| Subject: | Re: [Syncro] What I Found About Tire Height Calculations |
| From: | Brent Christensen |
| Date: | Wed, 31 Jan 2001 08:31:47 -0800 |
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/ >> 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 >> If you would like to modify your subscription settings (including >> unsubscribing), visit: >> http://groups.yahoo.com/group/Syncro >> Please remember that rude or offensive language is not allowed on the >> Syncro list. Please try your best to resolve differences privately and >> keep them off the list. Brent Christensen '89 GL Syncro Westy "Klaus" Santa Barbara, CA www.vanagon.com
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