This article finds our aspiring off-road driver, Bud, back at the club training area. The Instructor had promised him a lesson in mud driving and to this end, he had read up on everything he could find about mud, and how to drive through it. To his mind, he was well prepared for the lesson but there were some issues with when and how to use stability control, and he intended having them cleared up.
“Well, Bud, are you ready to get stuck in the mud today? The rain over the last few days has really softened up the mud patch so we can expect to have some fun today. What do you think?”
Bud did not really think that getting stuck in mud was the same as having fun, but if that was what it took to learn about mud driving, then so be it. Bud had also followed the suggestion the Instructor had made the previous week to drive the new 4×4 as much as possible and after a week of urban driving, he could more fully appreciate the differences in the handling characteristics of his 4WD vehicle when driven off-road as compared to driving it on-road. He no longer thought the 4WD was going to fall over every time he went around a corner and he was beginning to realise that the differences on how the suspension on his German SUV was set up, as compared the 4WD, was because of the need to handle different types of terrain. Bud had never been off-road before but was beginning to understand the demands that off-road driving places on a 4×4 vehicle, but he was still unsure about this Dynamic Stability Control business, especially since he had read that it hardly ever works in the off-road driving context.
Having arrived at the mud pit, the Instructor launched into a lecture on the nature of mud, the salient points of which are:
- Sand that does not contain clay particles becomes mud when water acts as a lubricant between the individual grains:
- sand that does contain clay becomes mud when the clay particles that form part of the sand absorb water, which then acts as a super-lubricant between the individual sand grains:
- that water logged clay is sticky by nature; thus the more clay there is in the sand, the more it sticks to tyres and the underside of 4WD vehicles:
- that mud should be washed off as soon as is practical because accumulated clay deposits can interfere with the correct working of the sensors of some electronic driving aids, such as the ABS wheel speed sensors that control inter alia, the traction, stability, and corner brake control systems.
“OK, Bud, now that you know what you are dealing with, I want you to drive into the mud at around 20 km/h in high range second gear in 2WD, wheels pointed straight ahead, and all electronic driving aids turned off. The mud here is only about 10 cm deep but it has very high clay content and is as slippery as anything you are likely to encounter anywhere in Africa. It gets progressively deeper but the purpose is to see how far you get into the obstacle before you lose traction or control, or both.”
“Now remember, directional and braking control are functions of traction, of which you do not have very much and any sudden control inputs like steering, braking, acceleration, deceleration and such could cause you to lose traction and therefore momentum and control. The best way to avoid all these issues is to have enough momentum to carry you through the obstacle, but this patch is too long to get to the other side, no matter what you do, so just see how far you get. We have the bull-dozer here with some kinetic straps to pull us out.”
Bud reversed a bit, remembered the bit about keeping the revs above 2500 rpm, and attacked the mud pit as instructed. The first 20 or so meters went well- the 4WD just ploughed through the mud, the mud tyres flinging mud in all directions but then the mud got slightly deeper and the back end of the 4×4 started sliding. Counter steering worked for a second or so but then the rear wheels lost their grip and they were stuck.
“Not too bad, Bud, but what are you going to do now? You can try retracing your steps in reverse but you have lost traction already; if you start spinning your wheels now, you are likely to dig yourself in even deeper. In your other 4×4, you would have a problem, but with this one, you have several options. For instance, if the other side of the obstacle is closer than the distance you have already covered, you could try 4WD in low range to get through, however, much of Southern Africa is one long muddy stretch in the wet season, so correct gear selection, and the most appropriate driving wheel configuration is critically important to keep you moving.”
“But in this case, I wanted to give you a feel for mud driving, so we have to get out of the mud, or carry on to the other side, but there is a safety issue here. On the one hand, you are alone in the sense that there is no other vehicle to pull you out, which is something you should never be in the bush in Africa. On the other hand, you do not know how deep the mud is likely to get if you carry on. To make matters worse, you are too far away from the trees to use any of them as an anchoring point to use the winch. Therefore, you are in trouble; you could wait for the mud to dry out but that is something that may never happen, or you could have gone around the obstacle, even if it took hours and an extra hundred kms. The other alternative is that since you got into the obstacle as far as you have, you might try to get out the same way.”
Bud thought about the various options and considered the traction control system to be the best solution. He had experienced its effectiveness in the sand the previous week, and if it worked there, it was bound to work here too. The Instructor invited him to try but the result was that he only got stuck a few metres further on because he was in high range 2WD, and since the rear tyres could not maintain traction on the muddy surface, both wheels started spinning and the traction control got confused, so he tried alternative methods such as:
4WD, Diff locks, Low range…
Using the rear diff lock:
This got him nowhere because both driving wheels started spinning, the back end just started slipping down the angled bottom of the mud pit, and he got nowhere.
