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LOW TIRE PRESSURE
WHY AND HOW
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By Harry Lewellyn - (straight outa my book,
SHIFTING into 4WD)
 This
worked for my Dad well over seven decades ago and it still works
today! In 1930, he owned a service station at sand’s edge in
Long Beach, California. In those days, picnickers would drive
out on the sand and enjoy the beach, then head for the homestead at
sunset. That was until they found the gentle uphill slope to
the pavement impossible to climb in the soft sand. After digging
and fussing, they’d go to Dad’s station a few hundred yards away for
help.
First, this horse trading Okie from Muskogee would insist on collecting his 50-cent fee
up front. Big money in those days! Then, he’d proceed to
air down the tires and drive the car out to his air pump. With
raised voices and shaking fists, foul would be claimed. That was too
easy!
But like the man that fixed the mill with a single
hammer strike, it wasn’t the blow, but knowing where to hit that
mattered. Daddy hit high tire pressure every time he encountered
any sort of soft stuff - sand, mud and snow.
AIR DOWN
The key to easy movement on sand, mud or snow is to give your 4X big, soft feet.
Mother Nature knows this, for take a look at polar bear, camel and
snow shoed rabbit feet. They're big! So giant tires come
to mind, but most of us don’t want the expense and inconvenience of
carrying and using two sets of tires.
The other alternative is to air down. But
before we go any further, let me tell you in no uncertain terms, not
airing
(back) up is dangerous. It is dangerous to your tires,
your wheels, your 4X and maybe even your life. Low tire
pressure at sustained highway speeds will cause all tires to
eventually fail. The extreme flexing causes heat build
up. This weakens the rubber and leads to blowouts!
What’s more, that assumes you’re going straight. High
speed turns at low pressure on full-traction surfaces can roll the
tire right off the wheel and probably result in one other roll, your
4X! I want it to be perfectly clear that safe use of the
following low tire pressure methods is a function of the driver and
not the technique. Believe it, don’t prove it!
Drive slowly and cautiously when aired down!
Meanwhile, back on the sand, I have never experienced
low-pressure tire problems. I drive at reasonable
speeds, avoid the rocks and go easy on the turns. A
couple of decades ago I trained a beach-community police department how
to drive on the sand. When they learned it took 10
minutes or more to air down, the chief decreed their beach 4X was
not a street pursuit vehicle. Therefore, it would be run
night and day, seven days a week at optimum sand pressure.
That way, they were always ready for the soft stuff. I have
yet to hear of a tire problem from them.
So what does lowering the pressure do and how does it
work and how much should you lower the pressure and besides, how do
you drive on sand anyway? Just hold on to your air
gauges and I’ll answer these questions one-by-one. In the meantime, figure it takes 8 to 12 PSI
to put you in the sand-pressure ball park. Anything more is a
waste of time!
Staun Automatic
Tyre Deflators make airing down a cinch.
TINY SAND MOUNTAINS
Picture your tires only three or four inches deep in sand. See
the sand hills built up in front of each tire? They
aren’t very big, and the 4X “appears” to be level.
Nevertheless, these are soft mountains to the 4X.
From the standpoint of the instant you try to get started, it’s the
same as a 1,000-foot mountain. The 4X is attempting to lift
itself up and over the rise. So poor traction must not only
try to move the 4X, but climb these (apparently) small hills as
well.
With hard tires, what actually happens is the 4X goes
through the motions of climbing, but hopelessly shoves the hill
ahead in an ever-losing battle. Typically, instead of gaining
elevation, you lose it and bury the 4X deeper and deeper. The
sand hills just get bigger and steeper.
Lower tire pressure increases the tires’ footprints.
A bigger footprint means you sink less. If you sink less, you
have a smaller, gentler slope to climb. A gentler slope means
it takes less force and traction, and you move along with greater
ease. Eureka, easy going!
By the way, this tire pressure trick works for all
pneumatic-tired things: 4X, 2X, or trailer. The only subtlety
is that you have to do all the tires regardless of how many are
powered, pushed or pulled.
AIR DOWN
Tire pressure brings lots of
questions and some reluctance. Lower tire pressure, in
summary, is plus three and minus one.
Lower tire pressure gives you a bigger footprint.
Mother Nature knows big feet work or why would She give camels and
polar bears such giant bottom features? It also gives you the
effect of slightly lower gearing due to a decrease in rolling
radius. Lastly, it reduces what I’ve named obstacle-rolling
resistance. This is the tire’s ability to mold over rather
than have to climb up a minor impediment.
The only disadvantage is that lower tire pressure
reduces ground clearance. That’s because the 4X’s weight
compresses the soft tire. This also means the 4X can bounce
easier and further compress the rubber.
75% RULE
I used to call my soft-going tire pressure guideline the 75% rule,
but that needs a modern tune-up. My rule is perfect for
15-inch wheels, but needs adjustment for 16-, 17-, 18-inch and
larger wheels. The newer, larger wheels and lower
“profile” tires complicate my old rule. For example, on
a P235/65 R16 size tire, the “65” (percent aspect ratio) is the
“profile.” I have not completely run larger wheel, lower
profile tire sizes through the Coyote’s analytical ringer.
For these, I’ll make an educated guess.
Assuming you’re not running 65 or less profile tires
(foolish in the dirt),
and you are using something in the range of 70 plus tires, consider
this estimate. I’d reduce the 75% rule by 5% for each
inch of wheel increase beyond 15-inches. For example, a
16-inch wheel should use 70%. Now follow along with the
figures and throw your percentage in when I use 75%.
Load your 4X like you typically use it.
Make sure it’s on the flat and level and ensure your tires are
inflated to your normal street pressure. Measure the
height of the wheel, straight up from the ground to the very bottom
of the wheel (rim). Note the front and back may differ.
Take 75% of this number. Now deflate the tire until the
wheel is at 75% of the normal pressure height off the ground. Finally,
re-measure the pressure. This is your soft-going optimum
pressure. It will most likely be much lower than you’re
comfortable with.
 So
what does the ol’ Coyote really do if the 75% number is so
surprisingly low? Mine is typically 6 or 7 PSI! I
usually run 9 or 10 PSI when fat feet are needed. I know
for a fact that much above 12 PSI is virtually useless for 15-inch
wheels in soft sand, snow or mud.
Overall, I’m looking for smooth, easy, cool-engine
sailing when I lower my tire pressure. I want to be able
to take off easily after I stop. I do not want to struggle to move.
With too much tire pressure, you may notice your engine heats up a
bit. Higher engine temperature in the sand is a sure
sign you need to reduce tire pressure still more. Has
the “Big Numbers Myth” struck?
The Big Numbers game is what prompts us to buy a
200-PSI tire gauge or portable compressor. Why do you
need 200 PSI other than the fact that the glitz guys have made you
think it’s better? I only carry 25 PSI in my tires.
A 200-PSI pencil gauge doesn’t even start to register until the
pressure hits 20 or so. How are you going to measure 10
PSI if the gauge doesn’t start to move until twice that?
I actually carry two gauges. One is a 0-60 dial gauge
and the pencil tops out at 20 PSI.
Reduced tire pressure really works. Be prepared
to air up or drive very slowly to the first available air pump when
you get back to the hard black stuff. It’s down right
dangerous to do otherwise!
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