Should it rather be measured from the ground to the top?
Zoltan
----- Original Message -----
From: "Brent Christensen" <brent@VANAGON.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, January 31, 2001 8:31 AM
Subject: Re: [Syncro] What I Found About Tire Height Calculations

I have been sitting on the sidelines on this one, but I suddenly felt
compelled to add my 2 cents' worth...

One of the things I have not heard anyone discuss is the difference in
diameter of a tire while standing vs. while traveling at 70 mph.  Surely
there must be a difference.  Also, if you think about the physics of it, a
tire is not actually *round* while it it turning on a vehicle, is it?  It
has a flat spot where it contacts the road.  This distortion could certainly
introduce errors into a formula based upon the "static" diameter of a tire
(i.e. measured while mounted on the car, or simply mounted on a wheel laying
on the ground)

Kinda make you go "Hmmmmmm....?"

Brent Christensen
89 GL Syncro Westy
Santa Barbara, CA

  >>  SUMMARY: The material below reaches the rather stunning conclusion
that at
  >>
  >>  4,000rpm in 4th gear

  >>  --the stock MXLs cause the van to travel only 70.94mph
  >>  --BFGs cause the van to travel only 74.32mph

  >>  The lesson here is that for gearing calculation purposes we cannot use
any
  >>
  >>  more any of the online tire height calculators and to be accurate you
have
  >>
  >>  to find out the REVOLUTIONS PER MILE as rated by the manufacturer. In
  >>   other
  >>  words, our own tire height measurements are in error as are all web
sites   >>  that attempt to translate tire size into height data.

  >>  This tire size thing is truly a mess.

  >>  ===================================================

  >>  The trouble I have with going with manufacturer data is that I don't
trust
  >>
  >>  the manufacturers' data!

  >>  For example, I measure my BF Goodrich 27 x 8.50/14 tires as being
26.15"
  >>  tall, but the manufactuer says it is 26.6" tall. Those tires are NOT
26.6"
  >>
  >>  tall, based on the measurement I actually made last weekend. If the
tire
  >>   is
  >>  26.6" tall, then it must have been measured at about 500psi or
something.

  >>  The diameter number the manufacturer gives is meaningless.

  >>  The revolutions per mile is the only meaningful number. Now, even the
  >>  bfroodrichtires.com web site is a contradiction. Aside from being 26.6
  >>  inches tall, the web site says that the tire has 782 revolutions per
mile.
  >>
  >>  One can translate 782 revolutions per mile into the actual diameter,
which
  >>
  >>  suggests the tire is only 25.8 inches tall!

  >>  =========================================

  >>  DANGER CALCULATION IN PROGRESS

  >>  782 revs per mile, which the manufacturer gives, comes out to

  >>  --to convert 1 mile into inches equals 63360 inches
  >>  (http://www.math.com/students/converters/source/length.htm)
  >>  --thus, the tire moves 63360 / 782 revolutions per mile = 81.0230
inches
  >>  per revolution
  >>  --to find diameter
  >>  http://www.math.com/students/calculators/source/circle-solver.htm
  >>  --81.0230 inches per revolution yields 25.8 inches of diameter
  >>  =============================================

  >>  This means that by this measure that John Wessels' 30X9.50 BFG Mud
  >>   Terrains
  >>  actually have a rolling diameter much smaller than would be implied by
its
  >>
  >>  size. That is, wherease the manufacture lists them as having 29.5
inches
  >>   of
  >>  height and John also reported that height, in fact their rolling
  >>   resistance
  >>  is much less--only 28.6 inches of effective diameter.

  >>  ===========================================
  >>  CALC ON THIS

  >>  BFG says these tires have 705 revolutions per mile

  >>  --to convert 1 mile into inches equals 63360 inches
  >>  (http://www.math.com/students/converters/source/length.htm)
  >>  --thus, the tire moves 63360 / 705 revolutions per mile = 89.8723
inches
  >>  per revolution
  >>  --to find diameter
  >>  http://www.math.com/students/calculators/source/circle-solver.htm
  >>  --81.0230 inches per revolution yields 28.607 inches of diameter of
real
  >>  rolling diameter.
  >>  ===========================================

  >>  I sent an email to Michelin asking about the stock Michelin MXLs. They
  >>  responded on the phone that the MXLs had revolutions per mile of 819,
  >>   which
  >>  translates into 24.625" of height.

