Tire Pressures revisited

[ADias]

My rear tires got down to the tread bars and I noticed that the wear was most on the center of the tires. I thought that meant over-inflation, and since I ran on 33PSI front and 41 PSI rears (PAG recommends 36PSI front, 4 PSI rear for my 996 C4) I thought I should drop those further. Wrong! My tire installer told me today that it is precisely the opposite - the tires were worn in the center due to under-inflation. That has to do with the fact that these tires have short, very stiff, sides (18" wheels), which buckle out when the tire is under-inflated, pulling the patch up, and more so in motion, aided by centrifugal force. This means uneven wear AND a smaller patch, therefore less available gripping surface.

So, there you have it. This is the price to pay to run high performance tires. Want performance and even wear? use the recommended tire pressures. Want more comfort and accept diminished performance? lower the tire pressures (but not by much for safety sake).

[Loren]

I run 32 front and 36 rear. I look for the best handling for my type of driving.

I do not worry about wear. I don't think there is anything you can do to get even wear with these cars and the amount of negative camber you need for performance. The rear tires will wear faster -- 911's have been this way since the mid 80's.

[nick49]

I just replaced my 18" rear Pirelli P-Zero Asymetricos. They were worn mainly on the outer 25%, then inner 10%, the centers had the least wear. I had a 4 wheel alignment when these tires were about 20% worn and ran the high recommended pressures.

[wdonovan]

ADias, interesting take on under-, over-inflation. I think that with a flat laying tire (i.e. no camber) this would apply. I think though with the heavy negative camber there is more to it than the tire dealer's advice. See my post in the last visitation of "Tire Pressures Revisited" from a few weeks back.

Loren, I'm looking for a way to get more rubber on the road and get maybe a few more miles from my rear tires.

My driving is never track, sometimes pretty frisky, a lot of just plain driving (less than 3 lateral g's cornering force). My theory is that the Porsche rear suspension essentially still acts like my old 356 swing axle and the astronomical negative rear camber is to get the tire flat on the pavement only during the limits of cornering with a large amount of body roll.

Do you think it is possible (practical) to get a heavier rear swaybar (my 02 C4 Cabriolet has I think an 18mm rear) to reduce body roll, reduce the amount of negative camber to maybe -3/4 degree (since less body roll should require less neg camber to compensate) and lower the tire pressures based on pyrometer readings, compromising between temperature balance after highway cruising and after some typical hard cornering? I think I could in this way get the pressure down to under 35# in the rear and have a good contact patch. I'd love to wear a pair of tires so that I can happily throw out the whole tire rather than wish I could keep half.

[wdonovan]

ttt. Anyone?

Edited May 16, 2007 by wdonovan

[Ranger]

I think that the role that negative camber plays in traction under lateral g's is not a HS physics problem. I mean that in the sense that there are some complex interactions occuring. I'm sure that there are models that predict this behavior well, but they must be complicated.

Which is a long winded intro to this idea: "There's a lot more going on then just the role of body roll". Was that a pun? There is a force vector going thru the car's center mass to the center of the traction patch. Under significant lateral g's, there's a hell of a horizontal component to the force vector. If the load is more then a g then there's more horizontal component then vertical. And since the force of friction is a function of the vertical (assuming flat road) force, that means that tires shouldn't be able to hold more then a g of lateral force. Yet we know that they can. Just one example of how tricky the dynamics of suspension is.

When talking friction, that vertical force is called the "normal" force. Normal means perpendicular to the surface.

Thought experiment:

Consider what is happening to a car's traction patches when it travels around a 45deg banked corner and is pulling 1 g. Intuitively we know that the banking provides additional grip. In this case the vertical vector associated with gravity and the lateral vector associated with turning, are both equal at 1g. Since they are equal the sum of the horizontal and vertical acceleration components is a 45deg acceleration vector. Since, in this example, the banking is at 45deg, the sum of the acceleration vectors are exactly perpendicular to the track. Therefore the entire force is pressing down on the track's surface and there's no horizontal component of acceleration left that the tire's traction patch has to fight.

Ok, now transition from that thought experiement to the word of camber. I read somewhere the other day that camber can be thought of as a taking turns and bringing your banking with you. There's some weaknesses in that analogy, but it's got some charms too.

