Ratings for Tires (fwd)

["Barton P. Chambers" <bchambers@atlantic.nos.noaa.gov>]

This is forwarded as a public service announcement.  For those of you wise
enough to heed the old soldier's wisdom about not reading the latest issue
of Excellence (an All Porsche Rag), be advised the Jim is one of the regular
technical contributors/Editors.  He speaketh wisdom worthy of consideration
of anyone who entrusts his or her life to 4 little contact patches...

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From: pasha@hal.com (James Pasha)
Date: Wed, 2 Nov 94 11:28:22 PST
To: porschephiles@tta.com
Subject: Ratings for Tires


Hi all,

In my post about tires, I mentioned speed rating. This was for a factor
most auto owners never consider, the car was designed to have a specific
tire to assure minimum advertised performance can be met. Then there are 
the TUV (German) standards that enforce replacement with specified speed
rated tires so the owner doesn't go cheap and spread himself over the 
countryside when the rag fails. Rag is an expression in the tire industry 
for a tire that isn't safe unless parked in the driveway (questionable
even there).

Look, I don't care what people put on their cars for tires, it's their 
wallet and car. But when you buy something that is capable of almost Warp
3 on the interstate (three times 65 MPH) consider this: You want control
at all times, the higher speed ratings give you a tire with better 
construction and more steps in the assembly/quality process. In most cases 
there is a definate improvement in low-speed handling as well.

Another consideration, it's now getting cold out. Tire failures will 
increase for a month. WHy? low tire pressure. Check the pressure weekly, 
especially after a freeze. Cold weather causes fluctuations with the 
temperature cycles. The air contracts, lowering the pressure, the opposite
of running on the track. Sidewall failure is the problem with low pressure.
The sidewall isn't supported correctly from the low internal pressure, it
flexes more, generating heat. The heat causes the radial sidewall to 
delaminate, finally rupturing. It takes some time to occur, but will fail
if the pressure isn't corrected.

Size. Well, on a Porsche the OEM delivered size for your wheel size will
work well. Be very careful when using aftermarket wheels. The offsets are,
in a few cases, just different enough to cause rubbing or interference 
on some models. An example: a 914/6 can run 205/60 15 tires on six-inch 
wheels with the standard 911 back-spacing (1965-1989). The same is true of 
a 914/4 if front wheel offset is corrected to clear the front strut. The 
914/4 has a narrower front track because it uses a different front strut/
hub than a 914/6 (this is the best reason I can think of for the five-bolt
conversion). If not corrected, the 205 will rub on the front shock cover
when cornering. (note, the rear fender lips should be rolled because
the bodies are not true from the factory).

So much more can be talked about when it comes to tires. *sigh*

Jim Pasha
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Cheers,

Bart Chambers
'77 Feline Varmint Felix, Gray Tabby
'86 Carrera Cabriolet, Indischrot
'87 Syncro (Stealth Quattro)