Stewart-Warner power brake fade

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I’ve been musing over the discussions over the years about the confidence and lack of brake fade apparent in the ’33 to ’35 Pierce-Arrows with the Stewart-Warner power brake system. It worried me there could be overconfidence in coming down mountain grades with the S-W system since the brake shoe linings are the same material as traditional drum brake systems, and the cause of brake fade is a reduction in the friction coefficient with extreme brake temperatures. The brake shoe and drum temperatures shouldn’t change with the method of applying the brake force since the energy dissipation remains the same.

After some cogitation I realized that what must be happening is that most drum brakes have a degree of self actuation where the geometry of the shoes lets them  “wind up” very slightly in a way that multiplies the force being exerted from the pedal. This multiplying effect increases how much more force has to be exerted on the pedal to try and stop with hot fading brakes when the friction of the lining helping to actuate the brakes is reduced. Because of the higher force of the S-W force booster, I think its brake geometry does not rely on self actuation in the drum system and the change in pedal force will be less dramatic with the reduction in lining friction from extreme temperature. The force on the shoes is being applied via the torque on the driveshaft acting on a wet clutch disc plate riding in relatively cool gearbox lubricant unaffected by the brake lining friction.  They are more like disc brakes in just requiring a stronger boost system to actuate the brakes without a mechanical force multiplier at the shoes. It doesn’t mean there won’t be brake fade, it might be less obvious.

I don’t think I will  be coming down long  grades in free wheeling in my ’35, even with the S-W power brake.

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