Without doubt, electronic driving aids or driver assist systems, as they are also known, have proven their worth in the city and urban driving environments by having prevented thousands of accidents, injuries, and fatalities. However, in the off-road context where vastly different driving conditions obtain, serious 4×4 or off-road drivers hold differing opinions on the benefits, effectiveness, and even the desirability of having electronic driving aids fitted to off-road vehicles.
During the design phase of a new passenger car, the effectiveness of for example, a traction control system can largely be predicted by making use of sophisticated mathematical models in which factors such as the weight of the vehicle, road surface(s), tyre condition and traction category, speed, and others are known to within narrow margins, but the off-road driving conditions in Southern Africa, or much of the rest of Africa for that matter, are such during the rainy season that no mathematical model can even attempt to predict the effect a particular electronic driving aid might have on the handling characteristics of a heavily laden 4×4 vehicle struggling through thick mud. If that mud happens to be on even a slight side slope, handling might become so difficult that not even the driver may be able to predict what happens next, if for example, a tree or some other obstacle suddenly came in range of say, the sensors of an autonomous braking system.
Nevertheless, not all electronic driving aids are undesirable and some have undoubtedly prevented serious, if not fatal accidents involving 4WD vehicles in the bush: however, given the fact that they were originally designed for use on family sedans and supercars, and not with off-road driving conditions, or the sometimes limited skills of some off-road drivers in mind, the result has been that inexperienced drivers of 4WD vehicles sometimes expect too much of electronic driving aids (i.e. expecting these systems to drive their vehicles for them) and as a result, many preventable accidents involving 4×4 vehicles have occurred, with rollovers topping the list.
By briefly explaining how the electronic driving aids that are most commonly fitted to 4×4 vehicles capable of surviving arduous overland expeditions through the extremely difficult off-road driving conditions offered by South Africa, Southern Africa, and all points north, throughout all of Africa work, what their main functions are, and why some are not always effective, this article hopes to educate especially inexperienced drivers on the use and ((in)effectiveness), if not the desirability or otherwise, of electronic driving aids.
In the off-road driving context, the loss of traction through whatever agency, excessive speed, excessive braking, or steering angle input is arguably the single most dangerous situation the driver of a 4WD vehicle can find himself in. However, while the advent of traction control systems has somewhat reduced the possibility of losing traction and perhaps ending up in an uncontrollable skid, the main function of traction control systems as they relate to 4×4 vehicles and off-road driving, is to prevent the loss of momentum through wheel spin, which can cause a heavy off-road vehicle to sink into a mud pools or deep sand in a matter of milli-seconds.
Although there are two main types of traction control systems, all are based on the characteristic that differentials have of allowing wheels to rotate at different speeds, with a commensurate increase in torque to the faster rotating wheel: thus, when one wheel loses traction all the available torque is channelled to that wheel, with the result that the other wheel stops rotating. The inevitable result is a 4×4 vehicle stuck in mud or sand, or worse.
Nonetheless, while a spinning wheel might find traction at the bottom of the mud or sand layer, wheels that have lost contact with the ground, in for instance wildly uneven terrain, means that the vehicle is stuck until it is towed off the obstacle, with possibly serious damage to engine, transmission, suspensions, and differentials resulting as it is towed across rocks and/or tree stumps. Limited slip differentials might be of some use in these cases but their limited capability to maintain traction unfortunately means that the chances of getting off the obstacle unaided are also limited.
Traction control systems however, are not infallible- nor do they give added traction: all a traction control system does is to assist an off-road driver in making the best use of the available traction. Over reliance on traction control systems can cause a 4×4 vehicle to get stuck as easily as not having traction control in the first place: a common mistake made by inexperienced off-road drivers is thinking that it is possible to cross all types of terrain and all obstacles because their vehicles are fitted with traction control systems. This is not true: because while traction control systems can often get a 4WD vehicle through mud and/or sand that a non-equipped vehicle might not have been able to cross, it is extremely unwise to enter a mud or sand obstacle without first making some attempt to establish how deep the mud or sand happens to be.
