Date: Wed, 31 Jan 2001 00:06:21 -0500
Reply-To: Derek Drew <derekdrew@RCN.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Derek Drew <derekdrew@RCN.COM>
Subject: What I Found About Tire Height Calculations
In-Reply-To: <f7.6ffee7a.27a72165@aol.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed
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/
To sign up for the Vanagon mailing list send email to
listserv@gerry.vanagon.com
with the body text SUBSCRIBE VANAGON Firstname Lastname
To sign up for the Syncro mailing list go to
http://www.egroups.com/group/Syncro
My refrigerator article:
ftp://gerry.vanagon.com/pub/technical/Refrigerator-Manual.txt
My article that shows how to deal with insurance companies:
ftp://gerry.vanagon.com/pub/auto-insurance-madness.html