Sand Driving

sand-driving - 4x4 AfricaIn the previous article we found Bud, the newest member of our off-road club, undergoing his first practical lesson in basic off-road driving, which did not go too well, however, where there is a will, there is a way and Bud is back for some more expert off-road driving instruction.
He had thought long and hard about what his Instructor had told him about electronic driving aids and his lack of off-road driving experience, and so, with the approval (and assistance) of his bank manager, he had bought a second 4WD vehicle. He had grown attached to the first one (even in the short time he had had it), and placed it in storage against the day he could confidently drive it right across Africa- without “training aids”, as the salesman so snidely (and ignorantly) referred to electronic driving aids- being an expert off-road driver himself.
But be that as it may: this second 4WD vehicle (of Japanese manufacture), did not have a solid front axle, but it was new and had a raised body (with a 1 ton carrying capacity) to accommodate the bigger than standard wheels shod with mud tyres. It also sported:

  • all the electronic driving aids money can buy; those did not come as standard equipment were fitted at some expense to be sure, but Bud now had at his command such things as
  • a non-brake based Traction Control system,
  • a state of the art Dynamic Stability Control system,
  • the ubiquitous ABS,
  • a Hill Descent Control system,
  • a Hill Start Assist system,
  • a Rollover Mitigation system,
  • a Corner Brake Control system,
  • an Adaptive Load Control System
  • a top of the range winch mounted on a
  • non airbag compatible steel bumper, and last but not least,
  • Electronically controlled diff locks and 4WD engagement, and
  • Professionally made and fitted skid plates under the body sills, engine and transmission.

If only he knew how and when to use them all.driving-in-sand-is-all-about-momentum

“Don’t worry about that Bud, we will get to all that a bit later.” The Instructor was impressed with the new 4WD vehicle but thought that while all the electronic driving aids fitted were a good thing, they might cause his pupil to become over confident but decided against saying anything about it now. “So, Bud, how do you like the ABS on this 4×4? Do you think it might help you in the sand?”

Bud was mightily impressed with the ABS brakes and said so.

“Well, this is not your luxury SUV and we are going to see if it works as well on the sand as it does on the highway, but do you know why sand is the way it is? Have you ever thought about why a vehicle, even a 4WD vehicle like this one that has all the bells and whistles, can get stuck in sand so easily?” Bud had to admit that he had never thought about why sand is the way it is but for the moment he felt vastly more confident on the gravel road in this new 4WD. The more flexible sidewalls of the mud tyres made it less bumpy, and the independent front suspension made the ride much more comfortable. Keeping his speed below 40 km/h, he found he had the time to choose a proper line around the rocks, ruts, and tree stumps in the farm road on the way to the sand pit.

Arriving at the sand pit, the Instructor told Bud not to enter it but to stop at the edge. “We made this pit to simulate the conditions you are most likely to encounter in the desert areas of Africa: there are different types of sand here and we made it so that the depth of the various types of sand varies as well. Some types of sand give a more solid footing at the same depth that other types will get you stuck in, especially if they get wet: sea sand for instance, will compact as long as it is damp while red sand that contains some clay will turn to mud as soon as you spit on it. That white stuff over there is the type of sand you often get in the north of Africa; it contains a lot of chalk and it does not need water to get you stuck. Entering that at the wrong speed will get you stuck in very short order because it is so loose and powdery- and it offers no support even to deflated tyres.”

“That black stuff at the end over there is like a bottomless pit and you would do well to avoid it when it is wet, no matter where you find it; you only attempt crossing that if you are absolutely certain you know how deep it is. It can be as solid as a tarred road when it is dry but because it contains more clay than red soils, it absorbs more water than red soils. Which means that it turns to mud much sooner than any other type of sand: a light shower that may just run off anything else will turn that to a greasy slime that could cause you lose control instantly if it catches you unawares. The mud you get from that stuff will stick to your wheels, suspension and under carriage like glue, and if you attack it with the wrong tyres or at the wrong speed, just the sheer weight of it sticking to your wheels can get you stuck. Traction control might get you across, but it might not, especially if it is deep and all four wheels start spinning. It may be good for planting onions in but it hates 4×4 vehicles and their drivers, so avoid it like the plague.”

