|The MGA With An Attitude
HOW DISC BRAKES WORK - BT-122
A prequisite for this course is BT-121 (How Drum Brakes Work), as that is discussing weight shift and brake balance. Also review BT-104 (Function of a Disc Brake Caliper).
Considering that the disc brake caliper pinches the flat rotor from the sides, it will work equally well going forward of backward. This may at first seem like a good advantage, until you consider the balance relationship between front a rear brakes and what happens when you back up. The idea of a strong forward bias for the brakes is still valid when driving forward. This is again achieved by using larger bore cylinders on the front axle.
This arrangement does create an interesting issue when backing up. Rear drum brakes with one leading and one trailing shoe work as well going either way. Front disc brakes also work as well going either way. This leaves the front brakes with stronger braking power going either forward or backward. When braking hard in reverse causes front to rear weight transfer, you end up with too much braking force on the lighter front end. This causes the front wheels to lock up long before the rear wheels reach full braking capacity. When you ease off the pedal pressure to avoid the lock up condition the rear brakes are still far short of full braking capacity, so this results in longer braking distance in reverse. The same applies to cars with four wheel disc brakes.
So why are disc brakes so popular? It is because they have one paricular endearing quality, that they resist fade under high usage conditions. When disc brakes get hot and incur various types of thermal expansion all of the friction surfaces remain parallel, so there may be very little brake fade with increasing temperatures. You can use disc brakes hard and often with little or no brake fade, as long as the pads don't melt and the fluid doesn't boil.