Reader Question: How to control oversteer in a 4×4…
Nobody wants to skid backwards into a ditch… so, how do you control oversteer in a 4×4 with mud-caked tyres? REad on…
QUESTION: I’d like to relate an event that occurred whilst touring/tenting with my wife in our Prado 4WD, towing nothing. Hopefully this will generate feedback from readers as to the correct method of tackling this problem.
Setting the scene – I have experienced 45 years plus of driving and recently completed two 4WD courses, driving in sand and basic 4WDing. We were enjoying Dalhousie Springs, SA when rain began; after 18hrs we decided to make for the nearest bitumen at Kulgera on the Stuart Highway about 319kms away. Tracks were quite slippery in places so care was exercised in speed and steering. When we reached Mt Dare, a popular meeting place; with tracks leading to the Simpson Desert, the old Ghan route to Alice, West to Kulgera and where we had just come from, it was like a quagmire – mud-covered 4WDs are a common sight. The roadhouse was virtually out of food. Very slow leaving here 20kph onward into NT heading for Finke, all due care taken and no problems.
Now the last stretch, a wide straight decent quality track 147kms to the Stuart Highway. Conditions varied from slushy, like melted snow on a firm base, wet soft mud or bull dust, also on a firm base, and other stretches that were just damp with quite good drainage and traction allowing 70kph. Not long on this route the car suddenly lost traction on all wheels – foot off throttle and don’t touch the brakes. The car started to spin to the right, I over-corrected to the left and it responded by coming round to the left too far so I corrected right and it came back round and I regained traction facing forwards, taking about 5 secs start to finish. Driving continued on concentrated high alert also rationalising cause & effect. About 15 minutes later exactly the same thing happened again, recovery being the same as before. Both occurrences were on a straight section just under 70km/h, from memory with a delicate touch on throttle movements on no sharp steering motions
With about 75kms to go it happened yet again only this time on the third steering correction I still could not get it under control and it continued rotating anticlockwise until the car had turned 180 degrees and we were sliding backwards, slowing and drifting down the camber towards the edge of the track with no pedals being touched and the steering wheel seemingly disconnected from the wheels even though I was holding it with the wheels in straight ahead position.
The driver’s side was slowly approaching a long shallow narrow bank about 200mm high and 1.5M wide sloping down away after this – but thank goodness the car was really slow now. The two tyres contacted the bank simultaneously at less than 10km/h, the tyres, with two tonnes of car moving, albeit slow, were pushed off the outer wheel rim seal so the two tyres instantly deflated and the car dropped by 100mm, the rims dug into the bank stopping the car movement with remaining inertia inexorably lifting the Prado offside delicately resting the car on its driver side without even pushing the mirror in.
Neither my wife nor I were injured, exiting the car was interesting; we had it back on all four wheels within 15 minutes with very little damage but all that is the second part of the story.
I checked the wheels and the wheel arches were completely full of thick mud as were the treads – two new tyres on the front two half worn on the rear. I have thought about this incident at great length and included as much pertinent information as I can recollect from my diaries and hope that somebody can offer a reason why it happened and offer experienced advice on the best possible action I, or anyone else in this situation, could have taken. I did think of putting on the handbrake so the rear wheels would act like rudders to keep going straight. A 4WD tag along operator I spoke to, after some thought, said he would have “gunned it” which seemed a bit risky to me.
I look forward to comments.
ANSWER: First off well done to the writer for documenting what must have been a scary experience so we can all learn. Now onto the analysis.
It’s surprising the road wasn’t closed as it was clearly very wet, and even if it wasn’t chances are it would have been damaged by vehicles in the wet so should have been closed. Nobody should have been using it. But we don’t know the circumstances, so, instead let’s look at the skid situation.
There are two car control aspects to consider here, prevention and cure.
The story recounts three separate spins. If you spin once then it’s likely to happen again, and twice… well, a third time is pretty likely too. So, as conditions deteriorate, the first prevention point is to slow down. It’s that simple.
When you’re going slower there’s less chance of making a mistake, and mistakes are easier to recover. Also, a tyre only has a certain amount of grip. The faster you go, the more of that grip you need to keep the car going straight and to turn and, thus, the less there is to keep the car straight – see what I mean?.
When conditions become slippery you need the maximum amount of sideways (lateral) grip. Slowing down also means the vehicle bounces less, so the wheels are in greater contact with the ground. And at slow speeds the vehicle can be steered around particularly poor patches of traction.
Also, the more you accelerate or brake the less lateral grip is available because you’re using some of that grip for brake/acceleration, so the next point is to keep the speed constant, not just slow.
Reducing tyre pressures would also be a good move. This would increase the tyre’s contact patch, and therefore increase grip in that low-traction situation. It would also decrease rolling resistance as clearly the terrain is soft, and that means there’s less need for power and less chance of wheel spin.
It is also critical to keep the wheels rotating, as rotating wheels can keep mud from clogging the tyre tread, offer some lateral grip and the front wheels can be used to steer. Locked wheels mean you’re now in a toboggan and control is lost. To keep the wheels rotating be gentle on the controls, and use gears to slow down. Lower gears also mean speed can be adjusted by fine movements on the throttle.
Then there’s vehicle setup. The Prado is a constant-4WD vehicle, which is a good start. Any part-time 4WD vehicle such as Patrols or most utes should be in 4WD. And if fitted stability control (ESC) would actually be good to enable in this situation as it would keep the vehicle on track, traction permitting. However, that advice depends on the calibration of the ESC – if it is very sensitive it will not permit any slip and slow things down too much. It would be a case of enable-and-see, but I certainly wouldn’t discount it.
