What Were They thinking?

What Were They thinking?

hey pick the size of the parts? What prevented them from making them bigger? What kind of tests did the parts and the vehicle have to endure to be judged ready for the market? These are all questions that go into the design of the brake system for the vehicle. These decisions are the responsibility of the brake system engineer.

This role can exist in either the engineering department of the auto company or at the brake system supplier. Brake system suppliers will also sometimes be referred to as “foundation brake suppliers.” This is just insider jargon for the braking components used for normal braking, such as calipers, rotors, boosters and master cylinders. Most of these suppliers also make a wide range of chassis control systems as well.

Some vehicle manufacturers source the entire brake system as a complete package and some make these decisions themselves and then shop for the particular components. Both have the potential to achieve an excellent system. This article will give some insight into how the engineer develops the brake system, picks the sizes and technologies used and the series of tests that a vehicle is put through to ensure that the brake system chosen performs on the vehicle.

The first task of the brake system engineer is to determine all of the performance requirements that the brake system must provide for the vehicle. These come from a variety of sources including:

1. Federal requirements (FMVSS 135 for passenger cars and light trucks;)
2. Pedal feel targets;
3. Thermal endurance;
4. Stopping distance;
5. Durability/life;
6. Noise and vibration (roughness, pulsation, squeal);
7. Budget and ability to meet vehicle schedule; and
8. Other vehicle performance requirements including CAFÉ, EPA and some occupant safety-related requirements.

Many times the expectations for each of these things conflict. Therefore the system engineer must prioritize which best meet the needs and expectations of the particular consumers that will purchase that type of a vehicle. In order to sell the vehicle the manufacturer must meet the federal requirements for the country in which they intend to sell. These requirements establish the sizing of many of the key components of the system.

FMVSS 135 does not impose a particular technology, but rather determines the minimum performance a vehicle must achieve to be safe and legal. In most cases, the section of FMVSS 135 that has the biggest influence on “system sizing” is the section titled Inoperative Power Assist. This section establishes the minimum ability of the vehicle to be stopped when the boost assist function has failed.

Recall that the boost assist multiplies the Driver Input Force by approximately six times and as much as 15 times. For example, if the driver pushes on the pedal with 25 lbs, the rest of the brake system will see 150 lbs and maybe as much as 350-375 lbs. So if that part of the system fails, it is important that the vehicle be able to stop in a reasonable manner with the original 25 lbs.

Specifically, the Inoperative Power Assist requires that a vehicle loaded to its maximum weight rating (Gross Vehicle Weight), travelling at 100 kph (62.5 mph) stop in less than 168 meters.

The driver can not push any harder than 500 N (112 lbs). Based on these requirements and Newton’s law, you can determine how much force the vehicle’s brakes must generate to stop the vehicle within the requirements.

As an example a full-size pickup with a gross vehicle weight rating of 7,000 lbs will need the brakes to generate 1,695 lbs.

Newton said the force required to stop the vehicle is equal to the mass multiplied by the deceleration:

Force Required = (7,000 lbs/ (32.2 ft/s/s)) X 7.8 ft/s/s) = 1,695 lbs of brake force.

The system must use 112 lbs of push and multiply it about 15 times. The component characteristics that accomplish this include the pedal ratio, master cylinder diameter, caliper diameter, lining friction level, rotor diameter and the tire diameter. The driver’s force is multiplied by a few key ratio’s that establish the size of the components. First is the pedal ratio.

The pedal acts as a simple lever and multiplies the force by generally about four times. Second is the hydro-mechanical ratio. This uses the pedal ratio multiplied by the ratio of the caliper piston area divided by the master cylinder piston area. For the front brakes, this ratio is generally in the low 20’s and for the rear brakes about 10.

For most vehicles, the front brakes do about twice as much as the rear brakes. The fronts are asked to do the bulk of the work to ensure that the vehicle will not have the tendency to lock or skid the rear wheels first. This is termed as “front brake biased.” Locking the fronts first limits the vehicle’s ability to steer in a skid condition, but prevents a spin.

Rotor diameter is generally maximized as much as the smallest wheel offered on the vehicle will allow. The last, but highly critical factor in determining the inoperative boost performance is the friction level of the brake pads.

The ability of the brakes to generate stopping force is directly proportional to this value. In the OEM environment, the available friction will generally range from about 0.30 up to as high as 0.45. Therefore, just within this range the output of the brake can increase by up to 50% just by selecting a pad of a different frictional value.

It is very tempting for the system engineer to select a material with a high frictional value. However, this is only one characteristic of friction material that determines its selection. Like any component in a vehicle, the variation must be considered. The brake system engineer must have high confidence that all vehicles made will meet the FMVSS requirements. Therefore there must be confidence that the vehicles at the low end of the friction range have enough braking power to still meet the federal regulations.

