Clutch
control
One of the most difficult challenges of learning to drive is
the correct operation of the clutch, which we know as clutch control. Many
learners opt to take an automatic driving test to alleviate the stress of
having to operate the clutch.
A significant problem with many learner drivers is lack of
preparation by not looking at what is happening in front. For example if you
have stopped in traffic, a learner will often wait till the car directly in
front starts to move before they think about operating the clutch. Ideally they
need to be looking further ahead for signs that the traffic has started to move
and to get they prepared to move.
How
does a clutch work?
To gain a better understanding of clutch control, you need a
basic understanding of how a clutch works. A car engine, even on tick-over is
constantly spinning. A clutch separates the car wheels from the engine. If the
car wheels where permanently linked to the engine, it wouldn't be possible to
stop the car without the engine stalling. Without a clutch, it would also be
very difficult for the car to move off from a stationary position. A clutch
also enables you to change gear.
Engine A In the diagram provided, we can see the two plates
that represent the clutch. Engine A has the clutch pedal depressed to the
floor, which in turn separates the clutch friction plates. When the clutch
plates are separated, it disconnects the car engine from the wheels, which also
prevents any power to the wheels.
One of the clutch plates is still spinning along with the
engine, as the engine is constantly spinning. A gear is then selected.
Engine B Now that the car is in-gear, the clutch pedal needs
to be raised, which in turn brings the clutch plates together. When moving off
from a stationary position using 1st gear, lifting the clutch pedal slowly
allows the clutch plates to come together slowly. This allows the plates to
slip and allow the car to move off smoothly until they are tightly pressed
together, joining the broken link between the wheels and the engine.
Clutch control is a technique that allows a driver to
control a cars speed. Learning clutch control is essential for passing the
driving test as you may be required to demonstrate hill starts to the driving
examiner, plus the driving manoeuvres require a high level of clutch control by
keeping the car incredibly slow.
Learning clutch control for the first time should be done on
a flat, even and quiet road.
Ensure the car is setup correctly so that you can depress
the clutch fully without overstretching, you are wearing your seat belt, the
engine is turned on and the handbrake is OFF.
1.
Depress the clutch fully and select first gear.
2.
Provide the engine with a little extra power by
very gently pressing the accelerator so that the rev counter reads around 1500
rpm.
3.
Ensure before your car moves that it is safe to
do so and now very slowly, raise the clutch.
When the clutch plates just start to meet, this is called
the clutch bite point. You will know it's the bite point at the car will slowly
start to move. The clutch bite point can occur at any point during the clutches
working travel as you lift it, as all clutches are different. The bite point
however will always be in the same place for that particular clutch. The key
here is with plenty of practice is to remember where the bite point is.
Now that we have found the bite point, the car is slowly
starting to move forward, now fully depress the clutch and gently brake to a
stop. Keep repeating the process by giving the engine a little power and very
slowly lifting the clutch till the car starts to move and fully depressing the
clutch. By repeating this, you will gain an understanding where the bite point
occurs.
Once you have got the hang of that, try it again but instead
of fully depressing the clutch once the car starts to move, press the clutch
just a small amount; try and think the thickness of a one pound coin. By doing
this you are slightly releasing the clutch plates, disengaging the wheels and
letting the car slow down. As soon as the car has slowed slightly, lift the
clutch around the thickness of a one pound coin again to move the car again.
Essentially what you are trying to achieve is to keep the car moving but at the
slowest possible speed. This technique is called slipping the clutch and is
required to perform the driving manoeuvres on lessons and the driving test.
Clutch
control techniques
More advanced clutch control techniques will need to be
mastered for holding the car steady on the clutch bite point. These techniques
are important for safely moving a car off on a hill.
Clutch
control on a hill
A driving test may involve the examiner requesting you stop
and move off again on a hill. This depends on the location of your driving test
centre as certain parts of the UK are very flat.
If you stop on a hill, the process is similar to above,
except you will require the use of the handbrake. Once stopped:
1.
Apply the handbrake and select first gear.
2.
Provide the engine with a little extra power,
again around 1500 rpm on the rev counter.
3.
Gently raise the clutch until you reach the bite
point. You will know when you have the bite point as the car may creak a little
or the bonnet may rise slightly as the car tries to move forward but cannot due
to the handbrake being applied.
4.
Provided you have the clutch bite point, the car
will not roll backwards when you release the handbrake.
Clutch
control tips
Practice learning clutch control until you are confident you
can move the car off in a reasonable time and without stalling.
