Auto Service World
Feature   February 1, 2009   by CARS Magazine

Science of Stopping

All pads are not created equal. Understanding what goes into a brake pad can prevent misunderstandings, and even – gasp! – comebacks.

Wouldn’t it be nice if all customers treated brake work as seriously as those who look after fleets of police cars?

These vehicles are usually treated to the best parts available. No wonder, too, because high speeds are common in emergency situations, and hard braking comes along with that.

Many customers, especially fleet customers, don’t believe in using premium brake parts for a variety of reasons.

If it were legal, many customers would surely try stopping their cars by dragging their feet on the ground, like in the old cartoons.

Customers often exert extreme pressure on garages to keep repair costs down, and using bargain parts can be tempting.

But there’s often a good reason to install higher-cost parts, and not just because the fleet administrator said to. Sometimes bargain parts simply cause more problems than they’e worth and will only lead to frustration and come-backs.

Here’s a quick look at the different brake material compositions, the best places to use them, what to avoid, and when being frugal might work out OK.

Even though asbestos isn’t as common in brake linings anymore, it’s important to understand what made it so popular and why it was phased out – because, frankly, you just never know what you’re getting when you open a mystery box of cheap brake parts.

Asbestos linings were strong, resistant to high temperatures and chemicals, and were also relatively cheap. But they weren’t such a good choice in high-temperature environments – remember, the rotor temperature is what dictates the pad’s lifespan. The hotter the rotor, the shorter the life of the friction material. Plus, asbestos pads didn’t transfer heat as well as other brake lining materials (like metallic ones).

The worst thing was the dust, released as linings wore down – along with leaving a dirty residue on wheels it could cause cancer and lung disease.

Bottom line: phased out, but be careful anyway.

Non-asbestos organic pads are quiet and effective, but don’t last very long under high temperature conditions like stop-and-go traffic. This makes them a poor choice for taxis or heavily loaded vehicles despite their many positive qualities. Additionally, NAO pads don’t transfer heat as well as other materials.

Bendix Brakes recommends this trick to see if a brake pads is made of an NAO material: see if a magnet sticks to the pad surface. If it does, it’s likely not an NAO style.

Bottom line: Low noise, low heat transfer, low dust, low wear – not for HD use.

These pads are standard equipment in about 30 per cent of vehicles on the road. Ceramic material is used in place of steel in the friction material, but they may also contain copper and brass. Ceramic pads don’t make much dust, so they won’t mess up fancy rims. And their low-noise characteristics make them appealing to drivers and technicians alike.

But they’re not perfect. They don’t wear well in heavy duty situations, and wouldn’t work well on fleet vehicles. Also, ceramic pads are a lot like fruit juice – you need to read the label to make sure of what your getting, since many products contain just a small amount of the good stuff. Some ceramic pads have as little as one per cent ceramic content. But be careful, since too much ceramic in the mixture leads to noisy brakes.

And actually, ceramic pads are really a type of NAO pad.

Bottom line: Low dust, less noise, good performance, higher cost, not for heavy loads or stop-and-go driving.

The most common type of friction material on the road, semi-metallic pads are standard equipment on 60 per cent of vehicles. Metallic pads are typically made of 50 per cent iron or steel, plus graphite composites and petroleum coke. The metal in pad allows heat transfer through brake system, so they last longer than NAO pads do.

These linings work hard, have very good high-temperature performance characteristics, they’re relatively cheap, and they can handle heavier loads. Police cars, fleets, and vehicles used for towing almost always use semi-metallic pads.

The downside, however, is the noise they can make.

Almost all brake manufacturers require a smooth rotor finish and strict break-in period when installing semi-metallic linings.

As with many other types of pads, there’s usually an assortment of grooves, shims, slots, and chamfers to change the frequency of the vibration (and therefore noise) of the pad into an inaudible one. Some pads even have a coating to reduce noise (don’t sand this off!).

Bottom line: Good heat transfer, good pedal feel, long-lasting, good for heavy loads, low cost, good for fleets, can be noisy.

These pads aren’t common, but it’s worth knowing when they’re used (namely, on some high-end European vehicles). They’re usually made up of about 20 per cent iron or steel, mixed with inorganic and organic fibers and lubricants. They make more noise and dust, but are used in some specialty applications.

Bottom line: Understand them, but remember they’re not common.

Sintered metallic
This is an FYI point, really, because most techs won’t encounter these pads very often. They’re mainly for off-road use and race cars. A regular vehicle using these pads may take several hundred feet to stop, so they’re not a good choice at all. These types of pads are also used on airplanes.

Bottom line: Not for normal use.

The important thing is making sure that the replacement linings are the same composition type as the ones the manufacturer intended. In other words, if the vehicle’s original brake pads were semi-metallic, the new pads should also be semi-metallic.

It’s possible, however, to install pads with a different type of friction formulation than the original, as long as you’re very careful. It’s never a good idea to replace semi-metallic linings with NAO linings (unless they’re ceramic and approved), and it’s usually OK to replace NAO pads with semi-metallic ones, but there are exceptions so you’ll need to check and make sure. The bottom line is, make sure the customer knows what they’re getting and that your behind is well covered. Make sure that saving the customer money doesn’t end up costing you money in the end.

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