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TRN-AUTO19-04 · Lesson 11 · 22 clock hours · lab

Tire Service: Inspection, Mounting, Balancing, and Rotation

Objective. Students will demount and mount a tire using a tire changer, perform static and dynamic balance using a computer wheel balancer, correctly identify four tire wear patterns and their causes, and execute a five-tire rotation pattern to OEM specification.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Overview

Tire service is one of the highest-volume tasks in any shop. Done correctly, it extends tire life, improves fuel economy, and keeps vehicles safe. Done carelessly, it can destroy an expensive wheel, cause a blowout at highway speed, or injure a technician. This lesson covers the four core skills: demounting and mounting, balancing, wear-pattern diagnosis, and rotation.

Safety First

Before any tire work begins, review these non-negotiable rules:

Demounting and Mounting a Tire

Start by releasing all air from the tire and removing the valve core. Lubricate both beads generously with tire bead lubricant — a water-based paste formulated for the purpose. Never use petroleum-based grease, which degrades rubber.

Bead breaking: Position the wheel so the bead-breaker shoe contacts the tire sidewall close to — but not on — the rim flange. Apply pressure until the bead breaks free. Repeat around the circumference and on both sides.

Demounting: Seat the wheel on the changer turntable and secure it with the cone or clamping jaws. Drop the bead into the center well (the drop center) on the opposite side from where you are working — this is the only way the bead has enough slack to come off the rim. Use the demount head and bar to work the bead over the rim flange. Never rush or pry aggressively against the rim; alloy wheels scratch and crack easily.

Mounting: Inspect the rim for corrosion, cracks, and bent flanges before mounting a new or rotated tire. Install a new valve stem every time the tire is dismounted — valve stems are inexpensive; blowouts are not. Lubricate both beads, seat the tire on the rim, and use the mount head to work the second bead over the flange, again using the drop center.

Inflation: Seat the beads by inflating carefully. After seating, set the tire to the pressure shown on the vehicle door-jamb placard, not the maximum sidewall pressure. Use a calibrated hand gauge to verify — tire changer gauges are not precise enough for a final reading.

Static and Dynamic Balance

An out-of-balance tire/wheel assembly causes vibration felt in the steering wheel or seat, accelerates bearing wear, and creates uneven tire wear.

A computer wheel balancer measures both types simultaneously. Mount the assembly on the balancer shaft using the proper cone and flange so the wheel is centered exactly as it sits on the hub. Enter the wheel width, diameter, and weight-placement distance (offset) as prompted. Spin the wheel; the machine calculates the amount and location of weight needed on each plane.

Install clip-on weights on steel wheels (and alloy wheels with a standard lip). Use adhesive (stick-on) weights on alloy wheels without a clip surface, or when appearance requires a hidden placement. Re-spin after adding weights. The target is 0.25 oz (roughly 7 g) or less of residual imbalance on each plane. If the reading is higher, recheck weight placement and re-spin.

Tire Wear Patterns and Their Causes

Reading wear patterns is diagnostic — a worn tire tells you what went wrong mechanically.

| Pattern | What You See | Root Cause |

|---|---|---|

| Center wear | Tread worn in the middle, edges OK | Chronic overinflation |

| Edge (shoulder) wear | Both edges worn, center OK | Chronic underinflation |

| One-sided wear | One shoulder worn more than the other | Incorrect camber or toe alignment |

| Cupping/scalloping | High-low wavy wear around circumference | Worn shocks/struts or wheel imbalance |

Measure tread depth with a tread depth gauge at three points across the tread: outer edge, center, and inner edge. New tires typically start at 10/32" to 11/32". The Uniform Tire Quality Grading (UTQG) wear indicators — molded bars in the grooves — appear at 2/32", which is the legal minimum in most states. Classify each tire: serviceable (above 4/32"), monitor (2/32"–4/32"), or replace (at or below 2/32", or with sidewall damage).