Bud was impressed and said so but he regarded the sand pit with some misgiving: around four hundred metres long and half as wide, the variously coloured sand looked like a trap waiting to embarrass him.

“It is not as bad as it looks Bud; once you know what you are dealing with, it is often just a case of allowing your 4×4 to do the work for you, and if do get stuck here, no problem, we just get the bull-dozer to pull you out. We have a little tracked bull-dozer to maintain the surface of the pit, so don’t worry, we won’t let you sink.”

“But as I was saying about sand: the reason a 4×4, or any other vehicle, sinks into sand is because the individual sand grains do not have corners anymore, except for sea sand grains which is generally younger than terrestrial sand and has not yet had their corners worn off. Through being blown about by the wind and trampled on for millions of years, terrestrial sand grains have become rounded like rough marbles: they do not have corners that can bind the mass together. If you put pressure on sand, the grains on top slide over and past the ones next to them. However, how much they slide, depends on the direction and the amount of pressure, as well as the area over which the pressure is applied. When you deflate your tyres to increase their contact area, the sand will either compact to some degree, or be displaced even further, which is why 4×4’s get stuck.”

“If the rate at which the top layer of sand is displaced is bigger than the momentum of the vehicle to carry it over the top layer, the sand simply moves away from the area under pressure. Which is why a train can run on a long pile of sharply angled stones: the individual stones have acutely angled corners that form a solid mass by interlocking with each other, which is the mechanism that enables it to support the train’s weight, which could be several thousand tons.”

ABS and Sand Driving

sand-driving-and-momentumNot being technically minded, Bud thought he understood this but kept quiet: better to wait for what came next. “So, now, we will see how effective ABS brakes are on sand: the sand at this side of the pit is only a few centimetres deep but it is enough to affect your brakes. So, reverse a bit and approach the pit at around 50 km/h in high range 2WD; as soon as you are on the sand, brake as hard as you can because I want you to see why using ABS brakes are not a good idea on sand or gravel surfaces.”

Bud was rather surprised at this but did as he a told, however, he was not prepared for happened as soon as applied the brakes: he expected a slightly longer stopping distance but was amazed at the ridiculously long distance it took him to stop: the 4×4 just rolled on as if it had no brakes at all. Why was this- the brakes worked fine before?

“I will explain what happened just now, but first, I want to disable the ABS if you don’t mind. I have to show you the difference in stopping distances with and without ABS before this will make sense to you.” With that, the Instructor dove in under the dashboard and pulled a fuse. “Now, try the same thing again, at the same speed, but close your windows first.”

Bud did, and was even more amazed at what happened: he came to a stop in a whole lot less than half the distance, in a huge cloud of dust. He found this difficult to understand and said so.

“It is simple really; ABS systems work because they measure the rotational speed of each wheel individually, and when it senses that a wheel is slowing down because it is locking up, the system will release the braking force on that wheel until it again rotates at the same speed as all the others. On a paved surface, this allows a driver to apply full braking force without having to worry about individual wheels losing traction because they are locked: locked-up front wheels have lost all traction and because of this, the driver has no directional control. So if the wheels are prevented from locking up, the driver has the ability to steer around obstacles without losing directional control.”

“But this only works on paved roads: while the system still works on sand and gravel, it works too well. On slippery surfaces tyres only have a small fraction of the traction they have on hard surfaces because the sand or gravel acts as a lubricant between the tyre and the surface, and because of this, the wheels lock up much easier than they would have done on a paved road. Therefore, every time the wheels lock up, the ABS releases the pressure to get them rolling again until they lock up again, and the system releases them again. In this way you end up with no effective braking force and if this happened on a steep mountain pass with a loose surface somewhere in Africa, you might end up driving over the edge.”

