Difference Between Primary & Secondary Brake Shoesby Richard Rowe
Brake drum systems seem like something of an anachronism in today's high-tech world of antilock systems and powerful disc brakes. But, like many archaic mechanisms, drum brakes are really quite clever, exhibiting dozens of subtle little design features that make the whole assembly more than it appears to be. But to understand what differentiates the primary shoe from the secondary, it helps to know how brake drums work.
Triggering the Primary Shoe
There are many different variations on the drum brake theme, but most work on a similar principle. The brake mechanism itself starts at the hydraulic servo, which uses fluid pressure -- supplied by the master cylinder -- to push a piston and rod. That rod generally connects to the top end of the front, or primary, brake shoe; the bottom end of the shoe attaches to a pivot. When the rod presses on the shoe, it pivots outward and presses against the inside of the spinning wheel drum. Friction between the shoe's friction material and drum slows the drum.
Self-energizing brakes take advantage of an interesting aspect of geometry within the drum brake mechanism. With the mechanism designed above (known as a duo-servo or self-energizing system), all of the braking force comes from the master cylinder, which means that either the driver has to supply all of the braking force, or that the car needs to use a power-assist mechanism to increase force. A self-energizing brake shoe uses a pivot slightly out of alignment with the axis and rotation of the wheel; so, when the servo pushes on it, the rotation of the drum wedges the shoe downward and against the drum. So, the brake shoe takes some of the energy from the drum and uses it to enhance braking power.
Triggering the Secondary Shoe
On smaller cars, the brake servo will sometimes use a two-sided piston that actuates the front and rear brake shoes, referred to as a "non-servo" or "leading-trailing" arrangement. But on the self-energizing systems that most vehicles use, the bottom pivot on the front shoe connects to a moveable rod or plate, which connects in turn to the bottom of the rear -- secondary -- shoe. The top of the secondary attaches to a pivot; when the leading or primary shoe hits the drum and wedges against it, its pivot shoves backward on the rear shoe (which may or may not be self-energizing) and forces it against the drum. Thus, the rear shoe does most of the braking and the front shoe's primary job is to engage the rear shoe.
What's the Difference?
Because the rear shoe does most of the braking, it needs to be a bit longer than the front shoe; other than that, the two are usually identical in terms of design and friction material. The shorter and less-grippy front shoe allows for a bit more progressive engagement, which is particularly important with the slightly more lock-up-prone self-energizing brake shoe. So, using a shorter, self-energizing front shoe and a longer, non-self-energizing rear shoe makes for brakes that provide good stopping power with more linearity than a fully self-energizing arrangement would.
Often times, non-servo or leading-trailing systems will use interchangeable front and rear brake shoes. The non-servo system can get away with this because it applies the same pressure to the both shoes. Alternately, a self-energizing system may use front and rear shoes that are the same length, but use different types of friction material to modulate the pads' engagement strength for either front or rear applications. For instance, a front shoe's friction material might contain a bit more copper than the rear, making it a bit more slippery and easier to modulate. But, because the shoe is longer than it would otherwise be, it can still provide the same amount of grip as it would if it were shorter but contained less copper. Lastly, the primary shoe isn't always on front; some applications place the primary shoe on the rear, and the secondary on the front.
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