ABS-Anti Lock Breaking System

Disc brakes do have a tendency to get locked due to higher efficiency level of braking of the system. While in motion when the disc gets locked, the wheel starts to skid on the road disbalancing the rider. Thus tangential resultant forces comes into effect on the whole motorcycle frame. Ultimately if this tangential force is not stopped in time, it forces the whole motorcycle to slide sidewise causing the riders to loose complete control of the vehicle & fall and may cause a major accident.

ABS on the other hand acts in such a way so as to stop the wheel but not completely locking & stopping the wheel. During a braking action the rider/s moves forward, thus moving the vehicle centre of gravity towards the front end of the vehicle. Thus stopping of the vehicle with the help of front wheel is more effective than stopping of the rear wheel/s.

Considering the vehicle is running at 90 Degrees angle with the road while brake is applied, if the front wheel is locked while the rear wheel is still rotating, there is a chance of vehicle getting overturned frontways due to shift of centre of gravity (towards front) and high inertia of motion and less load on the back side of the vehicle compared to the forward forces acting on the vehicle.

Considering the same above conditions (i.e. brake is applied & front brake got locked while vehicle in motion) when the vehicle is not running at 90 degrees angle with the road i.e. the vehicle frame is having an angle less than 90 degrees with the road (in either left or right side), then a radial force will act upon the rear wheel where the radious will be the length between the centre of gravity of the vehicle at that time to the point of contact of the rear wheel with the road. This radial force will try to move the rear wheel left or right – sideways (depending upon the < 90 degrees angle the vehicle frame made with the road surface during that time – either left side of the vehicle or the right side of the vehicle). Thus the rider will find (if at all it is possible for him / her to find it) that the rear wheels tend to move sideways more compared to forward movement.

During this time the front wheel also moves the opposite direction i.e. if the rear wheel moves left wards then the front wheel will move rightways, but the movement of the front wheel will be so little compared to the rear wheel that the rider will only feel about the rear wheel & not the front wheel. All this movement will happen in motion & keeping the center of such movement at the centre of gravity – which point will also moves due to the movement of the riders body movement due to inertia. Such complex movements of the moving mass (vehicle + rider) will eventually skid the vehicle sideways & for a two wheeler the rider & the vehicle will fall causing injuries & damage due to such accident.

With the above description we can clearly understand that why ABS system is necessary for any vehicle particularly a two wheeler. Simply because to avoid locking of disc braking system as they are very efficient in braking & quickly acts in stopping the wheel.

Compared to a disc system, the drum braking system in not very effetive & can not stop the wheels as effectively as a disc brake. That means that the time taken by the drum braking system in stopping a wheel is more compared to a disc system. The main intention of a braking system is to stop both front & rear wheels in the same time. This can be achieved mechanically if both front & rear brake system are of same type ( i.e. either disc or drum). However if any one of the above system is of differnt type then the question of ABS system arises.

Moreover if you consider that the front wheel is with a drum system & rear wheel is with a disc system (which is very unlikely, as front system should be an effective one as the momentum of the vehicle works towards the front end, as described earlier), there are chances that the rear wheel will get locked & stopped while the front wheel is still running. This will also create a sidewise redial force for the rear wheels, with centre of gravity as its center & eventually the vehicle will tend to skid sideways from the rear end. Thus this will also require a ABS system for the rear wheel. But even ABS will also not save the skid in such a system as we have considered a less effective braking system in front & more effective braking system at rear. Such a system, during braking, will normally generate a big difference in wheel rpm’s during braking & will always tend to pull sideways of the rear end of the vehicle causing a disbalance.

In a front & rear drum system if you install a ABS (I do not know if there is any) it has to be purely an electronic one. Rather than a mechanical system a electronic ABS system found in todays costly cars are the right one which actually takes into account many parameters like engine speed, throttle conditions, hydraulic pump, pressure developed in hydraulic system & wheel speed etc while braking both front & rear wheels.

The topic is very complecated in nature, but I hope it enlightens you about ABS.

In Detail,about the topic is given below...

TVS has already made their ability by introducing Apache 180 with ABS. ABS in not a new concept for four wheeler and two wheeler as well. In 1988, BMW introduced the first motorcycle with an electronic-hydraulic ABS: the BMW K100. Honda followed suit in 1992 with the launch of its first motorcycle ABS on the ST1100 Pan European. In 2007, Suzuki launched its GSF1200SA (Bandit) with an ABS.

In 2005, Harley-Davidson began offering ABS as an option for police bikes. In 2008, ABS became a factory-installed option on all Harley-Davidson Touring motorcycles and standard equipment on select models. Now let us enlighten how ABS works on bikes.

Skidding Mechanism

Skidding of a vehicle leads to disaster in many cases. Skidding starts when force applied by driver on the brake lever is more than the required. Skidding results when friction in brakes become more than the friction exists between tyre and road surface. That means wheel gets locked and start skidding on road surface. Less force leads to poor braking and more force leads to skidding. So to avoid the skidding of vehicle, the braking force should remain in limit.

In normal bikes, the brake lever is directly connected with calliper. The force applied by the driver on lever is directly exerted on calliper & disc without any interrupt. In the case of ABS, this braking force is exerted through ECU and Hydraulic valve.

The ABS prevents the wheels from locking during braking. It does this by constantly measuring the individual wheel speeds and comparing them with the wheel speeds predicted by the system. This speed measurement is done by individual speed sensors.

