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bajalou
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Tire pressure when off-roading
Rather than hi-jack Bajacat's thread about San Felipe trip, I figure I'd start a new one about tire pressures.
This is taken from Extreme 4X4 Trails and Ecological 4-wheeling Adventures.
I've followed this advice and believe it's great advice for getting through tough places with the least trouble. It points out things that I never
thought of and when I follow them - it works
Extreme 4X4 Trails
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Letting the Hot Air Out of Tire Talk
Tire Pressures for 4-Wheeling
by Harry Lewellyn
Experiences Playing in the Sand
I guess I should cut those reluctant to lower their tires down to sand pressure a little slack. My first time onto the beach in San Felipe, Baja, I
did the air pressure trick, with a little unwillingness, and wasn’t sure it was really necessary. Then, over the next three or four years, I would
first tackle the mellow white stuff at full street pressure and when all my power, skill and finesse failed, I would let ‘em down and move ‘em out.
Me, low range and low gear, would struggle along at a whole five MPH. Deflation they defied resistance as I sailed along in high range, OD, at
whatever speed I chose! Time and again, year after year, I tested the softies with the same undeniable results. Low tire pressure just plain works in
the sand!
What Affects Optimum Sand Tire Pressure
Optimum sand tire pressure is a combination of many things, of which old husband's tales are least productive. However, your tires, their construction
methods and materials, what your car weighs, how it is loaded, and wheel width all play into the sand-pressure tire formula with predictable results.
For this article, I will neglect tire temperature which I suppose could be critical if you test below freezing and play at +80°. Why low pressure
works and how to determine your best sand pressure follows.
Why Low Pressure Works
It’s a simple fact, which some diehards still deny, the bigger the footprint, the softer the stuff you can travel. Mother Nature knows it. 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. For those who
say skinny, hard tires are better for snow, mud or whatever, please tell me why they don’t use ten-speed bicycle-type tires on snowmobiles? Sand rail
people and mud boggers know big feet work better too! With that out of the way, lets take a look at tire pressure and footprints.
Tire Pressure and Footprints
Right next to your wheel, on the tire, find the small black print that specifies maximum load pressure. For example, my BFGoodrich Radial All-Terrain
T/A 30x9.50R15LTs state 1990 pounds at 50 pounds per square inch (PSI) cold. Most folks, generally including those who install tires, run them up
close to this, and neglect the actual term, "MAX. LOAD" pressure. The street pressure, TREAD FOOTPRINT figure below represents Coyote assessed over
inflation.
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 4X cruises at higher and higher speeds. Centrifugal force tries to increase the diameter of the tread. You only have to look at dragster tires
when they light 'em up to convince yourself of this tire reality. The sidewalls tend to hold the outer edges closer to their static diameter than the
center of the tread and hence the center of the tread spins into an ever increasing diameter with increased speed. In other words, while moving at
highway speeds, the tire is trying to run more on the center of the tread than the edge. That’s why I feel my full width, static contact test
(described below) is conservative. My tire life and wear pattern prove putting more rubber on the ground increases longevity.
Full tread width contact, street pressure
Since this article deals with sand tire pressure, see a back issue of Ecological 4-Wheeling for full details on how to safely determine your exact,
full tread width contact, street pressure. Summarizing Tire Pressure, April, 1990 4-Wheeling, you don’t want to overheat the tire and should only be
able to slip a business card under the outside and inside edges of the tread less than 1/4 inch. Reprints of past 4-Wheeling articles available for
$4.00 each.
Follow along with some arithmetic that scopes in the same results. Four tires times 2000 pounds each (1990 MAX. LOAD rounded up to simplify math)
equals 8000 pounds total (tire) load capacity. My Explorer weighs 4000 (3800 rounded up), or half the maximum capacity of all four tires combined.
That roughly says half the pressure should yield ample load capacity.
As a result of both methods above, I run 26 PSI, night and day, seven days a week, and typically get 60,000 to 80,000 miles out of a set of BFG T/As,
including sand runs at much lower pressure. I trust this information will add practicality to determining your street pressure and encourage you to
accept overall lower tire pressure. No figure is shown for the increase in street pressure footprint, but it should approach full tread width.
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
(gas 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 tires may differ.
This is your optimum sand pressure. As the TREAD FOOTPRINT figure shows, this typically results in more than a 250% increase. That is like having ten
tires where you only had four. This pressure is only valid for exactly what you tested. Change vehicle, tires, wheels or load and you have to retest.
