Question Of The Day: Why Do Semi Trucks Have 18 Wheels?


Question: How did 18 wheels become standard for trucks?

Answer: No matter where you go on the road, a huge truck will eventually roll past you with a clap of thunder. Whether it’s hauling electronics bound for the big box store, a fuel for the next truck stop, or vegetables enroute to a large distribution center, these rigs are some of the most important cogs in the industrial machine. Trains and planes may handle the most urgent shipping, but heavy trucks perform the daily grunt work and take the final step in a complex distribution network.

So, how did 18 wheels become a standard for semi trucks? One word: containerization.

Decades before 18 ­wheelers were blowing by on their way to Walmart, cargo was shipped around the world as break bulk. Instead of everything placed within a single container, break bulk shipping meant cargo would be loaded onto a boat or truck individually. The shipping method was tedious and labor­ intensive, and each time a ship would stop on its route, every item would be moved around to accommodate more cargo. Finally, off­loaded cargo would be stored in a warehouse before trucks came to bring those goods to their final destination. It was not the most efficient means by any measure.

Inklings of a different method ­­containerization ­­appeared as early as the late 18th century in the form of coal-­ferrying English box boats and wagon trains. Essentially, shipping companies designed big boxes that can be filled, stacked on a big ocean­going ship, and then off loaded. From there, the boxes can be placed directly onto the back of a truck and driven to its final destination.

The International Organization for Standardization came up with containerization standards between 1968 and 1970, that regulated the size, weight, and the standard focusing on container lengths. Five were chosen, and of those, most containers are either 20­ feet or 40 ­feet long, with the average 40-­foot container capable of carrying up to 60,000 pounds of cargo inside.

However, carrying heavy loads is taxing on pavement. The U.S. Department of Transportation generally restricts weight limits to 80,000 lbs for gross weight, 12,000 lbs for the steering axle, 20,000 lbs per single axle, and 34,000 lbs per tandem unit. To be able to move those containers to and fro over land, more wheels and axles are needed in order to comply with the regulations. That meant 60,000-pound trailers needed at least three axles and 12 tires. Heavier trucks required a third axle on the back for the trailer, and trailers gained a second axle of four wheels. All told, there are four axles with four wheels on each axle, and one axle—the steering axle—with two wheels on it, for a total of 18 wheels.

A coalition of trucking and shipping firms have pressed Congress to let states raise the weight limit for trucks to 91,000 lbs. on six axles, from 80,000 on five today, as a way to reduce costs and move more freight. Trucking firms contend as long as the extra weight is spread out, it will cause no additional road damage—and make 22 wheels, rather than 18, the new standard.

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