  >>  ============================================
  >>  CALC ON THIS

  >>  --to convert 1 mile into inches equals 63360 inches
  >>  (http://www.math.com/students/converters/source/length.htm)
  >>  --thus, the tire moves 63360 / 819 revolutions per mile =  77.362
inches
  >>  per revolution
  >>  --to find diameter
  >>  http://www.math.com/students/calculators/source/circle-solver.htm
  >>  --77.362 inches per revolution yields 24.625 inches of diameter of
real
  >>  rolling diameter.

  >>  ===========================================

  >>  What these calculations yield is the rather stunning conclusion that
at
  >>  4,000rpm in 4th gear

  >>  --the stock MXLs cause the van to travel only 70.94mph
  >>  --BFGs cause the van to travel only 74.32mph

  >>  The speedometer shows much higher speeds than these at those rpms and
the   >>  speedometer is wrong. Probably made that way to trick us into
thinking the
  >>
  >>  vehicle had more power than it actually does when the vehicle was new.
  >>   This
  >>  speedometer of ours must have had one of the highest error rates of
any
  >>  vehicle on the market ever introduced, and my guess is that the error
was   >>  intentional.

  >>  For the record and the benefit of the archives, the Michelin caller
said
  >>  the direct Michelin replacement for the MXL tire that was stock on the
  >>  Vanagon is the MXT, which is a reinforced passenger car tire with 836
  >>  revolutions per mile. (Hey!, that's even slower!--Derek) He said you
could
  >>
  >>  order these replacements by ordering Michelin part number 73568 and
that
  >>  they had them in stock, but not a huge number of them.

  >>  For the uninitiated, the list wisdom is not to buy any passenger car
tires
  >>
  >>  anymore, but at replacement time to try to shoot for light truck
tires.
  >>   You
  >>  can often tell these because they go up to 50psi. If your new proposed
  >>   tire
  >>  goes up to, say, 40psi this is a hint that you may be buying the wrong
  >>  tire. So in any case, the advice of the list and my personal
experience
  >>  would suggest that one not purchase these MXTs offered as replacements
for
  >>
  >>  the discontinued MXLs.

  >>  Thanks Frank for the tip.

  >>  At 02:41 PM 1/29/01 -0500, you wrote:
  >>  >Derek,
  >>  >
  >>  >I'm sure that you will have other opinions expressed, but I believe
that
  >>   the
  >>  >static numbers that you are requesting are fraught with error. The
real
  >>  >number needed is the dynamic rolling number which is strongly related
to
  >>   the
  >>  >tire construction detail, the inflation pressure and loading. Many
  >>  >manufacturers publish these numbers as the "number of revolutions per
  >>   mile".
  >>  >The data is available from all manufacturers with an eMail to
customer
  >>  >service. Comparing tires from Michelin, Dunlop, Yokohama and Goodyear
  >>   will
  >>  >show similar static heights and static loaded radii with differing
  >>   rev/mile
  >>  >stats. The rev/mile number is generally given for a specific
inflation
  >>   and
  >>  >load. The manufacturer can often give a curve with revs/mile vs. load
or
  >>  >pressure.
  >>  >
  >>  >Frank Grunthaner

  >>  ________________________________________________________
  >>  Derek Drew                              New York, NY & Washington, DC
  >>  derekdrew@rcn.com
  >>  Email me for Viscous Couplings
  >>  '90 Syncro Westfalia...
  >>  ...seen off-road at http://www.tiu.net/~des/vw/drew/index.html

  >>           Also seen at:
  >>                        http://4wd.sofcom.com/VW/snow.html
  >>                        http://4wd.sofcom.com/VW/Campers/Campers.html
  >>           Note: most valuable Vanagon sites on the planet (for owners)
are:
  >>
http://gerry.vanagon.com/cgi-bin/wa.exe?S1=vanagon
  >>                       ftp://gerry.vanagon.com/pub/
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  >>
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Brent Christensen
'89 GL Syncro Westy "Klaus"
Santa Barbara, CA
www.vanagon.com