Tires, Wheels, and Hubs Flashcards

When mounting dual tires on a truck rear axle, what is the MOST important matching requirement between the two tires on the same axle end?

Answer: Both tires must have matching overall circumference (rolling radius)

Dual tires must be matched for overall circumference (rolling radius) because both tires travel the same distance and rotate at the same speed. A significant circumference mismatch causes the smaller tire to scrub and the larger tire to drag, accelerating wear on both and stressing the axle components. Brand, tread pattern, and date code matching are good practices but circumference match is the critical functional requirement.

A truck wheel stud breaks during wheel removal. Upon inspection, the remaining studs show a characteristic 45-degree angular fracture surface. What is the MOST likely cause?

Answer: The lug nuts were under-torqued, causing the wheel to loosen and fatigue the studs

A 45-degree (diagonal shear) fracture on wheel studs is the classic signature of fatigue failure caused by a loose wheel. Under-torqued lug nuts allow the wheel to micro-move with each revolution, cyclically bending the studs. Fatigue cracks initiate at the thread root and propagate at 45 degrees to the stud axis until sudden fracture. Over-torque causes straight tensile fractures or thread stripping, not angular fatigue faces. Bolt-circle mismatch would be immediately obvious. Galvanic corrosion causes surface pitting, not angular fractures.

After replacing a wheel hub bearing assembly on a truck steer axle, the technician must adjust the bearing preload. Which method correctly describes the adjustment sequence?

Answer: Torque the adjusting nut to seat the bearing, back off to zero drag, then advance to the specified low preload torque while rotating the hub

The correct procedure for adjustable (non-unitized) steer axle bearings is: (1) torque the adjusting nut to the initial seating specification while rotating the hub to seat the rollers, (2) back off to zero drag, (3) advance to the final low preload specification while continuing to rotate the hub. This ensures rollers are properly seated and preload is within spec. Backing off a full turn produces excessive end-play. Using a pull force alone without a backing-off step may leave the bearing preloaded instead of adjusted. Finger-tight installation causes dangerous end-play.

A technician inflates a truck tire to the maximum pressure stamped on the tire sidewall instead of the pressure specified in the vehicle manufacturer's placard. What is the likely consequence?

Answer: Harsh ride, uneven tread wear, and potential overloading of wheel components

The maximum pressure on the tire sidewall is a structural limit, not a recommended operating pressure. Inflating to the maximum causes the tread center to bulge, concentrating wear in the center, stiffens the sidewall producing a harsh ride, and may subject the wheel, hub, and suspension to higher peak impact loads than they are designed for. The vehicle manufacturer's placard specifies pressures matched to the vehicle's load and suspension — always follow the placard, not the tire's maximum.

A truck's Tire Pressure Monitoring System (TPMS) warning light illuminates at highway speed but goes out after 15 minutes of driving. What is the MOST likely explanation?

Answer: Normal tire pressure increase from heat build-up bringing pressure back above the warning threshold

Cold-inflation pressure can be several psi below the warm operating pressure because air expands as the tire heats up from friction and flexing. If a tire starts slightly underinflated (close to the TPMS warning threshold), the light may trigger until the tire warms up and pressure rises above the threshold. This is why TPMS specs are based on cold inflation pressure — the vehicle should be parked and tires checked cold to confirm actual inflation status.

When using a torque stick (torque-limiting extension) to tighten truck lug nuts with an impact wrench, which statement is CORRECT?

Answer: Torque sticks are a convenience tool only and must always be followed by a calibrated torque wrench for final verification

Torque sticks (torsion-limiting bars) are calibrated to limit the torque transmitted at a rated value, but their accuracy depends on the impact wrench's hammering frequency, air pressure, and the wrench model — making them a useful time-saving tool but not a precision instrument. Industry best practice requires confirming final torque with a calibrated torque wrench. Re-torquing after initial driving is still recommended to account for wheel seating. Torque sticks do not 'reset' when used in reverse.