There is a good chance that if the mud is not deep enough for suspension parts or the differentials to start “ploughing” through it, a 4WD vehicle with a traction control system might be able to cross the obstacle successfully, however, once the differentials and suspension start ploughing, rolling resistance will inevitably become greater than the already tenuous grip the tyres have on the surface, and the vehicle will get stuck. Thus, while traction control can be effective; it cannot replace good judgement and common sense: or compensate for mistakes made in gear selection, tyre pressures, and speed.
Thus, depending on the degree of “stuckness” of a 4WD vehicle, for instance, when it is sunk into mud or sand up to its floor plates, the traction control system might become confused since all four wheels are spinning, but with that said, exactly how do traction control systems work?
• The Mechanics of Electronic Traction Control:
Since the wheels of a 4WD vehicle driven in a straight line all rotate at the same speed, the available torque is divided equally between them. However, if that same vehicle should encounter a deep mud patch and the wheels on one side loses traction and starts spinning (using all the available torque to spin), the traction control system will apply sufficient braking force, (by using the same sensors and hardware as the ABS system, to which it is closely linked), to those wheels to stop them spinning. This slowing down of the spinning wheels results in torque being channelled to the stationary wheels: (stationary because they have been deprived of all torque), which results in the off-road vehicle maintaining, or regaining, momentum.
This process takes place automatically, with no input required from the driver: while the traction control system is operation this switching of braking force, (and thus torque) between the wheels on both axles when 4WD mode is engaged, can take place several times per second, and in many cases, the driver will not even be aware that his momentum is entirely due to the effectiveness of the traction control system on his 4WD vehicle.
Notwithstanding this though, some traction control systems suffer from serious deficiencies, especially those that are ABS brake based, meaning those that depend on stored energy for their operation.
Brake based traction control systems
While design specifics differ, most brake based traction control systems work by means of a master cylinder that is connected to an “accumulator”, a gas-filled cartridge, or canister that is kept under pressure by means of an electrically operated pump.
The traction control system uses the energy stored in the canister to energise the brake system, and the traction control system will only work until this energy is depleted, and how long this takes depends entirely on the capacity of the gas canister and the ability of the pump to keep it pressurised, but around 30 seconds or so is the norm, depending on the number of brake actuations required to negotiate the muddy stretch.
The net result of this is of course that the traction control system might stop working while a 4WD vehicle is right in the middle of a long stretch of mud filled two-wheel track. However, some off-road vehicles can be fitted with optional, but very handy, electronically controlled differential locks that are integrated into the traction control system, a feature that significantly increases the effective operational time of brake based traction control systems.
Non-brake based traction control systems
An increasing number of 4×4 vehicle manufacturers have of late begun moving away from brake-based traction control systems to more sophisticated (albeit vastly more expensive) electronically controlled torque distribution systems to achieve the same end; the loss of traction on one or more wheels.
Instead of applying a braking force to spinning wheels via the ABS system, these systems apply a braking force to the spinning axle from within the differential casing, and while design specifics vary, these systems generally use a series of clutches within the differential casing to apply a braking force to only the spinning side shaft. This system must not to be confused with differential locks or limited slip technologies that lock the two side shafts together solidly: traction control systems based on torque distribution allows the 4WD vehicle to negotiate corners since the braking force is applied to only one side shaft at any one time, unless the sometimes integrated differential lock is engaged.
The advantage of this type of traction control system is clear, since the system will work without interruption for as long as it is engaged. On most 4WD vehicles with full-time all-wheel drive, activation is automatic and in some cases permanent, while on some vehicles with part-time 4WD, the system is not always permanently engaged, but can be activated via push-button type controls. The same principle has found its way into transfer cases as well, where the same arrangement of clutches can distribute torque between the front and rear axles.
Hill Descent Control
Opinion among seasoned off-road drivers on the desirability of HDC (Hill Descent Control) systems is divided, to say the least. Some off-road drivers maintain that normal ABS brakes control 4×4 vehicles on steep hill descents just as effectively as the complicated electronics of any HDC system can, while others maintain that there is no way that any off-road driver can surpass the integrated approach of a good HDC system.
Also known by various other names, HDC systems work in much the same way that a cruise control system does: turning the system on (HDC systems do not engage automatically), allows the off-road driver to select the speed, ranging from around 3 km/h to approximately 30 km/h, which he wants his 4×4 to maintain while going down a steep hill. Most HDC systems make use of the ABS, traction control, stability control, electronic brake distribution, and all and any other brake related safety systems to prevent wheel lock-ups, momentary (or worse, sustained,) loss of traction on any wheel(s), and in the case of automatic transmissions, HDC systems will prevent unexpected gear shifts as well.