It’s obvious a vehicle change would dictate retesting. Tires differ in number and stiffness of sidewall plys and rubber compounds, hence the need to
retest with a tire change, and in actuality, tire 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 tire 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 4X, tires, 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 PSI. Note I measured no height change from 50 to 37 PSI. From 50 to 20 PSI resulted in
only 3/8 inch drop in height. The drop from 20 to 12 PSI was about 1/2 inch and the drop from 12 to 7 PSI yielded more than 3/4 inch 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 16 or 18 PSI and still had trouble in the sand," so the claim goes. It
should be apparent from the curve, the last few pounds 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 tires are best? I believe not. Again the 75% rule is somewhat tire and wheel dependent, but at too low tire pressure, the
center of the footprint begins to well up, reducing the footprint and creating a small “traveling hill” in the center of the footprint. This hill
offers increased resistance to vehicle movement.
For my combination of 4X, tires, etcetera, my optimum sand pressure is six to seven PSI and I typically get by with eight to ten.
Tire Gages
The Great American Big Numbers Misconception
Before we move on, let's talk pressure measurement - tire gauges. I’ll leave the type - stick, dial or digital - up to you and a future article in
4-Wheeling. What I’m primarily addressing is the Great American 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 tire 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 50 PSI and my low pressure gauge goes to 20. It’s pretty hard to measure 10 PSI on a gauge
that starts at 20 PSI. Most 120 to 200 PSI gauges don’t even start until 20 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. 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 tires 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 at
"bringing ‘em back alive" - me, the car and the tires.
Four Affects of Low Tire Pressure
Low tire 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 figure shows the center of the axle is lowered by the decrease in wheel height. This results in lower
ground clearance and consider the softer tires 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 pullin’ power". See December
1988, Ecological 4-Wheeling, for a complete treatment of net moving force. Think of it this way: you know how bigger tires eat up low gearing and
smaller tires effectively give you lower gears? Flatter tires act like smaller tires and increase your pulling (moving) power.
OBSTACLE ROLLING RESISTANCE
Move on to the OBSTACLE ROLLING RESISTANCE figure below to learn how significant this sleeper is! Several years back, I was convinced a tire’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.
Face-off two 4Xs some 30 feet apart, on a flat, smooth surface. Take the winch of one and connect it to the other, with a dynamometer (a big fish
scale) in the cable. Put a pair of 2X4 wood blocks in front of the pulled vehicle and measure the force (pull) required for different towed vehicle
tire pressures.
A simple, lowly 2X4 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 tires.
The 2X4 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 tires started up over the obstacles (the 2X4s) was not precisely recordable with my crude equipment and test methods. With the
sand tire-pressure, it was obvious the dynamometer saw a gradual buildup in force as the tires smoothly conformed and crawled over the obstacles. What
this 40% difference means is you now have roughly six tires where you only had four. Add this to the "ten tires" of the pressure drop and you now have
16 where you had four. Any more questions about the effectiveness of lowering tire pressure?
There is a lot more to making this test perfect and I would be most happy to redo it using proper equipment and controls. Anyone have access to strain
gauge cells and strip chart recorders?
Rock Crawling
Actually, I suspect this is also why lower tire 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 soap box. Use Caution! Caution! Caution! with low tire pressure. The Obstacle Rolling Resistance factor works against you with speed and
in the rocks! Soft tires 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 tires back to street pressure when you hit the black stuff.
Even in 2WD
You 2-wheelers take note as well, for this trick works equally well in 2WD. But remember, you must still deflate all four tires even though you are
only putting power to two. Your hard front tires create hills that your rears must continually try to climb.
Treading Lightly
I also feel lower pressure is more ecologically compatible. With better traction you don’t have to spin the tires and hence leave Mother Nature
unscathed.
You’ll be doing yourself and Mother Nature a favor when you lower your tire pressure the next time you hit the soft stuff.
© Harry Lewellyn
Ecological 4-Wheeling Adventures
P.O. Box 12137
Costa Mesa, CA 92627
voice: (949) 645-7733
fax: (949) 645-7738
email: info@eco4wd.com
No Bad Days
\"Never argue with an idiot. People watching may not be able to tell the difference\"
\"The trouble with doing nothing is - how do I know when I\'m done?\"
Nomad Baja Interactive map
And in the San Felipe area - check out Valle Chico area |
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BigWooo
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Long article, but good info. I never knew there was a formula, I just let air out until it "looks good". We usually drop the pressure to smooth out
washboard though. That's more of a balance between smoothing the ride and not overexposing too much of the sidewall to the loose/sharp rocks.
