The answer is for safety.
With engine fitted in the front, the gravity center of most passenger vehicles especially for front-driven-vehicles, is placed more foward under the effect of "weight of powertrain".
When braking hard at medium to high travel speed, the gravity center of vehicles, will furtherly move forward in inertia increasing more work load to the front wheel brakes. To cope with the load onto the front, front wheel brakes must be able to provide greater braking forces, therefore the size of them will be much larger.
Some may doubt “what if providing same braking force to rear wheel brakes? Doesn’t it reach an overall better braking effect?” To answer this question, we need to talk about the influence of braking force allocation on vehicle trajectory.
For our old friends should know from our past posts something about antilock brake system ABS, which play the role of preventing the wheels from locking at braking .
Then what if the wheels are blocked?
First, when front wheels block at the time of braking for turn, the vehicle will lose the ability of changing direction. The vehicle under the effect of centrifugal force, will move away from flexural center along with the tangential direction of the original turning arc. This is what we called understeer.
If the rear wheels lock in the same situation, the rear part of the vehicle will move away from flexural center due to centrifugal force. While the front wheels are still rolling as before, the front part of the vehicle will move along with the turning track, and then the vehicle will drift. That is what we called oversteer.
From the introduction above we have learned that for most of passenger vehicles, with powertrain in front, the inertia and loading to rear wheel brakes is much smaller than those to front ones. Because the vehicle gravity center moves forward at braking, the front axle damper will be compressed, while the rear axle will tend to rise under the effect of leverage reducing the rear wheels adhesive force to the ground. Therefore if the rear wheels are set the same brake force as that of the front wheels, they will be more likely to be locked than the front wheels at braking, leading to oversteer and out of control.
Extended instruction: The steering character is intentionally set into understeer when the steering system is calibrated in auto manufactures. It well explains that oversteer is much danger than understeer in driving. Therefore it is a crucial setting for safety to allocate smaller brake forces to rear wheel brakes preventing the locked rear wheels from oversteering.
Even for super sports car with excellent performance such as Ferrari 488, their rear brake discs are also smaller than that of front ones. Most of super sports cars are in middle-engine and rear-wheels-drive set. Once the rear wheel brakes are locked to drift, the vehicle will be quite hard to control due to their back allocated gravity center, and the collision impact is bound to be more violent.
For rear-drive vehicles with high power, if the sliding and oversteering is caused by improper speed control, it will be evitable by honing driving skill and replacing into tires of good grip. When coming across to vehicle drift due to locked rear brakes, perhaps you can only leave the fate to God.
In summary, the properly calibrated rear brake force is an important premises to ensure the driving stability, so it is necessary to take account of the material formula and friction coefficient when the rear brake pads need to be replaced. It is prohibited to increase rear wheel brake force too much to cause rear brakes being locked.
Choose the braking products with reasonable performance to garantee the driving safety in maximum.
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