Descending from the Sani Pass in the Maluti Mountains of Lesotho, in Southern Africa, in deep snow is not for the faint hearted: it is generally considered to be one of the most dangerous mountain passes in the world and since it is an undisputable fact that computers are better at controlling vehicles, 4WD vehicles included, than people, inexperienced off-road drivers especially should avail themselves of the advantages of HDC systems whenever the opportunity presents itself since HDC systems apply braking force to each individual wheel separately. This fact is important because ABS systems supply braking force to all four wheels simultaneously, except in some cases, such as when 4WD is engaged, and only releases the brakes on wheels that lock-up, which might lead a 4WD vehicle to drive over the edge.
Slippery surfaces, such as snow and ice, cause wheels to lock-up almost continuously and given the fact that ABS systems prevent lock-ups, it means that the vehicle effectively has no brakes. In contrast, an HDC system keeps the wheels rolling by applying, and maintaining braking force to the wheels asymmetrically, and reduces brake pressure only enough to keep the 4×4 vehicle moving at the speed selected by the driver, thus preventing lock-ups and the loss of traction.
Specifics on the controls of HDC systems vary, but generally speaking, once the 4WD vehicle has reached an even keel, the HDC system can be de-activated either by the use of the accelerator or brake pedals.
Originally developed by Land-Rover, the terrain response system allows an off-road driver to change the way a 4WD vehicle responds to certain driving conditions. Provided the system is engaged before difficult terrain and/or driving conditions are entered, the terrain response system will change the way the ABS brakes, traction control system, stability control system, suspension, differential locks, transfer case, torque distribution, HDC, and even throttle response works.
All this may sound like a case of over-kill, and many off-road drivers consider it to be so: however, using the correct setting for any given condition, such as that for grass, gravel, and/or snow, where a slow and gentle throttle response is required, will allow an off-road driver to concentrate on the terrain instead of having to switch between various systems until he finds one that controls his 4WD vehicle better than any other.
Another useful feature of the terrain response system is that it has a setting especially suited to the rutted two-wheel tracks of which much of the road system in Africa consists. This setting allows the 4WD vehicle to immediately take the corrective actions required to keep the vehicle in the track. While normal traction control systems may prevent the vehicle from getting bogged down, the terrain response system will in addition to controlling the traction control system, prevent any harsh, or severe throttle inputs that can cause wheel spin and excessive yawing movements in the event the tyres suddenly, or unexpectedly, find traction under the mud layer.
Hill Start Assist
Design specifics vary, but from the off-road driving perspective, hill start assist systems are an effective way to prevent not only destroying clutches, it is also an effective way to prevent 4×4 vehicles from rolling back when driving off from a steep incline, which is its main function.
While a traction control system will prevent the wheel spin that is often a feature of starting uphill, a Hill Start Assist system allows a smooth start by holding the brakes and releasing them slowly when the system senses that enough torque is available to allow the 4WD vehicle to start off without stalling or jerking.
These systems use sensors to detect the angle of the incline, pedal positions, engine speed, and the ABS wheel speed sensors to confirm that the vehicle is stationary on an incline: when changes in the pedal positions alert the ECU that the vehicle is about to be driven off, the brakes are released progressively until a predefined wheel speed has been reached, upon which the system becomes inactive.
Although design specifics vary in relation to the time the hill start assist system holds the 4WD vehicle stationary, continued, and positive movements of the pedals cause this predetermined time (two to three seconds in most cases) to be cancelled, allowing the vehicle to be driven off without the driver having to wait for this time to elapse.
Anti-stall systems work by automatically adjusting the idle speed or throttle opening when it senses that the engine is about to stall, for example, when negotiating steep hills or sand.
However, these systems are mostly work in low range gears, and low range reverse in some cases. Making these systems to work only in low range has the probably unintended effect of preventing some off-road drivers to attempt steep inclines while a 4WD vehicle is in a higher gear in high range: a condition that will almost invariably cause a stall, regardless of whether an anti-stall system is fitted or not.
