Date: Wed, 31 Jan 2001 10:52:47 -0700
Reply-To: Karl Wolz <wolzphoto@WORLDNET.ATT.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Karl Wolz <wolzphoto@WORLDNET.ATT.NET>
Subject: Re: [Syncro] What I Found About Tire Height Calculations
Quite some time ago, I recall seeing a catalog from ( I believe) Michelin,
which gave number of revolutions per mile at different speeds. I don't
recall any actual numbers, but it was interesting to see that different
tires expanded at speed by quite different percentages.
Karl Wolz
----- Original Message -----
From: "Brent Christensen" <brent@VANAGON.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, January 31, 2001 9:31 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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>> My refrigerator article:
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>>
>> ftp://gerry.vanagon.com/pub/auto-insurance-madness.html
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and
>> keep them off the list.
Brent Christensen
'89 GL Syncro Westy "Klaus"
Santa Barbara, CA
www.vanagon.com
