Driving tips

for the ST-X Navara fitted with the five stage automatic transmission.

When driving off road we have to contend with ground contour and composition. By this I mean that it usually involves travelling uphill, downhill and over terrain with humps and hollows.

If we were to look at the three main forces that play a part in driving a vehicle in this environment, traction, momentum, and gravity we can see that both momentum and gravity can move the vehicle both in a controlled and uncontrolled way. Or put another way, either under the drivers guidance or in the case of a slide, uncontrolled.

Understanding traction is the key to safe off road travel. Traction can overcome gravity in the case of a climb or can be used in a controlled fashion to descend a hill holding the vehicle back from taking off (caused by gravitational forces).  

Momentum can be harnessed to cross ditches, or to overcome loss of traction on short climbs. If you use your imagination then, you can begin to work the relationship between the three forces to ensure you always have them working in your favour. Get the combination wrong and suffer the consequences!!!

So let’s explore traction in more depth. Not the whole picture but some basics.

For a tyre to gain traction it requires a downward force. This increases the friction between the ground and the tyre and allows the vehicle to move in a controlled fashion.

There are four wheels on a vehicle, and between these wheels there are differentials that allow the vehicle to turn corners without scuffing the ground excessively. By the very nature of the differential action a 4wd vehicle (with no modern traction aids) only has to skid one wheel on each axle to lose traction.

Now put this basic understanding of the vehicle into the environment. If we always drove surfaces as flat as bowling greens we would always maximize traction, (simply put) but introduce contour and you can see that as one wheel goes down into a low point on the ground it correspondingly raises the opposite wheel at the other end of the vehicle. This in turn leads to lower tractive forces being available to the two wheels with less down force (applied by spring pressure) and hence lower levels of traction. The wetter the surfaces the less down force variation is required to lose traction.

Contrary to popular belief the wheels do not have to be off the ground in an articulated situation to lose traction.

  So it goes that to maximize available traction one has to drive the contour trying to keep all wheels on as even a plane as possible. This basic concept works both going up and down hills. Remember going down a hill over uneven terrain can cause wheels entering lower sections of ground to lose (hold-back) traction and allow the vehicle to begin a forward slide. If this slide isn’t arrested as the wheels again reach even ground contour then the uncontrolled momentum (mentioned above) results in an uncontrolled slide.

What has all this to do with driving an automatic transmissioned vehicle off road? Everything really if we want to maximize vehicle control.

Let’s look at the basics of driving an auto on road. The law insists on the use of one foot for the two pedal controls. Brake and accelerator. I’m fine with this as who can think of a situation where you would want to go and stop at the same time?

On the road there is enough traction to utilize braking force alone to arrest vehicle speed and momentum, and only in severe situations do wheels completely lock up creating an uncontrolled slide. ABS braking usually ensures directional control so as to avoid a collision in this last scenario.

But off road in slippery conditions the rules change. How so? We know that while wheels are locked a sliding vehicle will follow the direction both momentum and gravity takes it. Why wouldn’t ABS prevent this lock-up? Well it does but only down to the shut-off threshold, usually around 5/6km. And remember excess speed introduces (uncontrolled? Momentum)

So in an off road situation the ultimate aim is to minimize the chances of wheels losing traction, sliding across the ground, and basically not taking the route the driver intended.

Let’s focus on descending hills. In a manual transmissioned vehicle with low range, first gear can be selected and so long as the driver chooses a route avoiding humps and hollows the vehicle will maintain a slow controlled decent with minimal input from the driver, while tractive forces are available. Note at this stage we haven’t introduced any braking.

Put an auto trans vehicle in the same situation and you will see that the holdback (engine braking) capability is not as strong. This means that the descent would be quicker. This in itself may not be an issue until we introduce bumps. As the suspension starts to react to these bumps the ground pressure under each wheel changes and this can lead to an early slide being induced.

Or we simply may need to turn half way down a descent and the increased speed could cause directional change issues (arresting and changing the direction of momentum)

So the goal then is to minimize any increase of speed while descending a hill. Why not simply utilize the brakes? The main issue with using the brakes in an auto is that a front wheel could lock up causing loss of directional control, and possibly leading to a longer slide.

If we were to utilize the brakes in a manual trans 4wd while not touching the clutch the engine would be forcing the wheels to turn up until the engine stalled, (and a modern common rail engine is damned hard to stall on idle under load). And by doing so would minimize the chances of brake lock-up. You with me so far?

So to descend a hill in an auto at the lowest practical speed it is desirable to provide drive to the wheels with light throttle while providing light brake force to arrest overall vehicle speed.

Now put the vehicle in uneven terrain and drive down hill. The wheels on the high points are having their speed arrested by traction and light brake force; the wheels in the low points would normally start to lock up under brake pressure but are forced to turn by the light throttle pressure.

If you were to put the vehicle into a failure to climb scenario and apply the same principals but in reverse gear your reverse descent speed and directional control is greatly enhanced.

I term this technique “overdriving” not “left foot braking” the latter being a mechanically unsound technique utilized to minimize wheel spin in mud thus sending traction to wheels still with traction. I can’t think of a better way to break CV joints on a modern 4wd!!!

“Overdriving” utilizes very light throttle activation and very moderate brake forces. Basically never put power into a drive system that hasn’t got a way to escape through the drive line.

If you can grasp the above concepts you are half way to maximizing off road travel in your ST-X.

Now this is fundamental stuff. Remember there are ABS brake forces and computer management programs to understand before total control is understood.

Some basic tips to consider.    Practice is everything. Always practice in a safe place with run-out areas. Practice in dry conditions where “feel” is available.