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Rgp . 02, 2024 17:02 Back to list

An 18 Wheeler! Which Is the Fifth Wheel?

An 18 Wheeler! Which Is the Fifth Wheel?

Which is the fifth wheel? The answer is “none”, if you thought you had to select among the 18 wheels that is.  Our articles are balanced so that we provide good information for both the newbie and the seasoned trucking dispatcher.  So, the fifth wheel is the plate above the tractor’s back dual wheels where the trailer’s king pin hooks to the tractor.

The Old Days

The name derives from the horse drawn carriage days where a trailer was attached to the “wheel” like attachment on the rear of the carriage.  A carriage had 4 wheels and so this was then called the “fifth wheel”.  So we’ve added a whole bunch of other wheels since those days, but the fifth wheel provides the same duty.
carriage

High quality JSK casting fifth wheel 37C

The King Pin is King

The king pin and the fifth wheel are the only two components working together to keep the trailer behind and attached to the tractor.  Of course this puts a lot of pressure, no pun intended, on keeping up with safety concerns.  Because the service plate on the trailer rotates on the fixed position of the fifth wheel, a lot of grease is used to keep this working smoothly. King Pin locks in different shapes and sizes are available that prevent trailer theft. This unfortunately is a big issue in the van trailer industry, so when parking a loaded van trailer you should really have it secured.
king pin lock

Adjusting the Fifth Wheel

A fifth wheel can also be slid forward or backward over the tractor duals in order to adjust weight distribution.  The same way you would adjust the trailer tandems to adjust for weight.  For example if you needed more weight on your tractor duals you could adjust the fifth wheel forward and that would shift more weight to the tractor duals.  A notch forward or backward changes weight distribution about 500 lbs.

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