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.
Note from Editor: This article appeared as a comment in one of our articles. I posted it here because this is a useful article which deserves better exposure. ‘BA’, please let us know your full name.