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Tyre Pressures are critical for both on road and
off road 4x4 driving. Having the correct tyre pressures on road are critical for
safety and handling of the Land Rover and it will also affect fuel economy.
Using the guidelines from Land Rover are a good starting point. However knowing
what is the best tyre pressure to run for your specific vehicle with a specific
load under 4x4 off road conditions is a little bit more tricky.
What Affects Optimum Sand
Tyre Pressure
Optimum sand tyre pressure is a combination of many things, of which
old husband's tales are least productive. However, your tyres, their construction methods
and materials, what your vehicle weighs, how it is loaded all play into the sand-pressure
tyre formula with predictable results. Why low pressure works and how to determine your
best sand pressure follows.
Why Low Pressure Works
It’s a simple fact, which some die-hards still deny, the bigger
the footprint, the softer the stuff you can travel on. Take a look at the feet of camels,
polar bears and marsh birds. They are big and spread out to distribute their weight over a
larger surface area. With that out of the way, lets take a look at tyre pressure and
footprints.
Tyre Pressure and Footprints
On the tyre sidewall, find the small black print that specifies
maximum load pressure. (Most modern tyres have these values) For example, my BFGoodrich
Radial All-Terrain T/A 30x9.50R15LTs state 902 Kg at 3.44 bar cold. Most people, generally
including those who install tyres, run them up close to this, and neglect the actual term,
"MAX. LOAD" pressure.
Do you really need maximum load
pressure?
Consider that standing flat and level (static), loaded the way you
normally run is one thing. Now further consider you lose full tread width contact as the
vehicle travels at higher and higher speeds. Centrifugal force tries to increase the
diameter of the tread. You only have to look at dragster tyres when they spin up up to
convince yourself of this tyre reality. The sidewalls tend to hold the outer edges closer
to their static diameter than the centre of the tread and hence the centre of the tread
spins into an ever increasing diameter with increased speed. In other words, while moving
at highway speeds, the tyre is trying to run more on the centre of the tread than the
edge. That’s why I feel my full width, static contact test (described below) is
conservative. My tyre life and wear pattern prove putting more rubber on the ground
increases longevity.
Full tread width contact, street pressure
A rough method of establishing your full street
pressure:
Assume your tyres have a full load pressure of 5 bar
Take your vehicle's mass, loaded as normal (Assume 2 000 Kg for the example)
Multiply the maximum carrying capacity of your the tyres by 4 (Assume 1 000Kg each for the
example)
This gives a maximum load potential of 4 000Kg for the vehicle at full load pressure.
Therefore, in theory, the Ideal pressure will be:
Ideal Pressure = (vehicle mass / maximum
load potential) x full load pressure
Which gives Ideal_pressure = (2000/4000) x 5 = 2.5 bar
This value can be offset by front and rear loading
changes:
Rear
pressure = (Rear load % / 50) x Ideal pressure
Front
pressure = (Front load % / 50) x Ideal pressure
If the front to rear loading of the vehicle is 40/60
then:
Front pressure = (40/50) x 2.5 = 2 bar
Rear pressure = (60/50) x 2.5 = 3 bar
Optimum
Sand Pressure
To determine your optimum sand pressure, perform the following test
on a flat, level and smooth surface, fully loaded as you would be for a sand run (Fuel
tank and passengers included). Measure the vertical height to the bottom of the wheel rim
from the ground. This is your 100%, street pressure, wheel height. Now reduce this height
by 25%. In other words, let out air until your wheel is 75% of the street height. Measure
and record this pressure and depending on your vehicle and loading scheme, front and rear
tyres may differ.
This is your optimum sand pressure. As the picture below shows, this
typically results in more than a 250% increase. That is like having ten tyres where you
only had four. This pressure is only valid for exactly what you tested. Change vehicle,
tyres, wheels or load and you have to retest.
It’s obvious a vehicle change would dictate retesting. Tyres
differ in number and stiffness of sidewall plys and rubber compounds, hence the need to
retest with a tyre change, and in actuality, tyre age/wear too. Wider or narrower wheels
influence how the sidewalls bulge, so this too requires doing the deed anew.
How did I measure the 250% increase? I measured the pressure,
painted the tread, let the tyre down onto a piece of paper and “printed” the
footprint for various air pressures. I could see the edge begin to make contact and
footprint increase with ever-decreasing pressure.
 The
results are dramatic, but carefully observe the PRESSURE-HEIGHT CURVE, and understand this
is not a universally applicable curve. It is specific to my, tyres, wheels and load. Wheel
height and footprint are obviously related. Putting the curve into words, the footprint
really starts to increase (wheel height decrease) with the last few drops in pressure.
Note I measured no height change from 3.44 to 2.55. From 3.44 to 1.37 bar resulted in only
9.5mm drop in height. The drop from 1.37 to 0.827 bar was about 12mm and the drop from
0.827 to 0.482 bar yielded more than 20mm drop in height. These last few pounds are where
the real effect takes place. Give them pudgy cheeks!
I’ve had people tell me the low pressure trick does not work. "I went down to
1.1 or 1.25 bar and still had trouble in the sand," so the claim goes. It should be
apparent from the curve, the last few bars really count! A good indicator other than ease
of movement comes by watching your engine temperature gauge. If you are heating up, your
pressure is still too high or you’re really in some tough stuff!
