The Stovebolt Site Forums The Parking Lot Big Bolts air disc brakes, input wanted

Steve, you appear to know what you're talking about, and you've obviously done your homework. However, I wonder if you could address some questions I have regarding performance of drum vs. disk brakes. I'm old enough to have driven new vehicles with both systems, and my experience tells me that disk brakes are superior to drum brakes.

Back in the old days, good drivers were very careful about applying the brakes too much because of brake fade. You knew that if you used the brakes too much, you'd lose them due to heat buildup. Also, the motion of the drum tends to push the shoes away from the braking surface, making it necessary to apply more force to get the same braking than is required with disks, which "drag" along the rotating surface.

Finally, racing enthusiasts (like myself) know that race cars adopted disk brakes long before personal vehicles did. I never thought of racing as a place where cutting costs was even a consideration. I think racing adopted disk brakes because they were superior to drum brakes.

Clearly, hydraulics allow you to apply a great deal more pressure than compressed air, but the amount of force applied is only part of the problem. You also have to be concerned about the braking surface area and the materials used in the friction material.

Disk brakes offer a great deal more surface area for braking than drum brakes do (10 times as much?). Furthermore, almost all of the drum brake surface is engaged in braking all the time, allowing very little surface area for heat dissipation. To make matters worse, the enclosed drum system does not allow the use of air flow to cool the brake system except on the outside surface of the drum.

That is why you had to worry about brake fade. Under extended use, such as going down a large grade, a perfectly good, well maintained set of drum brakes could fail completely due to heat failure. That's why so many highways in mountainous areas had big "sand traps" built to "catch" runaway trucks. We used to read about those runaway incidents regularly when I was a kid. I don't recall the last time I heard about a runaway vehicle.

Disks, OTOH, only engage a small percentage of the braking surface at any given time, allowing the rest of the braking surface an opportunity to cool. I'm not aware of any case, or any literature, that discusses brake fading with disk brakes. If you're a NASCAR fan like me, you know that they can run with disks glowing orange and still have full braking power. I doubt seriously you could do that with drum brakes.

So I'm having some difficulty accepting your position that drum brakes are superior to disk. (Or perhaps I misread what you are saying?)

In other areas, I agree with you. For example, I think the king pin setup on my '48 is as good, if not better, than the newer ball joint/control arm or Macpherson strut setup. That's why I haven't changed it. I'm also not convinced that rack and pinion steering is any better than the old drag link/pitman arm setup.

I look forward to your response.