From: Patrick Jankowiak (recycler@swbell.net)
Date: Fri Jun 02 2006 - 19:55:54 PDT
There can be a problem even when supposedly the same size tires are
used front and rear, although it won't break anything.
When doing 55, having the 1100x20 street tires on front and 1100x20
NDT on rear, I can engage and disengage the front ok, but in fact if I
disengage it for 1/2 second and re-engage it, it takes 4 seconds to
'click' in, meaning that the driveshaft speeds differ very slightly
due to the tire sizes, inflation at the time, and probably wear.
Does not the front engagement spline in the transfer have 6 teeth?
that would make it 4 seconds for one tooth, and because each tooth is
occupying 1/2 the size of the space of 1/6 of the circumference and
only has to slide half as far to click in, then there is 48 seconds
for one revolution, meaning that the rear driveshaft is turning
2500RPM and the front is turning 2500.0208333 RPM (assume faster for
the argument, same difference either way) disengaged.
The driveshaft difference is then 0.0208333RPM which is 7.499988
degrees per minute and also is 0.1249998 degrees per second.
The 6.72 gear ratio reduces this to 0.0186011607 degrees per second at
the axle.
An 11.00x20 tire (I think) has a circumference of 128.8 inches, and so
1 degree is 0.35 inch.
Therefore the front and rear tires are forced to slip relative to each
other by 0.00651 inches per second at 55MPH. I had a hard time
measuring this to confirm it officially because the calipers kept
getting knocked out of my hand as I had them down too near the road to
be sure and accurately measure the slippage along the tire's contact
patch.
I can see that this is a major source of friction anyway and I better
go back to the matched 11.00x20 NDT's on the front right away.
PJ
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