Drum vs. Disc: Today
In today's automotive pantheon, it's not uncommon to find four-wheel disc brakes as standard equipment on medium-priced, non performance-oriented models. The majority of new vehicles, however, continue to utilize a front-disc/rear-drum brake setup. What does this say about the current state of braking systems? Are these manufacturers sacrificing vehicle safety in order to save a few bucks by installing disc brakes on only the front wheels?
The truth is that today's disc/drum setups are completely adequate for the majority of new cars. Remember that both disc and drum brake design has been vastly improved in the last 20 years. In fact, the current rear drum brake systems on today's cars would provide better stopping performance than the front disc setups of the '70s. And today's front disc brakes are truly exceptional in terms of stopping power. Combined with the fact that between 60 and 90 percent of a vehicle's stopping power comes from the front wheels, it's clear that a well-designed, modern drum brake is all that's required for most rear wheel brake duty.
(Ref.: Karl Brauer, Editor in Chief, Edmunds.com)
ABS (Anti-lock Braking System):
Antilock brake systems use a computer to monitor and control the braking system. Essentially the computer monitors the rotational speeds of the wheels and then releases the brakes when the wheels are about to lock up. Wheel lock up is undesirable for two reasons:
1. The braking system including the tires is most effective, i.e., produces the most retardation, before the wheels lock up and start to slide. So it is beneficial to keep the wheels rotating at a rate of about 5%-10% of the free rolling velocity for the vehicle's speed. This improvement in braking efficiency, while present, is actually a secondary benefit of ABS however.
2. Locked wheels cannot provide for directional or rotational control of the vehicle. If the rear wheels lock, the vehicle will tend to spin out toward the end of the braking interval. If the front brakes lock up, the vehicle cannot be steered. It is probably this second aspect of the ABS effect on vehicle control that is most highly publicized, but the actual benefit of these systems for accident prevention or injury reduction is yet to be proven..
The vehicles most in need of ABS are light trucks or passenger cars with loads (like CNG cylinders) in their trunks. The braking systems of these vehicles are frequently biased toward the rear to accommodate the payload. But when the vehicle is lightly loaded, there can be too much rear bias resulting in early wheel lockup and loss of control. Pickups in particular are vulnerable to this problem.
Why Honda applied ABS on rear Drum brakes in their 1.3L compact sedan and not the front Disc brakes? Well, if you do this, then the rear just locks up while the front locks and unlocks (because of ABS on front). So it's like yanking the handbrake into a turn, if you try a lane change with ABS active, you will just go sideways. Although Honda doesn't recommend converting your car to natural gas; intentionally or un-intentionally; ABS on rear drum brakes goes to the benefit of CNG users with a heavy cylinder in their trunk. So either, the ABS has to be on all the four wheels, or it should be on the rear wheels (in case of rear drum brakes) as is found in the new Honda City.
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