Why Do Semi Trailers Have So Many Tires?

semi trailer

Semi trailers are also called 18-wheelers, but do you know why semi trailers have so many tires? I’ve been confused about this for a long time, so I searched a lot of trailer forums and finally figured out why. Today I will explain in this article why semi trailers have so many tires.

Carrying More Loads

  1. The U.S. Department of Transportation has standards about loading semi-trailers. They only allow 12,000 pounds for steer axles, 20,000 pounds for each individual axle and 34,000 pounds for tandem axles. Fewer axles means the trailers can carry less weight. Then drivers can only haul less load and make less money on each trip.

  2. In most states in the northern U.S., you can see bridge weight limits posted as “winter limits” and “non-winter limits,” which are usually lower than non-winter limits. This means that trailers can only haul a very small amount of weight on certain bridges and roads. By adding additional wheels, the weight of the trailer can be distributed over a larger surface than a standard tandem or tri-axle trailer, so the trailer may tow close to its full rated load year-round without causing damage to bridges and roads.

Better Stability

Unstable means the truck is either prone to skidding or tipping over.

  1. With these extra tires in contact with the ground on some rough roads, you can have a highly stable vehicle.
  2. When a semi-trailer makes a turn or needs to correct the vehicle quickly, it can cause a weight shift inside the trailer. If you take a standard pickup truck and suddenly shift the weight on its chassis to one side, the pickup will likely have two wheels off the road and can flip. But with an 18-wheeler, they have enough wheels to keep the semi-truck planted. So the semi-truck can stay balanced even if the cargo moves around.
semi trailer

Saving a semi-trailer during a blowout

Since semi-trucks usually carry very heavy weights, if the tires are not strong enough to support the weight of the semi-truck, they will blow out. To prevent this, one way is to use bigger, stronger and more expensive tires. Another affordable way is to use multiple tires. By having 18 tires working together, the weight is distributed over 18 tires. Even if a blowout occurs, another tire next to it will keep the vehicle lumbering along the side of the road and repair the blowout.

semi trailer

What do the wheels that don't touch the ground do?

semi trailer

Some people think that the tire floating on the ground of a semi-trailer is a spare, but it is actually a “drop axle“.

As you know, the more tires there are, the more weight the semi-trailer is allowed to haul. The trade-off is that the vehicle is less maneuverable. This means bigger turns, worse fuel economy, and more difficulty getting through parking lots.

To make semi-trucks easier to drive, some manufacturers have installed these drop axles. When the axle is up and in the “stored” position, these four wheels do not touch the ground. In this position, the semi-truck has the best maneuverability. If a heavier load is carried, then the driver will drop the axle and it will come into contact with the road.

To summarize, the driver can decide whether 18 or 14 wheels need to be in contact with the road by raising or lowering this axle. But no matter how many axles are in contact with the ground, the driver has to comply with the corresponding weight limits and other criteria.

The next time you pass an 18-wheeler with an axle in the air, you’ll know they have a lighter load and they’re choosing a more maneuverable transport device.

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