The advantages of this system is clear, especially since inexperienced off-road drivers often underestimate the amount of torque required to reach the top of a steep incline. Stalling on a steep incline could be positively dangerous since the brakes on off-road vehicles, (ABS or not) generally do not work as well when rolling backwards as they do when going downhill nose first. Maintaining control of a heavy 4WD vehicle as it rolls back down a long, steep incline can be very difficult, if not impossible, and many a perfectly good 4×4 has been destroyed in this way.
First developed by Bosch, and introduced by Mercedes-Benz on their passenger cars in 1985, Anti-lock Brake Systems measure the rotation speeds of individual wheels, and in the event one or more wheels start to rotate at a slower rate than the others because of incipient brake induced wheel lock-up, the system will reduce the brake pressure to the affected wheels until all wheels again rotate at the same rate.
Central to the efficient working of an ABS system is a modulator that divides the brake system in individual channels, one to each wheel. Although design specifics of modulators differ, they all have the ability to manipulate the volume, and thus the pressure, in each individual channel in very quick succession, which is felt as a rapid pulsation of the brake pedal when the system is in operation.
All vehicles, 4WD vehicles included, rely on the friction of the tyres relative to the road surface to maintain momentum in any given direction, and the loss of traction for whatever reason means an off-road driver is in danger of completely losing all directional and braking control of his 4WD vehicle.
The maintenance of full directional and braking control is vitally important, particularly in the often harsh off-road driving conditions of Africa, and it is in these conditions that ABS systems may be valuable, albeit not in all circumstances and/or conditions: by being able to steer around obstacles while applying full braking force, an off-road driver is potentially able to prevent many accidents caused by, for example, animals that can sometimes wander into the path of an oncoming 4×4 vehicle.
The ability of ABS systems, especially when they are linked to dynamic stability control systems to prevent wheel lock-ups even in situations where different wheels are running on completely dissimilar surfaces, is the main reason why a driver can maintain full control of a 4WD vehicle in an emergency situation: at least, that is the theory: the reality as it relates to ABS systems and off-road driving is somewhat different.
4×4 Driving and ABS
Of all the electronic driving aids fitted to 4WD vehicles, ABS systems are arguably the most contentious: because while ABS systems have clearly demonstrated their value by preventing many, if not most otherwise unavoidable accidents, injuries and fatalities, on hard, paved roads, they are generally regarded as being ineffective and hazardous in the off-road driving context.
Just as in most other places, 4×4 driving in South Africa, Southern Africa, and the rest of Africa can be dangerous: traction is very often at best marginal and the danger of wheel lock-up are very real. Sand and/or gravel strewn mountain passes, huge stretches of thin, greasy mud, and plenty of powdery sand of varying depths and consistencies can sometimes make it extremely difficult to bring a heavily loaded off-road vehicle to a stop in a safe and timely manner, and herein lays the problem with ABS systems in some off-road conditions.
The main function of any ABS system is to prevent the wheels from locking up under braking: however, with at best marginal traction an ABS system will always release the brakes each time a lock-up happens; which is very often, since greasy mud or sand acts as lubricants between the tyres and the road surface. In practice this means that the brakes are released every time the vehicle starts to slow down, and if the driver maintains pressure on the brake pedal, the wheels may lock up again, and the ABS system will release the brakes again, resulting in ridiculously long stopping distances.
If this cycle continues for any length of time, or if the driver maintains pressure on the brake pedal, (bearing in mind that the ABS system applies, releases, and reapplies the brakes automatically several times per second), the very expensive 4WD vehicle may drive off the side of a mountain because the ABS system actively prevented it from stopping.
The above scenario is not as far-fetched as it may seem: it has happened in the past and it will happen again, which is why almost all seasoned off-road drivers have installed ways and means of deactivating the ABS systems of their 4×4 vehicles in conditions where operation of the system may be hazardous.
EBD (Electronic Brake Distribution)
This system monitors, amongst other parameters, the weight distribution, (an important factor on 4×4 vehicles), road conditions, suspension travel on all wheels, and in some systems, the steering angle of an off-road vehicle.
These systems are electronic versions of the old-style mechanical load sensing valves, that in conjunction with the ABS system, determines the correct amount of braking force to apply to both the front and rear axles, based on the speed, road condition and weight distribution of a 4WD vehicle.
