Short summary: a wheel bearing lets the wheel rotate smoothly while supporting vehicle weight. On most Mazda3s the front wheel uses a sealed hub-bearing assembly bolted to the steering knuckle; older designs may use press‑in bearings. The job is: safely lift the car, remove the wheel/brake rotor/caliper, free the hub from the CV axle or steering knuckle, remove the old hub/bearing assembly, install the new unit (or press in the new bearing), reassemble and torque to spec, and check for play and proper operation. Below is a beginner‑friendly, component‑by‑component description, the why/theory, step‑by‑step procedure, safety notes, and common failure modes and mistakes.

THEORY — why bearings fail and what they do
- What a bearing does: it keeps the wheel centered on the hub and lets it rotate with low friction. Think of it like the bearings in a skateboard wheel: steel balls or rollers between an inner and outer ring carry the load while allowing rotation.
- Loads: bearings handle radial loads (weight and cornering forces) and axial loads (braking/acceleration impulses). They also must tolerate shock loads from potholes.
- Sealed hub assembly vs serviceable bearing:
- Sealed hub assembly: bearing, hub and wheel studs are one replaceable unit. No greasing. Common on modern Mazda3s.
- Press-in bearing: inner and outer race and rollers are pressed into the knuckle and the hub is pressed on. Requires a hydraulic press and bearing drivers.
- Why they fail: water/road salt contamination, loss of lubrication, corrosion, shock damage, overloaded conditions, or simply wear. Failure progresses from noise (growl/rumble), to looseness, to ABS faults, to wheel wobble or unsafe steering/braking.

COMPONENTS (every component you’ll touch or should know)
- Wheel and lug nuts: the wheel and fasteners that keep it on.
- Brake caliper: clamps the pads onto the rotor. Components: caliper body, pads, guide pins.
- Brake caliper bracket: holds the caliper and mounts to the knuckle.
- Brake rotor (disc): the rotating surface the pads clamp on.
- Brake pads: friction material inside the caliper.
- Wheel hub / hub assembly: the part that the wheel studs mount to; may include the bearing (sealed unit). If bearing is integral, replacing the hub assembly replaces bearing.
- Wheel studs: threaded studs pressed into the hub for lug nuts.
- CV axle (drive axle) and axle nut: transfers drive torque to the hub on front‑drive cars. The big axle nut holds the CV shaft to the hub.
- Steering knuckle / spindle: the cast iron/steel arm connecting suspension to hub; hub bolts to this.
- Dust shield/backing plate: thin plate behind rotor protecting hub from debris.
- ABS tone ring (reluctor): ring on hub or axle that feeds the ABS sensor; sometimes integral to hub.
- ABS sensor: reads the tone ring; looks like a small probe mounted to knuckle.
- Cotter pins/castle nuts and snap rings (where applicable): locking hardware.
- Bearings/inner/outer races, rollers/balls, seals (if not sealed hub).
- Fasteners: hub bolts, caliper bolts, axle nut, knuckle bolts, etc.

TOOLS & MATERIALS
- Jack and quality jack stands (never rely on jack alone).
- Wheel chocks.
- Lug wrench or breaker bar and socket for lug nuts.
- Torque wrench (essential).
- Sockets: axle nut socket (large, often 32–36mm on many cars), caliper bolts, hub bolts, etc.
- Ratchet, extensions, impact gun (optional), breaker bar.
- Pry bars, flat screwdriver.
- Rubber mallet or hammer (for stuck hub).
- Punch/center punch (to help remove cotter pin).
- Zip ties or wire to hang caliper.
- Penetrant (PB Blaster), wire brush.
- Bearing press and driver (if pressing bearings) or bench vice and drivers for simple press-ins.
- New hub/bearing assembly (OEM or quality aftermarket). If replacing bearing only, get bearings, seals, races and snap ring.
- New axle nut (recommended), new lock washers/cotter pins as required.
- Anti‑seize or light grease (on threads only).
- Brake pad grease (on pad contact points), shop rags, gloves, safety glasses.

