From Crankshaft Coalition Wiki
Jump to: navigation, search

by: Alittle1, Cobalt327, Crosley, Jon, Metzijndrie, Techinspector1
(Click here to edit this page anonymously, or register a username to be credited for your work.)

This article needs further development.

Please add content. See: How to improve articles.


[edit] Preamble

This article will introduce you to brakes and the components that are involved in a hydraulic brake system. We will try to break down the brake system in to its proper categories so that you can understand them better and apply that knowledge.

Many people ask frequently about braking systems, and what is better and what is usable. Everyone wants to go faster, handle better, braking is often ignored, mainly due to the price of major upgrades. After research and some experience most people will find that big brake kits, slotted, cross-drilled rotors have not made a huge dent in 60-0 or 70-0 stopping distances.

In this article we will look at what is important in stopping a vehicle and what factors are involved. The reader should remember the concepts behind braking in this article. These concepts are universal: no matter what vehicle we talk of, improving stopping distance is a matter of applying physics based on vehicle parameters, driving habits and driving conditions.

[edit] Caliper types

In general, there are two types of calipers, floating and fixed.

[edit] Floating

Floating calipers usually have one or two pistons located on the inboard side of the caliper and they squeeze the pads against the turning rotor using a thru-bolt as a guide for the pads.

[edit] Fixed

As the name implies, the caliper does not move. They have the pistons on both sides of the rotor, the equal force applied by the pistons centers the caliper without it needing to float. There may be as many as six pistons that apply pressure to the pads. They are usually a high performance type of brake, i.e. Baer brakes.

[edit] Rotors

Slotted and cross drilled rotors help cool the brake system with airflow across them and the calipers. Without sufficient airflow, having slotted or cross drilled brake rotors is hardly more effective than standard ventilated discs. The slotted/drilled rotors do help with brake pad out-gassing; when the pads reach their operating temps or higher they start to give off gas as they wear and the slots and/or holes gives this gas a place to go so it doesn't build up between the pad and rotor, causing a loss of brake effectiveness.

Typical vented rotor‎
Slotted and vented rotor
Slotted and drilled rotors‎

The downside to cross drilled rotors is reduce surface area, a decreased co-efficient of friction and they might be more prone to warping or cracking if not done correctly.

The advent of carbon metallic friction materials with their increased temperature stability and friction coefficient characteristics can mean slotted/drilled rotors are needed less than before. Typically, in original equipment passenger car applications these holes are cast then finish machined to provide the best possible conditions to resist cracking in use. But they still may crack eventually under severe circumstances. With a properly designed brake system, drilled ventilated discs can operate cooler than non-drilled non-ventilated discs, due the higher airflow rates through the vents from the supplemental inlets and increased surface area of the holes. The airflow is into the hole and out through the vent to the OD of the disc. If discs are to be drilled, the external edges of the holes must be chamfered or radiused.

[edit] Drum brakes

Leading/trailing shoe drum brake left; duo-servo type on right

[edit] Dou-servo drum brakes

When replacing drum brake shoes, if you have one shoe with less friction material than the other, the shoe with less material covering the steel shoe goes towards the front of the vehicle. This is commonly found on duo-servo type drum brakes. Because of how the brakes work in a duo-servo, the rear shoe sees more wear than the front. On the diagram above this can be seen. On the duo-servo drum brake, the front shoe is called the "primary" shoe, the rear shoe is the "secondary" shoe.

[edit] Leading/trailing shoe drum brakes

These are seen in lighter duty applications such as the rear brakes of a front wheel drive vehicle. On them, the bottom of the shoes are solidly held against a pivot point, and only the tops of the shoes expand out to contact the drum. This design has less braking ability than a duo-servo type brake because only one shoe has a self-energizing effect. In this setup, the front (leading) shoe may be the same as the rear (trailing) shoe as far as the amount of friction material covering the steel backing shoe, but the material may be thicker on the front (self-energizing) shoe. In other cases the front shoe will be smaller like the duo-servo type.

There has been years of confusion on the issue of which position the longer lining shoe goes in. The reason for the confusion is the Willys Overland company and it's successive owners. Ask almost any mechanic who works on vintage cars and they'll tell you "Bigger goes in Back" - B = B.
Old Willys vehicles are the major exception to this rule. All old Willys (Jeep) vehicles - CJ-2A, CJ-3A, DJ-3A, M-38, CJ-3B, early CJ-5, M38-A1, Wagon, Truck, Forward Control call for the longer shoe to be placed to the front. Any factory manual will confirm this. Don't trust aftermarket manuals, they may tell you to follow the convention that the rest of the automotive world followed for decades.

[edit] Other drum brakes

  • Two-leading-shoe drum brakes were used on motorcycles before disc brakes became popular. Also seen on large trucks.
  • Single-servo type brakes are used on trailers, etc. On them the brake cylinder plunger extends only from one side, bearing on the leading shoe.

[edit] Self adjusters

On vehicles having self adjusting brakes, the self adjuster mechanism (link or cable depending on the type) goes towards the rear of the vehicle.