Using the rear diff lock in low range:
The same thing happened- only slower.
Using 4WD in low range:
This got him moving but only for a little while because the left front wheel fell into a depression in the bottom. The traction from the rear axle could not get him out of it, since the right front wheel could not maintain traction with the left front in the trench the way it was. Because of the reduced contact between the right front wheel and the bottom, all the available torque was channelled to the rear axle, which caused the right front wheel to start spinning when he applied power. Even though 4WD was engaged, the traction control was switched off because he thought 4WD mode in low range would be enough to get him out of the hole.
Using 4WD in low range with the traction control turned back on:
This did the trick: the traction control started applying braking forces to the spinning wheels, channelling torque to the other wheels through the differentials, which got them moving again. The heavy 4WD lifted itself out of the hole the left front wheel had fallen into and remembering what the Instructor said about sudden control inputs, Bud started a wide, gradual turn to steer away from the deeper part of the mud pit. But then the Instructor leaned over and turned off the traction control- which got them stuck again immediately; all four wheels started spinning in the mud because of the 300 angle of the front wheels, which was just too much for the mud tyres to handle in the limited traction conditions. Bud started sweating- he thought he was never going to get out of the mud by himself.
“OK, Bud, so apart from the diff locks on both axles in high range, there is something else we can try- the Instructor made some notes on his clip board before speaking again:
“Do you remember that I told you that maintaining momentum was of critical importance in mud or sand? Well, we are going to see how it works in this situation: the mud here is not as wet as in other places, so it will compact to some degree, but it is as deep as your tyres are high, which means lots of rolling resistance. The wheels will not just roll over it; they must now push through it which means you will need all the power you can get, but we should have enough in high range.”
Bud thought this was strange, and said so. There was no way he could see high range getting him out of the mud.
“Yes well, you are right but only up to a point: low range is good for getting up steep hills or to prevent wheel spin in slippery conditions but some wheel spin is sometimes better at getting unstuck than anything else. Also, remember that this exercise is about momentum, which the low range will not give you in these conditions. As I said before, momentum is a 4WD vehicle’s tendency to keep on moving until some kind of force acts on it to stop or slow it down. Therefore, if you have enough momentum to overcome all the other forces that are trying to slow you down, the inevitable result is that you will keep on moving. So to demonstrate this, I need you to turn off all the electronic driving aids, select reverse in high range 4WD mode and straighten the wheels.”
“The object here is to clear a path in front of all the wheels without getting out and physically remove the mud around the wheels; there is an easier way. Just keep the revs up to 2500 or so, let out the clutch slowly until you start moving- this will compress the mud behind the wheels and you will have created a gap in front of them. Be careful not to let the wheels spin: you do not want to dig yourself into the mud. OK, that is fine, now do the same in second gear-, but try to move forward further than you did backwards without letting the wheels spin. We are on a slight incline which means you will roll back until you hit the mud behind the tyres again, which will make the gap in front of the tyres bigger. Ok, that’s fine; now just keep on doing this until you have free movement of around four metres or so; we have around 30 metres or so to get out of the mud pit so four metres should do it.”
Bud rocked the 4×4 back and forth under the Instructor’s direction until he had cleared a four-metre long path in front of the wheels and the Instructor had expressed his satisfaction.
“OK, Bud that should do it: now remember, I also said that momentum is a function of traction, so what we want to do here is build up as much momentum as we can by using all the available traction. We could use the traction control, but your other 4×4 does not have that, and you might be stuck in a similar situation with it, so this is the perfect opportunity to demonstrate the relationship between traction and momentum.”
“In this case you have around four metres in which to build up enough momentum to carry you 30 metres out of the mud, but remember you are on mud and by applying too much power you could lose traction, which will only get you stuck again. So, to prevent this, get the revs up to give you enough boost, and use second gear with a large throttle opening to make sure the revs stay at around 2500 or so. However, second gear could give you some wheel spin, so let the clutch out slowly, and once you start moving, keep on moving. Do not let up on the accelerator and do not use the clutch again until you are clear of the mud.”
“Because you have a four metre run-up, you will build up some momentum, which means that you have to rely less on traction alone to get you out, and because you are in 4WD mode, all the wheels can make full use of the available traction. The wheels might start spinning, but the tyres will fling off the mud because of the fact that they are spinning; this will clear them and make it possible for the edges on the tread blocks to grip the surface. Also, because you are moving already, most of the traction the tyres manage to find will be more easily converted to momentum, even if they are spinning some of the time.”