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bajalou
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Just remember, If you've dug a hole deep enough that your differential is on the ground, you have to dig it out before deflating.
Airing down works best before getting stuck, but if you do get stuck, stop immediately and air down.
No Bad Days
\"Never argue with an idiot. People watching may not be able to tell the difference\"
\"The trouble with doing nothing is - how do I know when I\'m done?\"
Nomad Baja Interactive map
And in the San Felipe area - check out Valle Chico area |
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elgatoloco
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| Quote: | Originally posted by bajalou
................, but if you do get stuck, stop immediately and air down. |
Worked for me a few weeks ago at Playa San Rafael. 
MAGA
Making Attorneys Get Attorneys
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Phil C
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18 to 20 lbs front, 20 to 24 ,depending on load, rear. Multipal Chevy Suburbans 10,000 Mi. off road in Baja. You do present the sidewall to the
terrain. I've used BFGs Ats. Not many problems. If you're realy off road everyone gets flats... Carry two spares.....
[Edited on 4-21-2009 by Phil C]
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woody with a view
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| Quote: |
Carry two spares.....
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what he said. the small amount of bed space required to carry a second spare is minimal. the satisfaction of knowing you can get 3 flats before you're
stuck is incalculable......
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David K
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Harry has a great book for four wheeling... He was a guest at Viva Baja 3...
My sand driving tips (beaches, arroyos, desert, dunes)
A 'easy' place to start your ideal sand pressure is drop to 50% of your normal street pressure... 30 psi street to 15 psi sand. I can get through the
sand/ crushed shell beach at 15 psi in my Toyota, but it does float better at 10 psi. What you want to do is create a buldge in the sidewalls that
acts like a life preserver or inner tube on water, in that it will cause the tire to 'float' on the sand and not spin and dig in. What will sink into
sand more: your pointed finger, or the palm of your hand?
Next is have some momentum to cruise over the dry deep sand and arrive at the wet, low tide part of the beach (provided there is any!)... This is like
driving on pavement compared to the dry sand, the tracks get erased by the next high tide, and you vehicle will run cooler !
Do not brake suddenly or turn sharply in the sand. When you come to a stop, do so slowly and have your vehicle pointed straigh... and downhill if
possible. That will make getting going again much less a problem.
Pacific Sand vs. Gulf Sand: BIG DIFFERENCE!
The Pacific beaches are mostly flat, small grain sand and if much easier to drive... even without lowering air pressure... Try it first at normal
pressure (4WD) . If your rig struggles or tires spin... STOP and deflate!
Gulf beaches are typically steep and have large grain sand and crushed shell or pummice mixed in... This is like quicksand, even to a 4WD! Don't even
try without deflating 50% or more of your air pressure.
With enough air out of the tires, even 2WD vehicles can drive in sand (not just dune buggies or rear engine VWs, but regular cars, vans, &
motorhomes! Just don't turn suddenly and pop the tire off the rim (usually could only happen below 8 psi).
Have a good dial gauge the reads below 10 psi!
Remember, do not beach drive in populated areas or in sea turtle nests (in season)... A properly deflated tire will not go as deep as a human
footprint, in the sand, so they aren't the danger as some would have you think!
NOTE: Like driving Hwy. 1 in the desert ,over 50 mph (80 kph), beach driving is also 'illegal'!
[Edited on 4-22-2009 by David K]
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BajaWarrior
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| Quote: | Originally posted by woody in ob
| Quote: |
Carry two spares.....
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what he said. the small amount of bed space required to carry a second spare is minimal. the satisfaction of knowing you can get 3 flats before you're
stuck is incalculable...... |
SpareSpare.
Got lucky found this identical spare to my spare on CraigsList, even in my own community!
$35.
I thought they guy was nuts to get rid of it, it's brand new! First to be used, second spare riding comfortably underneath.
Haven't had a bad trip yet....
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boe4fun
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What's the rule for towing a small boat trailer? I'll be towing a 13 1/2' Gregor down next time (Sept-October) and plan on going to Gonzaga via
Coco's Corner, Agua Verde, and San Roque on the return leg. Any hints on how much to air down? Thanks in advance, Paul Boe
Two dirt roads diverged in Baja and I, I took the one less graveled by......
Soy ignorante, apático y ambivalente. No lo sé y no me importa, ni modo.
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bajalou
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Take a look at the track the trailer makes in the sand. If it's cutting into the sand for any depth 2-3 inches, then airing down would probably help.
