How to Tell Which Brake Pads Are Worn Without Removing the Wheel

By Core Brake Parts Tech Team — Circleville, OH warehouse

You can usually judge brake pad wear without pulling a wheel: look at the outer pad through the wheel spokes with a flashlight, listen for the squeal of a wear indicator tab, watch for a brake pad warning light, and notice which corner the noise or pull comes from. If the friction material you can see looks thinner than about 1/4 inch, or the center wear slot is gone, the pads on that axle are due. The catch is that the pad you can see is often not the one that wears fastest, so a quick look narrows the problem down but does not replace a full inspection.

How to See the Pads Through the Wheel

Wagner Brake notes that on many vehicles you can see the brake pad through the openings in the wheel. Here is how to do it well:

1. Let the brakes cool. Wagner advises waiting at least an hour after driving before inspecting or handling brake parts. Rotors and calipers get hot enough to burn skin.
2. Turn the steering wheel to full lock. On the front axle this swings the back of the caliper toward you and often gives a better angle between the spokes.
3. Roll the car a few inches if needed. Line a wide gap in the wheel up with the caliper so you are not squinting past a spoke.
4. Shine a flashlight at the caliper. Find the outer pad, which is the steel backing plate with a block of friction material pressed against the outside face of the rotor.
5. Compare friction material to the backing plate. The backing plate is steel and does not wear. The friction block is what you are measuring. Wagner's guideline is that pads thinner than 1/4 inch may be due for replacement.
6. Look for the wear slot. Many pads have a groove cut down the middle of the friction material. Wagner says that if the slot is gone or barely visible, it is time for new pads.

Wide-spoke alloy wheels make this easy. Solid steel wheels, hubcaps and many truck wheels hide the caliper completely, and rear drum brakes cannot be checked this way at all because the shoes sit inside the drum.

Why the Pad You Can See Can Fool You

Most cars and light trucks use a floating (sliding) caliper. The piston sits on the inboard side and pushes the inner pad into the rotor; the caliper body then slides on guide pins to pull the outer pad in from the other side. Because the piston acts directly on the inner pad, it normally wears a little faster. The ASE study material from Free ASE Study Guides puts that normal difference at roughly 2 mm.

The trouble starts when the guide pins corrode or lose their grease. According to that same ASE material, seized pins keep the outboard pad from sliding fully into contact with the rotor, so the inside pad does most or all of the braking. A worn piston seal that does not let the piston retract can also make the inner pad drag. In both cases the outer pad you see through the spokes can look healthy while the inner pad is down to metal.

So treat a through-the-wheel check this way:

• Outer pad thin: the pads on that axle are worn. Replace them.
• Outer pad thick but you hear grinding or squealing on that corner: assume the inner pad is worn and get the wheel off to confirm.
• Outer pad thick and no symptoms: probably fine for now, but have the inner pad measured at your next tire rotation.

Noises, Lights and Other Clues That Point to a Corner

When you cannot see the pads at all, your ears, the dash and the steering wheel tell you which pads are going.

The wear indicator squeal

Many pads carry a small metal tab on the backing plate. NAPA describes it as a small metal protrusion that contacts the rotor before the pad wears out, producing a squeal. It is designed to be annoying. Roll a window down in a quiet parking lot and brake gently; the side that squeals is the side with the tab touching.

Grinding or growling

Wagner says grinding or growling when you press the pedal typically means the pads have worn through and are grinding into the rotors. NAPA calls this metal-on-metal contact, with the steel backing plate rubbing the rotor. At that point the rotor is being damaged with every stop and the car should not be driven until it is fixed.

The brake pad warning light

Some vehicles use an electronic wear sensor instead of a squeal tab. Brembo explains that the sensor monitors pad thickness and triggers a warning light when the pads reach minimum thickness. Many vehicles only put a sensor on one wheel per axle, so the light tells you which axle needs pads, not necessarily which corner is worst. Replace pads on both sides of that axle.

Pulling, pulsation and cleaner wheels

Wagner lists pulling to one side and brake pedal pulsation among the signs of worn or unevenly worn pads. It also points out a clue most people miss: brake dust drops off as pads wear, so wheels that suddenly stay cleaner can mean the pads are near the end.

