Date: Wed, 7 Apr 2004 22:43:58 -0400
Reply-To: Derek Drew <derekdrew@RCN.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Derek Drew <derekdrew@RCN.COM>
Subject: Re: Revs per Mile Calculations
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 09:57 PM 4/7/2004, you wrote:
>Five or six weeks is perfect. How do I figure out revs per mile. Is it
>calculated by the tire size printed on the side wall? Ideally I would like
>to go with either BFG AT's, Bridgestone Dueller AT's, or maybe consider a
>Michelin tire. Anyway it would be easier to research if I could calculate
>the revs per mile myself.
>
>Thanks
The revs per mile number is one that you would get from the tire
manufacturer. The number is valid when it is reported at a particular miles
per hour. They typically use 40mph, but ANY mph is fine -- just as long as
they report that number. Otherwise, there is a chance that they generated
the figure mathematically from the tire's diameter. Figures which are
generated mathematically are always way off and should be ignored. The way
you know you have a mathematically generated number is if you input the
tire's height or circumference into one of the tools you can find on the
web which allow you to do that and which then calculate the revolutions per
mile. Other tools allow you to input the tire's published tire height and
tell you the tire's revolutions per mile. If the numbers pretty much match
the predictions from one of these tools then you know you have a bogus
number there if the manufacturer did not report a mph figure to you when he
reported revolutions per mile. Luckily, all tire manufacturers have valid
revolutions per mile numbers in their databases and can tell in on request.
In fact, about half the tire manufacturers publish the revolutions per mile
numbers online. The others shoot it back quickly when asked.
So, I guess what I am saying is never to calculate revs per mile! Always
make sure it is an empirically observed number. The difference is large
enough that you shouldn't cut corners on this and be tempted to calculate.
The reason this is so important is that all our benchmark targets for
gearing issues are all based on valid revolutions per mile numbers. If we
had built those guidelines based on the mathematically derived numbers,
then it would be important to stick with that basis and not change the
basis in the middle of the calculation. In that case, I would have told you
to use a direct calculation off either the circumference or tire height
using, well, I would just point to one of the many tools on the web that
allow you to make those calculations.
In fact, we would have done that, but we found in examining revolutions per
mile data that the tire revolutions per mile varied greatly depending on
what tire it was. Two different tires with the same published height or the
same published size, were found to have very different revolutions per mile
characteristics. Tire sizes are only a guideline for the tire
manufacturers, but there is no reason for them to make the tires to rigid
tolerances (across different brands). They are, of course, made to close
tolerances for tires of the same brand, type, and tire size spec.
One tire that seems to run oversize is the BF Goodrich All Terrain Radial,
which has lower revolutions per mile numbers than other tires of similar
height simply because BFG builds their tires larger in diameter as a
general rule. Why, I cannot say. BFG is in the middle of working to make
improvements in the BFG All Terrain tire at this time and I am only hoping
that those improvements won't change the revs per mile numbers very much or
those of us who have syncros won't want to mix the new types in with the
old if we had a reason to. Our list member Steve Best met the fellow in
charge of that change a few weeks ago and he spilled the beans about it to
Steve. Last time BFG rebuilt their All Terrain tire was to change it to the
KO version, which, among other things provided more of a sidewall to
protect the rim from getting scraped. I spent a lot of time calculating the
revs per mile differences of that KO tire vs the old version of the BFG AT
and found that the difference that the gears would see was an acceptably
small fraction of how much the tires wear through their life that I
determined for syncro that it was OK to mix and match KO with pre-KO versions.
Another place to get revs per mile numbers is from the CV-900 article near
the bottom, and there are a lot of (confusing, sorry!) tires mentioned there.
I have a brand new BFG At 27x8.50/14 for sale by the way. It still has the
sticker on it.
>Derek Drew <derekdrew@rcn.com> wrote:
>The wheels were delayed slightly because some were diverted to Spain at
>the last moment (by me).
>But the shipment is back on track and the wheels are being loaded onto a
>ship as I write.
>So.... it is starting to look like the projected arrival date would be...
>well, perhaps four or five weeks from now.
>I still plan to work on some of your tire issues in more detail after
>mid-April when my time schedule will allow that.
Derek Drew Washington, DC
derekdrew@rcn.com Phone 202-966-7907
Email me for viscous coupling s (Cell 703-408-1532)
'90 Syncro Westfalia...
...seen off-road at
http://users.rcn.com/derekdrew/Syncro_Madness_Area.htm
...seen with 30.5" tall tires at
http://users.rcn.com/derekdrew/vanagon/images/dereks245_75_16.jpg
Note: most valuable Vanagon sites on the planet (for owners) are:
http://gerry.vanagon.com/cgi-bin/wa.exe?S1=vanagon
http://gerry.vanagon.com/files/
http://groups.yahoo.com/group/Syncro
http://www.vanagon.com
http://www.syncro.org
Many of my Syncro list postings consolidated
http://groups.yahoo.com/group/Syncro/messagesearch?query=212-580-6486
http://groups.yahoo.com/group/Syncro/messagesearch?query=202-966-7907
My refrigerator article:
http://www.vanagon.com/info/articles/Refrigerator/Reefer_Madness.htm
All About 16" Wheels And The CV-900.htm
http://users.rcn.com/derekdrew/vanagon/All_About_16-Wheels_And_The_CV_900.htm
All About Viscous Couplings
http://users.rcn.com/derekdrew/vanagon/viscous_couplings_vanagon_syncro.htm
How To Live With Heavy List Volume--Mail Filters
http://groups.yahoo.com/group/Syncro/message/7403
My article that shows how to deal with insurance companies:
http://gerry.vanagon.com/files/auto-insurance-madness.html
To read some material about how to add a front differential lock to your syncro
http://groups.yahoo.com/group/Syncro/message/13105
To view Wolfgang's incredibly informative wheel article
http://users.rcn.com/derekdrew/fitbigtiresonvanagon.htm
To view Tim Smith's incredibly useful gearing calculator
http://users.rcn.com/derekdrew/syncrotireandgearratios.xls
To view two articles about how to think about improving your gears at
transmission rebuild time
http://groups.yahoo.com/group/Syncro/message/7207
http://groups.yahoo.com/group/Syncro/message/7221
To view an overview article about choices in larger sized tires see
http://groups.yahoo.com/group/Syncro/message/11535
To view some 16" Trailing Arms that enable much larger tires see
http://groups.yahoo.com/group/syncrolist/files/Wheels%2C%20Tires%20%26%20Gearing/Wheel%20%26%20Tire%20Photos/16_Inch_Trailing_Arm.jpg
My article about belt protectors and CV joint protectors
http://groups.yahoo.com/group/Syncro/message/6112
SYNCRO OWNERS WHO DRIVE OFF THE ROAD:
You only get to change your ring and pinions once every 10 years
because that's how long the average syncro transmission goes before
its next rebuild. If you want bigger tires and more ground clearance
start planning your higher number ring and pinion sets right now so
you
can take advantage of your next trans rebuild.
