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Thread: Airing Down

  1. #1
    Join Date
    Mar 2008
    Last Online
    August 9th, 2012
    User Name
    Real Name
    Pete Brody

    Default 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
    There are also some disadvantages:
    • 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
    Take the need to air up when leaving a trail seriously. It is very 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 buildup.
    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 downright 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.

    Author: “Airing Down” from a street/highway pressure of 32psi to 25psi isn’t really considered ‘airing down’ and the benefits of such a low pressure reduction will probably not be felt in any way. Airing down to 20-22psi from a street pressure of 32psi will result in a ‘softer’ ride on a bumpy trail, but down to 16 or 18psi will be more noticeable. 16psi is suitable for most difficult trails, including snow, sand and mud.
    On harder trails, airing down to 10-12 psi will provide a noticeable increase in traction. Below 8psi, you will stand a good chance of popping a tire off the bead (rim) unless you have special ‘bead locks’ to keep the tire on.
    All of this is based on a standard sized rig, running around 32psi on the highway. Many heavier rigs run 50psi or more on the highway and reducing the pressure down to below 20 isn’t really a good idea. The inverse is true for ultra light weight rock buggies, though they never see the highway. Some of these are so light that running 0-4 psi is enough off road.

  2. The Following 3 Members Say Thanks to Brody For This Post:

    JandDGreens (October 6th, 2015),Jurgen60 (April 7th, 2017),SOS-1999TJ (March 19th, 2018)

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