While ABS systems do much the same thing, an important distinction between the two systems is the fact that ABS systems generally do not take into account the weight distribution of a 4WD vehicle: it merely senses wheel speeds and will prevent individual wheels locking up, regardless of how the weight of the vehicle is distributed across the two axles, which is important from the perspective of the traction- and stability control systems, to which EBD systems are closely linked.
EBA (Electronic Brake Assist)
Underestimating the amount of brake pressure required to stop a heavy off-road vehicle in an emergency situation is an easy, and common, mistake for inexperienced off-road drivers to make.
By monitoring the speed, steering angle, suspension movements, and initial brake pressure applied by the driver, an electronic brake assist system will determine if an off-road vehicle is in an emergency situation or not: if it determines that an emergency exists, it will autonomously apply full brake pressure in conjunction with the ABS and other brake based safety systems to prevent wheel lock-ups and loss of directional control.
Until the system decides that the emergency has been dealt with, the driver is deprived of brake and throttle control, automatic transmissions are prevented from executing gear shifts that could aggravate the problem, and in some cases, the seatbelt pre-tensioners (where fitted), are activated.
The theoretical advantages of these systems lay in the fact that if they are unable to prevent a crash, they at least reduce the crash impacts to levels that are less likely to cause injuries and fatalities. However, given the fact that off-road vehicles usually operate in conditions where traction is likely to be very much less than optimal, the overall effectiveness of an electronic brake assist system may be severely compromised.
Dynamic Stability Control
Based on the ABS system, DSC systems detect individual wheel speeds, steering angle, vehicle speed, suspension movements, and the readings from a jaw sensor, lateral acceleration sensors, rollover warning sensors and throttle settings: also known under many different names such as Vehicle Dynamic Control, Electronic Stability Program, Vehicle Stability Assist and many others, DSC systems are essentially electronically controlled auto-pilots that have the function of correcting the differences between the path an off-road driver intends his 4WD vehicle to go, and the actual path the 4WD vehicle is following.
Excessive speed combined with an acute steering angle may produce an under steer condition and to correct this, the DSC system will apply braking force to the wheels that are rotating at a lower speed, thus channelling torque to the opposite wheels. This action forces the vehicle into the turn; however, depending on readings from the other sensors in the DSC system, braking may be applied to all wheels, albeit at different pressures and rates but braking will always first be supplied to the wheels that are calculated to correct the dangerous situation by having braking force applied to them.
Conversely, DSC systems correct over steer conditions by supplying braking force to the wheels that will pull the 4WD vehicle out of the turn, also by manipulating braking force to individual wheels through the ABS system.
During corrective actions, DCS systems will also reduce or increase engine torque to assist in restoring stability: inadvertent or sudden throttle inputs by a driver may upset a vehicle in a critical situation even further and for this reason the driver is temporarily deprived of throttle control, while automatic transmissions are prevented from executing gear shifts that could compound the problem.
In the case of a roll over danger, braking force will first be applied to the wheels on the opposite side to which the 4WD vehicle is leaning: by slowing those wheels down and thus channelling torque to the wheels on the side to which the vehicle is leaning the imbalance is corrected: these actions take place in quick succession- typically several times per second, and in many cases the situation is averted even before the driver realises that an accident had been imminent.
However, dynamic stability control systems cannot overcome the laws of physics; careless and reckless off-road driving may lead to situations which no electronic driving aid(s) can correct because they are limited to working within pre-defined parameters: exceeding these parameters could lead (and has led) to accidents, injuries, and fatalities.
Driving in off-road conditions can sometimes lead to excessive body roll and pitching: load control systems refer more to the way suspension loading affects vehicle handling characteristics, than the way 4WD vehicles are loaded. Load control systems have the ability to alter the characteristics of other electronic driving aids such as dynamic stability control, traction control, ABS, and others during periods of high suspension loads.
While actual vehicle loads play a role in how a 4WD vehicle reacts to uneven terrain and steep inclines, adaptive load control systems analyse these effects and by changing the way suspension and driver assist systems react to road surfaces and uneven terrain.
The net result is that off-road vehicles are easier to control in uneven terrain, with excessive body roll and pitching largely dampened out.
