Grass Driving

4x4 Africa - grass drivingDriving on wet and long grass

The next lecture was on the subject of driving on grass, something Bud was particularly interested in because of the fact that the traction control system of his luxury SUV had a dedicated setting for driving on grass. Of course, he had no idea why this was, nor had he ever driven on grass, but he was confident the Instructor would explain the matter. Therefore, he turned to a new page in his notebook, and mentally sharpened his pencil to take notes.
“OK, for those among you that have never driven over and/or through grass of whatever length, remember this: driving on grass is more than a little tricky because grass is slippery, and it can set fire to your 4×4. However, there are relatively few places in the world where you can drive on grass without breaking the law: one is in the Australian Outback, which is covered in spinifex grass; another is in the Masai Mara Game Reserve in Tanzania, where the tourist infrastructure is very poorly developed, and maybe in some private game parks or reserves in other parts of Africa. Most, if not all, hunting farms and game reserves in Southern Africa prohibit the use of private vehicles on their property except in getting to hunting camps and camp sites, and then only via established roads and tracks.”

How to drive on grass

driving-on-grass“In Africa, it does not often happen that one has to deal with short, slippery grass obstacles, but where and when it does happen, the best way to deal with it is to maintain just enough momentum at the lowest possible speed to get across the obstacle. Short grass, especially when it is wet, can be extremely slippery, so, avoid any sudden control inputs. Some new 4WD vehicles have dedicated settings on their traction control systems to deal with short, wet grass. These settings change the way the traction control and other driver assist systems work; for instance, by preventing large or sudden throttle inputs that could cause wheel spin.”

“You should not be able to get wheel spin with the traction control engaged, but wet grass is so slippery that a sudden throttle input could cause all the driving wheels to start spinning, which is something traction control systems cannot deal with. However, if you do not have such a dedicated setting, use it in high range and 4WD mode in a low gear to keep your speed down, because the stability control system might not be able to correct dangerous slides because it relies on traction to work, which you do not have much of on wet grass. It should not be necessary to use low range on level surfaces but if you do have to use it, remember that you will not give you more traction: all you will get is the benefit of the front wheels helping you to make the best use of the available traction.”

Grass Stopping

“Grass, regardless of whether it is wet or dry, will also significantly reduce your ability to stop: your ABS will behave in much the same way it does on sand- which is badly. ABS often does not work in low range 4WD mode anyway: but where it does work on the rear axle, remember that on grass you cannot lock the wheels to let them dig in as you can on sand. Therefore, just keep your momentum low enough to cross the obstacle but high enough to get over any uneven patches such as small inclines, without having to apply large amounts of torque because you do not have enough momentum. Suddenly applying lots of power will only cause wheel spin and loss of traction.”
“If you have to stop on short, wet grass, the best way of doing it is just to let go of all the pedals: letting up on the accelerator will generate some engine braking force, so just let the 4WD coast to a stop by itself. If you actively apply the brakes, you will lock the wheels, you will lose all traction, and depending on whether you are on a level surface or a slope, you might end up in an uncontrollable slide, which the stability control system might be unable to correct because you do not have enough traction for the brakes to force the 4WD back into line.”wet-and-tall-grass-driving and stopping
“Much the same thing applies if you are going downhill on grass, wet or dry: hill descent control systems might work well on reasonably solid surfaces that will allow the brakes to control your speed, but the limited traction on grass might cause the wheels to lock, which will cause the ABS to release the brakes on the affected wheels. If this happens in quick succession, you will be in the same situation as trying to stop on sand with the ABS in operation: you will end up with no effective braking force, and thus no way to control your speed effectively.”
“So, what you do is this: you do not use the HDC system. Instead, you use engine braking in 4WD mode in a high gear, but this depends on how slippery the slope is. If it is very slippery, engine braking might cause the rear wheels to lock up, which could force the back end into a slide but by using 4WD, chances are the added traction of the front wheels might be enough to prevent the wheels from locking up. This configuration will also allow the front wheels to rotate, which is what you need to be able to maintain directional control.”