“The quickest way to stop on sand is to lock the brakes to allow the tyres to dig in, which will build up little walls of sand in front of the tyres. The rolling resistance you build up in this way will eventually overcome the momentum of the 4WD vehicle, and you will stop.”

Diff Locks and Sand Driving

sand-driving-with-jeep-wranglerBud though he had just learned lesson number two: “OVER RELIANCE ON ELECTRONIC DRIVING AIDS CAN BE DANGEROUS!” He was glad of the object lesson but before he could make an intelligent comment, the Instructor spoke again. “So now you know how to stop, but when you are driving off-road, especially in Africa, where there are no bull-dozers to pull you out, you have to be able to maintain your momentum, and this is where a diff lock is important. So, reverse back out of the sand and see how far you get by driving over the sand in 2WD high range with all your electronic driving aids turned off. See if you can hit the sand at about 40 km/h in third gear; that should give you enough engine speed for the turbo to develop enough inlet boost pressure to keep you going for a bit.”

Bud entered the sand at 40 in third gear; – and were stuck hardly 30 metres in when the engine stalled. He silently cursed the salesperson: he was led to believe that his very expensive 4WD could cope with anything, which it clearly cannot.

“So you are stuck, but you only stalled the engine because the Anti Stall system does not work in high range and you could not maintain momentum. However, we will just turn on the diff lock on the rear axle and get you going again. Anything you do now will just cause either one or both driving wheels to start spinning because the traction control is turned off. Therefore, once you have restarted the engine, select high range first gear, turn on the diff lock, and see what happens when you move off, but keep the wheels straight ahead to keep rolling resistance to a minimum and to prevent possible damage to the diff. If the wheels start spinning, select second gear and try again, but always keep the RPM’s above 2500 to keep the turbo spinning fast enough to give you enough boost to develop all the torque you need.”

Traction Control and Sand Driving

Bud followed these instructions and to his amazement, his 4×4 moved off without hesitation.

“Keep going until you either stall or get stuck again: when you do, don’t panic- we will try something else.” Which happened soon after- the front wheels encountered a low wall of red sand and the rear wheels dug themselves into the sand, both spinning. “OK, now turn on the front diff lock and the 4WD in high range and see what happens, but keep the wheels pointed straight ahead as before.”

Again, Bud was amazed at the ease with which his 4×4 moved off, and they covered about half the length of the sand pit when they were stuck again. “Ok, not a problem Bud, just turn the diff locks off and engage the 4WD in low range. We are now in the deepest part of the pit and the chalk in this sand could perhaps beat us yet, but the traction control will also be working after I replace its fuse, so just select low range second gear, and see what happens. If it does not get us going, just turn on the diff locks again. You might find the steering feels different but don’t worry about it, it should feel that way.”

For a third time Bud was amazed at the ease with which his 4WD extricated itself: it just started; just like when he drove it off from the showroom. For the first time he became aware of the power of the 4×4 and the forces at work in keeping him moving, and for a moment he imagined himself on a Cape to Cairo off-road expedition- his mighty Japanese 4WD conquering all obstacles in its way, but they had reached the end of the sand pit. This was the life, and he wanted to know all about it; he wanted to learn about the mechanical workings of his 4×4- for instance, how did the diff lock get them moving when they were stuck the first time? Thus, he started asking questions.

Sand Driving Explained

South Africa sand-driving-trailsThe Instructor made some notes on the overall performance of the 4WD and its owner before speaking: “We would have had virtually no chance of crossing this sand obstacle in your other 4×4, because it has none of the electronic driving aids this one has. It may have diff locks and 4WD but those are not always the answer. Southern Africa, in fact, the whole of Africa is full of places like this: different types of sand that require different methods of dealing with them.”

• Diff locks:

“Take diff locks for example: it is because of the way a diff is made that it allows a vehicle to be steered around corners. The wheels on either end of it can rotate at different speeds, with the inside wheel rotating slower, but with less torque because of it. The outside wheel, rotating faster, gets the most torque but if you are driving straight, like on this sand pit, both wheels get the same amount of torque until one wheel loses traction and starts spinning: the other wheel cannot do any work because all the torque is being diverted to the spinning wheel. So, the diff lock is a mechanism that locks both axles together so that both rear wheels receive the same amount of the available torque.”

