| Airing Down for Off Road Driving |
How We Air Down
for Testing
for Testing
Don Alexander
AIRING DOWN
One of the important tools for off roading is airing down tires.
Reducing tire pressure for off road driving has several benefits:
important for safe highway driving to air back to the proper
inflation pressure. While you may be able to drive for a mile or
two a very low speeds to a gas station with an air compressor,
greater distances and speeds over 25 MPH present an
increased risk of tire damage and failure due to heat build up.
The only way to assure safe passage on the highway is to air
up before hitting the asphalt. This requires an on-board
compressor system or an air tank with high volume for airing up
large tires. A compressor is the best bet. Several systems are
available, including low cost, but very slow 12 volt compressors
from auto parts and large chain stores. A better solution is a
system designed for on board air compressing. We use and
highly recommend the Viair on board compressor and tank
system, which allow much faster airing up from low tire
pressure than the hardware store compressors.
WHY AIR DOWN?
The major factor is improved traction. As the distance between
the ground and the rim (sidewall height) is reduced as the
pressure is lowered, the tire contact patch length (footprint)
increases considerably at lower pressures. At the same time,
the width of the contact patch also increases slightly as the
crowning of the tread is reduced. At very low pressures, the size
of the contact patches can increase as much as 250%. The
contact patch area is one factor determining the traction
available from a tire. The down side is, as mentioned, the tire
unseating from the wheel rim, and, increased heat build up at
higher speeds, especially at highway speeds. This can cause
catastrophic tire failure over time.
An additional factor is tire reliability. Many easy to moderate 4x4
trails are strewn with sharp rocks, which can easily puncture a
tire tread or sidewall. Reducing the pressure reduces the risk of
puncture, in the same way a soft balloon is less likely to “pop”
from a sharp object than one at a much higher pressure. And
finally, as long as the tire is not bottoming out on the wheel rim,
the ride is improved.
HOW LOW A PRESSURE FOR OFF ROAD DRIVING?
Many factors contribute to the minimum tire pressure which can
be safely utilized for off road driving. These include:
Sidewall stiffness
Tire aspect ratio (vs.wheel diameter)
Driving speed
Trail surface
On a smooth trail where higher speeds can
be maintained, say 30-45 MPH, higher
pressures are needed. For most trails with
steep climbs on loose surface, much lower
pressures can be used. As a rule of thumb,
the lowest pressure can be used on smooth
surface with little traction like snow, ice, mud
and sand. Speed in these conditions are
normally very low, about 3 to 10 MPH, so the
rotational inertia of the tire is low as is
traction and the tire is not likely to rotate on
the rim, causing the bead to unseat.
Cornering forces are also very low in these
low traction situations, again reducing the
chances of a bead unseating or “peeling”
of the wheel.
For hard surfaces and rock crawling, higher
pressures are needed to help keep the tire
bead seated on the wheel rim, and to protect
the sidewall and tire bead from bottoming
on the rim, which would likely damage the
tire or even bend the wheel rim.
It is important to remember what I consider
to be the most important goal when four
wheeling – to reach your destination or
return home safely. Flat tires, for any reason,
are counterproductive to this goal, But then
so is a loss of traction. If you don’t have the
traction to climb a steep slope, or worse,
descend a steep slope, not only will you not
make it to your destination, but you very well
could find yourself in a precarious situation,
stressful at the minimum and down right
dangerous at the extreme. So the answer to
the how low should you go is like walking a
tight rope. Too low and you could unseat the
tire bead from the rim, causing a flat; too
high and you could reduce traction too much
or have a sharp rock pierce the tread or
sidewall.
A few years ago, Harry Lewellyn, a four
wheeling author, instructor and tour leader
and a very sharp guy, devised a simple
procedure for determining the minimum tire
pressure for soft surfaces. We have been using this method for
some time now, and applied it to our all terrain tire tests. It has
never failed and for the tires in our all terrain test, we never had
a problem either with punctures or with unseated tire beads.
