G'day,
In answer to Redneck's original question, I offer the following. Vehicles slow down due to four main facors, wind resistance, rolling resistance, off-accelerator engine drag and the friction generated by applying the brakes.
Automotive companies look at all four when designing a car, even a truck these days. No vehicle manufacturer releases a car for sale that has not been tested in a wind tunnel. Low drag bearings and low viscosity oils are used to minimise rolling resistance. Compression ratios have come back up from the seventies due to more efficient fuel and spark delivery.
But the biggest advance in stopping a car is in the braking department. Stopping a car (or truck) relies on the amount of friction generated by applying a stationary friction pad to a rotating piece of cast iron. The bigger the size of the pad, the more friction there is, but this is all dependent on the amount of force applied to the pad.
On the same size and weight car, a drum brake will generally have more square inches of friction material than that on a disc braked car. Therefore less force is needed to be applied to the pad in order to generate the same amount of friction, and therefore stopping distance. That is why virtually all cars up until the sixties and early seventies had non-power drum brakes.
That was all that was needed for the traffic conditions of the day. However, as the post war affluence filtered through to the car manufacturers and they made more cars and the cars they made got heavier, stopping distances increased. To overcome that, the driver had to apply more pressure to the pedal, and this was felt more by the so-called weaker sex, whose leg muscles were in many cases, less than those of a male.
This is why vacuum power boosters were offered, first as an option and then as standard. This worked well for a while but, as the wages went up and the relative price of the cars came down, the amount of road traffic went up. More cars on the same amount of roadway meant that the gap between cars on the road became less and less. Motorists were starting to use their brakes more often and this is where the drum brake's major disadvantage began to show itself.
Brake fade! This condition is caused by a lack of heat dissipation. Science tells us that as a metal heats up, it expands. Brake drums do exactly that when they get hot and they expand AWAY from the friction material. As well as that, the friction co-efficient of a hot cast iron brake drum is LESS than that of a cold one.
So, when a drum brake gets hot, its ability to slow the car becomes less and less. A drum brake also takes longer to cool down, as there is no natural way for it to dissipate its heat, other than by radiation of that heat, and that takes time.
However the disc brake, particularly the ventilated variety, solves all of that. As well as having two friction surfaces instead of one, the disc takes longer to heat up. Then, due to the hollow nature of the vents in the disc and the naturally occurring centrifugal force of the spinning rotor, cold air is "sucked" from the centre of the disc up through the vents, cooling the cast iron and expelled as hot air.
This is the main advantage of discs over drums. Heat dissipation.
The only real downside is that the driver needs to exert a lot more force on the pads for them to slow the car, and that means a booster is required. And this adds cost and complexity to the car. However, most people agree that this is a small price to pay for a far superior overall braking system.
There is only one area where a drum is far superior to a disc and that is as a parking brake. Much less effort is required to apply a drum parking brake than for a disc parking brake (of a similar amount of friction material) for the same amount of cold static friction.
That is why the 1965 Corvettes had the most advanced braking system in the world when introduced. They had huge ventilated discs on all four wheels, but still had small drums incorporated into the rotor for the parking brake in the rear hub assembly.
As an aside, I drive buses for a living, and all of the new buses we get have disc brakes at each end. And they are all power boosted, but by air pressure rather than vacuum or hydraulic assistance.
I am not a certified mechanic, and all of the above is based on my many years of working on cars and the knowledge I have picked up from people I've met who knew more than I did. And I'm still learning.
Regards from Down Under.
aussiejohn