Date: Wed, 31 Jan 2001 03:46:14 EST
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Re: What I Found About Tire Height Calculations
Content-Type: text/plain; charset="US-ASCII"
Derek,
Just a few additional comments. For my application, 2wd Westfalia, 15 inch
Ronal alloys, I concluded that the Yokohama Geolander AT+ 215/75R15C tires
were the largest I could fit. These gave a revs/mile number of 755. This
compares to the stock reinforced 185R14 r/m rating of 836. This reduces the
engine revs per mile (or the effective gearing) by 10.7%. My 5.87 rear axle
becomes 5.30 effectively.
Compared to the 235/75R15C Geolander AT+ at 723 r/m, it seemed a good
compromise.
Note that the rev/mole numbers are measured with about 3 significant figure
accuracy. If you examine the Yokohama tire catalog for the various light
truck tires available in the 30-9.50x15 size you will find numbers such as
705, 709 and 704 for different sidewall and tread designs. Within the
Yokohama brand group, the 235/75R15 tire has r/m numbers varying from 723 to
744.
These numbers are also strongly related to sidewall strength, as shown on the
Dunlop Tire page. Compare the Dunlop P235/75R15 (passenger tire) with r/m of
753 to the LT235/75R15 (light truck tire) with r/m of 744 (Radial Rover AT)
or 741 for the Radial Rover RV.
Finally, as has been pointed out before, at the bottom of the speedometer,
VDO has marked the calibration point of the unit in revs/mile (US
deliveries). On my 82, this calibration point is 805 revs/mile. In my
measurements, this has proven to be correct to within 0.6%. For the
pathologically inclined, I connected the speedometer cable to an electrically
encoded calibrated motor. I found 60 mph indicated on the speedometer 801
shaft rotations per minute (801 revs/minute, or 1 mile per minute and 60
miles per hour). Most people report dead accurate speedometers when running
27-8.50R14C tires (rated at 795 revs/mile as Dunlop Radial Rover RV's, and
801 for the BF Goodrich Comp T/A). Unfortunately the range of revs / mile for
the 27-8.50R14C tire varies from 785 to 831 for the range of available brands.
During my 14 inch wheel period, I found the BFG's to be superb and dead on!
I should also mention that there are excellent videos with a strobe camera
showing the flexure of the tire sidewall and profile while moving at speed on
a regular surface, while cornering, while braking and while running over
rougher terrain. Frankly, I'm amazed that they work at all!
Good luck on further calculations,
Frank Grunthaner
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