Pressure to size ratio? - Page 2 - Alfa Romeo Bulletin Board & Forums
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post #16 of 19 (permalink) Old 06-21-2006, 03:16 AM
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strange I have never had a tire wear in the center even on the back runing max PSI stamped on it or even a bit above that.
"Higher front pressure makes oversteer, lower makes understeer and higher rear pressure makes understeer while lower makes oversteer."
let think about this
"Higher front pressure makes oversteer"= the front stick more so the back slides out
"lower makes understeer(front)"= less stick so the front slides
"higher rear pressure makes understeer"=back sticks so front slides
"lower makes oversteer.(back)"=back sticks less so front slides out
so if any of this hold true more PSI = sticking.

somone talked about using one of them laser therometers to read across the tire after a hard run that seems like a good way. but I think if you also pumped up past the point where the center started to wear you would also see that rule of "Higher front pressure makes oversteer, lower makes understeer and higher rear pressure makes understeer while lower makes oversteer." should switch

1987 black Milano Verde, 1987 (made in 1986)blue $200 gold with ABS and LSD
1972 White spider 2000 Veloce

Last edited by slyalfa; 06-21-2006 at 03:18 AM.
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post #17 of 19 (permalink) Old 06-21-2006, 06:07 AM
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Cord: Yes ....same.

Cord: I agree:""...the proper tire pressure should provide uniform downward pressure across the entire width of the contact patch."

In rain I run higher pressure. Hydroplaning concerns me. I run about 33 if anticipating a long ride in the rain. Otherwise its 30
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post #18 of 19 (permalink) Old 06-21-2006, 06:46 AM
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Just looked in the owners manual of my 92.

14" 24/26 as previously discussed
15" 27/28

IMHO tire pressures like these are promoted by manufacturers to make the car drive very predictably with lots of understeer/plowing and to make the car ride smoother.

Obviously...you go autocross the car with those pressures....you'll be riding on the side wall. You take curves at speed often...you'll prematurely wear the edges.

How I do it... if you are driving spirited....bump it up in the low 30s. Its not as cushy. More cowl shake. But handles better. If you are autocrossing....start at 40 and watch the edge contact.

Best Regards,
John M

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1978 AR Spider Veloce 2000....the first and still here...Bianco
1984 AR Spider Veloce............the second & gone to the parts bin....Rosso
1992 AR Spider Veloce............the third and still here...Bianco
1991 AR 164L........................the fourth and traded on the SS....Rosso
1965 AR Sprint Speciale..........the fifth...in boxes....sold....Bianco
1978 AR Spider Veloce 2000....the sixth gone to be a parts car...slow black
1967 AR Giulia Sprint GT.........the seventh with Gregg and sold......Rosso
1993 AR Spider Veloce............the eighth with Gregg and still here....Verde Inglese
1982 Ferrari 308 GTSi.............the first and still here....Azzurro
1999 Ferrari 355F1 Spider........the second and still here....TdF Blue[/SIZE]
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post #19 of 19 (permalink) Old 06-21-2006, 07:31 AM
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The handbook for my '67 Duetto is also 24/26 psi.

I have had the center of tires wear - from over inflation. Note that many cars have a different pressure spec for light/max cargo load (max GVWR), including my 4200 pound Mercedes. I've had tires wear in the center when inflated for trailer towing (not an Alfa) - more load strength from the higher pressure, but worse contact patch uniformity.

The Tire and Rim Association (http://www.us-tra.org/) has recommendations for rims and tire pressures that differ from the auto manufactures. They are oriented to maximum safety (tire frailure from overloading is the major problem they see); auto makers balance comfort, tire wear, performance and handling, rain, dry, etc. for their recommendations, adn different makers have different balance points.

For racing with somewhat larger than stock tires, I've ended up with higher tire pressure than the book values. On the track, its all about lap times. Use a tire pyrometer to measure across the tread, adjust pressure (and about a hundred other variables) to get uniform temp, or at least similar right and left side temps. I've used pressures in the 30's (32 to 38) for 205/60/14 Kuhmos or Hoosiers, lower for my old favorite A008's.

I have several high quality (means expensive) tire guages, and some cheep ones. All sorts of readings from any of them. The only thing that matters is that you can get the same reading from one guage several times, over several days. (That's hard enough). I've found 5 psi variations among very good guages with careful techniques.

How you take a tire pressure reading can affect the pressure reading by several psi, so you must establish a repeatable technique.

Cold or warm maters. Before or during runs especially affects what the number is. I have tires that read up to 8 psi higher after a few laps. No extra air added. Tha means how you drive matters too.

Some racers are now using Nitrogen gas to inflate their tires. Better heat transfer, less sensitive to tire and track temperature.

And all this discussion about the difference between 26 and 32 psi! That's less than one cu ft of AIR!!!

Robert
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