Brake Pad Friction Ratings Explained (Mu and Edge Codes)

By Core Brake Parts Tech Team — Circleville, OH warehouse

The two letters printed on the edge of a brake pad, such as FF or GG, are its friction rating. The first letter grades the pad's coefficient of friction (mu) at normal temperatures and the second grades it when hot, on a scale that runs from C (least grip) to H (most grip). They are useful for matching a replacement pad to the one you took off, but they say nothing about noise, dust, pad life or stopping distance.

What Mu Actually Means on a Brake Pad

Mu is the coefficient of friction: the friction force a pad generates divided by the force clamping it against the rotor or drum. A pad with higher mu makes more stopping torque for the same pedal effort, which is why a high-friction pad feels more aggressive under your foot.

In theory the number runs from 0 (no friction) toward 1.0. In practice, street pads sit well below that. A trade summary of a Hot Rod technical article notes that decades ago street pads were lucky to reach the high 0.20s, that even original-equipment pads today are well into the 0.30s, that top-tier performance street pads land around 0.40 to 0.45, and that some race compounds run in the high 0.60s or more.

Friction is not a fixed property. The same pad changes grip with temperature, humidity, wear and age, which is why the rating system grades it at two temperature bands instead of one.

How to Read the Friction Letters (Edge Codes)

The letters come from SAE lab practice J661 (J688 is the truck counterpart) and the coding system in SAE J866. Each letter covers a band of measured friction:

The first letter is the normal friction level, taken in the range of roughly 200 to 400 degrees F. The second letter is the hot friction level, taken in the range of roughly 300 to 650 degrees F. So an FF pad measured 0.35 to 0.45 in both bands, while an FE pad drops one band when hot.

How the two letters relate tells you more than either one alone:

• Letters match (FF, GG): friction stays in the same band cold and hot. For a street car this consistency is usually what you want.
• Second letter lower (FE, GF): grip falls off as the pad heats up, which points to a pad that may fade under hard or repeated stops.
• First letter lower (EF, FG): the pad grips better hot than cold, so it may need some heat in it before it bites fully.
• ZZ: the material could not be classified by the J661 test. These codes typically show up on very aggressive performance compounds.

The Rest of the Edge Code

The friction letters sit in the middle of a longer string. The exact layout varies by maker, but the pieces are consistent:

1. Manufacturer code. The first two or three letters identify who actually made the friction material, which is often not the brand on the box. The Automotive Manufacturers Equipment Compliance Agency (AMECA) keeps the list of these codes.
2. Application and formulation. A sequence of four to eight characters that may mirror a catalog application, followed by characters identifying the friction formulation.
3. Friction letters. The cold and hot pair described above.
4. Environmental compliance letter and year. Since 2014, the last three characters are a compliance letter (A, B or N) plus a two-digit year of manufacture. N19 means an N-level formulation made in 2019.
5. Optional date code. Some pads add a four-digit batch code. In one example, 1660 means the pads were produced on the 60th day of 2016.

A simple older example is SCB 115 FF: SCB is the manufacturer, 115 identifies the lining material, and FF is the friction rating.

The A, B and N letters line up with the LeafMark symbol on the box. According to Washington's Department of Ecology, Level A means asbestos, cadmium, chromium, lead and mercury are limited; Level B adds copper under 5 percent by weight; Level N meets all requirements of the law, which phases copper down to under 0.5 percent by weight for pads made after January 1, 2025. An X or WX marking indicates an exemption.

How the Friction Rating Is Tested

The J661 test has its roots in a 1960s Ford procedure, and it still runs on a simulated drum. A Chase machine spins an 11-inch drum at a constant 417 rpm while a fixture presses a 1-inch square sample of friction material against it with a normal force of 150 pounds. Heaters can take the drum from 200 to 650 degrees F, and a load cell records the friction force. The sequence of 11 tests includes baseline runs, two fade cycles, two recovery cycles and wear measurements.

Because mu is friction force divided by normal force, a sample averaging a 42-pound reading at 150 pounds of load works out to 0.28, which is an E. If it reads that in both temperature bands, the pad is marked EE.

That design explains the limits. The test was built as a fast, repeatable quality-control check on incoming lining shipments, not to predict how a pad behaves on your car. It uses a small coupon, not a whole pad, and a drum, not your rotor and caliper.

