ADias

Can you explain how that correlates to lower rear pressures on the 4WD?

alexb76

Correct, but they are heavier upfront, and not in the rear! So, again... to speculate, it could be the lower pressure in the rear would create a better balance vs. heavier front. Also, I believe the swaybars are different between RWD and AWD so that could also play a role.

Mike in CA

Here's my take. One of the primary reasons for the big differential in tire pressures and tire sizes on the C2's is Porsches' attempt to control the tail happy lift throttle nature of the 911 platform. We all know it's there even though it's been tamed. Keeping in mind the following discussed :HERE

To Decrease Understeer and Increase Oversteer:
Raise Front tire pressure, Lower Rear tire pressure
To Increase Understeer and Decrease Oversteer:
Lower Front Tire Pressure, Raise Rear tire pressure

Porsche uses higher rear pressure to give more rear grip and help control oversteer, not least for liability reasons. The more "stable" AWD platform of the C4S doesn't require as much help in this regard and so the differential between front and rear pressures can be reduced for the sake of better tire wear and comfort, as well as just being a better match for the handling of the C4's.

mdrums

Mike, you're probably on to something.

alexb76

I think you got the best explanation, just one correction, high rear pressure is actually to give it *less* rear grip.

ADias

Yeah, I think it is reversed, and in light of that it makes sense to reduce the pressure differential front-to-rear (reducing the rear pressure to 37 from 39) on the 4WD given the extra front grip those cars have.

Then, on that note, if the 4S can ride with 37PSI in the rear (cold) so can the 2S, as long as the front drops to 31/32PSI (cold).

alexb76

Hard to say, but generally, increasing pressure (to a point), reduces the footprint, hence grip, while underinflation also reduces grip. In this case, you wanna have more understeer in 2S hence more pressure in the rear.

I am not sure what you're asking, I find it that on my 19 tires, if I lower to less than 33, it rolls too much and probably will have excess tire wear, not sure if it is recommended.

Mike in CA

All due respect, but you guys have it wrong. Again, please refer to the link below. The Tire Rack in particular knows a thing or two about tires and handling and their info is consistent with my experience and what I've read on the subject. Increasing pressure, to a point, increases grip which makes sense in the context of why Porsche requires higher pressure at the rear to give more grip and reduce oversteer. Only above a certain point, will tire footprint and grip be reduced.

The Tire Rack on Handling

sjfehr

There is a sweet spot for grip for tires. Grip will continue to increase as you increase pressure until it plateaus, at which point it will drop off again. My general observation is that Porsche intentionally WAY underinflates front tires for grip, but sets the rear tires pretty close to optimal grip. Same with camber. The recommended alignment and tire pressures are all wrong for performance as they induce horrible boring understeer.... but boring understeer is what keeps trophy wives out of ditches and Porsche's reputation as a "safe" car safe. They're not keen to revisit their reputation of old.

Fortunately, both pressure and alignment are easily remedied

Edit: now, with link!
http://books.google.com/books?id=cr4...page&q&f=false

This is an excellent book, btw.

Mike in CA

Which is why I qualified my statement with "to a point". I think you're spot on with your comments.

alexb76

It never says anything about *grip* of the rear tires, but how to reduce oversteer which I completely agree with you, higher tire pressure in the rear reduces oversteer!

Also, I said generally *up to a point*
There's a sweet spot, when pressure increase upto a point you gain grip, after a point you lose when footprint is reduced! Where's that sweetspot on our cars, I have no idea! I'd say around 38-42 PSI...

Mike in CA

Sounds like were on the same page, Alex.

racer

Oversteer and understeer are conditions that develop when one has, or doesn't have, grip .. you guys are talking about the same thing

I imagine traction benefits of AWD allowed for Porsche to change slightly the stock pressures, without impacting the cars grip / understeer-oversteer concerns.

I also think for nearly 99.5% of owners, being able to discern, in street use, a 2psi difference would be near impossible.

gpjli2

What they said above. I would not expect a 4wd to run the same pressures as a 2wd car Tire loading is very different if you consider the sharing of drive responsibilities. The forces on a tire are determined by more than just weight distribution.

kyrocks

My cat's breath smells like cat food.