“We were able to get moving the first time because the wheels were not dug in very deeply and because all the available torque was divided between the driving wheels and therefore the traction of the driving wheels overcame the rolling resistance. The diff lock kept you moving up to the point where rolling resistance became greater than the traction of the driving wheels, which was when you got stuck for the second time.”

“But don’t forget, since the axles are locked together, the driving wheels will not be able to rotate at different speeds, and you will therefore not be able to go around corners on hard surfaces. You can only use a diff lock in conditions where tyre scrub can be absorbed, like the sand in this pit for instance, to prevent damage to the diff because the wheels cannot rotate at different speeds.”

• ABS:

“I have already explained why ABS brakes could be dangerous in the off-road driving context, but it would be foolish to disable it altogether. There are many places in South Africa where even if you are off-road, an ABS system could save your life, but it is a good idea to have some sort of switch installed that is within easy reach with which you can disable the ABS when it might be dangerous to have it in operation. You have seen the effect it can have on stopping distances, so I would look into that if I were you.”

• Traction Control:

“Traction control does much the same thing a diff lock does, only it does it better, it does it all the time, and you can still go around corners. Some traction control systems, like the one on your SUV, use stored energy to activate the ABS to apply braking force to a spinning wheel: slowing down spinning wheels channels torque to the other wheels and in this way the system keeps the 4WD vehicle moving. However, the system only works until the stored energy is depleted; which can happen in around 30 seconds or so.”

sand-driving-with-traction-control“But it is important to remember that no traction control system gives you extra traction: traction is a function of the friction between the tyres and the surface you are driving over, so all a traction control system does is helping you make the best use of the available traction. The other type of traction control system, like the one you have on this 4×4, uses torque distribution via a series of clutches in the diff casing to apply a braking force to spinning wheels directly to the drive shaft. This system does not involve the ABS in any way, except to use its sensors to detect differences in wheel speeds.”

“The advantage of this system is that it works as long as it is needed, and since it does not involve the brakes, it makes your brakes last longer which is a good thing in Africa, because you could go from Cape Town to Alexandria in Egypt without ever finding replacement parts.”

“It is also important to remember that traction control systems are not infallible: you still need to choose the correct line through mud and sand obstacles. The object of off-road driving is not to see how much abuse a 4×4 can take before it breaks down: in Africa you are dependent on your 4WD not breaking down because there might not be anyone to rescue you. It is a big mistake to charge into all and any mud or sand obstacle thinking your traction control will get you through every time. Traction control will only work as long as there is some traction, and it definitely will not work if you are sunk into a mud pool up to your floor plates because you did not take the time to choose a safer line around the obstacle.”

• 4WD:

“You are wondering why 4WD works? Well, for one thing, it increases total traction by 100%, in ideal conditions at least. It cannot increase the amount of traction, but it makes the available friction between the front tyres and the surface available for conversion into movement, or momentum. By using the diff locks and traction control with the 4WD function, you make optimal use of the traction available to all the wheels, but the downside of this setup is the fact that fuel economy goes way down. You only want to use this setup for as long as it is required because fuel is not always available throughout much of Africa, and if you do get it in some remote spot, it may be a dangerous mixture of substandard diesel and paraffin.”

“Again bear in mind that you cannot go around corners using this configuration on hard surfaces: diff locks prevent wheels from rotating at different speeds and if you do not have sand or mud to allow the tyres to scrub, you run the risk of destroying both differentials.”

