The best equipped vehicle doesn't make you a capable off-roader. It is skill that counts, whatever vehicle you drive.
Most
of my off-roading is done at tick-over speeds and power gently applied where
necessary. The most innocent of inclines can be an obstacle and reading the
surface which you drive on (or are about to) is the single most important
skill to come to grips with. If nothing else, do read the Seven Golden Rules
Four wheel drive, or 4x4,
means that all four wheels on a vehicle are driven or at least, that is the
common assumption. In fact it isn’t quite as simple as that, and to understand
why, an understanding of how differentials work is important.
Every conventional car has
a differential on its driven axle. Without a diff you couldn’t take a bend
because it compensates for the fact that an outside wheel travels further
than an inside one under cornering.
It does this by relaying
engine torque, or turning power, to the wheels in the easiest way it can
find. This means that if one driven wheel is on a slippery surface like ice,
all the torque will be directed that way. The wheel will continue to spin
hopelessly, but there will be no power going to the other wheel - you’re
stuck!
When four wheel drive is
engaged, the front and rear axles of a vehicle are effectively linked through
the centre differential or viscous coupling. If the wheels on one of the
axles remain unaffected by the slippery surface, at least one of them will
be able to grip the road and keep the vehicle moving. So what 4x4 really
does is reduce the odds of losing all traction.
There is a further complication.
When cornering, front axles have further to travel than those at the rear.
This doesn’t matter in two wheel drive, but when both axles are driven, cornering
ability is compromised. The solution, applied on many Jeeps such as automatic
Cherokees and most Grand Cherokees, is to fit a differential between front
and rear axles. Just as differentials apportion engine torque to the most
receptive wheel, a centre differential directs it to the most receptive axle.
With a centre diff fitted it is possible to drive in 4x4 on dry roads, because the vehicle can corner normally. Jeep Wranglers and manual Cherokees don’t have a centre diff, so should be put into 4x4 on road only in slippery conditions, when maintaining traction is the primary consideration.
To overcome the problem of
all the engine torque being fed to a slipping wheel, or to one axle alone
by the centre diff, some type of locking device that can distribute torque
evenly between the axles would seem sensible. Lock the wheels together solidly
and you lose the whole point of having a diff, so the answer is a limited
slip diff or viscous coupling.
How
does a Jeep translate these principles into practice? It uses four types
of 4x4 transmission systems: Command-Trac, Selec-Trac, Quadra-Trac and Quadra-Trac
II, which is a developed version of Quadra-Trac.
1
Command-Trac is a part time system. It is used in Jeep Wrangler and manual
transmission Cherokee, as well as 97 model year Grand Cherokee Turbo diesel.
(Check your handbook if you aren’t sure which system your Jeep uses).
In
normal road conditions Command-Trac is a two wheel drive system, powering
the rear wheels only. A limited slip differential operates on the rear axle
(apart from Wrangler automatic and Cherokee 2.5 litre petrol).
On
a slippery surface you can "shift-on-the-fly" into high range 4x4 for added
grip. You can do this at up to 55mph, though you should be as cautious as
conditions dictate.
On
loose surfaces only, you can use low range 4x4. Select low range with the
gearbox in neutral, and your Jeep moving at no more than 3 mph. The low range
gears provide more than double the torque at the same engine speed you would
use in high range.
2
Selec-Trac is the system used on 4.0 litre Jeep Cherokee. It is an advanced
part time 4x4 system with front, centre and rear differentials. As with Command
Trac, the rear wheels alone can be powered in normal road conditions but
because Selec-Trac has a centre diff, high range 4x4 can be selected on any
road surface, slippery or dry, to boost the Jeep’s roadholding capabilities.
With
Selec-Trac you can choose to use high range 4x4 with the centre diff unlocked
(full time four wheel drive) or locked to give maximum grip on slippery roads
(part time four wheel drive).
Like
Command-Trac, Selec-Trac uses a transfer box to provide low range transfer
gears that more than double engine torque. With the centre diff locked, low
range is for off road use only or for use on snow and ice.
3
Quadra-Trac is the sophisticated permanent 4x4 system used on all Jeep Grand
Cherokees other than the ‘97 model year 2.5 litre TD. In normal use, a centre
viscous coupling unit feeds most of the engine torque to the rear wheels,
but if one of these starts to lose traction torque is fed to the front axle
to help pull the Jeep out of trouble.
Once again, a transfer box provides low ratio gears for serious off roading.
The
all-new Grand Cherokee uses an advanced development of Quadra-Trac called
Quadra-Trac II. Under normal driving conditions, most of the engine’s power
is delivered to the rear wheels.