No Bad Days
\"Never argue with an idiot. People watching may not be able to tell the difference\"
\"The trouble with doing nothing is - how do I know when I\'m done?\"
Nomad Baja Interactive map
And in the San Felipe area - check out Valle Chico area |
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mtgoat666
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| Quote: | Originally posted by David K
Pacific Sand vs. Gulf Sand: BIG DIFFERENCE!
A properly deflated tire will not go as deep as a human footprint, in the sand, so they aren't the danger as some would have you think!
NOTE: Like driving Hwy. 1 in the desert ,over 50 mph (80 kph), beach driving is also 'illegal'!
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You are always good for a laugh, DK 
using your tire/footprint logic, it seems you would rather someone drive over your foot than step on your foot. this sounds like a good study to
conduct next time you visit shell island and have drunk 10 shots of tequila
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Curt63
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BFG All Terrains
Thanks for the informative post. I am surprised that you run 26 lbs on road. My local Community College Auto Shop teacher suggested I run max.
pressure on the road.
There is a great reason why so many of us run All Terrains for on road and offroad - TRIPLE LAYER SIDEWALLS!!!!!!
If you are going to air down and/or rock crawl your tire sidewalls bulge and become far more vulnerable. Shredding the sidewall becomes a real
possibility and can't be fixed. Triple layer protection is huge.
And they run quiet on the road!
Saludos
No worries
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bajalou
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| Quote: | Originally posted by Curt63
Shredding the sidewall becomes a real possibility and can't be fixed.
Saludos |
I have a tire man in San Felipe that will vulcanize to repair sidewall punctures.
No Bad Days
\"Never argue with an idiot. People watching may not be able to tell the difference\"
\"The trouble with doing nothing is - how do I know when I\'m done?\"
Nomad Baja Interactive map
And in the San Felipe area - check out Valle Chico area |
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Curt63
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Is there anything those Mexicans can't fix?
No worries
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David K
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The sidewalls on BFGs aren't as bulletproof as they make them sound... I have experienced a sidewall cut/ leak on a sandy road on new T/As (must have
been a rock somewhere)... I have seen many T/As with blown out sidewalls... I have had 3 ply sidewalls (Cooper tires) and 2 ply sidewalls... never a
cut/ leak in any. However, 3 ply sidewalls are much more difficult to drive in sand and require letting more air out to get them to 'buldge'.
Also, more aggressive mud tires do poorly in sand, where as street treads or smooth tires do great.
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Curt63
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Yeah yeah yeah, BUT how many of those tires say "Baja Champion" on them.
That's kind of cool!
Seriously, if you are aware of a better on road, offroad combo tire, I'd love to know the make and model.
Saludos
No worries
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Mulegena
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Thanks, Guys...
This is a really interesting thread. I'm not an off-roader like you but it seems I've done my share of adventuring, and I've got a couple of questions
to throw out to you all :
1. I've heard that lowering your street psi makes for a better ride, but wonder if there is a formula for figuring it, just as the sand formula was
presented here; how low can you go on pavement for efficient everyday driving? How does a lower psi figure in miles per gallon?
2. I'm in the market for another car and am asking for suggestions. Here's the wishlist: In the interests of ecology and economy I'd like one that
would give good mpg and have a low carbon-footprint. It'd be my daily driver but I'd like it to be sturdy and have clearance to go some of the rugged
roads in Baja (like run over the hills from Mulege to San Juanico for instance), 4x4 standard/automatic optional.
Thanks, Mulegena
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Curt63
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Get a Taco (Toyota Tacoma) or a 4 Cyl Toyota Four runner (avoid the 3.0 liter V6). I love my Tundra, but its a gas hog.
Ask Ironman Ivan Stewart, Toyotas do OK in Baja.
No worries
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David K
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Toyota Tacoma is numero uno in Baja... If you want to be more 'green', then might I suggest a Subaru AWD or older 4WD Wagon... You will not have the
ground clearance to get to Mision Santa Maria, but with careful placement of the tires, most everywhere else should be reachable!
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Mulegena
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| Quote: | Originally posted by David K
Toyota Tacoma is numero uno in Baja... If you want to be more 'green', then might I suggest a Subaru AWD or older 4WD Wagon... You will not have the
ground clearance to get to Mision Santa Maria, but with careful placement of the tires, most everywhere else should be reachable!
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Ok, how 'bout a Tracker or Samurai or even VW bus?
and do you recommend a 4x4 or would 2x2 do?
Thanks un tiempo mas, Mulegena
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