# 18-wheelers

What is an 18-wheeler? I’ve seen tractor-trailer and semis on US highways, but have yet to see one with exactly 18 wheels. Do you count the “tractor” (“cab”) ? Or just the trailer (semi)? What is the wheel configuration I should be looking for? Are there tractor-trailer combinations with more than 18 wheels? What’s the upper limit? And how often are they disabled by flat tires?

The wheels are arranged like this:

One pair of single wheels at the front of the tractor = 2
two pairs of double-wheels at the rear of the tractor =8
two pairs of double-wheels at the rear of the trailer = 8

2 + 8 + 8 = 18.

I think most trailers do have 18 wheels - two steering wheels, two drive axles with 4 wheels each, and two axles on the trailer also with 4 wheels each.

Yes you can have more than 18 wheels. You’ll find 98-wheeler “land trains” in Australia.

Two steering wheels on the front of the tractor.
Two driving axles on the rear of the tractor, each with two sets of dual wheels, giving eight wheels.

Two axles on the rear of the trailer, each with two sets of dual wheels, giving eight wheels.

2 + 8 + 8 = 18.

Top view:

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The “18 wheels” thing has been explained.

Many tractor-trailer combinations WILL have more than 18 wheels, often far more; I’ve seen combinations with 28 wheels. On a standard tractor-and-one-trailer combination the most I’ve ever seen is 28; beyond that there’s hardly room for more axles. Generally speaking, you will see more wheels when the trailer is carrying a particularly heavy load. Tankers and gravel trucks, you will notice if you look, tend to have extra axles.

Trucks have weight restrictions on both the load as a whole AND load per axle. A load that is legal as a whole can be illegal on one or more axles. A standard 53-foot trailer combination might be allowed to carry, say, 55,000 pounds as a unit, but if it has only two trailer axles might exceed the allowable weight on those axles. (The exact legal weights are hideously complicated to figure out because it depends on a lot of the vehicle’s specific dimensions and the state or province you are in.) It’s in cases like that that the trailer needs more axles, and hence more wheels. The normal 18-wheel configuration is suitable only for low to medium-weight loads, like foodstuffs or furniture or crap like that. Heavier loads require more axles. If you look like a flatbed trailer carrying heavy equipment or steel or something, you’ll notice it has a LOT of wheels.

Of course, trucking companies will try to use trailers with the minimum possible number of wheels and axles, adding them only when absolutely necessary. Having too many wheels reduces fuel efficiency (because of the added friction; the wheels cost energy) and makes the trailer harder to steer, plus it adds to the cost of the trailer’s purchase and upkeep.

In some jurisdictions, trailers may be fitted with lift axles, wheels that can be raised or lowered as the need dictates, customarily with pneumatic pressure. These are illegal in some places, however, because it makes it easy for drivers to lower the wheels for weigh scales, making the trailer legal, then raising them once they pull onto the highway, making them illegal.

A truck generally will not lose control if any rear tire blows out - in fact, a full combination can lose several tires. A steering axle tire blowout, however, could lead to disaster.

Slight nitpick. They’re called “road trains”.

In Australia, the two most common form of “semi trailer”, as they’re known here, are the 18 wheeler (in the configuration Q.E.D. mentioned), and the 22 wheeler, which has three sets of double axles at the back. The 22 wheeler may even be the slightly more common one here. The raisable axle is sometimes seen as well. Often, this is used to reduce cost on tollways which charge heavy vehicles by the axle.

Some more road train pics are here. They are amazing things. Because of their size, they are not only banned in cities, but also in most rural areas, and only operate long haul runs across desert and extreme outback areas. Here in Sydney, it’s common though to see “B Double” rigs operating on certain approved main arterial roads. These have two trailers, with the rear of the forward trailer having a turntable behind its load for coupling of the rear trailer. This means the centre eset of axles is shared by the two trailers, though belonging to the first. I’m not sure of the exact number of wheels, but I think it’s in the order of 28 or so.

There are some very impressive trailers used in construction. For road construction, for example, they sometimes bring in 200-300 ton pre-built girders for some of the spans, and these need specialized equipment.
http://www.goldhofer.de/eng/html/body_news.html

If you look about one page length up from the bottom they have a 20-axle trailer, for a 92 total tire rig with only one trailer. It’s European, but I’ve seen simlar stuff in America as well.