The Trailer Alignment Procedure

Before you start the trailer alignment procedure, be sure you have followed all of the instructions on prep for a trailer alignment and have all of the necessary tools laid out. Now you are ready to perform the trailer alignment procedure.

Kingpin Extension

Install the kingpin extension. The kingpin extension must be perfectly level. When the kingpin extension is set up correctly it transfers the exact position of the kingpin to a lower position. If you use the new style kingpin extension, it will self align directly below the kingpin so you can take an accurate measurement. There are no moving parts that break and no figiting with levels. Just install and measure. It's that easy.

Bazooka Tube Installation

The next step in the trailer alignment procedure is to remove all of the hubcaps. Be sure to have a container handy to catch the oil if your trailer is on oil. Many mechanics just quickly flip the hubcap down to catch the oil from the hub. If your trailer is on grease, you can remove the hub caps without a container. Just be sure to cover the caps with a shop rag so that no contaminants get into the grease.

Install the axle extension tubes (bazooka tubes). Do not cross thread the extension tubes when installing them on the axle ends. Screw them on until they just bottom out on the axle threads. Install all 4 bazooka tubes the same way. This ensures that the distance from the center of the axle to the end of the extension tubes is the same.

Trailer Alignment Basics

We are going to take 5 basic measurements: A, B, C, D, and E. The "E" measurement should be taken first. It will measure to make sure the kingpin and the axles are in the center of the trailer body. If the kingpin or one of the axles are not within 1/8" of the centerline of the trailer body. Do not perform a trailer alignment. You must first fix the centerline problem.

Thrust Angle

If the measurement "E" is within tolerance, continue the trailer alignment procedure by attaching a 50' steel tape measure to the kingpin extension or kingpin adapter. Extend the tape to the end of the curbside axle tube extensions on the front axle. Use a tension measuring device to note the tension used to during measurement "A" on the ground infront of the curbside front axle. This is the exact tension required at all 4 axle ends when taking the measurement. Always measure to the front center of the axle tube extension. Write measurement "A" on the ground in chalk.

Move to the roadside axle end of the front axle. Extend the steel tape measure and achieve the same tension on you marked on the ground for measurement measurement "A". Remember to measure to the front center of the axle extension tube. Write measurement "B" on the ground infront of the roadside front axle.

Subtract the smaller measurement from the larger measurement. For example if measurement "A" is 418¼" and measurement "B" is 417¾", subtract B from A and you have a difference of ½". The maximum tolerance is 0.1° or 1/ 8". Because measurement "A" is the curbside and the curbside is non-adjustable, you must move measurement "B" (the roadside) back towards the rear of the trailer to make the measurement longer by ½". Move the axle as per the manufactures instructions and re-measure both A and B. Repeat until the measurement is exactly the same. I know that the tolerance is 1/8". However, by following this trailer alignment procedure we will maximize the fuel mileage per gallon. You will achieve the highest mile per gallon by having the lowest rolling resistance.

Scrub Angle

Now that measurement A and B are identical, the next step in the trailer alignment procedure is to measure the scrub angle or the angle difference between the front axle and the rear axle(s). Because we are measuring from center of axle to center of axle, we have to measure from front of axle extension to front of axle extension. This is the equivalent of finding center to center. It would be too difficult to eye ball the center of the hollow axle extension tube. First measure the curbside which is measurement "C". Use the tape tensioning device. Write that measurement on the ground in front of the curbside rear axle. Next measure distance "D" with the tape measure and the tensioning device. Write measurement "D" on the ground in front of the roadside rear axle.

Now just like we did for the front axle, take the larger distance and subtract the smaller distance. The allowable tolerance is 1/16" for all scrub angles. You should however get the difference in measurement to zero for the goal of lowest rolling resistance. Remember the curbside is a fixed position. So you will have to move the rear axle on the roadside either forward or backwards to align the scrub angle.

If you have multiple axles behind this, remove the rear axle tube extension and install them on the next axle in line. Remember, all scrub angles are measured from the front trailer axle. Set the alignment of all remaining axles.

Finishing touches on the trailer alignment

All of the axles are now in alignment. Remove all of the axle tube extensions. Clean and reinstall all hub caps. Be sure to use new hub cap gaskets. Make sure to add oil to the full level or top off the grease to a full level.

This trailer alignment procedure should be a part of your regularly scheduled maintenance program.

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