No grass rules

wet-grass-driving“However, these are not hard and fast rules: wet grass is one of the most slippery surfaces you will ever have to deal with, so treat these comments as guidelines only. No one can tell you what to do in all possible conditions, but bear in mind that the final responsibility of getting down a slippery slope rests solely with you, the driver. No two grass slopes are the same, which means that you, the driver, have the responsibility to assess the situation accurately before you start a descent. However, if your gut feeling says you might not be able to cope with a given situation, do not attempt it.”
“There is no shame in finding an alternative route: as I said before, grass is one of the most difficult surfaces to cope with, so if you feel uncomfortable dealing with it, just do not do it. Rather be safe; spend as much time and fuel as you have to in finding another way down the hill, and arrive at your destination in one piece.”

Watch out for the grass…

“However, there is another type of grass obstacle- long grass. In Australia, you have spinifex- tall, hard grass with spiky seeds that can clog a radiator. To deal with this you need to have a screen of some sort over the grill to keep the seedpods away from the radiator, which can become so clogged that no airflow through the core is possible. If this happens, you are seconds away from overheating the engine, which could have fatal consequences for you and your passengers if you are not rescued in very short order.”
“But that is only one issue: the other is that the spinifex stalks could -and do- wind themselves around the prop shafts, and if you do not regularly check for this, the wound-up stalks can rip oil seals from the differentials and/or the transmission. This is just as bad as overheating the engine: if you lose your gear oil, you are stranded, which is no joke in the Outback. Without enough water, you could be dead in three days or less, so if you are ever in the Australian Outback, check your prop shafts every few minutes, and remove all the spinifex stalks you find, every time.”

grass-4x4-driving“But there is still another problem: Africa may not have spinifex grass, but just like in Australia, tall grass can become entangled in the hot exhaust system. This is not such a big problem on older diesels whose exhaust systems do not get nearly as hot as that on a petrol engine, but most new diesels have DPF’s, or Diesel Particulate Filters, which is a device that collects all the soot in the exhaust gas, where it is stored until sensors in the DPF decide that is clogged, and needs to be burned clean. In most cases, this happens through raw fuel that is injected into the filter and ignited. This cleaning process can heat the DPF to the point where it glows red from the heat, and if you had any grass entangled in the exhaust system, you will now have a burning 4WD, which chances are you will not be able to put out with your fire extinguisher, since the fire will be well established by the time you get your extinguisher to bear on it.”

“Many off-road vehicles have been destroyed in this way, in both Africa and Australia, and everywhere else in the world where off-road vehicles are driven through dry, long grass. The problem with DPF’s is that this cleaning process is automatic and autonomous: the driver has no control over it, which means that while you are driving through long grass, you could catch fire at any moment or start a fire that could potentially destroy thousands of acres of grass and brush, and although most types of grass and brush needs to burn to rejuvenate, they need to burn at very specific times of the year.”

“If you are going to be driving through long grass in the dry season of wherever you are going, have your entire exhaust system wrapped in some sort of fire-proof material to create a heat shield around the hottest parts of the system, which is the part closest to the exhaust manifold and the area around the DPF. Exhaust bandage works well but you need a lot of it, but never mind the cost; off-roading is expensive as it is, but if the relatively small expenditure of having your exhaust fire-proofed can prevent damage to property, the effort will have been well worth the expense.”

“However, DPF can be removed, but with possible negative consequences for your warranty, therefore, if you do not want it removed, do not drive through tall grass, ever. You cannot control its self-cleaning process- which means that the only options available to you is to either fire-proof and keep it, or completely avoid situations where you have to drive through long grass, and that goes for the long grass that grows in the raised middle of two-wheel tracks as well. However, given the fact that much of the road system in Africa consists of two-wheel tracks with tall grass growing in the middle, you would do well to have the DPF on your new 4WD removed.”

Grass driving in Africa

africa-grass-and-scrubBy this time, Bud was convinced that off-road driving in Southern Africa or anywhere in Africa was not for the faint hearted, but just then lunch was announced- a welcome break during which Bud called his dealer to make an appointment to have the diesel particulate filter on his new 4WD removed. There was no way he was going to catch fire in the middle of the savannah because of something he had no control over, and although he might be penalized on his warranty, he felt he could live with that, because he was sure his new 4WD was not going to break down any time soon. However, if it did break down while he was in the bush, he was sure he could fix it since he was enrolled to do the bush mechanics course as well.

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