in assuming that 'ET 52' means there is 52mm
from the plane of the hub at the rear to the plane of the inside rim.
Yes, I am considering custom wheels too.

ebaker

ET 52 means there is 52mm from the plane of the hub to the plane of the rim midline.
The inside will be very close to the spring mount (with stock spring size). The ET52 wheels will be 14.5mm further inboard than the stock 8" ET50 wheels. The 9x18's come in soild and hollow spoke. I think the hollows weigh about 22# or 2# more than the stock 8" hollow spokes..

Joe Weinstein

Thanks again. So let's lay a wheel on the ground, face up, so it's
inside rim is flat to the ground. In a 9" wheel the plane of the rim
midline will be 4.5" above the ground. The inner hub face will
typically be above the midline for a front wheel, so an ET52 wheel
will have the hub face 52mm higher of the ground (4.5" + 52mm).
An ET45 will hav it's hub face lower, making the track wider by
7 mm per side, for a 1.4cm total increase of front track.
Do you by chance know of the tire brand(s) and size(s) that this
setup has run without problem? I'll look up those tires and get the
section width and diameter so I can compare them to the tire I would
want to run. Did the front fender need any rolling?

Thanks again for your time and experience.
Joe Weinstein

ebaker

Turn your front wheel in and check the clearance at the front, it is very close to the radiator cover/wheelwell liner.

bigsky

Here is a link to Tirerack diagram of the above explanation for those, like me, who need the visual:
www.tirerack.com/wheels/tech/offset.htm

Tim Wasmer

Here is a little tool to check your tire sizes etc.

Size figure calculator

Joe Weinstein

Hi and thanks all. Tim, thanks, but I want to warn you away from
such tire size calcualtors. They depend on the assumption that the
tire size code is accurate, eg: that a 245/45-16 tire has a 245mm
width and a sidewall height of 0.45 * 245, on a 16" wheel.
The truth is that they are only very approximate. I have seen two
types of tire with the same designation differ in diameter and width
by .5 to .75"! And these were from the same manufacturer! Just different
models. The only way to really know whether a tire will fit is to read the
published tire specs.
Joe

Tim Wasmer

Joe:

I have used this calculator before. I just now re-found it.

Oddly enough I did a comparisson of the original tires on my C4 (225-40-18 and 295-30-18) vs. going with a 255-35-18 on the front with my new 9in rim.

According to what I see the 255 is closer in spec (revs per mile etc.) than the originals.

Here is the data.

Tire 1 Tire 2 Difference 225-40-18 and 295-30-18
Sidewall Height: 3.54 3.48 .-6
Section Width: 8.86 11.61 2.75
Overall Diameter: 25.09 24.97 -.12
Circumference: 78.81 78.44 -.37
Revs per mile: 829.02 833.00 3.98

Tire 1 Tire 2 Difference 255-35-18 and 295-30-18
Sidewall Height: 3.51 3.48 .-3
Section Width: 10.04 11.61 1.57
Overall Diameter: 25.03 24.97 -.6
Circumference: 78.63 78.44 -.19
Revs per mile: 831.00 833.00 2.00

The revs per mile are closer with the 255 as well as the rest of the numbers. I plan to go with a ContiSport so that is a Porsche tire. I will also keep my original rears on the car and just swap the fronts.

Joe Weinstein

Tim, I hope everything works out for you, but the calculator may be
wrong. Even the page says:

"The results of this calculator are based on the mathematical equations
of the sizes entered, not the actual tire specs provided by the tire
manufacturers. Please refer to the guides supplied by manufacturers for
exact specifications".

You aren't going to be able to know the revs per mile, or any other
number, to any degree of accuracy from that calculator.

This calculator is even more ridiculous than some others, because it
has the temerity to even 'calculate' the section width, which varies
widely for different brands of the same size tire, depending on the
stiffness of the sidewall.

Eg:
Published specs for the tires I run:

Hoosier AS03 P245/35ZR-18 - Hoosier publishes a diameter as 24.5"
The calculator says it would be 24.75".

Kumho Ecsta V700 305/30-18 - Kumho says it's diameter is 25.0".
The calculator says it's 25.2"

Continental ContiSportContact 2 - Continental publishes it's diameter as 25.2"
The calculator is wrong, saying it's 24.97"

For some cars, this sort of difference isn't material, but for anyone
trying to get the maximum, or for anyone with an AWD PSM car like
ours, this magnitude of difference can cause PSM to malfunction. I
have seen a 996tt's PSM almost bring a car to a halt on a curved
onramp at pedestrian speeds because tire diameter differences
fooled the PSM into thinking the fronts were spinning when in fact
they were just a couple percent smaller diameter than the rears.

Spec Sources:
http://www.tirerack.com/tires/Spec.j...SportContact+2
http://www.hoosiertire.com/specrr.htm

Tim Wasmer

I found several other "calculators" online as well and the results of all were close. Wether that means they are all using the same math I am not sure.

I will just go by the tire store and have them pull the stats (as they are professionals) and see what happens..