“You might start slipping and sliding but that is normal and to be expected, however, do let up on the accelerator and do not use the clutch: with some luck you will have built up enough momentum to carry you through the mud all the way to solid ground. If you start slipping and sliding, do not panic, just correct the slides as much as you can but allow the 4×4 to find its own way. If you over correct the slides, you might start building momentum at a tangent, or angle to your intended line of travel, which means that the sideways momentum of the tangent reduces your forward momentum by the amount of the sideways movement. OK, Bud? Are you clear on all this? OK, if you are sure there are no rocks or other obstacles close by that you can slide into or collide with, let’s do this and see what happens.”
Bud was not sure what to expect but he was surprised at the way the 4×4 hauled itself through the mud. He had expected much more slipping and sliding but there was not much, and the little of it there was, he easily managed to correct. Once or twice, he thought he was going to be stuck again but remembering what the Instructor said about swinging the steering wheel through a narrow angle, he tried it and both times the front wheels managed to find some traction, which kept him moving.
However, what amazed Bud the most was the way the 4×4 found its own way through the mud. He did not have directional control but he found that by manipulating the accelerator very slightly every time the 4×4 started slipping sideways on the uneven bottom of the pit, he could control the wheel spin and therefore the amount of slippage. However, before he could figure out why this was, he reached the solid edge of the mud pit and stopped on solid ground.
The Instructor nodded his approval and congratulated Bud on the way he controlled the 4WD: “Very well done Bud, especially the way you used the accelerator to control the wheel spin, but there are several things that you need to remember in situations like this; let us go through them one by one:
Your overall weight:
Because you were not carrying anything, the 4×4 managed to build up enough momentum in the cleared channel, but because it was so light, less force was needed to stop it or impede its momentum. Less weight generally equates to less traction, which means that more initial momentum is required to negotiate obstacles like this because wheel spin is more likely to occur. On the other hand, let us say you had been carrying a ton: the extra weight translates into more kinetic energy, in other words, the 4×4 would have required a lot more force in the form of rolling resistance to slow it down. But then again, it would also have required more input energy to build up enough momentum in the cleared channel, so first gear would have been a better choice if you were carrying weight.”
“There are no real hard and fast rules to describe the relationship between weight and traction when you are dealing with mud or sand, but generally speaking, the heavier you are, the more difficult it becomes to control wheel spin with the throttle. Nevertheless, you will find that the heavier you are, the less you need to control wheel spin because in most cases, the extra weight pushes the tyres down through the mud onto a reasonably dry layer where traction is better. Of course, there are exceptions but this is what mostly what happens with mud that is not liquid, but deep mud is not always liquid, nor is liquid mud always deep.”
“So, whenever you encounter large mud pools, always try to determine the depth of it before you enter it. If there are no tracks from other 4×4’s that had recently crossed it that you can follow in, try to get a feel for the obstacle by walking through at least a good part of the route you intend to take through it. By pushing a stick into the mud, you can usually get a good idea of the firmness of the bottom. This is what will determine your gear selection and run-up distance if you are sure you can make it to the other side, but again speaking generally, mud of whatever consistency that is more than around 30 cm deep is potentially dangerous and needs some careful thinking about before entering it.”
“There are no bull-dozers in Africa just waiting under the nearest tree to pull out stuck 4WD vehicle, and even if you are a party of two or three 4WD’s, all of them could get stuck trying to pull each other out of the mud. If you come across obstacles like this, and if you have the time, wait for it to dry out, or of you do not have the time, first try to find a way around it. This mud pool is easy: there are no hidden rocks to break differentials or cut tyres, no tree stumps and dead branches to be stuck on or deep gullies. So, at the risk of repeating the obvious, it is dangerous and reckless to barge into any sort of obstacle without investigating it first: be safe, use some common sense, and arrive in Cairo in one piece.”
Low range vs. High range:
“If you were sure about the firmness of the bottom of a mud obstacle, and if you were carrying a load, low range might be an option but low range limits you to around 3 or 4 km/h in first gear, which might not be enough the carry you all the way through. For instance, should the mud suddenly get much deeper, or if your suspension suddenly started ploughing through the mud, double your normal amount of torque might just start the wheels spinning against the increased drag, even in 4WD mode. On the other hand though, if you were doing say, 15 or 20 km/h in high range 4WD mode, you might just have enough momentum to get you through the deep patch, without getting stuck.”
2WD vs. 4WD:
“4WD is almost always the better option on slippery surfaces because it effectively doubles the available traction, as opposed to 2WD. But always bear in mind the 4WD does not give you more traction; all it does is use the available traction more efficiently.”
