Pre-Trip Inspection: Tires
12 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.
Sources: 49 CFR 396 Appendix A, 49 CFR 570.62
What you need to know
Steer tires get 4/32, double the 2/32 allowed everywhere else.
A steer tire failure takes your steering with it, so the margin is larger where the consequence is worse.
The tread shall be not less than four thirty-seconds of an inch deep on each front tire of any vehicle other than a trailer and not less than two thirty-seconds of an inch on all other tires.
49 CFR 570.62
2/32 on everything that is not a steer tire.
The usual check is a tread wear indicator, which sits level with the tread at exactly this depth.
not less than two thirty-seconds of an inch on all other tires
49 CFR 570.62
Half an inch.
A larger tire in a dual pair carries more than its share and scrubs the smaller one, so a mismatch destroys both tires and overloads one bearing.
dual tires shall be matched for overall diameter within one-half inch
49 CFR 570.62
A bulge means the structure inside has already failed and only the rubber is holding.
Tread depth is irrelevant: this tire is out of service regardless.
Tires shall be free from chunking, bumps, knots, or bulges evidencing cord, ply or tread separation from the casing.
49 CFR 570.62
The rule says 'or when cuts on the tire are probed'.
A cut that hides exposed cord is still a defect, so the standard expects you to check inside it rather than glance at it. Exposed cord counts whether or not you can see it at a glance.
Tire cords or belting materials shall not be exposed, either to the naked eye or when cuts on the tire are probed.
49 CFR 570.62
The marking is decisive and there is no distance or axle exception.
These tires are built for low speed and will fail at highway speed.
Tires marked “Not For Highway Use” or “Farm Use Only” or other such restrictions shall not be used on any motor vehicles operating on public highways.
49 CFR 570.62
570.62 names all three and none of them depends on tread depth.
A tire can measure legal and still be unfit on any one of these counts.
Tires shall be free from chunking, bumps, knots, or bulges evidencing cord, ply or tread separation from the casing.
Tire cords or belting materials shall not be exposed
Tires marked “Not For Highway Use” or “Farm Use Only” or other such restrictions shall not be used on any motor vehicles operating on public highways.
49 CFR 570.62
Exposed ply or belt puts a steer tire out of service outright.
The structure has failed, and pressure proves only that the inner liner is still holding.
Has body ply or belt material exposed through the tread or sidewall.
49 CFR 396 Appendix A
They deform differently under load, so the pair never shares the work evenly.
The axle is out of service regardless of how well the two tires look individually.
Mixing bias and radial tires on the same axle.
49 CFR 396 Appendix A
Regrooving cuts into rubber the manufacturer left to protect the belts, so it is prohibited on a steer axle outside the narrow urban exception.
Regrooved tire except motor vehicles used solely in urban or suburban service
49 CFR 396 Appendix A
Constant rubbing wears through the stem, and a steer tire losing air suddenly is among the worst failures you can have.
The appendix names it specifically. A steer tire deflating suddenly is among the worst failures there is.
Tire flap protrudes through valve slot in rim and touches stem.
49 CFR 396 Appendix A
Any ply repair rules a tire off a steering axle.
A repair restores the air seal, not the strength of the carcass it was made through.
Boot, blowout patch or other ply repair.
49 CFR 396 Appendix A
Check yourself
- The minimum tread depth on your steer tires is ______.
- The minimum tread depth on your drive and trailer tires is ______.
- What is the tolerance within which the overall diameter of duals must match?
- What is indicated by a bulge in a tire sidewall?
- How should you deal with a shallow-looking cut in a tire?
- May you run a tire marked Farm Use Only on the highway?
- What is it that puts a tire on your vehicle out of service?
- A steer tire with body ply showing through the sidewall is ______.
- May you run a bias tire and a radial on the same axle?
- Is a regrooved tire acceptable on your steer axle?
- Why is your tire flap touching the valve stem a defect?
- What is the meaning of a blowout patch on a steer tire?
Answers are in the practice test for this topic.