SAFETY PRECAUTIONS — read and follow
- Park on level ground, set parking brake and chock opposite wheels.
- Use rated jack stands; never work under a car supported only by a jack.
- Wear eye protection and gloves.
- If pressing bearings, be careful: improper pressing can break parts and cause injury.
- If you smell burnt grease or see heat discoloration on a new bearing, stop and diagnose — that indicates improper preload/installation.

GENERAL STEP-BY-STEP PROCEDURE (front hub/bearing replacement — typical Mazda3)
Note: Exact bolt sizes and torques vary by year/engine. Always verify torque values and any model‑specific procedures in the factory service manual.

1) Prepare and loosen lugs
- Park on level surface, chock rear wheels, set parking brake.
- Slightly loosen (break free) the wheel lug nuts while car is on ground.

2) Raise the car and secure it
- Jack up the car at the recommended jacking point and support on jack stands. Remove the wheel.

3) Remove the axle nut
- Locate the axle nut at center of the hub. Remove any dust cap. While the car may be supported by the jack, it’s safer to support the hub so the wheel can rotate when loosening the axle nut; some prefer to have the wheel lightly on the ground to prevent hub rotation while breaking the nut, then finish removal with wheel off.
- Break the axle nut free with a long breaker bar or impact; remove it. Note: axle nut is torque‑to‑yield in some cars — replace with new.

4) Remove brake caliper and bracket
- Unbolt caliper guide bolts. Hang caliper from suspension with zip tie/wire—do not let it hang by the brake hose.
- Remove the caliper bracket (if necessary) to allow rotor removal. Keep bolts and note orientation.

5) Remove rotor
- Pull the rotor off. If stuck from rust, use penetrating oil and a mallet on the hub face or through the rotor hat to free it.

6) Disconnect ABS sensor and any wiring
- Unplug ABS sensor or remove sensor retaining bolts to free the harness. Be gentle with the sensor; it’s fragile.

7) Remove hub assembly bolts or hub-to-knuckle fasteners
- Many Mazda3s have the hub assembly bolted to the knuckle from the back side with three bolts. Remove those bolts (may be accessible from the back of the knuckle). Apply penetrating oil and use impact or breaker if needed.
- If the hub is rusted, hit the hub flange face with a mallet to break the rust bond. Some hubs have tap holes to push the hub out; consult manual. If hub is press-fit, a puller/press may be needed.

8) Separate the CV axle from hub (if necessary)
- After removing hub bolts, the hub and wheel hub assembly should slide off the splines of the CV axle. You may need to pry slightly; be careful not to damage CV boot. If hub is stuck on the axle splines, you can use a pry bar carefully between knuckle and hub. Do not hammer the CV shaft; you can risk joint damage.
- In some designs you must remove a C‑clip or push the axle inwards and out of the hub to free it.

9) Remove old hub/bearing
- Remove the hub assembly from the knuckle. Inspect knuckle, wheel studs, and tone ring for damage or corrosion.

10) Clean mating surfaces
- Clean the knuckle where the new hub seats, remove rust and debris. Clean threads and apply small amount of anti‑seize to threads only (avoid getting grease on bearing surfaces).

11) Install new hub/bearing
- If using a sealed hub unit: slide new hub onto the CV axle splines and into the knuckle. Align bolt holes and install hub bolts finger tight, then torque to the manufacturer specification in a star pattern.
- If pressing a bearing in (serviceable bearing): use a hydraulic press and proper sized drivers to press bearing into knuckle and install races/hub as required. The force must apply only to the bearing race being pressed; applying force to the wrong race will damage the bearing. If you’re a beginner, buy the sealed hub assembly to avoid pressing.

