Carlo_Carrera

The manual my say it is okay but I wouldn't trust my viscous coupling to survive that set up for long.

xmaciek82x

This was helpful. I did the math on the oem summer tires the 996tt came with and rears are slightly smaller. It makes sense for the winter tires to follow the same logic. I'm sure using the winter setup Porsche recommends that causes the fronts to be slights bigger won't have any real negative effects either, but when the time comes for me to finally own a 996tt, I will follow the summer setup route and run slightly smaller tires in the rear.

Carlo_Carrera

Sounds good.

xmaciek82x

Last question around winter setups... I promise

If I go aftermarket route, then best offset for 18x8 wheels to run in front is 50mm, what offset should I be looking for when searching for rear 18x10? 40mm?

breakfast

It's perfectly ok. There is slack in the design for either side.


I do happen to run with the slightly smaller diameter in the rear in the winter but I wouldn't lose any sleep if it was the other way.

Carlo_Carrera

I would lose sleep, and I could not in good faith recommend someone else try a set up I would not use.

Thomas42

A few burn outs should fix it.

doubleurx

So you guys trust your tech, more than the engineers that designed the car?

I've run the recommended winter tire set up in the manual for 6 years without issue. Am I do for a failure? I don't think so.

breakfast

absolutely not.

Carlos is making an issue about nothing.

breakfast

thats not how it works. There isnt enough of a speed differential(with the factory spec'd set up) for this to happen just cruising around.
stop spreading misinformation

Carlo_Carrera

Porsche's engineers also thought glueing coolant pipes and using a water pump with plastic blades were good ideas.

I am not spreading misinformation and rather than us going back-and-forth about our opinions how about I lay out how the all-wheel-drive system works and we can let everyone decide for themselves.

A viscous coupling works very similarly to a torque converter. There is no direct mechanical link connecting the input and output. It is basically two finned disks housed closely together in a fluid bath. When one disc rotates it's rotational force is transferred to the other disc via the movement of fluid.

With the recommended summer tire sizes on the car because the rear tires are smaller and rotating faster than the fronts the viscous coupling system is always sending a small amount of power to the front wheels. If I remember my technical specs correctly I think it's something like 10%. That causes the all-wheel-drive to always be active and engaged.

With the winter tire sizes and the fronts being smaller the viscous coupling is now working in reverse. The rotational force from the front tires is now trying to increase the rotational speed of the transmission and engine. So not only does that sound wrong it's also negatively charging the all-wheel-drive system. Instead of it being active all the time like you want on snow or ice the fronts are actually in-active until the rears spin. And that negatively charged state means the viscous coupling basically has to reverse itself every time the car loses traction.

I am not the only one here who believes the factory recommended winter sizes are not ideal. I was asked for my opinion and I am practicing what I preach.

If you folks feel differently that is your prerogative.

Carlo_Carrera

LOL, that will work.

Berra

It's 5%. Under normal use the split is 95-5 rear/front.

Carlo_Carrera

Thanks for clarifying.

jeanmarcboilard

It doesn't matter which tire is bigger or smaller. As long as its within 3%, the viscous fluid is cool and not engaged. If either input, rear or front, spins faster, the fluid hardens and locks the coupler, then power is transmitted to the front regardless of which input caused it to harden.
That's how I understand it.