[edit] Brake system

A high performance brake system will often use specific brake fluid for the application, stainless steel hard brake lines, teflon lined braided flexible brake lines, air ducting and venting, along with all the other necessary items to aid in controlling brake component temperatures.

The picture below shows a typical hotrod brake system which utilizes an adjustable proportioning valve, residual pressure valves, a master cylinder, and a vacuum brake booster. This is an under-floor brake system setup.

Hot rod brake system.jpg

[edit] Proportioning valves

An adjustable fluid valve that allows you to set the amount of pressure being applied to a wheel cylinder by increasing or decreasing the fluid by way of a adjusting valve.

[edit] Residual pressure valves

A one-way valve that allows fluid to flow through it at any pressure, but limits the amount of return pressure to a certain amount by way of a spring loaded check-valve. Usually comes in 10 psi (red color) for drum brakes and 2-3 psi (blue color) for disc type brakes. These valves are mainly used in under-floor systems where the calipers are higher than the master cylinder reservoir and to compensate for the return spring tension in drum brake systems. RP valves eliminate excessive brake pedal travel in both systems.

On a normally functioning brake system having the MC above the calipers, there is no need for a RP valve. If there seems to be a need for a RP valve in a disc brake system having the MC above the calipers, look for the root cause, rather than putting a 2 psi band aid on it.

The reason a RP valve shouldn't needed is because disc brake calipers have no retraction mechanism like a drum brake. A drum brake has springs that pull the shoes away from the drum, calipers don't. It would take 'vacuum' applied to the brake fluid to cause the caliper piston to retract- and that's what happens when the MC is BELOW the calipers and the fluid tries to run "downhill" to the MC.

If you have a soft pedal w/a 4-wheel disc non assisted brake system it could be due to air in the lines- bleeder screws on the bottom (calipers swapped side to side) will cause air to be trapped and a soft or spongy pedal. A too-high pedal ratio, a defective MC, or too small MC bore will also cause it.

[edit] Master cylinders

A dual reservoir master cylinder (MC) is used in any modern vehicle and should be used on older vehicles that had single reservoir master cylinders.

Usually a drum brake master cylinder will not work for disc brakes because the bore size is too small, and the drum brake MC may have a built in residual valve.

[edit] Power master cylinder diameter and pedal ratio

Most times, a master cylinder for a power brake system should have a pedal ratio of about 4:1 when using a master cylinder bore diameter of 1-1/8".

[edit] Manual master cylinder diameter and pedal ratio

Most times, a master cylinder for a manual brake system should have a pedal ratio of about 6:1 when using a master cylinder bore diameter of 1". As a rule of thumb, a pushrod attachment point on the brake pedal about 1" above where the power brake attachment point was (closer to the pivot point) will be about right for a manual brake system.

If using a bore larger than 1" on a manual system, the brake pedal effort can become very high.

Pedal ratio guidelines from

The problem sometimes encountered when using a disc/drum manual MC on a 4-wheel disc system is inadequate volume to the rear disc brakes. Calipers take more volume than drums. If there's not enough volume, the pedal will bottom before the brakes are fully applied and the pedal will not be firm.

To use a disc/drum MC on a 4-wheel disc system, the MC needs to be capable of about 1200 psi and has to have adequate volume to operate the rear disc calipers. The MC bore needs to be about 7/8" to 1", and the pedal ratio needs to be around 5:1. Be sure to remove the residual pressure valve to the rear drums. An adjustable proportioning valve can be used to adjust the front-to-rear brake bias.

[edit] Pedal ratio/bore size vs. pressure output
Pedal RatioBore SizePounds InputApprox. PSI Out
6:11 75575
5:11 75475

[edit] Brake boosters

In most cases the vacuum required to operate a power brake vacuum booster should be at least 18" for best results. In most cases anything less than 14" of vacuum will not be enough. Using a vacuum reservoir is not a very good substitute for inadequate vacuum. In cases where there's not enough vacuum, a vacuum pump may be used, or the system changed to manual brakes.

If space is a problem, a dual diaphragm booster might be enough smaller in diameter to help, as long as the booster has sufficient pressure to do the job.

[edit] Some guidelines from MP Brakes:

A midsize GM car with disc brakes in the front and drum rear brakes will require at least 900-1,000 psi to the wheels to lock them up. The pressure output of the booster is directly proportional to its diameter, the larger the booster the greater the power assist.

The following assumes 18 in/Hg of vacuum at idle, and 120 psi of pedal force:

  • Four wheel disc = 9" dual diaphragm (1200 psi)
  • Front disc/rear drum = From the 9" dual (1200 psi) down to a minimum of a 7" dual (900 psi)
  • The 8” dual diaphragm booster makes 1,000 psi
  • 9” single diaphragm 900
  • 7” dual diaphragm 900
  • 7” single diaphragm 800

[edit] Brake lines and fittings

SM brake fitting01.jpg

[edit] Power to manual brake conversion

[edit] Resources

  • Technical support from MP Performance Brakes. Includes FAQ, Configurations, Troubleshooting, and Installation Guides

This may be one of the best brake articles I've ever seen....

Personal tools