Now to lock the centre diff or not? Again, vehicle dependent. If there is a purely open diff such as that in the 80 Series, Defender or similar I’d lock it so you don’t have a problem of the power being sent to a single wheel. If, on the other hand the diff is more intelligent, and let’s say it is the Prado which has a Torsen centre, then I’d leave it unlocked as long as you’ve moving reasonably fast (20km/h+) as locking the centre diff promotes understeer which is what you don’t want. Still, that’s somewhat vehicle dependent so if in doubt, lock it up. Certainly lock it for slower work.
Now we come to the part at which the vehicle has begun to spin – the back overtakes the front, an oversteer situation, and despite all your prevention it’s happened and time for the cure.
Clearly, the tyres have lost traction, otherwise this rear-end rotation wouldn’t be happening. The loss of traction is simple – the grip ask has exceeded the grip available. So if you add power, you’ll need even more grip… yet the tyres are already way over their limit, hence the slide. The result will be even more of a spin.
But what about the front wheels, being a 4WD? Could they “pull you straight”? Well, clearly the fronts still have some traction relative to the rear otherwise the car would be understeering, ploughing straight on. However, if we now ask the front wheels to accelerate chances are we’ll ask for more grip than is available and they’ll spin too. So we’ll have all four wheels spinning, losing traction, and we’ll spin even quicker. Ideally, you’d want to have only the front wheels driving and the rear freewheeling and then there would be an element of “pulling straight”, which is one reason why front wheel drive cars are inherently safer and easier to control than rear drive. But this is a 4WD, and you don’t want to make the rear grip situation any worse than it already is.
The “power out” approach perhaps has its roots in roadcar racing. If you have a lightweight high-performance car with a responsive, powerful engine you can shift weight by throttle control – as you delicately balance on the limit of traction. However, here we’re talking about a heavy 4WD with a much less powerful engine, chances are it’s a slow-responding diesel and it isn’t balancing on the limit of traction, it’s well past. So the weight-shift-by-throttle argument isn’t appropriate. And if you watch a racecar driver recover a car heading into a spin they don’t “gun it”. When you see rally cars and the like powering out of corners you’re seeing them initiate and maintain a slide by power. They are not, repeat not, recovering from an unintentional slide by means of power. If a car is over-rotated it has to be caught by gentle throttle and opposite lock, then you power out.
So how do you recover from a rear-end skid? All at the same time:
- Look where you want the car to go, and look well ahead.
- As the back end steps out, turn the steering wheel to keep the wheels pointed where you want to go. Do this quickly and positively, don’t assume the car will correct.
- Maintain the gear you’re in.
- Slightly reduce, or maintain throttle. This leaves as much of the tyre’s grip as possible for lateral grip as you aren’t using any grip for accelerating for braking. A slight reduction slows the car down, and less speed is good in skid recovery. But not a quick deceleration which destabilises the vehicle. While a weight shift to the rear under acceleration is unlikely with a heavy 4WD at speed, the brakes are powerful and you can unweight the rear end sufficiently to exaggerate a skid.
- So now you’ve corrected with the steering wheel, and if you’re lucky and quick the rotation will stop. Get ready for the next part.
- The car’s back-end will then swing back in line. This is where you need to be very, very quick with the counter-steer and accurately return the steering wheel to centre, or sometimes a bit beyond. Again, don’t accelerate or brake.
That’s it. More on skid recovery here.
No powering out required, and certainly no slamming on the brakes. Less dramatic than the “gun it” approach, but more effective.
If you want to prove this for yourself just find a skidpan and a rear- or all-wheel drive car, drive at perhaps 30km/h, turn, apply the power sharply and the back end will come out. At that point try “gunning it” and see what happens. Then try the technique described above and see what happens. You can also try applying the handbrake and braking.
The tricky part of this car control technique is steering – firstly turning the steering wheel quickly, and secondly turning it accurately. Most people can get the wheel turned pretty quick, but lose track of rotations and cannot return it to centre accurately, ending up with what’s known as “spaghetti arms”. There are several techniques which can be used, and one of the least effective is the classic shuffle steer.
The best method I’ve found is to grasp the wheel at quarter to three and hold the hands there until the wheel is turned 90 degrees – then keep the hand at the bottom (6 o’clock) stationary, letting the wheel slide through it as you turn with the other hand. When the wheel reaches around 120 degrees take the hand at the bottom and use it to grasp the other steering wheel spoke which is now around the 2 o’clock mark, and pull down. This takes some getting used to, but if it’s good enough for the BMW driver trainers it’s good enough for me and I’ve found it to be very effective for quickly and accurately turning a steering wheel. Remember, in a skid situation the steering wheel will not self-centre, and even if it does it won’t do so anywhere near quickly enough. All this is explained in our steering article.
What about the role of ABS in all this? Not relevant, as the brakes are never used in the technique described above! But if you needed to slow down, you’d do so by applying the brakes as hard as you could, steering to keep straight and changing down at the same time.
Finally, getting to the last part of your question. The car is sliding backwards into a ditch. What can you do? The easiest thing is to lock the brakes and hope, as locked wheels will slide but also brake better in slippery conditions than rotating wheels. The “gun it” approach might work…or might land you in worse trouble, depending mostly on the conditions and your ability to control a car from slewing sideways. Like most such situations, by the time you’re heading for disaster the it’s too late, which is why I’ve concentrated on the factors leading up to this situation.
As for the parkbrake – if the centre diff was locked then pulling the parkbrake on would lock all four wheels.