ABS
Introduction
ABS stands for
“anti-lock braking system,” or “advanced braking system,” depending on whom you
ask. It does, pretty much, what it says
in the title, but as we shall see, there are several different types of ABS
that have been, and currently are, available in the market. ABS was originally used in aerospace
applications - specifically, to reduce wear and tear on aircraft tyres after
landing, caused by flat spots. Now if
you consider the replacement cost of a single Boeing 747 tyre, then consider
how many tyres they have...
ABS is usually
bundled together with EBD, Electronic Brakeforce
Distribution.
What Does It Do?
In simple terms, ABS is designed to help
the driver maintain control during an emergency-braking manoeuvre. It does this by either preventing the wheels
from locking, or if they do lock, by releasing and then reapplying the brakes
once more. This is because when a tyre
is locked, it provides very little directional stability. On other words, you cannot steer with a
locked front wheel, and a rear wheel will tend to go wherever inertia sends it.
In effect, it
is a mechanical way of cadence
braking (except it may be either controlled by a mechanical or
electronic system). There are two
advantages of using ABS over non-ABS brakes for the typical driver. One is that the ABS system is able to “pump”
the brakes on and off much quicker than the driver’s leg, and the other is that
it requires no skill or experience - the car does all of that for you.
When you do an
emergency stop in a car, it is important to dump the clutch as soon as you hit
the brakes, rather than delaying it (as you were probably taught in a
non-ABS car that you took your driving test in). If you keep the clutch in, the ABS system has
to cope with both the forward momentum of the car and the engine at the same time.
ABS Helps You Stop?
In simplistic
terms the braking effort on the wheel when ABS is working varies from a locked
wheel (or a nearly locked wheel) to no brakes being applied at all. Even if the system goes from one extreme to
the other many times a second, the average braking effort will be less than
“full on.” Therefore, do not assume that ABS always
shortens stopping distances.
To go into
slightly more technical detail, the maximum possible braking effort is achieved
just as the wheel decelerates rapidly compared to the road speed, called the
retardation point. In other words, a
very slight skid. Most ABS systems
detect that a wheel is locking by detecting this rapid wheel deceleration (which
is in no way similar to a wheel under ordinary braking and so relatively easy
to detect). The system then releases
the brake for that wheel.
ABS systems
optimised for racing track performance are designed to release the brake just
enough to start the wheel accelerating once more before reapplying the
brake. They are able to keep the wheel
at the retardation point and so may maximise braking efficiency. The compromise with this form of ABS is that
the driver has almost no more steering control compared to a locked wheel –
these ABS systems are designed to stop the car in as short a possible distance
and so are of less use for the road.
Thus,
conventional ABS systems as fitted to road going cars do not stop the car any
shorter than non-ABS brakes. A skilled
driver will be able to pull up the non-ABS car in a shorter distance compared
to the driver using ABS. However, there
is almost no skill involved when performing an emergency stop using ABS. Any driver is able to stop as quickly as
possible whilst keeping the car pointing in their intended direction.
It is possible
to fool the ABS system if you are reckless enough, but you do have to be doing
something rather foolish and I’m certainly not going to tell you how to do it
here. J
Mechanical and Electronic Systems
As you would
expect, mechanical ABS use a “nuts and bolts” method of detecting when the
wheel has slowed down too much, and electronic systems use clever little
gizmos. Mechanical systems are less
sophisticated, and usually do not work at very slow speeds - one example is the
mark three Fiesta, which only works above 20
mph. They also tend to require more
space inside the car, which is a consideration for all cars, but more so the
smaller ones. Finally, mechanical systems
do not tend to work on all four wheels.
There may be a split front / rear system or both individual front wheels
have their own sensor and both rear wheels use the same one.
On the plus
size, mechanical systems tend to be more robust, and for high speed
applications where space is not so pressing (such as the aforementioned
Jumbo Jet), they are useful.
Electronic
systems do not require so much space and are much more sophisticated - they can
work at much lower speeds. Kermy’s
handbook reports that his ABS works at all speeds above 4 mph but it works
right down to just before you stop. All
current production cars sold in the
ABS
Kicks Back!
Different cars
do different things when the ABS kicks in, but in most cases, the brake pedal
kicks back. If you have never
experienced this, it is quite an unusual experience - and some drivers are
reputed to ease off the pedal at this.
With ABS in an emergency situation, this is precisely the wrong thing do
to (and one of the reasons why many manufacturers use Emergency Brake Assist). In some cars, the system makes a racket,
which can also be off-putting. The Ka’s
ABS sounds a bit like the brakes are grinding against gravel!
Over-Enthusiastic ABS?
I hammered those
brakes - note how the nose is rather low!
