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Voodoo Brake Science: How Brakes Work

Dodge Truck Driver Side Shot
Rick Péwé
| Four Wheeler Network Content Director
Posted March 1, 1998
Photographers: Tom Morr

Understanding Pedal Ratios, Bore Sizes, And Line Pressures

Brakes are rather simple when you think about it; you stop if you have them and crash if you don't. Somewhere between these two extremes lay the majority of our own brakes, and for the most part, they usually function pretty darn well.

But for those of you who swap brake parts to build custom creations or are saddled with a project that wasn't done right, this story will give you the basics on the three least-understood aspects of a hydraulic braking system that have the greatest impact on how well it works: pedal ratio, master cylinder bore size, and hydraulic line pressure.

One of the biggest braking problems occurs when a stock vehicle, after it's had 1,000 pounds of accessories added and bigger tires slapped on, just won't stop like it used to.

If you've ever noticed the drum and rotor differences between a 1/2-ton and 3/4-ton truck, you know that size does make a difference. The heavier-rated truck is capable of carrying a heavier load, and the larger brakes are properly designed to safely stop that theoretical maximum weight.

Does that mean your overweight Suzuki needs 1-ton-rated brakes for the ultimate in stoppage? No, because the extra weight of the components themselves would negate the advantages. While the ideal situation is to figure out what you need in stopping power, most people are unaware of how to do it. The standard answer is to replace drum brakes with discs, but disc brakes do not necessarily stop better than drums, due to the larger frictional area of drum brakes and their self-energizing feature. However, disc systems definitely work better in the 'wheeling world, because they offer consistent operation in high-heat and wet situations and are much easier to maintain and adjust.

Whether you go with discs or drums, safe, effective braking is the ultimate goal. Achieving that requires either the hit-or-miss approach of parts swapping or a real understanding of the relationship between your foot pressing on the pedal and the tires grabbing the ground.

Probably the three most misunderstood factors that affect braking performance are pedal ratio, master cylinder pressure, and master cylinder volume. If these three items are correct, other issues, such as the type of lining material and brake-biasing procedures, can easily be handled. For example, if the custom brake system on a truck doesn't stop it sufficiently many owners will just throw on better brake pads, when in reality the pedal ratio or the pressure or volume to the calipers may be wrong.

In addition to explaining these braking mysteries, we have also included a few tricks and tips, along with some associated formulas for figuring out how to stop your rig in the real world, that we've learned through our own hit-or-miss upgrades.

But remember, the braking system on your vehicle is one of the most important aspects regarding the safety of yourself and others. If you can't modify the braking system correctly, don't do it at all. It's far better to leave this type of work to the experts than to end up in a ditch, or worse.

Pedal Ratio
Most vehicles have a brake pedal that hangs under the dash on a pivot, with a rod attached beneath the pivot that goes into the master cylinder. The distances between the foot pad, the pivot, and the pushrod are designed to provide leverage. This allows a small amount of foot force (referred to as pedal pressure) to translate into the high amount of hydraulic line pressure needed to stop the truck. On most production vehicles the pedal ratio is preset and nonadjustable; generally it's about 5:1 for manual brakes and 3:1 for power brakes, since they require less effort to activate. But on custom installations these ratios can be completely out of whack, leading to a hard pedal if the ratio is low, or excessively long travel if the ratio is high.

Also important is the stroke of the master cylinder. Most master cylinders have 1 to 2 inches of stroke; the pivot point and the pedal ratio must be adjusted to match the stroke of the master cylinder, otherwise full pressure or complete retraction may not be accomplished. For example, if the pedal is in the full up position against the pedal stop but the master cylinder piston isn't fully retracted, the brakes may still be applied and they may drag. Likewise, if the brake pedal hits the floor or another stop before the master cylinder is fully depressed, full braking pressure may not be available.

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Sources

Tri County Gear
Pomona, CA 91766
909-623-3373
www.tricountygear.com
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