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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Preventive Maintenance for Diesel & Heavy Trucks › Lesson 10
TRN-DIES20-06 · Lesson 10 · 14 clock hours · lab

Tires and Wheels: PM Inspection and Inflation

Objective. Students will inspect all tires on a Class 8 tractor for tread depth, irregular wear patterns, sidewall damage, and valve stem condition, measure and record inflation pressures against load-based OEM specifications, and check all wheel-end fasteners for correct torque.

⚠ 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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Why Tire and Wheel PM Matters

Tires are the only part of a Class 8 tractor that touches the road. A single tire failure at highway speed can cause a loss of vehicle control, damage the truck, and injure or kill people. Federal Motor Carrier Safety Administration (FMCSA) regulations set minimum standards for commercial vehicle tires in service, and many carriers set internal standards that are stricter. During preventive maintenance, your job is to catch problems before they become failures.

Safety Before You Start

Before inspecting tires and wheels, apply the parking brake and chock the drive axle wheels you are not working on. If the truck is on a lift, confirm it is seated on rated jack stands—never rely on a hydraulic jack alone. Wear safety glasses and steel-toed boots throughout. When using shop air to inflate tires, never stand directly in front of or behind the tire; cracked or damaged tires can separate violently under pressure. Use a clip-on chuck and a long hose so you can stand to the side. Never inflate a mounted tire beyond the maximum pressure molded on the tire sidewall.

Tread Depth Inspection

Insert the tread depth gauge probe straight into the major groove—not on a wear-bar platform—and press the shoulders flat against the tread surface. Read the gauge and record the result in thirty-seconds of an inch or millimeters, depending on your shop form. Measure at three points across the tread width at each tire position: inboard shoulder, center, and outboard shoulder. Three readings per tire capture wear that a single center measurement would miss.

FMCSA regulations require that steering axle tires have a minimum of 4/32 inch of tread depth in every major groove, and all other tires must have at least 2/32 inch. Many carriers pull tires at 4/32 inch across the board to build in a safety margin. If any reading is at or below the actionable limit, flag the tire for replacement.

Irregular wear patterns tell a story:

Mark any tire with irregular wear with chalk, note the pattern and probable cause on the PM form, and flag for alignment or mechanical inspection.

Tire Inflation Inspection

Check inflation pressure only on cold tires—tires that have not been driven for at least three hours or less than one mile. Heat from driving increases pressure; a hot reading will be higher than the true cold setting and will cause you to bleed out air incorrectly.

OEM and tire manufacturer inflation tables give target pressures based on the rated load on each axle. For a typical Class 8 steer axle carrying around 12,000 lb, the recommended pressure for a common steer tire size is often in the range of 100–110 psi; drive and trailer axles carrying greater loads often require higher pressures—consult the specific table for the tire size and axle rating you have in front of you. Never guess.

Attach the clip-on chuck, stand to the side of the tire, and read the gauge. If pressure is low, add air in short bursts and re-check. If pressure is high, use the gauge bleed valve to release air in small increments. Record the as-found and as-left pressures on the PM form.

Inspect each valve stem: the cap must be present (it is the primary dirt and moisture seal), the stem must not be cracked, cut, or bent, and metal stems must not be corroded to the point of seizing. Replace any missing cap before the truck leaves the shop.

Wheel and Fastener Inspection

Look at each wheel for cracks, especially around the pilot bore (center hole) and at the lug seat area where the nut bears against the wheel. Cracks in these areas are cause for immediate wheel removal and replacement. Check for rust streaking from lug holes, which can indicate movement between the nut and wheel.

To torque-check lug nuts, use a calibrated torque wrench fitted with the correct heavy truck socket (commonly 33 mm or 1-1/2 inch for many hub-piloted wheels, but verify for your application). Work in a star pattern to load the wheel evenly. OEM specifications for hub-piloted disc wheels on Class 8 trucks are typically in the range of 450–500 ft-lb for a ten-stud wheel, but always use the manufacturer's published value for the vehicle you are servicing. Apply torque until the wrench clicks; if a nut moves before the click, note it as under-torque and re-torque. Record the result for every nut on the PM form.

Documenting Findings

Every measurement—tread depth, inflation pressure, and torque status—goes on the PM inspection form with the date, mileage, technician name, and unit number. Any defect that meets an out-of-service criterion under FMCSA regulations must be communicated to the supervisor immediately and the vehicle must not return to service until corrected. Clear, accurate documentation protects the carrier, the driver, and you.

Key terms8

Tread depth gaugeA small probe instrument used to measure the remaining depth of tire tread grooves, typically graduated in 32nds of an inch.
Cold inflation pressureTire pressure measured after the vehicle has been stationary for at least three hours or driven less than one mile, before heat from operation raises internal pressure.
Cupping (scalloping)An irregular wear pattern where the tread shows alternating high and low spots around the tire circumference, often caused by worn shock absorbers or wheel bearing play.
FeatheringAn irregular wear pattern in which each tread rib is worn with a sharp edge on one side and a rounded edge on the other, commonly caused by toe misalignment.
Hub-piloted wheelA disc wheel mounting system where the wheel is centered on the hub pilot diameter rather than on the lug nuts; standard on most modern Class 8 trucks.
Torque wrench (calibrated)A wrench that signals or limits output to a set torque value; must be periodically calibrated to remain accurate and traceable.
Valve stem capThe threaded cover on a tire valve stem that acts as the primary seal against dirt and moisture; required to be present during commercial vehicle inspection.
Out-of-service criterionA FMCSA-defined condition that requires a commercial motor vehicle or component to be removed from service until the defect is corrected.