Slipping
the clutch
Slipping the clutch is referred to when the clutch is
lowered and raised in the bite point area to keep the car moving slowly.
Slipping the clutch is used when moving very slowly in traffic or when
performing driving manoeuvres such as the turn in the road.
Certain drivers may leave the car in gear and slip the
clutch whilst stationary in traffic. This practice significantly increases wear
on the clutch plates. Replacement clutches are expensive so if stationary in
traffic either fully depress the clutch and apply the handbrake or if for a
long period, place the gear lever into neutral and release the clutch.
The clutch must be raised slowly to allow the two plates to
join slowly else the static plate connecting to the rotating plate will make
the engine stall or wheelspin will occur. This form of clutch control is called
slipping the clutch.
Riding
the clutch
Whilst driving at a steady speed on a clear road, ideally
the driver should remove their foot from resting on the clutch and place it on
the foot well floor or a dead pedal (place to rest your foot) if the vehicle
has one. If a driver rests the foot on the clutch whilst driving, this is
referred to riding the clutch.
Clutches contain a clutch release bearing which applies
pressure to the pressure plates to aid releasing the clutch plates. Resting
your foot on the clutch can cause wear on the release bearing and too much
pressure can cause the plates themselves to slip, causing significant wear to
the clutch plate friction material.
Do
automatic cars have a clutch?
Automatic cars do not have clutches and instead use a device
called a torque converter. A torque converter is a circular object that rotates
with the engine and uses oil to drive the transmission which turns the wheels.
As the engine spins faster more oil is released at higher pressures into the
torque converter. Centrifugal force drives the oil onto fins, which provides
the energy needed to the transmission.
Is
slipping the clutch bad?
Slipping the clutch is bad in the sense that it wears the
clutch plate out. Slipping the clutch is what it's designed to do however and
is part of the process of driving. Slipping the clutch is essential when for
instance moving in extremely slow moving traffic where the car needs to move
slower than the actual natural revolutions of the engine and where you would
need to park the car in a difficult location and very slow speeds are needed.
Cases such as this would require partially disengaging the clutch plates to
reduce speed and gain accuracy. Riding the clutch is different however as it is
the driver unnecessarily wearing the clutch out.
Part of the first driving lesson involves providing a
learner driver with an explanation of what the clutch biting point is and also
how to physically find it. The bite point is usually around half way through
the clutches working travel (around mid-way from fully pressed to fully release).
The does however vary a little from car to car.
An experienced driver will have little need to use the bite
point except for very slow moving traffic and moving off on a hill. A learner
driver may find that using the bite point in general driving helps to reduce
stalling.
How to
find the biting point
Practice on a quiet, flat road, ensure the handbrake is on,
fully depress the clutch pedal and select first gear. You will need to provide
some power to the engine by gently pressing the accelerator to around the width
of a £1 coin. The bite point revs should be roughly 15000 rpm on the rev
counter. Try and remain at roughly 15000 rpm and very slowly raise the clutch.
You may notice that if you accelerate fast in a low gear,
the engine revs high but little is made in terms of actual acceleration. You
may also notice a strange burning smell coming from the engine. This could be
the clutch plate slipping and producing significant heat to which will result
in burning.
Testing
a car clutch
You can test if your clutch is worn or damaged. Turn on your
vehicle and select 3rd gear. The handbrake can be either released or applied,
it doesn't matter. Gently release the clutch and the engine will stall if the
clutch is in good working order.
If the clutch is damaged or worn to the point it is
slipping, the engine will not stall and continue to rev.
Biting
point meaning
As a simple explanation, if you imagine the clutch as two
circular plates, one of the plates is connected to the drive wheels and the
other to the engine.
When the driver places their foot and depresses the clutch
pedal, the plates separate and so disengage the engine from the wheels allowing
the car to stop or change gear. When the pedal is released, the plates are
forced together, connecting the engine to the drive wheels.
Listening to the sound of the engine, as the bite point
starts to engage, you will hear the engine tone lower. This is because the
clutch plates are just starting to join and connect the engine to the wheels
and the engine is trying to drive the car forward.
The car will not move as the hand brake is on, so as soon as
you hear this change in engine tone, hold the clutch in this position. If the
car creaks or groans too much, slightly press the clutch. You have now found
the car biting point.
Benefits
of using the bite point
Finding the bite point before moving the car off from a
stationary position considerably reduces the chances of stalling. Many drivers
prone to stalling, especially learner drivers have a greatly exaggerated fear
of stalling the car, most of which is alleviated by finding the biting point.
Further information can be found in How to stop stalling a car,
Source: drivingtips.biz