For soft surfaces like ice, snow, sand and mud, at very low
speeds on 4x4 trails, we use Harry’s guidelines of inflating a
tire to near maximum pressure for maximum load. On the three
A/T tires we tested, this is 80 PSI at a maximum load around
3400 pounds per tire. Now this is very high, both for load and for
pressure. We have found that the difference in sidewall height
from 80 PSI to 50 PSI is very small. We use 50 PSI for our
measurement. At that pressure, we measure the sidewall
height from the ground to the bottom of wheel rim. We then
deflate the tire until the measurement is 75% of the starting
distance. For a six inch sidewall height at 50 PSI, this
measurement would be 4.5 inches. All of our test tires were just
over 6.5 inches tall at the sidewall at 50 PSI. At the 75% height,
just under a 5 inch height, each tire had a different pressure,
which indicates different sidewall stiffness. The pressures to
reach the 75% height were:
BFG – 6 PSI
Hankook – 9 PSI
Toyo – 8 PSI
This indicates that the BFG has the stiffest
sidewall while the Hankook has the softest.
A softer sidewall is more desirable for off
roading since the tire can maintain a higher
pressure at the target height which reduces
tire bead unseating and provides better
protection from the tire bottoming against
the wheel rim.
Keep in mind that these pressures are only
intended for very soft, slippery surfaces –
ice, mud, snow and sand – at very low
speeds. For harder surfaces and rock
crawling, we increase pressures between
5 and 8 PSI to improve bead seating and
provide additional distance between the rim
and tire tread. When driving over rocks with
sharp edges, the load on the tire tread and
sidewall increases since the contact patch area is reduced,
sometimes dramatically. For our all terrain tire test, we used a
five PSI increase on rocky and rock crawling trails since we
were testing the limits of the tires in several different ways. We
never experienced a problem with bead unseating or slippage
or with punctures. None of the tires showed any signs of
sidewall damage from bottoming against the wheel rim.
We devised an additional test to see just how much the
sidewalls of each tire compress when the tread is on a sharp
edge, as would occur in rock crawling situations. We placed
the load tire on top of an 2” by 2” aluminum angle resting on a
board. The distance from the rim to the peak of the angle was
reduced about 50% in each case. The trend, as you would
expect, was the same as on the flat surface. This graphically
shows why increasing inflation pressures is necessary for
harder, rocky surfaces. A five pound increase is the minimum
we feel will provide adequate protection against bottoming of
the tread against the rim.
For easy trails that are fairly smooth, higher pressures work
fine, but some airing down will improve ride and reduce the risk
of punctures. Remember that we can only relate our
experiences with these three all terrain tires from BFG, Hankook
and Toyo. While we have experience with many other tires, but
we have not tested to the extremes we have with these tire. So
use caution and decide for yourself what the best tire pressures
are for your vehicle and driving conditions. If you air down on the
trail, be prepared to re-inflate your tires before you drive on the
highway. Low pressures on the highway and interstate can be
very dangerous. And a good set of deflater valves is a good
investment. We use and highly recommend the Staun Tire
Deflaters, which are distributed in the US by Harry Lewellyn. A
good tire pressure gauge is also a necessity. As always use
caution and be extra careful and aware when you first try airing
down on a trail. Check your tires often for signs of problems
until you are confident and comfortable at your target trail tire
pressures.
AIRING DOWN
One of the important tools for off roading is airing down tires.
Reducing tire pressure for off road driving has several benefits:
- Larger tire contact patch or foot print for improved
traction - Improved ride quality over bumpy, rocky surfaces
- Less risk of punctures from sharp objects like rocks
- Reduced ground clearance due short sidewall from
less pressure - Increased possibility of the tire bead unseating from the
wheel rim - The need to air back to recommended highway tire
pressures for your vehicle
important for safe highway driving to air back to the proper
inflation pressure. While you may be able to drive for a mile or
two a very low speeds to a gas station with an air compressor,
greater distances and speeds over 25 MPH present an
increased risk of tire damage and failure due to heat build up.
The only way to assure safe passage on the highway is to air
up before hitting the asphalt. This requires an on-board
compressor system or an air tank with high volume for airing up
large tires. A compressor is the best bet. Several systems are
available, including low cost, but very slow 12 volt compressors
from auto parts and large chain stores. A better solution is a
system designed for on board air compressing. We use and
highly recommend the Viair on board compressor and tank
system, which allow much faster airing up from low tire
pressure than the hardware store compressors.