Worked examples4

1. You measure the tread depth on a steer axle tire at three points: inboard shoulder = 5/32 in, center = 6/32 in, outboard shoulder = 3/32 in. The FMCSA minimum for steer tires is 4/32 in in every major groove. Is this tire serviceable?
  1. List every reading: 5/32, 6/32, 3/32.
  2. Compare each reading to the minimum of 4/32 in.
  3. 5/32 ≥ 4/32 — pass. 6/32 ≥ 4/32 — pass. 3/32 < 4/32 — fail.
  4. Because at least one groove location is below the minimum, the tire fails the inspection regardless of the other readings.
Answer: The tire is NOT serviceable. The outboard shoulder reading of 3/32 in is below the 4/32 in FMCSA minimum for steer axle tires. Flag for immediate replacement and document on the PM form.
2. A drive axle tire on a truck with a rated rear axle load of 34,000 lb (dual tires) is found at 90 psi cold. The OEM inflation table for this tire size at that axle load specifies 100 psi. How much pressure must be added, and what is the percent deviation from specification?
  1. Required pressure = 100 psi. Measured pressure = 90 psi.
  2. Pressure to add = 100 − 90 = 10 psi.
  3. Deviation = (100 − 90) / 100 × 100 = 10 / 100 × 100 = 10%.
  4. Add air in short bursts with the clip-on chuck, standing to the side, until the gauge reads 100 psi. Re-check and record as-left pressure.
Answer: Add 10 psi. The tire was 10% below the specification. Record as-found (90 psi), as-left (100 psi), and date on the PM form.
3. You are torque-checking a ten-stud hub-piloted drive axle wheel. The OEM specification is 450 ft-lb. Your torque wrench is set to 450 ft-lb and you work the star pattern. Three nuts click at or before 450 ft-lb; two nuts turn before the wrench clicks. What do you do with the two nuts that moved, and how do you document this?
  1. Any nut that moves before the wrench reaches the set value was under-torque.
  2. Continue applying torque on each of the two under-torque nuts until the wrench clicks at 450 ft-lb.
  3. After tightening those two, re-check the remaining eight in the star pattern to confirm none were disturbed.
  4. On the PM form, record that two nuts were found under-torque at positions (note location, e.g., top-left and bottom-right), corrected to 450 ft-lb, and re-verified.
Answer: Re-torque the two under-torque nuts to 450 ft-lb, then re-check all ten in sequence. Document the as-found condition (under-torque) and as-left condition (450 ft-lb) on the PM form. Flag for supervisor review per shop policy.
4. A tire on the trailer position is measured at 5/32, 5/32, and 5/32 across the tread width—uniform wear. FMCSA minimum for non-steer positions is 2/32 in. The carrier's internal pull-out standard is 4/32 in. Is the tire in compliance with federal regulation? Does the carrier pull it?
  1. Federal minimum for non-steer tires: 2/32 in. All three readings = 5/32 in. 5/32 ≥ 2/32 — federal standard met.
  2. Carrier internal standard: 4/32 in. All three readings = 5/32 in. 5/32 ≥ 4/32 — carrier standard met.
  3. The tire passes both standards but is approaching the carrier pull point.
  4. Record the readings on the PM form. Note in the remarks that the tire should be monitored closely at the next PM interval.
Answer: The tire is legal under FMCSA regulations and meets the carrier's 4/32 in internal standard. Keep in service but flag for close monitoring. Document all readings and remarks.
In class: Using a tread depth gauge, measure tread depth at three points across the width of each tire position and record; identify and describe any irregular wear pattern (cupping, one-side wear, feathering) and name a likely cause for each pattern found. · Using a calibrated dial or digital tire pressure gauge, check and record cold inflation pressure at every tire position; compare to OEM inflation table for the rated axle load and adjust as needed using the shop air supply and a clip-on chuck. · Torque-check all accessible lug nuts on one drive axle using a calibrated torque wrench to OEM specification; inspect valve stems for caps and condition; inspect wheels for cracks at the pilot bore and at lug seats; document all findings on the PM form.
Materials: Class 8 tractor, tread depth gauge, calibrated tire pressure gauge, torque wrench with heavy truck socket, shop air supply with clip-on chuck, OEM inflation table, PM inspection form, chalk or tire marker
Check: PM inspection form reviewed by instructor: all tread depths and pressures recorded and correctly compared to specification, all lug nuts torque-checked, pass = no undocumented defects and correct torque on all checked fasteners.

Practice check8

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

ASE T-series – Preventive Maintenance
1. According to FMCSA regulations, what is the minimum tread depth required in every major groove of a steering axle tire on a commercial motor vehicle?
ASE T-series – Preventive Maintenance
2. A technician checks tire inflation on a truck that just completed a 200-mile highway run. The pressure reads 10 psi higher than the OEM cold specification. What is the correct action?
ASE T-series – Preventive Maintenance
3. During a PM inspection, you observe that a drive axle tire shows heavy wear on both shoulders with the center tread relatively full. Which condition is the MOST likely cause?
ASE T-series – Preventive Maintenance
4. Which wear pattern is characterized by tread ribs that feel sharp on one side and rounded on the other when you run your hand across the tire?
ASE T-series – Preventive Maintenance
5. When torque-checking lug nuts on a hub-piloted wheel, a technician finds that two nuts rotate before the calibrated torque wrench reaches the specified value. What does this indicate?
ASE T-series – Preventive Maintenance
6. Where on a wheel should a technician look most carefully for cracks during a visual wheel inspection?
ASE T-series – Preventive Maintenance
7. A tire valve stem cap is missing when you check a trailer tire during PM. What is the correct response?
ASE T-series – Preventive Maintenance
8. A scalloped (cupped) wear pattern is found on a steer axle tire. Which mechanical condition is MOST commonly associated with this type of wear?
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