Date:         Wed, 31 Jan 2001 19:02:49 -0800
Reply-To:     Mark Drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Mark Drillock <drillock@EARTHLINK.NET>
Subject:      Tire Height, revs per mile, pressure effects
Content-Type: text/plain; charset=us-ascii

This topic has crossed over to the Vanagon list from the Syncro list. It
may seem out of the blue here as the prior history was mostly on just
the Syncro list. Since I earlier said I would I am posting my results
here also. It is of less interest to the 2WD crowd but here it is.

Mark

I did a test to see the effects of tire pressure on tire revs per mile
and by extension effective gearing.
Also, some have suggested that in a pinch Syncro owners could adjust for
a mismatched tire by matching the tires static radius through pressure
adjustments and thereby reduce the stress on the VC.

Test done with 82 Westy 2WD, new 215/75/15 Michelin LTX M+S, 35 psi at
start.
Level smooth paved street, sunny, dry, 70F, 5 mph est. speed max

Using a plumb line at the center of each left side wheel, mark the tire
and the ground with yellow marker.
Drive straight ahead, counting 10 revs of the tire below the driver.
Mark the ground location where the tire mark lines up for each tire.
Measure the distance from the starting marks to the finish marks for
each tire.   834.5 inches

Return to the starting position and let most of the air out of the
driver's tire.
Measure from the ground to the center of the wheel for each tire.   left
rear = 12 7/8", left front 11 5/8"
Repeat the above test and mark the ground after 10 revs of each tire.
Measure the distance from start to finish for each tire.   left rear =
834.5 inches, left front = 831 inches

Measured revs per mile at 35 psi = 759
Measured revs per mile at ~7 psi = 762
Measured Difference = 3 revs per mile, 00.4%


Predicted revs per mile at 35 psi = 783
Predicted revs per mile at ~7 psi = 868
Predicted difference = 85 revs per mile, 10.9%

Clearly, tire pressure has little effect on tires revs per mile at slow
speeds.
Clearly, static radius measurements are useless for predicting tire revs
per mile.

Tire pressure adjustments have little effect on tire rotational speeds
at low road speeds. How they effect rotational speeds at high road
speeds is yet to be determined. This may be important to know for VC
protection purposes. Does anyone have any data showing revs per mile at
a given speed at varying pressure?

Mark