WHY AIR DOWN?
The major factor is improved traction. As the distance between
the ground and the rim (sidewall height) is reduced as the
pressure is lowered, the tire contact patch length (footprint)
increases considerably at lower pressures. At the same time,
the width of the contact patch also increases slightly as the
crowning of the tread is reduced. At very low pressures, the size
of the contact patches can increase as much as 250%. The
contact patch area is one factor determining the traction
available from a tire. The down side is, as mentioned, the tire
unseating from the wheel rim, and, increased heat build up at
higher speeds, especially at highway speeds. This can cause
catastrophic tire failure over time.
An additional factor is tire reliability. Many easy to moderate 4x4
trails are strewn with sharp rocks, which can easily puncture a
tire tread or sidewall. Reducing the pressure reduces the risk of
puncture, in the same way a soft balloon is less likely to “pop”
from a sharp object than one at a much higher pressure. And
finally, as long as the tire is not bottoming out on the wheel rim,
the ride is improved.
HOW LOW A PRESSURE FOR OFF ROAD DRIVING?
Many factors contribute to the minimum tire pressure which can
be safely utilized for off road driving. These include:
Sidewall stiffness
Tire aspect ratio (vs.wheel diameter)
Driving speed
Trail surface
On a smooth trail where higher speeds can
be maintained, say 30-45 MPH, higher
pressures are needed. For most trails with
steep climbs on loose surface, much lower
pressures can be used. As a rule of thumb,
the lowest pressure can be used on smooth
surface with little traction like snow, ice, mud
and sand. Speed in these conditions are
normally very low, about 3 to 10 MPH, so the
rotational inertia of the tire is low as is
traction and the tire is not likely to rotate on
the rim, causing the bead to unseat.
Cornering forces are also very low in these
low traction situations, again reducing the
chances of a bead unseating or “peeling”
of the wheel.
For hard surfaces and rock crawling, higher
pressures are needed to help keep the tire
bead seated on the wheel rim, and to protect
the sidewall and tire bead from bottoming
on the rim, which would likely damage the
tire or even bend the wheel rim.
It is important to remember what I consider
to be the most important goal when four
wheeling – to reach your destination or
return home safely. Flat tires, for any reason,
are counterproductive to this goal, But then
so is a loss of traction. If you don’t have the
traction to climb a steep slope, or worse,
descend a steep slope, not only will you not
make it to your destination, but you very well
could find yourself in a precarious situation,
stressful at the minimum and down right
dangerous at the extreme. So the answer to
the how low should you go is like walking a
tight rope. Too low and you could unseat the
tire bead from the rim, causing a flat; too
high and you could reduce traction too much
or have a sharp rock pierce the tread or
sidewall.
A few years ago, Harry Lewellyn, a four
wheeling author, instructor and tour leader
and a very sharp guy, devised a simple
procedure for determining the minimum tire
pressure for soft surfaces. We have been using this method for
some time now, and applied it to our all terrain tire tests. It has
never failed and for the tires in our all terrain test, we never had
a problem either with punctures or with unseated tire beads.
For soft surfaces like ice, snow, sand and mud, at very low
speeds on 4x4 trails, we use Harry’s guidelines of inflating a
tire to near maximum pressure for maximum load. On the three
A/T tires we tested, this is 80 PSI at a maximum load around
3400 pounds per tire. Now this is very high, both for load and for
pressure. We have found that the difference in sidewall height
from 80 PSI to 50 PSI is very small. We use 50 PSI for our
measurement. At that pressure, we measure the sidewall
height from the ground to the bottom of wheel rim. We then
deflate the tire until the measurement is 75% of the starting
distance. For a six inch sidewall height at 50 PSI, this
measurement would be 4.5 inches. All of our test tires were just
over 6.5 inches tall at the sidewall at 50 PSI. At the 75% height,
just under a 5 inch height, each tire had a different pressure,
which indicates different sidewall stiffness. The pressures to
reach the 75% height were:
BFG – 6 PSI
Hankook – 9 PSI
Toyo – 8 PSI
This indicates that the BFG has the stiffest
sidewall while the Hankook has the softest.