Inspect sidewalls for bulges (internal belt or ply separation — replace immediately), cuts, and weather cracking (ozone checking). Any bulge is an immediate replace classification regardless of tread depth.

Five-Tire Rotation Patterns

Regular rotation equalizes wear across all tires, extending overall set life. OEM rotation intervals and patterns vary — always follow the vehicle's service manual. A five-tire rotation includes the full-size spare (same size and construction as the four road tires).

A common five-tire pattern for non-directional tires moves the spare into the right-rear position, the right-rear to the right-front, the right-front to the left-rear, the left-rear to left-front, and the left-front becomes the new spare. Document the pattern in the rotation diagram on the worksheet, confirm with the service manual, and record mileage. After remounting wheels, torque lug nuts to OEM spec in a star pattern using a torque wrench — never an uncalibrated impact gun alone for final torque.

Key terms8

Drop centerThe recessed channel in the center of a wheel rim that provides the slack needed to work a tire bead on or off the flange.
Static imbalanceA single heavy spot in one plane of the tire/wheel assembly that causes vertical hopping; corrected with one weight at the heavy spot.
Dynamic imbalanceA rocking or wobble caused by heavy spots in two different planes of the assembly; requires weights on both inner and outer planes.
Bead lubricantA water-based paste applied to tire beads to allow them to slide over the rim flange without damage during mounting and to seat evenly during inflation.
Vehicle placard pressureThe recommended cold inflation pressure printed on the door-jamb or fuel-door label, specific to that vehicle's load and handling requirements — not the maximum sidewall pressure.
Tread depth gaugeA calibrated tool that measures remaining tread depth in 32nds of an inch, used to classify a tire as serviceable, monitor, or replace.
Cupping (scalloping)An irregular, wavy wear pattern around the tire circumference caused by wheel bounce from worn shock absorbers or struts, or by severe wheel imbalance.
Five-tire rotationA rotation pattern that includes the matching full-size spare tire, cycling all five tires through each wheel position to equalize wear.

Worked examples4

1. The computer balancer shows the outer plane needs 1.75 oz and the inner plane needs 0.25 oz. You have clip-on weights in 0.25 oz increments. What weights do you install, and does the inner plane meet the acceptance standard before you add weight?
  1. Check the acceptance standard: 0.25 oz or less is acceptable on each plane.
  2. Inner plane reading is exactly 0.25 oz — this is at the limit. To be safe and meet the ≤0.25 oz standard, install one 0.25 oz weight on the inner plane at the indicated position.
  3. Outer plane reads 1.75 oz. Divide by the increment size: 1.75 ÷ 0.25 = 7 increments. Install seven 0.25 oz clip-on weights at the indicated position, or combine into larger weights that sum to 1.75 oz (e.g., one 1.50 oz + one 0.25 oz).
  4. Re-spin the balancer. Both planes must now read 0.25 oz or less to pass.
Answer: Install 1.75 oz on the outer plane and 0.25 oz on the inner plane at the balancer-indicated positions, then re-spin to confirm both planes are at or below 0.25 oz.
2. A tire tread depth gauge reads 3/32" at the outer edge, 5/32" at the center, and 2/32" at the inner edge. How do you classify this tire and why?
  1. Compare each reading to the classification thresholds: replace at or below 2/32"; monitor between 2/32" and 4/32"; serviceable above 4/32".
  2. Outer edge: 3/32" — falls in the monitor range (above 2/32" but at or below 4/32").
  3. Center: 5/32" — serviceable (above 4/32").
  4. Inner edge: 2/32" — at the legal wear-indicator limit; classify as replace.
  5. The overall tire classification is determined by the worst reading: inner edge at 2/32" means the tire must be classified as replace.
Answer: Classify as REPLACE. The inner edge has reached the 2/32" legal minimum, which triggers an immediate replace classification regardless of the better readings elsewhere. The one-sided wear also indicates a possible alignment problem (camber or toe) that should be investigated.
3. You are inflating a tire after mounting. The sidewall shows MAX 51 PSI. The door-jamb placard shows 35 PSI front / 33 PSI rear. This tire goes on the left front. What pressure do you set, and what is the consequence of using the sidewall number instead?
  1. Identify the correct pressure source: the vehicle door-jamb placard is the OEM specification for safe operation of that vehicle.
  2. The left-front placard pressure is 35 PSI. Set the tire to 35 PSI and verify with a calibrated gauge.
  3. The sidewall maximum (51 PSI) is the tire's structural limit, not a recommended operating pressure.
  4. Inflating to 51 PSI on a vehicle specified for 35 PSI would cause center tread wear, reduced contact patch (reduced grip), harsher ride, and greater susceptibility to impact damage.
Answer: Set the tire to 35 PSI as shown on the door-jamb placard. Using the sidewall maximum pressure would overinflate the tire, causing center wear, reduced traction, and potential handling problems.
4. After a five-tire rotation, lug nuts must be torqued to 100 ft-lb. Your torque wrench clicks at 100 ft-lb. Describe the correct tightening sequence for a five-lug wheel.
  1. Hand-tighten all five lug nuts to seat them evenly.
  2. Use a star (pentagram) pattern — tighten one lug, then move to the lug approximately opposite (not the adjacent one), continuing around in a star sequence.
  3. Apply torque in at least two passes: first snug all five to roughly 50 ft-lb in star pattern, then torque all five to the final 100 ft-lb in the same star pattern.
  4. Confirm the wrench clicks on each nut at 100 ft-lb. Advise the customer to have lug nuts re-checked (re-torqued) after the first 50–100 miles of driving.
Answer: Torque all five lug nuts to 100 ft-lb using a star pattern in at least two passes, ending with a final pass at full torque. Recommend a re-torque check after 50–100 miles.