• Low range:

“The speed and power available to the driving wheels is directly related to the ratios between all the gears in a driveline setup. The reason a vehicle, this 4×4 included, has the lowest speed in first gear, but also the most power is because the difference in the diameters of the gears in first is the greatest: it needs this big ratio to overcome the inertia, or tendency to remain stationary until some force acts on it. Once the 4×4 starts moving, its momentum, or its tendency to keep on moving until some other force acts on it to stop it, progressively takes over the need to continuously apply a force to keep it moving, and so the ratios, or relative differences in the diameters of the higher gears become smaller.”

“Because of the cumulative acceleration, less power is required to keep the 4×4 moving, until the highest gear approaches a 1:1 ratio, or close to it. In this gear the torque from the engine is enough to overcome weight, gravity, and rolling resistance and so the vehicle keeps moving at a constant rate, unless some kind of force acts on it to stop or slow it down.”

“The final output torque from the transmission is multiplied by the difference between the diameters of the pinion and a crown wheel gear in the differential but this ratio remains constant, irrespective of the gear selected- it also remains constant irrespective of whether high range, which is the normal set of ratios, or low range is selected.”

“Low range is a reduction that taps off the transmission after the reduction between gears and it effectively doubles the final output torque of the transmission, irrespective of which gear is selected. At any given engine speed, the low range also reduces the speed at which you can travel by about 50%. However, the main advantage of the low range is that it greatly reduces the chances of the wheels starting to spin due to loss of traction because the input speed is so much lower: wheel spin can still happen but the chances of it happening are much smaller. Low range and traction control will often get you out of trouble but they will not if you are careless: choosing your line and having enough momentum before you enter an obstacle are still the best ways to prevent getting stuck.”

• Tyres:

“The mud tyres on this 4×4 are often the best compromise between all the various types of off-road tyres, although you may have gotten further on the sand the first time if you had sand tyres fitted. Mud tyres are the way they are not because they look macho; they are that way because the big gaps between the tread blocks allow mud to be flung off more easily. Tyres with smaller gaps get clogged up very quickly, which means that traction is lost much sooner.”

“For the most part, off-road tyres rely on the number of edges it has in the tread for traction and in the off-road context, the surface area of the tread blocks determines the number of edges that can grip the surface. Therefore, because sand does not cling to the tyre, the more edges it has to grip the surface the better, but it also has to have the ability to fling off the sand that does become trapped in the grooves, which means that the voids between the tread blocks on sand tyres are smaller than the gaps on mud tyres. This means that there are more edges to grip the sand, while still allowing for the tread to clear itself of sand that does get into the grooves, So, if you used normal on-road tyres on sand, the much smaller grooves will become clogged because they are so narrow, and once they are clogged, you lose all the edges you need for traction.”

“On-road tyres work on the same principle but there is no sand or mud on highways that can clog the grooves: all an on-road tyre has to do is be able to drain water through its grooves and they usually manage because there are so many grooves. Have you heard of aquaplaning? No? Well, this is when there is too much water to be channelled through the grooves in the tyre and it effectively becomes clogged with water, just as mud will clog the relatively narrow grooves in a sand tyre. So in these cases you have a tyre that rolls over the intervening medium, with no edges available to grip the surface, which means that you will have lost all traction and control of the vehicle.”

Sand Driving in Southern Africa

South African club-event-sand-driving-trailBud could never have imagined that there could be so much involved in driving across a patch of sand and he said so. His time was up for the day but he mulled over all he had learned about sand driving on the way back to the clubhouse. He had planned a long off-road excursion through Southern Africa in about three month’s time but he had to admit that he still had too much to learn. Although he was not disappointed in the efficiency of his electronic driving aids, he realised that he first needed to learn to use them effectively before setting off on his epic off-road trip.

His idea that all it took was getting into a 4WD vehicle and driving across Africa was thoroughly shattered. Issues like choosing a proper line through obstacles, momentum, turbo boost, low range, diff locks and when to use them, 4WD, and traction control, not to mention dynamic stability control, which he had not even tried yet, all seemed like a lot to learn in a limited time. Nevertheless, Bud was nothing if not persistent and he was determined to drive across Africa, therefore he was sure to be back for a lesson in mud driving the following week to make his dream come true a lot sooner.