What Friction Ratings Do Not Tell You

Every trade reference on this topic makes the same point: the edge code is a matching tool, not a performance grade. It does not tell you:

• Stopping distance. A G pad is not guaranteed to stop your car shorter than an F pad. Real-world performance depends on the whole brake system design and operating conditions.
• Noise. The code carries no information about squeal or groan.
• Pad life or rotor wear. Wear is measured during the test but not shown in the code. More aggressive, higher-mu compounds may also increase rotor wear.
• Dust or pedal feel. Neither is graded.

For whole-vehicle performance, the industry uses dynamometer procedures instead. SAE J2430 tests front and rear linings together and is designed to show how a set compares to the federal FMVSS 135 requirements for new brakes. The Dual Dynamometer Differential Effectiveness Analysis (D3EA) compares aftermarket linings against the original linings on a given vehicle platform. Some performance brands skip mu letters and publish their own scales. Hawk, for example, rates its motorsport compounds by torque level and optimal temperature range, from HPX (low torque, 100 to 600 degrees F) to DTC-70 (high torque, 800 to 1,200 degrees F).

Using Friction Ratings When You Buy Pads

The practical rule is to match, not maximize. If the hot and cold friction levels of a replacement pad do not match the originals, it can upset the braking balance of the vehicle. Keep these points in mind:

• Read the letters on the pads you remove and look for replacements in the same band.
• Replace pads in axle sets, and keep the same compound left and right.
• For a daily driver, matched letters such as FF are a sensible target. Our ceramic brake pads for daily drivers are a good place to start for quiet street use.
• If you tow or drive long mountain grades, a pad whose hot letter does not drop below its cold letter matters more than a high first letter.
• Avoid ZZ or track compounds on a street car unless the maker says they suit street use; some are built to work at temperatures a commute never reaches.
• Friction is only half the interface. Put new pads on true rotors from our brake rotor collection, and renew clips and shims with a brake hardware kit so the pads slide and seat evenly.

If you would rather not match compounds piece by piece, the kits in our Power Stop brake kits collection pair pads and rotors meant to work together. You can also browse every compound in our brake pads collection.

Frequently Asked Questions

What does FF mean on a brake pad?

FF means the pad's coefficient of friction tested between 0.35 and 0.45 in both the normal temperature band and the hot band. It is a consistent, mid-range street rating.

Is a GG brake pad better than an FF pad?

Not automatically. GG means higher measured friction, so the pad feels more aggressive, but higher-mu compounds can increase rotor wear, and a big jump from the original friction level can upset brake balance. Better means matching what the vehicle was designed for.

What does ZZ mean on brake pads?

ZZ means the friction material could not be classified by the SAE J661 test. It usually appears on very aggressive performance compounds.

Where is the edge code on a brake pad?

It is printed on the edge of the friction material or on the steel backing plate. Look for a string of letters and numbers with two friction letters in the middle and a compliance letter plus two digits at the end.

Can I have different friction codes front and rear?

Front and rear pads are separate parts and can use different formulations, so a different code between axles is not a problem by itself. What to avoid is mixing compounds on the same axle, or moving one axle far away from its original friction level.

Does the edge code show when the pads were made?

On pads made since 2014, yes. The two digits after the A, B or N compliance letter are the year of manufacture, and some pads also add a four-digit batch or date code.

Sources

1. Tomorrow's Technician - Undercover: The DA Friction Code - C, D, G, H and Z coding bands, J661 Chase machine test parameters, 42 lb example, J2430 and D3EA procedures
2. Tomorrow's Technician - Brake Edge Code Updates Explained - E and F coefficient ranges, normal and hot temperature bands, A/B/N markings, SCB 115 FF and date code examples
3. Brake and Front End - Decrypting Brake Pad Edge Codes - C to H aggressiveness scale, ZZ unclassified codes, edge code structure, limits of edge codes
4. Brake and Front End - Brake Pad Edge Codes - AMECA manufacturer code list, matching hot and cold friction to preserve brake balance, N19 example
5. Counterman - Deciphering Edge Codes: What They Do and Don't - mu definition, J661 and J688 purpose, wear, noise and stopping distance not indicated
6. The BRAKE Report - Disc Brake Pad Friction Codes Explained - typical mu of OE, performance and race pads, meaning of mismatched letters, rotor wear tradeoff
7. Washington State Department of Ecology - Better Brakes Law - LeafMark Level A, B and N definitions and copper limits by date
8. Hawk Performance - Motorsports Compounds - torque level and optimal temperature ranges used instead of mu letters