The math in this lesson1

Learners choose 0.25 oz clip-on weights and compare tread depths in 32nds with replacement limits.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Students demount and mount an assigned tire on a tire changer following correct bead-breaking and lubrication procedure, then inflate to the vehicle placard pressure using a calibrated gauge — not the changer's built-in gauge alone. · Students mount the wheel on a computer wheel balancer, spin to measure imbalance, install wheel weights in the correct positions (clip-on or adhesive per wheel type), and re-spin to confirm balance within 0.25 oz or less. · Students inspect a set of five tires for wear patterns, tread depth with a depth gauge, and sidewall condition, classify each tire as serviceable, monitor, or replace, and complete a rotation order diagram to OEM specification using the service manual.
Materials: Tire changer machine, computer wheel balancer, clip-on and adhesive wheel weights, tire lubricant (bead lube), calibrated tire pressure gauge, tread depth gauge, replacement valve stems, torque wrench and lug nut socket set, OEM tire rotation diagrams, tire inspection worksheet
Check: Instructor observes bead drop and tire-changer operation for safety compliance throughout; final balance reading must be within 0.25 oz on the balancer display; tread depth measurements and wear classification reviewed on worksheet — all three items must pass for lab credit.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

A4 Steering and Suspension / Shop Safety
1. A technician is about to seat a tire bead during inflation. What is the safest body position?
A4 Steering and Suspension
2. Which inflation pressure should be used when setting a tire after mounting it on a vehicle?
A4 Steering and Suspension
3. A tire shows excessive wear on both outer edges while the center tread is relatively deep. The most likely cause is:
A4 Steering and Suspension
4. A cupped (scalloped) wear pattern on the tread circumference most directly indicates:
A4 Steering and Suspension
5. The computer wheel balancer shows the outer plane requires 2.00 oz and the inner plane requires 0.00 oz of correction. This type of imbalance is best described as:
A4 Steering and Suspension
6. A technician measures tread depth at 2/32" across the full width of a tire. Which classification applies?
A4 Steering and Suspension
7. When mounting a tire on an alloy wheel that has no clip-on lip, which type of wheel weight should be used?
A4 Steering and Suspension
8. During a five-tire rotation that includes a full-size matching spare, what is the primary benefit over a four-tire rotation?
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