On my first trip to Chobe, my Nissan Hardbody initially had a lot of trouble driving in the very dry sand. Matter of fact, after the first morning’s game drive, where I was nearly stuck twice, I was almost in a cold sweat at lunchtime while thinking of the challenges of the upcoming evening game drive. It is hard to get excited about game viewing when you know that you cannot get out of a stuck vehicle due to predators and rocking back and forth only marginally helps.
Before I start this article, I must admit that there were actually two issues that solved my problems. One was clearly learning about how to adjust tire (BTW, I acknowledge that Africans and Europeans spelling is tyre but I will use the US convention for this article). The other issue is that I was attempting to drive in four wheel high rather than four wheel low range.
The Key: Understanding What Affects Optimum Sand Tire Pressure
Optimum sand tire pressure is a combination of many things. Factors that play into the optimal sand driving tire pressure including tire construction methods and materials, what your car weighs, and how it is loaded. Why low pressure works and how to determine your best sand pressure follows:
Why Low Pressure Works
It’s a simple fact that the bigger the footprint, the easier it is to travel on softer stuff. Mother Nature knows it. Take a look at the feet of elephants, camels, polar and marsh birds. They are big and spread out to distribute their weight over a larger surface area.
Tire Pressure and Footprints
Right next to your wheel, on the tire, find the small black print that specifies maximum load pressure. For example, BFGoodrich Radial All-Terrain T/A 30×9.50R15LTs 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 following image helps one understand street pressure vs. sand pressure:
Full tread width contact, street pressure
Follow along with some arithmetic those 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. A Toyota Land Cruiser 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 the 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 plies 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 was the 250% increase measured? Actually, someone measured the pressure, painted the tread, let the tire down onto a piece of paper and “printed” the footprint for various air pressures. Trust me, one can 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 a specific set of four wheel drive 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!
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 most combinations of four wheel drives, tires, etc., optimum sand pressure is between 0.5 and 1 bar (7 to 16 PSI). Since I also drive on tar going and coming from the park, I set my tires closer to 1 bar and have had no problems.
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”. 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?
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.
But of course no solution is perfect. Use 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! While operating at low pressure, I did suffer a punctured tire … and there was an on-coming Cape buffalo herd 500 strong (I sat for 2 hours until the herd safely passed, bumping my vehicle on the left and right as they did pass.) Also obviously one needs some way to reinflate the tires back to street pressure when you hit the tar roads heading back to civilization.
Conclusion and my final recommendation
I want to enjoy my photo safari and not worry about tires. On my July 2009 re-visit to Chobe, the sand was not as dry. Regardless of that fact, before I pulled out from the lodge, I lowered the tire pressure in anticipation of sand. A few of my South African buddies laughed and said this wasn’t needed – maybe not, but you know what? I didn’t slip during my 9 days at Chobe and was much more relaxed behind the wheel. So if you are headed out for a self-drive safari at Chobe, while you are packing power adaptors and other accessories, throw in that seldom used tire gauge. It is small and you want regret the safety.
NOTE: Images in this article supplied via Harry Lewellyn.