Corner Brake Control
Most off-road drivers are familiar with the effect under inflated tyres can have on straight line driveability: CBC or Corner Brake Control systems use this “drag” effect as the basis for the corrective actions they take to correct situations that sometimes occur when vehicles are braked while cornering.
Based on the ABS system, a CBC system senses when a certain pre-defined lateral acceleration is approached, in other words, a 4×4 vehicle is not following the path dictated by the steering angle and is attempting to carry on, while not in a straight line, along a line that deviates from the radius of the curve, and because of this, the load on the outside tyres is increased while the load is lessened considerably on the inside tyres, which may lead to severe, if not uncontrollable over steer, and a possible rollover.
By applying braking force only to the wheels on one side of the vehicle, a “drag” is created and the 4WD vehicle is forced onto the line dictated by the steering angle. CBC systems also work in conjunction with the ABS system during violent evasive manoeuvres, where tyre loads can shift from side to side several times during a short space of time. Cornering brake control systems can also apply braking force only to the rear axle in situations where the uneven tyre load could cause both rear tyres to lose traction, which may result in a potentially uncontrollable slide, or skid of the back end.
Roll overs are arguably the most commonly occurring accidents involving 4WD vehicles, mainly because of their high centre of gravity. However, terrain has as much to do with 4×4 roll overs as poor driving techniques and overloading of off-road vehicles.
While existing stability control systems have the ability to prevent rollovers to some extent, a dedicated roll over mitigation system has several advantages over a stability control system operating alone: using most of the existing electronic driving aids, (and their sensors), a roll over mitigation system uses an additional sensor that measures lateral accelerations, and will trigger a series of corrective actions independent of the driver or any control inputs a driver may make.
For instance, if the system determines that a roll over may occur as a result of excessive cornering speed it will first apply a braking force to the outside front wheel, then to one or both inside wheels. In addition, it will make steering angle corrections, as well as corrections or adjustments to the throttle opening. Since rollovers have been described as accidents that occur because vehicles cannot support their own cornering forces, these corrective actions (which takes place in fractions of seconds) compensate for imbalances that come about when these cornering forces are wildly out of equilibrium.
Nonetheless, most roll overs occur when a vehicle is “tripped up”, for instance, when two wheels on the same side come into contact with obstacles, like in the case of 4WD vehicles, are often rocks, ruts, and other unseen objects while cornering. This type of situation is especially dangerous on account of the fact that 4×4 vehicles are often fitted with loaded roof racks that significantly decrease stability by altering the centre of gravity, which as all experienced off-road drivers know, is a point on the vertical axis around which a 4WD vehicle pivots as it rolls over.
Some ROM systems counter this by activating the suspension on the impact side while lowering the suspension on the opposite side. This produces a counter movement and the roll over is prevented in many, if not most cases. However, this type of system only works on air suspensions that have the ability to lift or lower a 4WD vehicle.
Are Electronic Driving Aids Safe?
While some electronic driver aids have obvious advantages, they cannot overcome the laws of physics. Over reliance on any driver assist system(s) fitted to any 4WD vehicle, especially those that are engaged in difficult expeditions through any part of Africa, whether South Africa, or any other part of Southern Africa, could very well bring about the accidents they were meant to prevent.
Off-road drivers should bear in mind that traction control systems do not create traction- they merely help to make the most of what there is, and dynamic stability control systems cannot prevent the loss of directional control in conditions that exceed certain pre defined operational parameters: while it might prevent a skid on loose gravel at 35 km/h, it cannot do so if that same corner is taken at 95 km/h.
Roll over mitigation systems can similarly not prevent the roll over that is bound to happen if an inexperienced off-road driver should choose to cross a 35 degree slope at right angles, rather than going down it nose first. South Africa, and the whole of Africa has much to offer off-road drivers of all skill levels and inclinations, but it is unforgiving of carelessness: repair facilities are few and far between, and medical rescues of careless drivers from the harsh, inaccessible areas of Africa north of say, Zambia, or Malawi, are expensive, time consuming, and may or may not happen, depending on the insurance company involved.
Electronic driving aids are meant to prevent mishaps during “normal” off-road driving conditions and recklessness and an over reliance on them spells disaster, just as surely as not having any form of electronic driving aids does.