A softer sidewall is more desirable for off
roading since the tire can maintain a higher
pressure at the target height which reduces
tire bead unseating and provides better
protection from the tire bottoming against
the wheel rim.
Keep in mind that these pressures are only
intended for very soft, slippery surfaces –
ice, mud, snow and sand – at very low
speeds. For harder surfaces and rock
crawling, we increase pressures between
5 and 8 PSI to improve bead seating and
provide additional distance between the rim
and tire tread. When driving over rocks with
sharp edges, the load on the tire tread and
sidewall increases since the contact patch area is reduced,
sometimes dramatically. For our all terrain tire test, we used a
five PSI increase on rocky and rock crawling trails since we
were testing the limits of the tires in several different ways. We
never experienced a problem with bead unseating or slippage
or with punctures. None of the tires showed any signs of
sidewall damage from bottoming against the wheel rim.
We devised an additional test to see just how much the
sidewalls of each tire compress when the tread is on a sharp
edge, as would occur in rock crawling situations. We placed
the load tire on top of an 2” by 2” aluminum angle resting on a
board. The distance from the rim to the peak of the angle was
reduced about 50% in each case. The trend, as you would
expect, was the same as on the flat surface. This graphically
shows why increasing inflation pressures is necessary for
harder, rocky surfaces. A five pound increase is the minimum
we feel will provide adequate protection against bottoming of
the tread against the rim.
For easy trails that are fairly smooth, higher pressures work
fine, but some airing down will improve ride and reduce the risk
of punctures. Remember that we can only relate our
experiences with these three all terrain tires from BFG, Hankook
and Toyo. While we have experience with many other tires, but
we have not tested to the extremes we have with these tire. So
use caution and decide for yourself what the best tire pressures
are for your vehicle and driving conditions. If you air down on the
trail, be prepared to re-inflate your tires before you drive on the
highway. Low pressures on the highway and interstate can be
very dangerous. And a good set of deflater valves is a good
investment. We use and highly recommend the Staun Tire
Deflaters, which are distributed in the US by Harry Lewellyn. A
good tire pressure gauge is also a necessity. As always use
caution and be extra careful and aware when you first try airing
down on a trail. Check your tires often for signs of problems
until you are confident and comfortable at your target trail tire
pressures.
| To measure sidewall height for optimum low tire pressure for soft surfaces, start at 50 PSI |
| Measure the sidewall height from the ground to the bottom of the wheel rim, then reduce tire pressure |
| Until the sidewall height measurement is at 75% of the height at 50 PSI. This the lowest tire pressure you can run on soft surfaces like sand, snow, ice and mud, at least on the tires we tested here. |
| This tire has been aired down to minimum pressure |
| At the lowest pressure, the angle iron under the tire tread distorts the contact patch. When driving over sharp objects like rocks and stones, adding 5 to 8 PSI to the lowest pressure will help to keep the sidewall from pinching against the wheel rim. |
Crawling over rocks requires some airing down so that the tread
conforms to the surface, but too much can allow the tire tread to
be pinched against the rim or possible allow the bead of the tire
to become unseated from the wheel rim.
conforms to the surface, but too much can allow the tire tread to
be pinched against the rim or possible allow the bead of the tire
to become unseated from the wheel rim.
| The above photos show well the Hankook RF10 conforms to the rock surface when aired down to 14 PSI. This is the minimum soft surface pressure plus 5 PSI, which we test extensively for hard surfaces and rock crawling with no problems and excellent traction. If you air down for off roading, be sure to air back up to the recommend highway pressure before driving on paved roads at speed. |
| .We use and recommend the Staun Tire Deflator Valves for quick, easy and accurate airing down. The valves are adjustable to any pressure and screw onto the valve stem, shutting off at the preset pressure. |
Airing up is even more important when you leave the trail
for the highway. An on-board air compressor and tank is by
far the easiest way to quickly air up. We use the Viair
system which works exceptionally well.
for the highway. An on-board air compressor and tank is by
far the easiest way to quickly air up. We use the Viair
system which works exceptionally well.
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Copyright 2009 Don Alexander
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