Some consideration is also given to how the vehicle will be equipped in the aftermarket, since it is common for the brake pads on the front axle to wear out first (remember they do about 65% of the work).

Hopefully, the vehicle will be re-equipped with a pad material that meets OEM specifications. If the brake system engineer relied heavily on a very high friction pad as his silver bullet for FMVSS performance, the vehicle would have the potential to be rear skid biased if the vehicle was serviced with a significantly lower front friction value in aftermarket service. As a result of these issues and many other factors, most vehicles tend to select friction levels in the range of 0.33 – 0.40.

Care should always be taken in aftermarket service to ensure that a replacement friction is selected that is in the same edge code range as original equipment.

As I mentioned before, everything is a tradeoff. Virtually all of the component ratio’s that contribute to the failed boost legal requirement lead to additional pedal travel. While it would be easy mathematically to design a system that meets the FMVSS requirements, the result would lead you to very long pedal travels. Therefore, it is typical to design to meet the federal requirements with enough margin to protect for product variation but not degrade the pedal travel of the vehicle to unacceptable levels. This is a delicate balance for the system engineer. This becomes the first step towards determining all the critical sizing characteristics of the Brake System.

Many other parameters will be developed from the other requirements as we can explore in future articles.

Engineering the Complete Brake Job

By Larry Carley

When brake linings are replaced, always follow your friction supplier’s guidelines. As a rule, linings should be replaced with ones made of the same basic type of material as the original linings (or better). Replace semi-metallic with semi-metallic, ceramic with ceramic, and nonasbestos organic (NAO) with NAO — or upgrade to ceramic. The best advice here is to recommend premium grade linings as opposed to standard or economy grade linings. Premium linings will usually give your customer the best performance, longevity and overall value.

If you are installing a loaded caliper assembly on one side of a vehicle only, make sure the pads have the same approximate friction characteristics as the ones on the opposite side. If a different grade of friction material is used, it can increase the potential for a brake pull.

Unless the rotors and drums are in near perfect condition (no scoring, minimal runout, etc.), resurfacing should be considered a must to restore an optimum friction surface. Rotors should be resurfaced to OEM specifications.

If rotors are worn to minimum thickness, or can’t be resurfaced without exceeding the minimum or discard specs, they must be replaced.

Rotors should also be replaced if they are cracked, have hard spots or are severely corroded.

It’s the same story with drums. If a drum is cracked, has hard spots, is bell-mouthed, or the inside diameter exceeds maximum specs or a drum can’t be resurfaced without exceeding the limit, it must be replaced. Also, both drums should have about the same amount of wear. If the difference in wear is greater than about .040 inches, both drums (or rotors) should be replaced even if only one is at or near the discard limit.

As for drum hardware (self-adjusters, return springs, shoe springs, etc.) and disc hardware (caliper slide pins, bolts, bushings, sleeves, etc.), anything that is obviously worn, damaged or badly corroded should always be replaced.

If a return spring or shoe spring is stretched or discolored, it has probably suffered heat damage and must be replaced. But many brake experts say it’s a good idea to replace springs anyway when doing a complete brake job regardless of the spring’s appearance to assure like-new brake performance and to minimize the chance of a comeback.

The cost of new springs and other hardware is only a small portion of a complete brake job. Yet it can save you many dollars in lost labor revenue if you end up having to do the job over because you reused old hardware and it failed.

The MAP guidelines say it is not necessary to rebuild or replace calipers or wheel cylinders unless they are leaking, cracked or damaged. But as we said earlier, many experts believe replacing or rebuilding these components on high mileage vehicles is well worth the extra cost to minimize the risk of future problems.

As for brake fluid, every brake job (complete or not) should include a fluid change. But surveys have found that half of all cars and light trucks that are 10 or more years old have never had their brake fluid changed!

You can’t really judge the condition of the brake fluid by its appearance. Brake fluid may darken as it becomes contaminated with moisture, but some fluid does not. The most accurate way to check the condition of the fluid is to use an electronic tester that boils a small sample of fluid, or to use chemical test strips that react to the corrosion inhibitors and trace copper in the fluid.

Tests have shown that after only a year of service, the brake fluid in the average vehicle can contain as much as 2% water. After 18 months, the level of contamination can reach 3%, and continue to climb to as much as 8% or more as time goes on.

As for the ABS system, no parts should have to be replaced unless something isn’t working properly. Wheel speed sensors can sometimes give bad readings if their magnetic tip becomes contaminated with metallic debris from the brakes. A simple cleaning may be all that’s needed to eliminate the problem. A major ABS problem such as a failed modulator or control module, on the other hand, will be a major expense to replace.

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