Tire wear in the center... pressure too low?

OK,

I wanted to get everyone's thoughts on this.

I am running Fuchs 7 & 9s with 205/55 and 245/50s Yoko AVSs on my '85 Carrera.

I know this is common for the rears to wear in center however, I thought it was because the light car versus the large (over inflated) tires. Therefore, I began lowering my rear tire pressures to 34 PSI (as opposed) to the Porsche specified 36PSI. This seems to have induced a bit of under steer when coming out of the corners.

I was in my castle on the throne reading Merlin's Tech Notes II, when I re-read the article about tire pressures and why the "newer" radial tires wear on the inside. Bruce Anderson specified how a low tire pressure actually makes the tire wear on the inside.

Huh?!? I thought the opposite was true?!

Bruce demonstrates this in the article specifying how the "newer radial" tires will now wear on the inside when there is too little tire pressure. He says this is due to the Radial tires sidewalls actually collapsing, there-by, bulging the center tread area outward and wearing the tread in the center. He goes on to say the BIAS tires would wear on the inside if there were too much pressure, and reiterates how the opposite is true for the radial tires.

Is this correct? I thought this to be true if you run too small a rim. I understand the 9-inch rim is almost too big for 245 tires.

Does anyone else concur?

TIA


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Nick Hromyak
'85 Carrera

I thought the same thing. However, I remember reading something from Bruce Anderson that the reason the center of the rear tires wear quicker is because of the change in camber during ordinary driving conditions. And it makes sence. As I picture it, the center of the rear tires is always on the road, whereas the tire sides, be it outer or inner, will alternate depending on whether the car is squatting or lifiting.
As far as the difference between the radials and not, I have noticed that the outers always wear quicker regardless of whether the tire is over or under inflated.
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Paul
78SC Targa

Based on my RACE experience, I use a pyrometer to judge my correct tire pressures. After a hot lap, I jump out and measure tire temps on both edges and the middle.

If the middle is higher than the outsides, I lower the pressure, lower temp, raise the pressure.

If the outside temp and inside edge temps vary much, I look into camber adjustments.

The same thing can be done on a spirited run in the country. A pyrometer is a inexpensive tool.


As a note for the motorcycle riders....the tire that got used up in the center and still had tread on the outside was a tire that was not used correctly....8^)

[This message has been edited by mikez (edited 04-10-2001).]

PBS 911, your last statement says "As far as the difference between the radials and not, I have noticed that the outers always wear quicker regardless of whether the tire is over or under inflated."

My center has always worn quicker. Really strange things...

I have always (in the passed anyways) agreed with MIKEZ regarding changing pressures. Given the information from Bruce A. in Tech Notes II, I am beginning to wonder.. ?

Anyone else?


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Nick Hromyak
'85 Carrera

[This message has been edited by nhromyak@yahoo.com (edited 04-10-2001).]

To add to what Mike said, this is esp the case on the right side of the car where the tires are close to a hot oil tank on the rear and a hot oil cooler on the front. I have noticed on a hot track check that the right side pressure always ends up higher.
Nick.

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_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

I have always understood that heat is what wears tires. I still believe that. A pyrometer would be a fun toy. I mean tool, fun tool. I believe you could regulate your tire wear with it.

Oh, one more thing. LOW pressure makes heat, not high pressure. Heat is created by tire flex.

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'83 SC



[This message has been edited by Superman (edited 04-10-2001).]