I'd attempted to wax poetic on the weaknesses of the "bringing your banking with you" idea, but it seems that they've been trumped by 2 beers.

Hopefully that made sense. If it didn't, have another beer.

Oh, and I'm running AD07's with 30/36. Rear wear looks good, but I think that the fronts can roll over a couple millimeter more. So Fri-Sun I'm going to be experimenting with 29/36.

Edited May 17, 2007 by RangerGress

[k-vic]

I think that the role that negative camber plays in traction under lateral g's is not a HS physics problem. I mean that in the sense that there are some complex interactions occuring. I'm sure that there are models that predict this behavior well, but they must be complicated.

Which is a long winded intro to this idea: "There's a lot more going on then just the role of body roll". Was that a pun? There is a force vector going thru the car's center mass to the center of the traction patch. Under significant lateral g's, there's a hell of a horizontal component to the force vector. If the load is more then a g then there's more horizontal component then vertical. And since the force of friction is a function of the vertical (assuming flat road) force, that means that tires shouldn't be able to hold more then a g of lateral force. Yet we know that they can. Just one example of how tricky the dynamics of suspension is.

When talking friction, that vertical force is called the "normal" force. Normal means perpendicular to the surface.

Thought experiment:

Consider what is happening to a car's traction patches when it travels around a 45deg banked corner and is pulling 1 g. Intuitively we know that the banking provides additional grip. In this case the vertical vector associated with gravity and the lateral vector associated with turning, are both equal at 1g. Since they are equal the sum of the horizontal and vertical acceleration components is a 45deg acceleration vector. Since, in this example, the banking is at 45deg, the sum of the acceleration vectors are exactly perpendicular to the track. Therefore the entire force is pressing down on the track's surface and there's no horizontal component of acceleration left that the tire's traction patch has to fight.

Ok, now transition from that thought experiement to the word of camber. I read somewhere the other day that camber can be thought of as a taking turns and bringing your banking with you. There's some weaknesses in that analogy, but it's got some charms too.

I'd attempted to wax poetic on the weaknesses of the "bringing your banking with you" idea, but it seems that they've been trumped by 2 beers.

Hopefully that made sense. If it didn't, have another beer.

Oh, and I'm running AD07's with 30/36. Rear wear looks good, but I think that the fronts can roll over a couple millimeter more. So Fri-Sun I'm going to be experimenting with 29/36.

I found your post interesting and like to make my point.

It’s been mention that there is negative camber but the tire wears on the inside so I don’t see camber as the problem.

(I read somewhere the other day that camber can be thought of as a taking turns and bringing your banking with you. There are some weaknesses in that analogy, but it's got some charms too.)

This comment intrigues me. I believe Camber should be set to maximum the footprint width on maximum load conditions for best traction while providing the correct toe to minimize friction.

This comment has a lot more analogy than weakness.

But at this point we are talking about why tires wear on the inside.

Tire pressures can cause problems when low because the sidewall can collapse but with high tire pressure the tire is supported. I have a problem believing that tire pressure can force a steel belt to buckled.

One reason I believe for center tire wear is the tire construction and can easily been seen by bring your eyesight level down to the footprint and looking for air gaps between the road surface and footprint. You should see the tire centre sits higher and the main friction area for tire wear.

Your banking method is great and would love further discussion maybe the correct cambers for banking?

[Chuck Jones]

"Thought experiment:

Consider what is happening to a car's traction patches when it travels around a 45deg banked corner and is pulling 1 g. Intuitively we know that the banking provides additional grip. In this case the vertical vector associated with gravity and the lateral vector associated with turning, are both equal at 1g. Since they are equal the sum of the horizontal and vertical acceleration components is a 45deg acceleration vector. Since, in this example, the banking is at 45deg, the sum of the acceleration vectors are exactly perpendicular to the track."

HUH???? I think I'm just going to buy new tires when I see the safety bars..... :)

[wdonovan]

Yes. 45 degree, 1 g lateral force, 1 g gravitational force, the resultant vector is normal (perpendicular) to the track.

So...... is the big negative camber wearing the tires out?