Date:         Tue, 25 Feb 97 16:22:17    
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         mdstockr <mdstockr@neo.lrun.com>
Subject:      RE: more on tires/pressures 

Sean, to answer your questions ....


>On my van I have Michelin XZX tires, size 185 R 14.  All the information
>I have is from what's printed on the tire sidewall.  It says Max Load is
>1540 lbs at the Max Cold Inflation Pressure of 40 psi.  I can find
>nothing on the tire that refers to a load rating (like passenger car or
>light truck) but there is a marking that says REINFORCED Radial.  Now, I
>assume then, that the Reinforced marking indicates a light
>truck/reinforced sidewall tire?

We are dealing with several different standards when dealing with bus tires.  
Your 185R14 tires are designed to meet ETRTO (European) standards.  Reinforced 
does not mean that this is a light truck (LT) tire.  For example, my Audi 90's 
sticker specifies 195/70HR14 reinforced tires and high inflation pressures (42 
psi) when loaded with 4 passengers and luggage.  Note, I use conventional 
standard load tires on my Audi.


>Also a question regarding the load rating, the 1540 lbs is at the max
>pressure.  Now obviously the load rating goes down as the pressure goes
>down, but I was curious as to whether the relationship is linear or not?
>Probably not, but just checking.  I know mdstockr can help here :-)

Yes, maximum load rating does relate to tire pressure, for truck tires.  Ask 
your tire dealer to see his copy of the Tire and Rim Association handbook 
(most don't have one).  This book lists the specifications (dimensions, rims 
and load) for all tires produced in North America.  In this book you will see 
charts relating tire pressure to maximum rated load for each truck tire sizes. 
 I do not know if the relation is linear.  But, for your purposes assuming 
that it is linear over a 10 psi range should be OK.


>In addition, here is the information that I obtained from the tire
>pressure sticker in the doorjamb of my 1981 Vanagon Westy.  The
>information probably doesn't apply to you unless you have an aircooled
>Vanagon Westy.
>
>Tire Size     Pressure, Front (psi)      Pressure, Rear (psi)
>---------     ---------------------      --------------------
>7.00-14 8 PR         33                          48
>
>185 R 14 C 6 PR      38                          55
>
>185 SR 14 Reinf.     33                          40
>
>I assume by my tires, I should go by the third listing, for the
>reinforced tires.  That is the pressure I have them inflated to now, 33
>psi front and 40 psi rear.
>
>Could someone decode the tire size/ratings above, mainly the first and
>second ones?  What's the PR about, does that mean passenger car tires?

7.00-14 8PR
  - Biased ply construction
  - 7 in. at widest point (varies with rim width)
  - 14 in. rims
  - 8 PR load rating.  This relates to old tire designs that used to use
    cotton cord as a reinforcement.  An 8 PR (ply rating) means that it had 8
    layers of cotton reinforcement. Tires have not used cotton cord for a very
    long time (30-35 years?), so that standard was translated into a specified
    maximum load capacity.  This is an old standard and I don't know it
    relates to load ratings today.

185R14 C 6PR
  - 185 mm (7.28 in.) at widest point
  - R radial ply
  - 14 in. rims
  - Load range C or 6 ply rating

185SR14 Reinf.
  - 185 mm at widest point
  - S speed rating
  - R radial ply
  - 14 in. rims
  - Reinforced construction

Note, when you go out to look for tires you will see tires sizes specified 
differently than those on your bus's sticker.  Here is how you decipher North 
American tire sizes:

P195/70R14 90S
  - P passenger
  - 195 mm at widest point
  - 75/100 aspect ration (the height of the tire is 75% of the width or
    5.8 in.)
  - R radial ply construction
  - 14 in rims
  - S speed rating

LT205/70R14
  - LT light truck
  - 200 mm at widest point
  - 70/100 aspect ration (the height of the tire is 70% of the width or 
    5.5 in.)
  - R radial ply construction
  - 14 in rims

UTQG Treadwear 200 Traction A Temperature B

  - Uniform Tire Quality Grading, a US gov't standard

  - This tire's tread will last twice as long (200%) as a special reference
    tire used by the US government.  When comparing two tires, one rated at
    230 and one rated at 280, the 280 rated tire should last 22% longer for
    tread life than the 230 rated tire.

  - Good traction (A is best).  This does not relate to traction in snow/mud.

  - Fairly good at resisting temperature buildup (A is best).  An A tire is 
    cooler running than a B tire.  Safer.  Will last longer.

Hope I have not confused things further.
________________________________________________

  Michael Stocker <mdstockr@neo.lrun.com>
  North Canton, Ohio, USA     Time: 4:22:17 PM
  LiMBO / GASS                Date: 2/25/97
  1989 Audi 90 Quattro
  Wanted: Nice 1968->? bus or Westi
  Previous VW's: '66-T3, '67-T1 & '70-T2 Westi
________________________________________________