If, during braking, the measured wheel speed deviates from the system‘s predicted wheel speed, the ABS controller takes over, correcting the brake force to keep the wheel at the optimum slip level and so achieving the highest possible deceleration rate.

This is carried out separately for each wheel. Controller is nothing but an ECU with appropriate programming. This program avoids the rotational speed of wheel to become zero (Locking). This is done by temporary releasing the brake force by shutting off the valve in oil reservoir.

The ECU constantly monitors the rotation speed of each wheel. When it detect that any number of wheel are rotating slower than the other (this condition will bring the tyre to lock), it moves the valves to decrease the pressure on the braking circuit, effectively reduce the braking force on that wheel.

The wheels turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this is causes characteristic pulsing feel through the brake pedal.

Figure show major parts of Antilock-Braking System. Basic of antilock braking system consists of three major parts;

1. Electronic Speed Sensor: This sensor will measure the wheel velocity and vehicle acceleration. LOCATION: On wheel Hub
2. Toothed Disc: It helps the speed sensor to read the speed of wheel. LOCATION: With Brake Disc
3. Electrical Control Unit (ECU). ECU is a microprocessor based system contains program. LOCATION: Under the Driver’s Seat
4. Electrically Controller Valve. This controller valve will control the pressure in a brake cylinder. LOCATION: With ECU

The following are the 3 major benefits of ABS

1. Stopping Distance

As the braking force is controlled and applied electronically, the stopping distance reduces considerably in comparison with normal bike.

2. Sudden Braking

In the case of ABS, the braking is intermittent in nature. So vehicle remains easily steerable during braking also. Below figure shows the comparison of normal bike and ABS bike at sudden braking.

3. Braking on Slippery surface

Most of the riders must have experienced this condition with their bikes and also know the results. ABS provides equal distribution of braking force on each wheel and provides straight line stopping of vehicle.

Some Interesting Facts about ABS

Donovan Green, United States, Department of Transportation had performed some experiments on bikes with and without ABS in 2006. Following bike were selected by him for his test.

• 2002 Honda VFR 800 with ABS
• 2002 BMW F650 with ABS
• 2002 BMW R 1150R with ABS
• 2002 BMW R 1150R without ABS
• 2004 Yamaha FJR1300 with ABS
• 2004 Yamaha FJR1300 without ABS

He had performed two types of tests: 1. Dry Surface Tests 2. Wet Surface Tests. Following are the results of his experiments.

Dry Surface Tests

On the ABS-equipped motorcycles, the operator was tasked with braking sufficiently to assure the operation of the ABS. The measured stopping distance values were corrected to compare data from the speeds of 48 km/h and 128 km/h, except for the BMW F650 data, which was corrected to 48 km/h and 117 km/h, the latter figure limited by that model’s top speed of 157 km/h (i.e. 75% of 157 km/h).

In the ABS-enabled mode, for each load/speed/brake combination, the stopping distances were very consistent from one run to another. In this mode, the braking force was applied in a controlled and consistent manner by the ABS mechanism. With the exception of having to react to the possibility of the rear wheel becoming airborne under high deceleration, the rider did not require significant experience or special skill in order to achieve a high level of performance.

In the ABS-disabled mode, the stopping distances were less consistent because the rider while modulating the brake force, had to deal with many additional variables at the same time. Up to six runs were allowed for the rider to become familiar with the motorcycle’s behaviour and to obtain the best stopping distance.

Test results from non-ABS motorcycles were noticeably more sensitive to rider performance variability.
Despite being compared to the best stopping distances without ABS, the average results with ABS provided an overall reduction in stopping distance of 5%.

The stopping distance reduction was more significant when the motorcycle was loaded (averaging 7%). The greatest stopping distance reduction (averaging 17%) was observed when only the rear foot pedal was applied to stop the motorcycle from 128 km/h.

Wet Surface Tests

The original test procedure called for wet surface braking tests to be conducted at 48 and 128 km/h. However, for safety and stability reasons, all low-friction surface tests were performed in a straight-line maneuver, from an initial speed of 48 km/h. The tests were repeated with and without ABS. The test track was wetted by a water truck, and the wetting procedure was repeated every three stops.

With ABS-equipped motorcycles, the rider was instructed to brake sufficiently to assure that the ABS was fully cycling by applying as much force as necessary to the brake control device (no restrictions on force application).

The front and rear wheel brakes were operated simultaneously when the initial test speed was reached and then were operated individually when the front wheel and rear wheel were tested separately. During braking, the engine remained disconnected from the drive train.

A steering operation was allowed to keep or correct the running direction of the motorcycle during the test. Below vehicle speeds of 10 km/h, wheel locking was permitted.

For motorcycles not equipped with ABS, the test procedure was the same except that the rider was instructed to apply as much force as required on the brake control device in order to get the shortest stopping distance without losing vehicle control or having any wheel lockup above a speed of 10 km/h.

As with the dry surface tests, practically no learning process was required for the operator to achieve the best performance with the operation of ABS. In the ABS-disabled mode, the stopping distances improved as the rider became more familiar and comfortable with the braking system.

On the wet surface, the overall average stopping performance with ABS improved on the best non-ABS stopping distance by 5.0%. The stopping distance reduction with ABS was more significant when both brakes were applied, with an overall improvement averaging 10.8% over the best stops without ABS.

The greatest stopping distance reduction with the use of ABS was observed when the motorcycle was loaded and both brakes were applied, averaging a 15.5% improvement over the best stops without ABS