Vehicle Brakes – Part–Four

ABS The Limitations & The Advantages

What do you know about ABS? How do you use ABS properly? Have you ever experienced ABS when its been working? What do the letters A–B–S actually stand for?

Okay, let’s start with that name, ABS. It stands for Antilock Braking System, and what that means is that no matter how hard a driver may stamp on the brake pedal, the road wheels are never supposed lock into a skid, which for most driving conditions is true.

So How Does It Help, Not Locking The Wheels?

Well, for a start, you need to understand that each of your vehicle tyres has one allocation of grip, and let us say that overall it is 100%. What you also need to understand is that whatever the demand you place upon that tyre you are going to use up some of that grip–allocation. Whatever is left will be the total amount available to use for other tasks.

Consider this. For cornering purposes, if you are using 80% of the grip–allocation to get your vehicle around a bend, you will only have 20% left for braking – or any other task for that matter.

Taking the above into account, and when driving a car not equipped with ABS, if you brake so hard, or so harshly, as to lock your wheels into a skid, you have none of your grip allocation left with which to change direction, because the skidding tyre has used it all up. Simply you have nowt left for steering. And that’s what ABS is all about.

ABS V’s Raw Skill

ABS was introduced, primarily, to cover up for a lack of driver skills, and this was (and still is) necessary as almost every driver, when faced with an emergency that required both urgent loss of speed and change in direction, would crash into whatever it was they were trying to avoid. This is because the instinctive reaction for nearly all will always be to try and press the foot through the floor whilst vainly heaving on the steering wheel with all their might to avoid impact. All to no avail.

With ABS there is no locking of wheels
and therefore some directional control is still available.

So, the primary benefit of ABS, for normal road surfaces, is to increase the ability of the driver to maintain control of the vehicle, rather than go into a skid. Despite this, loss of control still remains likely on loose surfaces, like gravel, or slippery surfaces like snow or ice. On a very slippery surface, such as sheet ice, poor wet road surface, it is possible to lock multiple wheels at once, and this can defeat ABS, which relies on detecting individual wheels locking.

Threshold Braking

This is a term that some readers may well be aware of, and describes the technique where a driver finely calibrates the amount of brake pedal pressure to apply, so as to maintain a degree of braking effect at the wheels that is but a hair’s breadth inside the point of wheel lock–up. Thhis is finding the threshold between a rotating wheel under braking and a locked wheel under braking.

It is well reported that Lewis Hamilton, current Formula One world champion, has a particular skill for knowing where the braking threshold of his car will be at any time and in any conditions, and this has been one of the elements leading to his success. He is well in tune with his car and is able to work with it well.

However, the availability of the ABS of a vehicle should not deter drivers from learning to master threshold braking, as driver will tune to machine more accurately this way. As already noted above, the maximum braking effect is achieved when the tyres are at the limit of friction. ABS works by releasing the brakes as the tyres break traction, so a skilled driver, who is in tune with their car, should be able to exceed the braking performance of an ABS system under most circumstances.

ABS – The Pedal Will Pulse & Vibrate

When activated, the ABS causes the brake pedal to pulse or vibrate, which is very noticeable, and this is the partial release and reapplication of braking effect at the wheels many times per second. As most drivers rarely or never brake hard enough to cause the system to activate, and a significant number rarely bother to read the manual of the vehicle they drive, the effects may not be discovered until a real emergency is encountered.

It is usually during an emergency when a driver will encounter the pulsing of the pedal for the first time. Many will react by reducing pedal pressure and thus lengthening braking distances, contributing to a higher level of road collisions than the stopping capabilities of ABS would otherwise promise. In short, many driver’s first time experience of the rumbling pedal is quite alarming.

The recommended technique for non–expert drivers, when in a typical full–braking emergency and using an Antilock Braking System, is to apply maximum brake pedal pressure and maintain it, whilst steering to avoid collision as necessary. This is the one for all solution to most situations, but to do this effectively training will be required.

Don’t wait for a real emergency before you experience the sensation through the pedal, but try it out in controlled circumstances first, and then come the time you have to use it, the sensation won’t come as such a shock. Perhaps in an empty car park, on a quiet country lane, on some private land, but only do it at very low speed, and when there is no one else around to be caused alarm.

Shorter Braking Distances

As already stated, driving on high–traction surfaces, such as bitumen or concrete, whether wet or dry, a non ABS–equipped car, depending upon who is the driver, will be able to attain braking distances better (i.e. shorter) than those that would be easily possible with the benefit of ABS.

An alert and skilled driver without ABS should be able, through the use of techniques like threshold braking, to match or improve on the performance of a typical driver with an ABS–equipped vehicle. However, for a majority of drivers, in most conditions and in typical states of alertness, ABS should at least reduce their chances of a pile up, or reduce the severity of impact if nothing else. At least that’s the theory.

Target Fixation

Target fixation is where a driver, fearing he or she will hit something, actually ends up locked on to it, and as a consequence, WILL hit it! It’s an ‘old brain’ function. I remember Julian of Ride Drive, during a discussion on this topic, told me how he, when in his previous role as a traffic patrol police officer, had actually seen this happen for real on the public roads and how he had investigated collisions leading up to which a driver had ‘chased’ the victim across the road to the point of impact.

Firstly, be aware of Target Fixation, as the only way you will overcome this is through training. I experienced this myself with a group of others during a training exercise. Initially there was a 100% failure rate, and I had excellent skid control experience already!

Secondly, ABS should have significantly reduced the chances of a skid and subsequent loss of control and yet all of us crashed into what we were trying to avoid. However, to master the extra steering input required to turn the vehicle under ABS, again there was 100% failure rate within the group.

ABS Can Increase Braking Distances

However, on gravel and in deep snow, ABS tends to increase braking distances, not reduce them, but why? On these surfaces, locked wheels dig into the gravel or snow, building up a wedge of material ahead of the tyre, which helps to slow the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock, which is better, but not ideal.

Some manufacturers have therefore implemented something called Brake Assist systems that determine if the driver is attempting a panic stop and will maintain braking force in this situation. Nevertheless, ABS can significantly improve safety and control for drivers in on–road situations if they know not to release the brakes when they feel the pulsing pedal.

Vehicles equipped with braking systems without ABS have been shown to out brake vehicles equipped with ABS, and the British car magazines Evo and Autocar both conduct periodic tests of vehicles comparing their ability to accelerate from a standing start to 100mph, and then brake to a stop. This is the so–called 0–100–0 test. They repeatedly find that the cars without ABS will outperform comparable cars equipped with ABS.

The biggest advantage that ABS has over a non–antilocking system is the modern ABS systems will compensate for each of the four tyres being on four different surfaces. In this scenario the system will monitor and apply different degrees of braking pressure to each wheel according to the level of tyre adhesion available. This is something a human controlled non–antilock system can’t achieve, as with the non–ABS vehicle, the maximum overall braking effect will always be limited by the one tyre out of four that is experiencing the lowest level of grip.

So there you have it. ABS is great for many situations, but not the answer to all. And, to get the best from it you need some training. Otherwise it just remains a Marketing Point in a glossy sales brochure.

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Part–1 Effective Braking Techniques
Part–2 How a Car Braking System Works
Part–3 Vehicle Stopping Performance
Part–4 Effective Braking Techniques

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This page was last updated
Thursday, 27-Jan-2011

Vehicle Brakes – Part–Four