So does this mean flat tyres are best? I believe not. Again the 75%
rule is somewhat tyre and wheel dependent, but at too low tyre pressure, the centre of the
footprint begins to well up, reducing the footprint and creating a small “travelling
hill” in the centre of the footprint. This hill offers increased resistance to
vehicle movement.
For my combination of vehicle, tyres, etc, my optimum sand pressure
is 0.41 to 0.48 bar and I typically get by with 0.55 to 0.69 bar
Tyre Gauges
Before we move on, let's talk pressure measurement -
tyre gauges. I’ll leave the type - stick, dial or digital - up to you. What I’m
primarily addressing is the Big Numbers Misconception; the bigger the number, the better
it is.
As a kid, I remember one of my first automotive questions was,
"What does the speedometer go up to?" Not how fast does the car go, but how big
is the number at the end. This end number on both speedometers and pressure gauges has
little, and sometimes a negative bearing, on gauge usefulness.
Actually, I have quite a few tyre gauges. Several fell prey to the
Great Numbers Misconception. I carry and use two. One for everyday street use and another
for low pressure. My street pressure gauge goes to 3.5 bar and my low pressure gauge goes
to 1.5 bar. It’s pretty hard to measure 0.68 bar on a gauge that starts at 1.5 bar.
Most 8 to 13 bar gauges don’t even start until 1.5 bar or more. These are useless for
sand pressure measurements. Don’t waste your money on high pressure gauges! Consider
having two and perform the above tests and field air-down measurements with the same low
pressure gauge. The exact accuracy of the reading or value is not as important as
repeatability.
And something to plan for is the eventuality of no gauge or a
failure of the gauge. This is easily handled by knowing how long it takes to air down from
street to sand pressure. I use the one-thousand-one, one-thousand-two, second counting
method. I can let 50 seconds worth of air out of my tyres before I need to take a reading.
It’s usually another ten more seconds before I reach my optimum. Given a gauge
failure, I will still stand a fair chance of getting the pressure right.
Four Affects of Low Tyre Pressure
Low tyre pressure changes four things: footprint; ground clearance;
rolling radius and what I call Obstacle Rolling Resistance. Footprint was covered above.
Common sense and the TREAD FOOTPRINT picture shows the centre of the axle is lowered by
the decrease in wheel height. This results in lower ground clearance and consider the
softer tyres also flex and give more resulting in compression loss of ground clearance
too. But on the other hand, ground clearance is not that important in the sand.
Rolling radius is part of the equation which contributes to your net
moving force; your overall gear ratio; your "stump pulling power". Think of it
this way: you know how bigger tyres eat up low gearing and smaller tyres effectively give
you lower gears? Flatter tyres act like smaller tyres and increase your pulling (moving)
power.
Obstacle Rolling Resistance
Move on to the OBSTACLE ROLLING RESISTANCE picture below to learn
how significant this sleeper is! Several years back, I was convinced a tyre’s ability
to conform to obstacles played a big part in ease of movement, but I had no idea how
significant it was until I measured it. Here’s my experiment.
Take two vehicles about 10m apart, on a flat, smooth surface. Take the winch of one and
connect it to the other, with a strainguage (big fish scale) in the cable. Put a pair of
50x100mm wood blocks in front of the pulled vehicle and measure the force (pull) required
for different towed vehicle tyre pressures.
A simple, lowly little wooden block offers more of a climb angle
(hill) than you might initially think. It is roughly 25° to 30°. Knowing this, it’s
now easy to see why not so big rocks require the thrash and bash technique to climb with
hard tyres.
The wood block tests showed a 40% difference between street and sand
pressure! And it may be worse than that because the initial burst of pull required to get
the street pressure tyres started up over the obstacles was not precisely recordable with
my crude equipment and test methods. With the sand tyre-pressure, it was obvious the
dynamometer saw a gradual build-up in force as the tyres smoothly conformed and crawled
over the obstacles. What this 40% difference means is you now have roughly six tyres where
you only had four. Add this to the "ten tyres" of the pressure drop and you now
have 16 where you had four. Any more questions about the effectiveness of lowering tyre
pressure?
Rock Crawling
Actually, I suspect this is also why lower tyre pressure is very
effective for rock crawling. The Obstacle Rolling Resistance factor, as I call it, plays
an even more crucial role when rolling through the rocks.
Caution!
Now comes the problem:- Use Caution! Caution! Caution! with low tyre
pressure. The Obstacle Rolling Resistance factor works against you with speed and in the
rocks! Soft tyres easily bend and break wheels. Drive with caution when back on hard
ground or the rocks! And you obviously need some way to reinflate the tyres back to street
pressure when you hit the black stuff.
Treading Lightly
I also feel lower pressure is more ecologically compatible. With
better traction you don’t have to spin the tyres and hence your Land Rover
will leave Mother Nature
unscathed.
You’ll be doing yourself and Mother Nature a favour when you
lower your tyre pressure the next time you hit the soft stuff.
- Harry Lewellyn - USA (Slightly edited and "metrified"
by Landyonline)
More on
Land Rover tyres
and treads.
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