The
moment a wheel loses traction, a speed variation occurs between the front
and rear axle causing a pump within the transfer box to apply hydraulic pressure
to a multi-disc clutch pack and send power to the front axle. The system
allows the vehicle to maintain traction and -control seamlessly and within
fractions of a second.
So
that’s how 4x4 works and how Jeep provides options for on road driving in
both normal conditions and for serious off road driving. You have a "go-anywhere"
vehicle, so it’s time to go off road. If it's your first time, don't go on
your own if there is a chance that you can not recover yourself!
Your
first off road experiences should be under carefully controlled conditions
and supervision, though you will soon take responsibility for yourself. However,
there are important preparations to make before you venture off tarmac -
not just the first time, but every time. Damage to your Jeep could be costly,
damage to you or your passengers far more so!
Off
roading is not to be undertaken lightly. Take a pride in being thorough with
your preparations. If this is your first time off-road, or if the vehicle
is unfamiliar to you, your first task is to get to know it.
Check:
That
you are familiar with the controls — how to shift between two wheel drive
(where appropriate), high range 4x4 and low range 4x4, and that you can slip
into reverse quickly if necessary.
That
you have a clear mental picture of the layout of the Jeep. Walk around it
and note its length, wheelbase, track and ground clearance, and the approach
and departure angles between the base of the wheels and the front bumpers.
You’ll appreciate knowing how much overhang the vehicle has when you’re nosing into a ditch later on!
That
you know where all the "dangly bits" are relative to the ground — axles,
exhaust, sump, gearbox, cross-members and so on. Are skidplates fitted? Wranglers
have them, Cherokees don’t, and Grand Cherokees have a tank guard. If not
standard, they might have been fitted as optional extras. If you should hear
a scraping noise later, it’s good to know whether it’s a protective metal
plate or a vital component!
That
towing points are securely fitted to the frame, both front and rear. Towing
hooks are best, two at the front and two at the back ideal.
That
you know where the air intake is situated under the bonnet. You must always
ensure that it takes in only air — not water or mud!
That
you know the extent of your insurance cover. Your insurer might be less than
sympathetic if you end up with an engine full of water!
Every time out
Don’t
take anything for granted. This applies if you are setting off across a wilderness
or, as is more likely, round a dedicated circuit or down a farm track.
Check:
- Oil, water, battery fluid and radiator hoses. Make sure the battery is secured.
- All tyres for tread depth and signs of damage. Don’t forget to check the spare.
- That you have a jack and wheelbrace, a suitable towrope with shackles and a first aid kit.
- That people know where you are going and when you expect to be back.
- That you have enough fuel. Consumption can be considerably higher off road.
The recce
Are
you ready to drive off? No, now you walk! Do what the professionals do and
cover the route on foot first to find the easiest way through, note obstacles
and make contingency plans. Assess which gears to use, where there are safe
run-offs if an attempt at a section fails — uphill, downhill and on sideslopes
—and how you would recover a stranded Jeep.
On
a well driven route, note previous tyre tracks, but remember that just because
a 4x4 has gone that way before, it doesn’t mean that you can. That vehicle
might have different dimensions, loading, tyres, engine and so on. Back at
your Jeep, make sure everything inside it is secured, including passengers,
and adjust your seat and mirrors.
Never
go off road alone. If yours is the only vehicle, make sure someone is in
yours with you, if only for moral support! Now you’re ready to go.
1 Play chess
Forward
thinking is the most golden of rules in off road driving. Never barge into
any situation. Get out of the vehicle, assess the next step and the combination
of gears and power needed to tackle it and make contingency plans in case
Plan A fails. If in any doubt, don’t do it at all!
2 Slow as possible, fast as necessary
Grip
and momentum — it’s your job to balance those two forces, so drive as slowly
as you can, consistent with maintaining steady forward progress. Remember
that it’s engine torque, not brute speed, that overcomes obstacles. Taking
your time allows you to select a clear route ahead, and might avoid damage
caused by colliding with an immovable object. Usually expect to stay in low
range and low gear, but you might need to take a run at a hill in a higher
low range gear or, perhaps on flat sand, need to keep moving in high range
to avoid bedding down.
3 Left foot down on the floor
When
you first learn to drive, you are warned against "riding the clutch." It’s
particularly important not to slip the clutch off road, especially when going
downhill under engine braking, because you could lose control.
4 Spare the brakes too
Control
your speed off road by using the braking power of engine compression. In
most cases lifting your foot off the accelerator will provide all the braking
you need, so treat the brake pedal as a last resort. This is especially important
on a descent. The type of Jeep does have a bearing on this it is easier to
control a diesel manual Jeep without using the brakes than it is a petrol
automatic. If you have to use the brakes, be light footed — dab, don’t stab’
Remember that you’re not trying to stop, but stop the vehicle from speeding up.