When a Quick Look Is Not Enough

A through-the-spokes check is a screening test. Pull the wheel, or have a shop do it, when any of these apply:

• You hear grinding, or a squeal that the outer pad does not explain.
• The car pulls when braking or the pedal pulses.
• The outer pad looks tapered, thicker at one end than the other. The ASE guide notes that a single stuck guide pin wears the pad at an angle.
• The wheels hide the caliper and you have no other clues.

With the wheel off you can measure both pads at several points, check that the caliper slides freely on its pins, and measure the rotor. Brembo notes that rotors carry a minimum thickness stamped on them as MIN TH, and should be measured with a caliper at no fewer than four points, using the lowest reading.

The Fix: Which Parts to Buy

Pads are always replaced in axle sets, both wheels on the same axle, so braking stays balanced side to side. From there:

• Pads: choose a set matched to your vehicle from our brake pads for cars, trucks and SUVs. For daily drivers that want low dust and quiet stops, ceramic brake pads are a common choice.
• Rotors: Brembo's general rule is to replace brake discs at every other pad replacement, and to replace them on both sides even if only one is below minimum. If the pads ground into metal, plan on new brake rotors now.
• Hardware: new abutment clips and fresh synthetic brake grease on the guide pins keep the caliper sliding. A brake hardware kit is inexpensive insurance against the uneven wear described above. The ASE material warns that installing pads without servicing a sticking guide pin or piston will produce the same uneven wear again.
• Calipers: if a pin is seized in its bore or the piston will not retract, replace the caliper. Browse replacement brake calipers.
• Wear sensors: Brembo recommends replacing electronic wear sensors with every pad replacement where the vehicle uses them.

How Hard Is the Job?

Checking pads through the wheel takes a minute and a flashlight. Replacing front disc pads on most cars is a moderate DIY job with basic hand tools, a jack, jack stands and a C-clamp or piston tool. Torque the caliper bolts and lug nuts to the values in the factory service manual, pump the pedal until it is firm before moving the car, and bed in the new pads according to the pad maker's instructions. Rear calipers with electronic parking brakes may need a scan tool to retract the piston, so check your service information before starting.

Frequently Asked Questions

Can I check my rear brake pads without taking the wheel off?

If the rear axle has disc brakes and open-spoke wheels, yes, the same flashlight method works. If the rear uses drum brakes, the shoes are inside the drum and cannot be seen without removing the wheel and drum.

How thick should brake pads be before I replace them?

Wagner's guideline is that friction material thinner than about 1/4 inch may be due for replacement, and a missing wear slot means replace them. Your vehicle's service information may list a specific minimum.

Why is only one side squealing if pads wear together?

Often only one pad in the set carries the wear tab, and pads rarely wear perfectly evenly. A sticking caliper on one side will also wear that side faster. Either way, replace the pads on both sides of the axle.

Is it normal for the inner pad to be thinner than the outer pad?

A small difference is normal on floating calipers because the piston pushes the inner pad directly. A large difference, or a tapered pad, points to sticking guide pins or a dragging piston that should be fixed along with the pads.

Do I need new rotors every time I replace pads?

Not always. Measure the rotor against the MIN TH value stamped on it. If it is under that value, scored from metal-on-metal contact or causing pedal pulsation, replace the rotors in pairs.

Sources

1. Wagner Brake - How to Check Your Brake Pads for Wear - checking pads through the wheel, 1/4 inch guideline, wear slot, cooling time, brake dust, grinding and warning lights
2. Wagner Brake - Signs of Worn Brake Pads - pulling, pedal pulsation, tapered and uneven wear symptoms
3. Brembo - Electronic Wear Sensors - how electronic pad wear sensors trigger the warning light and replacing sensors with pads
4. Brembo - Minimum Brake Disc Thickness - MIN TH marking, four-point measurement, replacing discs every other pad change and in pairs
5. NAPA Know How - Noisy Brakes - how the metal wear indicator squeals and what metal-on-metal grinding means
6. Free ASE Study Guides - Sticking Brake Caliper Guide Pins - normal inner pad wear difference, seized pins leaving the inner pad to do the braking, synthetic brake lubricant
7. Free ASE Study Guides - Uneven Brake Pad Wear - tapered pads from one stuck pin and why pads alone will not fix uneven wear