981 wheel diameter questions???
 

ok, having difficulty sourcing nt01 tires in 18 inch

according to nt01 specs, 245/40-18 and 275/35-18 and exactly the same diameter, 25.55 inches

would these be ok to run on a 981 cayman?

i last ran nt01 on a 987 cayman and ran 245/40-18 and 265/40-18, which actually means the rear wheel is a bit larger diameter than the front

265/40-18, 26.34 inches seem nowhere to be found!

275/40-18, 26.65 inches are available also, would these work?

i am going to call paul a radial tire tomorrow to see what he can get me

You know you got the title of this post wrong? All 18” wheels have pretty much the same diameter (18”)

BTW, if the width fits the the same diameter tire will work fine

245/40-18 & 275/35-18 are typical for 981 track tires.

well, I guess my concern is not rim diameter, but wheel diameter, and its impact on brake bias, etc

anyway, I found a cool site last night

and if you go here

https://tiresize.com/tires/Porsche/Cayman/2014/S/

you can compare OEM fitment to alternatives, and it seems those combos highlighted in green are "OK"

it makes sense that manufacturers would be building tires to an industry spec, so this type of comparison can be done like this

so, the interesting thing I see, in my particular fitment with the nt01 tires is that the appropriate front tire would be a 245/45-18 front, and 265/45-18 rear

this would in theory keep outside wheel diameter to spec, or at least not far off percentage wise, and not far off front to rear ratio

those don't exist in the nt01 line

and as the 265/40-18 nt01 is not available, my best option is this

245/40-18 front
275/40-18 rear (not the 275/35-18)

these are still shorter than oem spec 19 or 20 inch rim wheels

and now I can see why Mark at og racing has said that pagid yellow rs29 in front, and black rs14 rear has been working for the 981 Cayman. I have even seen a few other sites that only have this combination for selection under 981 Cayman parts. that rs14 pad tames the slightly larger wheel diameter of the rear wheel. especially if you are running r-comp or r-dot tires.

now, if anyone on here can shoot this down, please speak up!

Mike,

why not the 275/40-18?

is the front/rear ratio truly too great?

Damn, Otto here you go overthinking again
Dirk

LOL

it is about the only thing that entertains me these days!

And the rest of us, thanks Otto, found your true purpose in life!! LOL
Dirk

My understanding is that combination is used when there is too much front brake bias in an effort to move some of it rearward. It’s not related to tire or wheel size. I use that combination for that reason, though not on a cayman.

I’m not following why tire diameter is being mentioned with brake bias. Brake bias is based on a host of other things. Here is an interesting article on it: https://www.brakes-shop.com/brakeped...nd-performance

For tires, usually you want the same diameter tires so the nannies don’t get in a tizzy, especially ABS and TC. If the computer sees one end spinning at a different rate due to diameter difference, it could think there is a skid or spin happening and work to control something that’s not an issue. Staying near OEM sizes is so the speedo and odo stay accurate. Maybe I am outdated and those are recorded differently on modern cars though.

good point, and thanks for the link

i will check it out

oem is 26.4 inch front, 27.3 inch rear - 0.9670

for nt01's...

245/40-18 is 25.55 inches

275/35-18 is 25.55 inches - 1.0000

275/40-18 is 26.65 inches - 0.9587

oh, and let us not forget that after a weekend at the track, all these numbers change a bit