Diff locks vs. open differentials:
“Diff locks are meant to prevent individual wheels spinning through loss of traction but they do not prevent the possibility of both wheels on one axle spinning through loss of traction. While some 4WD vehicles have torque vectoring, a system that transfers all the torque to the non-spinning axle, not all 4WD vehicles have this function, which means that you could potentially be stuck, even with both differentials locked if both wheels on one axle lost traction at the same time. This is not a problem on 4WD’s that have centre differentials because a locked centre diff will channel torque to both axles equally, irrespective of traction, however, yours does not have a centre diff, so if you are not sure about how and when to use the diff locks, rather use momentum with 4WD and traction control, but only where it is safe to do so because high momentum translates into long stopping distances.”
“Front diff locks can also interfere with steering; it can double or even triple your turning circle, which means that you could conceivably be unable to avoid obstacles like trees and rocks, which means that if you have to turn off a diff lock to be able to steer more effectively, you could lose traction and get yourself stuck. Diff locks definitely have their uses and can get you out of trouble but they can sometimes create more problems than they solve, so rather use the traction control if you are not sure about using the diff locks to get you through an obstacle.”
“Ok, when we started you said you were not sure about how stability control works in mud and sand. Stability control systems are designed to work at high speeds to correct both over steer and under steer conditions by using the ABS to apply a braking force to individual wheels to force a vehicle onto the path dictated by the steering angle to prevent a possible skid. Depending on the condition, this braking force can be applied either to the inside or outside wheels to create a drag to correct the situation, but in mud, a skid or a slide can start independently of the steering angle. The yaw sensor, which measures lateral acceleration and is linked to the steering angle sensor, will determine the amount of corrective action that is required to correct the situation, which could include cutting power to the driving wheels.”
“Therefore, say you were slipping and sliding through a mud pool and you started slipping sideways even though you made no steering input; your first reaction would be to counter the slide by counter steering. The stability control system will sense this and determine that an under steer condition exists, because the 4WD vehicle is not following the path set by the steering angle, which it will try to correct by applying a braking force to the wheels it thinks will correct the problem.
“Now, on a paved road where traction is not an issue, this braking force will force the vehicle into line, but on mud, it could very well destroy all your momentum, which could get you stuck. Worse, depending on your speed and the steering angle, it may also cut the power by closing the throttle until such time the system decides that the dangerous situation has been dealt with because that is what it is designed to do in dangerous situations. However, this will also deprive you of the ability to control the slide by manipulation of the throttle.”
“The sort of speeds at which stability control systems work best is much higher than those you are ever likely to use when driving through mud or sand. However, even if you were on a graded dirt road with a loose surface and the stability control system came into operation, the braking forces might be so small as to be almost useless, because the ABS system will prevent the wheel lock-ups that will almost certainly occur on the loose surface, especially when you are travelling at high speed.”
“All of this means that the system cannot do what it is meant to do, which is to apply sufficient braking force to individual wheels to create a large enough drag to correct the path a vehicle is taking through a corner. The most effective stability control system in high-speed off-road driving is based on the principle of a driver adapting his speed to any given situation. The over-riding principle of maintaining sufficient momentum at the lowest possible speed is by far the most effective method of maintaining control of a 4WD vehicle.”
“However, despite using common sense and good judgement, some slipping and sliding is sometimes inevitable and/or unavoidable when negotiating an obstacle. But then again, an off-road driver should never be in the position of having to rely on electronic driving aids to correct his errors in judgment on the one hand, or to compensate for his lack of off-road driving skill on the other, or worse, both.”
Bud’s thoughts on mud driving
Even just two weeks ago, Bud would never have thought there were so many issues involved in mud driving, and he would have scoffed at the idea that his first 4×4 could not have done it, or at least, not have been able to cross this particular mud pool without being stuck numerous times.
At one point during the mud-driving exercise, the Instructor had told him that there are very few absolute rules in mud driving and that each obstacle presented its own problems and challenges, but the one thing they all had in common was that they were all potentially dangerous. Mud driving requires a high momentum to get across an obstacle safely, but at the slowest possible rate to prevent, or minimise, damage to the 4×4 because of hidden rocks, ditches, tree stumps, and other unseen dangers.
“OK Bud, you did well today, but we need to do more mud driving in the coming weeks because I want you to be more comfortable with this 4×4: you need to learn more about just how powerful it really is on the one hand, and how to use that power on the other.”
“Mud driving has less to do with brute force than effectively using the available power, and I think you need more practice in relating traction to power. Therefore, next week we will be doing some hill climbs and maybe some hill descents, but before we get there, we had better get this mud washed off your brakes and ABS sensors at the clubhouse.”