12) Reinstall CV axle nut
- Install new axle nut and tighten to the correct torque specified by the factory. Some axles require a specific preload/cotter pin seating; follow the service manual. If equipped with a cotter pin and castle nut, align hole and install new cotter pin.

13) Reinstall rotor, caliper bracket, caliper, and ABS sensor
- Put rotor back on. Reinstall caliper bracket and torque bolts to spec. Reinstall brake pads and caliper; torque guide bolts to spec. Plug in ABS sensor and secure harness.

14) Reinstall wheel and torque lugs
- Reinstall wheel, snug lug nuts by hand. Lower the car slightly so wheel contacts ground and torque lug nuts to the specified torque in a star pattern.

15) Final checks and road test
- Spin wheel by hand to check for free rotation and absence of noise or binding.
- Check for play: with car lifted, grasp wheel at 12 and 6 and 9 and 3 and check for axial/radial play. Any discernible looseness suggests problem.
- Short, slow test drive listening for noise and checking ABS light. Re‑check axle nut and lug torque after a short drive.

COMMON FAILURE MODES & WHAT CAN GO WRONG (and how to avoid)
- Continued noise after replacement:
- Wrong part installed (front/back or wrong side), damaged tone ring, warped rotor rubbing.
- Bearing damage or premature failure:
- Over‑torquing axle nut (preloads bearing excessively) or under‑torquing (excessive play). Always use torque wrench and correct value.
- Contamination during installation: touching bearing surfaces with dirty hands, allowing water in, or using the wrong grease. Sealed assemblies reduce this risk.
- Damaging CV joint/boot:
- Pulling axle out roughly or pounding on CV shaft can tear boots or ruin CV joint. Use careful prying and support.
- Damaging ABS sensor or tone ring:
- Snapping sensor wires, magnetizing tools too close, or dropping the sensor can cause ABS faults. Inspect tone ring for cracks or missing teeth.
- Pressing bearings incorrectly:
- Applying force to inner ring when needing to press outer ring (or vice versa) will ruin the bearing. If you don’t have correct press/drivers, buy a hub assembly.
- Wrong hardware or reusing single‑use parts:
- Some axle nuts or lock nuts are torque‑to‑yield or single‑use—replace them. Use new cotter pins.
- Unsafe jacking or inadequate supports:
- Never get under a car on just a jack.

TIPS & ANALOGIES FOR BEGINNERS
- Analogy: the hub/bearing is like a ferris wheel axle: the bearing is the smooth mechanism that lets the wheel rotate under heavy load. If the bearing is rough, the ride gets noisy and the wheel wobbles.
- If the hub looks like one sealed unit (bearing + studs together), buy the whole unit — it’s easier and safer than trying to press bearings.
- Label bolts and take photos as you go if you’re unsure where something fits.
- Keep a clean workspace. Bearings hate dirt.
- If you’re unsure about torching, punching, or pressing, take the car or parts to a shop for the press work — improper pressing is the most common DIY failure point.

MAINTENANCE & DIAGNOSIS BEFORE REPAIR
- Diagnosis checklist for a bad bearing:
- Noise that changes with vehicle speed (growl/rumble), louder in turns.
- Play in the wheel (you can feel it if the wheel wobbles when rocked).
- ABS light or trouble code (if tone ring damaged).
- Heat at the hub after a drive (careful to check).
- If you hear a noise only on turns, test which corner by listening and road testing (left turn loads right bearing, etc.).

FINAL NOTES
- Always get the correct part for your exact Mazda3 year and axle/drivetrain. Sealed hub assemblies are easiest for beginners.
- Use a torque wrench for axle nut, hub bolts, and lug nuts. Improper torque is the main cause of premature failure or immediate problems.
- If any step requires pressing or destructive techniques you don’t have tools for, buy the hub assembly or get professional help.

If you follow the safety rules, use the correct replacement part (sealed hub recommended for beginners), and torque everything to factory specs, you’ll substantially reduce the chance of problems.
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