One tire of a dual is flat. How far can the truck go on it?

The short answer, and the rule behind it

A flat dual is not a tire you finish the run on, and it is not a tire you nurse to the next truck stop. It is a tire you get off the road on, slowly, to the first place where the truck can stand clear of traffic, and then you stop.

The federal rule is plain. Under 49 CFR 393.75(a), no motor vehicle may be operated on any tire that is flat or has an audible leak. The same paragraph also bars running on a tire with ply or belt material exposed, a tread or sidewall separation, or a cut deep enough to show the ply. The rule does not say a flat is acceptable because it has a partner beside it, and it has no allowance for limping to the next exit.

Moving a truck out of a live lane after a tire goes flat is a matter of immediate safety. Driving on for miles because the truck still feels normal is a different thing, and it is the choice that turns a tire call into something far more expensive. How a regulation applies to your particular stop is a question for your carrier's safety department or the officer on scene, not something this post can settle for you.

What happens to the partner tire

A dual assembly exists so two tires share the load of one wheel position. Each tire in the pair is rated for the load it carries as part of a dual, and that rating assumes its partner is doing half the work. When one tire goes flat, that assumption is gone. Here is what follows:

• The survivor takes the whole load. The good tire is now carrying roughly what two tires were built to carry, well beyond what it is rated for. A tire loaded past its rating flexes far more than it was designed to.
• Flexing turns into heat. Every revolution bends an overloaded sidewall, and that bending builds heat inside the casing. Heat is what separates plies and belts from one another, which is why an overloaded tire can fail without warning after miles of feeling fine.
• The damage may not show. A partner tire that survived being overloaded can have internal heat damage nobody can see from the outside. That is why a tire that ran alongside a flat is looked at carefully, and often replaced, rather than assumed to be good.
• A second failure is worse than the first. Once both tires on a position are gone, the wheel is running on its rim, and the truck is no longer something that can be driven to a safe spot at all.

What happens to the flat tire, the wheel and the truck

The flat tire is not simply along for the ride. A tire running with little or no air is folded under the load on every revolution, and it destroys itself quickly. Rubber and cord break down, the bead can come off its seat, and chunks of tread can tear loose and hit the brake lines, air lines, wiring and mudflaps behind it, or the vehicles following you.

The wheel suffers too. A flat tire lets the rim get close to the road and to the partner tire, and a wheel that has been run that way can be bent or have its bead seat damaged. Loose or overheated wheel hardware can follow.

Then there is fire. A flat tire dragged along under load generates a great deal of heat, and a tire hot enough can smolder and ignite. Tire fires are hard to put out with a cab extinguisher, and they spread to the trailer and whatever it is carrying. If you smell hot rubber or see smoke from a wheel position, stop as soon as you safely can, get clear of the truck and call 911 before you call anyone else.

How to tell an inside dual has gone flat

An outside dual that is flat is usually obvious. An inside dual is hidden behind its partner, and the truck rides almost normally on the survivor, which is exactly why inside flats get missed at a walk-around and found at a scale. These are the signs worth knowing:

• The outer tire looks wrong. When the inner tire loses air, the outer one takes the load and its sidewall bulges more than the outer tires on the other positions. Compare it with the same position on the other side.
• The two tires are touching. A flat inner tire sags toward its partner. Sidewalls rubbing or pressed together at the bottom of the assembly mean one of them has lost air.
• Heat and smell. A hot-rubber smell at a stop, or a wheel position that is noticeably hotter to stand near than the others, is a warning. Do not grab a tire or wheel to check; hold the back of your hand near it instead.
• A tire pressure alert. If the truck or trailer has tire pressure monitoring, a low-pressure warning on a dual position is worth stopping for rather than dismissing as a sensor fault.
• A thump or a shimmy. A flapping or breaking-up tire can show up as a rhythmic noise or vibration that changes with road speed.
• A gauge reading. The only reliable check is a pressure gauge on the valve. Thumping a tire with a bat will not reliably find a soft inner dual, because the outer tire hides it.

What to do once you know

Once you know a dual is flat, or suspect it, work through these steps in order:

• Slow down gently. No hard braking and no sudden steering. Signal early and move toward the shoulder or the next place with real width.
• Stop somewhere that protects you. A wide shoulder, a ramp with room, or a lot is far better than a narrow shoulder beside a live lane. Get there at low speed, and do not pass a safe stopping place looking for a better one.
• Flashers, then triangles. Hazard flashers go on as soon as you stop, and the warning triangles go out as 49 CFR 392.22 requires. Our post on where to place warning triangles covers the distances.
• Look from the safe side. Check the position from the side of the truck away from traffic if you can. Never crawl under the truck or trailer on a shoulder.
• Read the size. The size is molded into the sidewall. If the flat tire's sidewall is damaged, its partner almost always carries the same size.
• Note anything else hit. Look for dangling air lines, damaged wiring, a torn mudflap or a hub that is smoking or much hotter than the rest.

What the service truck does at the roadside

Our mobile tire service replaces steer, drive and trailer tires where the truck is standing, inside and outside duals included, with commercial tires in the common sizes carried on board. On an inside dual the outer wheel comes off first, the flat is replaced, and the assembly goes back together with the wheel nuts torqued properly.

Because a flat dual loads its partner, the partner tire is inspected as part of the job rather than assumed to be sound. A tire that has run flat is replaced, not aired back up: its casing can be damaged in ways that are not visible, and a weakened casing can fail violently when it is reinflated. A clean puncture in the tread of a tire that was caught before it ran flat is sometimes repairable, and we will tell you honestly which you have.

Broken studs, damaged nuts and the air lines, wiring and lights that a flailing tire takes with it are handled at the roadside as part of the call. Our post on how long a roadside tire replacement takes explains what decides the time on scene.

When it is not a roadside tire job

Some flat duals turn out to be more than a tire. If the hub or wheel bearing has run hot, with burned grease, a discolored hub or a wheel end too hot to stand near, that is shop work: the wheel end has to come apart, and the truck is towed rather than asked to carry the load any further. The same goes for a wheel end that has been damaged badly enough to need parts nobody carries on a service body.

A wheel that has been bent or cracked by running flat should not carry a new tire. Say so when you call, because it changes what the job involves. And to be clear about the rest: we do not retread tires, run a walk-in tire counter or do wheel alignment. If the tire is hard to identify, our guide to reading semi truck tire sizes shows what to read off the sidewall, and if the tire failed suddenly at speed, what to do after a blowout covers the first minute.

Sources

• 49 CFR 393.75, Tires
• 49 CFR 392.22, Emergency signals; stopped commercial motor vehicles

Related reading

Semi Truck Tire Sizes Explained: 11R22.5, 295/75R22.5 — In 11R22.

How Long Does a Roadside Semi Tire Replacement Take? — The tire change is usually the shortest part of a roadside tire call.

Truck Tire Blowout on the Highway: Do These Five Things — Do not brake hard and do not fight the wheel with sudden corrections.