Some people
have complained that the ABS in their car seems a bit too keen to kick
in. These individuals are often used to
a car without ABS and are assuming that there is more grip available from the
tyres than they have. In most front
wheel drive cars it is very difficult to detect a rear wheel starting to lock
up until it turns into a proper skid, whereas the ABS spots this.
It is not
beyond the realms of possibility that ABS manufacturers deliberately set up
their ABS to be a little bit more enthusiastic than is strictly necessary,
however, if we ignore this at present the ABS is a warning that conditions
under the wheels are not as rosy as they may appear. Whilst some modern car designs do appear to
paint a vivid picture of what the road is like, in reality, they isolate the driver
from the road condition (ask any racing driver!).
We may
not feel the skid itself, just feel the ABS kicking in – and that’s probably a
good thing.
ABS and Winter
Conditions
Some cars had
an ABS override switch, several Audi models for example, whereby the driver
could manually turn the system off.
Why would you
want to do this? There are two reasons -
one being that an experienced driver might want to eke out a bit more braking
performance at a track day (see below), and the other being that you were
driving on a snow covered road (or indeed, on gravel driveways). This is because snow building up in front of
a locked wheel improves braking performance.
Of course, with ABS, this does not happen since the wheel is not locked
for long enough! For some (foolish)
drivers, the perceived dismal braking performance of ABS equipped cars can come
as a big surprise in snowy conditions!
This picture right illustrates what the Ka’s ABS does
(not just in winter, but it’s easier to see on snow). When the wheel locks, the system releases the
brake – each wheel is braked independently.
The darker patches show when the wheel locked. Note that the Ka “wiggled” during braking –
but it kept pointing in the right direction.
You’ll all see
some Dervy Footprints, but you can’t see where Dervy fell over – the flash
didn’t quite reach that far!
To watch
Kermit’s ABS in action on snow, go here.
Many people consider
ABS to be a handicap on a track day or for competition use because it makes the
braking inefficient (in other words, you take longer to slow down). For almost cars, this is quite accurate – the
ABS system is optimised to retain car control over ultimate braking power and
this is certainly true over all Fords I’ve driven including the Ka.
An anti-lock
system can be designed with competition use in mind – that is, for maximising
braking effort. Systems designed for
road-going cars are compromised in this respect.
The Ka’s ABS System – Under
Pressure
For the
purposes of writing this article, and for testing how resistant the Ka’s brakes
are to fade, I took Kermit down a steep section of road. Visualise a twisty two mile section of road. Now imagine it has an average gradient of 25%
and 
your objective is to go down this as
quickly, and as safely, as possible.
That means keeping your speed down so that you can stop in the distance
that you can see . . . and to make some of the tighter corners, of course!
This road
passes from the top of Sutton Bank, near Thirsk in
Hacking down
this hill, braking heavily on the approach to each corner, the Ka’s ABS wasn’t
doing anything – apart from the approach to the tightest corner, which happens
to be after the longest strait. We
approach at 60 in third. There is a
large bump on the approach to this corner and I’m anticipating the ABS to kick
in once we’re the far side of it. We
reach it under hard braking, the nose low, and as we go over, the ABS does its
thing. It’s stopped pulsing as we reach
the corner, snatch second and around we go.
Kermit’s
ABS on Standard Suspension
Kermit’s ABS is
a four channel electronic system. I
don’t make a habit of using it, actually before we had the Ford Racing Suspension fitted I can
count on one hand the number of occasions when I triggered it (away from me
deliberately triggering it on snow or ice).
With the
standard suspension set up the Ka’s nose drops quite a bit under firm
braking. Drive over a rough road
surface, a grill or a white line and if any wheel is going to trigger the ABS
under firm braking it will probably be at the rear. As the driver, one would very rarely feel
this happen but you’d hear the clicking noise as the ABS cycles the brakes.
There is
an exception to every rule. When we
first put the XR2i wheels on the kid
they were wearing Wynstar tyres. As I have alluded to elsewhere in this
website, in wet weather his behaviour was a little untoward. Under
just moderate braking the ABS would kick in on any wheel as it lost grip. This was one of the main reasons why I
decided to get rid of the Wynstars and replace with Falken ZE502s!
Kermit’s ABS on Ford
Racing Suspension
By the time we
put the Ford Racing Suspension on
Kermit, we also had the strut brace
fitted and the XR2i wheels were wearing
Falken ZE502 tyres. With these modifications and under ideal
conditions, the lad’s ability to handle acceleration (as in changes in
velocity, so acceleration, braking and cornering) were very much
improved. I was able to brake harder and
longer without triggering the ABS and when it did kick in, it would either be
the front or the rear wheel that needed the assistance rather than almost
always the rear wheel. Using the Ford
Racing suspension has effectively balanced out the braking effort with the
ability for that corner to decelerate the Ka.