5 Thumbs up
Drive
with your thumbs held up and away from the steering wheel, in case you hit
an obstacle suddenly and put the wheel into a powerful spin. Thumbs have
been badly bruised or even broken by wheel spokes, on vehicles with and without
power steering. If the wheel does buck around in your hands, resist the temptation
to make constant corrections. And remember that it is acceptable to cross
your hands on the wheel off road if it helps you respond faster.
6 Read the ground
Get
in the habit of memorising the terrain you see approaching through the windscreen.
It takes practice, but eventually you’ll be taking mental snapshots and then
driving them as a matter of course. If you’re not sure of what’s ahead, stick
your head out of the side window — or get out and take a proper look.
7 Don’t go where you’re not wanted
Don’t
get off roading a bad name by driving across land not intended for the purpose.
Stick to areas where it is permitted. There are many purpose designed off
road courses involving different types of terrain, ideal for learning the
basics and for advanced driving too. If you do have permission to drive on
a farm or estate, respect the countryside and do your best to leave it as
you found it. "Tread lightly!" is the watchword.
Hill climbing
Recce, don’t wreck
It
might be a slog, but if you can, climb the hill on foot first. Study the
ground conditions. If it’s wet or muddy you can lose control and slide, while
sandy or loose, dry soil might put your wheels in a spin. Make sure there
is a safe run-back should your planned ascent fail — and while you’re up
at the top of the hill, plan the descent too.
Discretionary power
On
a dry hill you can crawl up in first gear, low range. On a slippery, grassy
hill you might want to take a run at it to maintain traction, using second
or even third gear low range, though mindful of the danger of obstructions
like large stones or stumps that could be hidden from view. Be ready to slow
as you go over the top.
Straight up
Reverse, don’t turn
If
you feel that the Jeep is not going to make a climb, don’t wait until the
last moment and risk coming to a stop with your wheels spinning. Apply the
clutch (with manual transmission) and brake together, select reverse, take
your feet off the pedals, hold the wheel at 12 o’clock and back straight
down. With automatic transmission, brake, select reverse and descend the
hill under control. Don’t try turning the vehicle round and risk a rollover.
If you did your homework, you will have room for a safe run-back down the
hill. If the vehicle should slide while reversing, accelerate gently into
the line of the skid.
Smooth starter
If
you stall on a hill in a Jeep the procedure is similar, but you need to move
fast if it is slippery. Apply clutch (if manual) and brake together, select
reverse, feet off the pedals, restart the engine and drive straight back
down. Try to do everything in one flowing movement! Later model manual Jeeps
have an interlock on the clutch which you must depress to start the engine.
Anchors away
Where
possible avoid using the brakes while reversing out of a failed climb if
the wheels lock, gently accelerate until you regain control. In some circumstances,
however, you might have to dab the brakes lightly if you are driving an automatic.
Know your limits
Don’t attempt a steep or difficult hill unless you feel confident. Practise on less challenging slopes first.
At the top
Stop at the top
As you crest a steep hill change down into first gear if necessary, ease off the
power and be ready to stop or you might find yourself airborne! Don let the
ground bring you to a halt - you must make the decision if you have reconnoitered
you will already know what lies ahead but you might need to make a fresh
assessment before going over!
Pivotal movement
This is where forward planning really pays off. Be acutely aware of your ground clearance and wheelbase if you are about to top a sharp ridge or peak. If the worst happens you could belly out and end up with the jeep balanced on its frame and the wheels in the air. If this happens, and you can't back out or dig yourself out a standard jack might be used to lift the vehicle; pack stones under the wheel first. You may need to call for a recovery vehicle or other assistance.
Downhill all the way!
Stay low
Going
down a Steep hill might look daunting but it is often simpler than going
up - if you do it right You should always be in first gear low range well
before you start descending and stay in that gear.
Time to unwind
Check that there is sufficient run-off at the bottom of a long hill especially if the surface is slippery. You don't want to plough straight into the next hill!
Cut the angle
As
when going uphill, you must descend in a straight line. Driving across the
face of the slope could result in a slide and possibly a roil-over.
Throttle only
Rely
on the engine to limit your speed – don’t use the brakes on a manual jeep
and never touch the clutch. Very light brake pressure might sometimes be
needed on an automatic which doesn’t benefit from compression braking to
the same extent- Rather than braking, gentle acceleration to maintain momentum
can be helpful downhill especially on a long, slippery descent. If you feel
the Jeep moving sideways don’t brake - accelerate gently and steer lightly
into the skid.
Rock steady
On
steep dry rocky descents - rarely encountered in the UK - the clutch must
be engaged and the brakes used gently, allowing the vehicle to creep forward.
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