xmaciek82x

My current setup:

Front: 19x8.5 with 49 offset
Rear: 19x110J with 40 offset

Front: 245/35-19 Pzero
Rears: 295/30-19 Pzero

With the current setup, my rears are bit taller then front. I'm thinking I should change the fronts out to make the rears smaller. What would be the best chose to go with on the 8.5 front wheel?

http://www.tirerack.com/tires/tires....reModel=P+Zero

I'm guessing 235/40 will be a better choice.

My rears are rubbing so I really want to do 285/30 in the rear but don't want to go narrower then stock tire recommendations.

Macster

If you venture off the reservation regarding tire choices that is elect to use something other than the factory sanctioned tires you have the concern about mis-matching tire sizes that can cause the PSM system to act up or even worse can cause premature failure of the viscous coupling. I assume you have not removed this and converted the car to just RWD.

This viscous coupling relies upon the different rotation speeds of the rear tires vs. the front tires to heat by friction special fluid that as it becomes hot(ter) becomes more viscous. As it becomes more viscous it transfers more torque to the front wheels.

If the front and rear tires sizes are too close to the same size torque transfer to the front wheels is reduced. I'm not sure what the down side is to this.

But if the front and rear tire sizes are too far apart the speed difference between the rear and front can cause the viscous coupling fluid to run too hot -- IIRC it already runs at around 350F -- and this over temperature condition can ruin the viscous coupling which is part of the front diff.

My advice is to use the factory sanctioned tire sets as a guideline as to what tire size differences, front vs. rear, are acceptable and if you then want to select tires that are different from those sanctioned by Porsche choose tires that stay within the size difference ratio of the factory sanctioned sets of tires.

xmaciek82x

Thanks Macster. Seeing my car came with 19" wheels and the front is 8.5" vs 8", I'm not able to use stock recommended sizes.

What happens if the rear tires are larger then fronts? I been read about this and i'm left confused. Half the people say it is fine, other half say it is not fine.

Here are the calculations using stock tires size, my current tire size, potential future tire size:

Stock Porsche Sizes (-.4% size difference, rear is smaller)


My current Tire Size: (+.8% size difference, rear is larger)


My proposed tire size (-1.5%, rear is smaller)

Macster

With the factory tires the rear tires turn more revs per mile than the front tires. This means at the viscous coupling the discs turned by the rear diff spin faster than the discs that are turned by the front diff. This speed difference heats the fluid and causes it to be more viscous. The end result is some torque is transferred from the discs turned by the rear diff to those turned by the front diff.

BTW, the amount of torque transmitted varies from a low of 5% to a high of 40%. My info is vehicle speed has to be in the vicinity of 150mph+ for this 40% torque transfer to occur.

With the current setup the rear tires make fewer revs per mile. While there is a difference in speed at the discs the torque will be transferred from the faster spinning discs to the slower ones. This acts to brake the car.

With the proposed setup it is similar to the factory setup in that the rear tires turn more revs per miles vs. the front tires but the difference is 4 times of -0.4% difference. Is this enough to exceed the safe operating temperature of the viscous fluid, add too much heat load the front diff and it lubricating fluid?

I can't say but that kind of an increase in difference in the number of revs the rear tires turn vs. the fronts has to be relying upon some pretty hefty margins in the heat load the viscous coupling and the front diff can tolerate.

There is more: As I touched upon above with the stock tire setup at approx. 150mph the torque transferred is 40%. If one approaches that speed with the proposed tires fitted that can more than overheat the fluid. It can exceed the material strength of the discs and shred these. And because the exceptional tire size and rotational speed difference even if you promise not to ever never ever reach 150mph the speed at which this can possibly happen can be quite a bit less than 150mph.

RngTrtl

if memory serves i recall reading in the manual or the essential 996 book that as long as you stay under 3% difference then all is well. Also, try on the 996 n/a forum as well since the 4s has the same set up and there is much more activity over there.