While inspecting the running gear of a three-axle commercial straight truck prior to depar… — Commercial Driver's License (CDL) General Knowledge Exam

While inspecting the running gear of a three-axle commercial straight truck prior to departure, a driver takes physical measurements and checks wheel components across all positions. Which of the following findings complies fully with commercial vehicle safety regulations and allows the truck to remain in service?

A
The steer axle tires are original-tread radials measured at $3/32\text{ inch}$ tread depth in all major grooves.
The drive axle dual tires are evenly spaced without touching and have a measured tread depth of $3/32\text{ inch}$ in all major grooves.Answer
C
The front steer tires have $2/32\text{ inch}$ of remaining tread depth and exhibit no fabric exposure or sidewall damage.
D
A drive axle dual set has an outer tire measuring $4/32\text{ inch}$ while the inner companion tire measures $1/32\text{ inch}$ in one major groove.

Answer

The drive axle dual tires being evenly spaced without touching and having a measured tread depth of

3/32\text{ inch}

in all major grooves.

Commercial motor vehicle regulations mandate a minimum tread depth of

2/32\text{ inch}

in every major groove for drive and trailer tires, along with adequate spacing between dual tires to avoid friction and heat buildup. Because

3/32\text{ inch}

exceeds the

2/32\text{ inch}

minimum and the tires are properly separated, this condition is fully compliant.

Step-by-Step Solution

Identify the minimum legal tread depth standards for commercial motor vehicles by axle position.

Steer axle (front) tires require a minimum tread depth of

4/32\text{ inch}

in every major groove. All other positions (drive axles and trailer axles) require a minimum tread depth of

2/32\text{ inch}

in every major groove.

Federal Motor Carrier Safety Administration (FMCSA) and CDL inspection regulations establish different safety thresholds based on the critical directional control provided by steer tires.

Evaluate the steer axle observations against the

4/32\text{ inch}

minimum standard.

Tread depths of

3/32\text{ inch}

and

2/32\text{ inch}

on the steer axle are both below the mandatory

4/32\text{ inch}

threshold and represent safety violations.

Insufficient steer axle tread depth drastically reduces traction and increases the risk of hydroplaning and steering loss.

Evaluate the drive axle conditions against the

2/32\text{ inch}

standard and dual assembly requirements.

The drive tires with

3/32\text{ inch}

tread depth exceed the

2/32\text{ inch}

minimum requirement, and their proper spacing prevents heat buildup from duals rubbing together. In contrast, any tire measuring

1/32\text{ inch}

fails the non-steer minimum requirement.

A vehicle component is only compliant when all tires on that axle meet or exceed their respective depth minimums and physical separation criteria.

Key Concept

Minimum Commercial Vehicle Tire Tread Depth and Dual Assembly Standards

Estimated Time:1m 30s

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