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Position Of The Trailer Tandems

Sliding your trailer tandems toward the front or back of the vehicle will primarily change the weight distribution between the tractor's drive axles and the trailer tandems. Sliding the tandems can also affect the weight on the steering axle, but to a rather small degree. For the purpose of focusing exclusively on positioning the trailer tandems, we're going to ignore the minor changes to the weight of the steer axle. It's rarely a factor when deciding where to place your trailer tandems.

By sliding the trailer tandems forward, you will put more weight on the trailer tandems and take weight off the tractor's drive axles. Conversely, by sliding the trailer tandems toward the rear of the trailer, you will take weight off the trailer tandems and put more weight onto the tractor's drive axles.

Amount Of Weight Moved Per Hole

tractor trailer slide rail for sliding tandems

The trailer tandems lock into place with a set of locking pins that slide into holes drilled into the tandem slider rail. You can see the holes in the slider rail in the picture to the left (click to enlarge).

The distance between the locking pin holes on the trailer will be the main factor affecting how much weight is moved per hole. This distance varies between the different trailer manufacturers, and the larger the distance between the holes, the more weight will be shifted with each hole.

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing. This will help you estimate how many holes you'll have to slide the trailer tandems to move the proper amount of weight necessary in order to get the axle weights legal.

Example 1:

You scale your truck at a truckstop and receive your scale ticket. The slider rail hole spacing is 6 inches which will move 400 pounds of weight per hole, and let's assume the steer axle weight won't change. What would you have to do in order to get your truck legal?

  • Steer:
    11,590
  • Drives:
    34,700
  • Trailer:
    32,100
  • Gross:
    78,390

In this example, the drive axles are overweight (34,000 pound maximum) by 700 pounds. Assuming each hole in the tandem slider rail moves 400 pounds, you would have to slide your trailer tandems toward the front of the truck a minimum of two holes to transfer enough weight from your drive axles to your trailer tandems in order to get your axle weights legal. The end result would be approximately:

  • Steer:
    11,590
  • Drives:
    33,900
  • Trailer:
    32,900
  • Gross:
    78,390
  • Steer: remains unchanged for this example
  • Drives: 34,700 - 800 (2 holes x 400 pounds each) = 33,900
  • Trailer: 32,100 + 800 (2 holes x 400 pounds each) = 32,900
  • Gross: Remains the same

Example 2:

You scale your truck at a truckstop and receive your scale ticket. The slider rail hole spacing is 4 inches which will move 250 pounds of weight per hole, and let's assume the steer axle weight won't change. What would you have to do in order to get your truck legal?

  • Steer:
    11,320
  • Drives:
    31,300
  • Trailer:
    35,050
  • Gross:
    77,670

In this example, the trailer tandems are overweight (34,000 pound maximum) by 1,050 pounds. Assuming each hole in the tandem slider rail moves 250 pounds, you would have to slide your trailer tandems toward the rear of the truck a minimum of five holes to transfer enough weight from your trailer tandems to your drive axles in order to get your axle weights legal. The end result would be approximately:

  • Steer:
    11,320
  • Drives:
    32,550
  • Trailer:
    33,800
  • Gross:
    77,670
  • Steer: remains unchanged for this example
  • Drives: 31,300 + 1,250 (5 holes x 250 pounds each) = 32,550
  • Trailer: 35,050 - 1,250 (5 holes x 250 pounds each) = 33,800
  • Gross: Remains the same

So as you can see, this is really simple math. Again, we're assuming the weight won't change on the steer axles for these examples, but in reality it might change a little bit. Most of the time the change in weight of the steer axle will have little or no affect on where you'll put the trailer tandems. The weight on the steering axle is significantly altered by the amount of fuel you have and the position of your 5th wheel, both of which we'll cover soon.

There it is folks - the #1 most important concept in redistributing weight across axles. Sliding the trailer tandems forward adds weight to the tandems, sliding them back removes weight from the tandems.
Memorize these numbers. You will be quizzed about this and you'll have to remember these in order to estimate how far to move the trailer tandems.

Review Questions - Click On The Picture To Begin...

What are the two most common tandem slider rail hole spacings, and roughly how much weight is transferred per hole?
  • 6 inches apart - 250 pounds per hole
    10 inches apart - 500 pounds per hole
  • 6 inches apart - 350 pounds per hole
    8 inches apart - 550 pounds per hole
  • 4 inches apart - 450 pounds per hole
    6 inches apart - 200 pounds per hole
  • 4 inches apart - 250 pounds per hole
    6 inches apart - 400 pounds per hole

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

There will be some variance here, but generally speaking these are very good estimates and they work very well. You'll learn to recognize the spacing by glancing at them eventually, but in the beginning you might want to measure quick so you know how far to slide the tandems.
Next
Sliding your trailer tandems toward the front or back of the vehicle will primarily change the weight distribution between which sets of axles?
  • The steer axle and the trailer tandems
  • It only removes weight from the trailer tandems
  • The tractor's drive axles and the trailer tandems.
  • The tractor's drive axles and the steer axle

Quote From The CDL Manual:

Sliding your trailer tandems toward the front or back of the vehicle will primarily change the weight distribution between the tractor's drive axles and the trailer tandems.
Prev
Next
You receive a scale ticket and your drive axles are 35,400 and your trailer tandems are 31,100. Based upon the two most common tandem slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and in which direction?
  • 6 inch spacing - slide tandems back 5 holes
    4 inch spacing - slide tandems back 7 holes
  • 6 inch spacing - slide tandems forward 5 holes
    4 inch spacing - slide tandems forward 7 holes
  • 6 inch spacing - slide tandems forward 4 holes
    4 inch spacing - slide tandems forward 6 holes
  • 6 inch spacing - slide tandems back 4 holes
    4 inch spacing - slide tandems back 6 holes

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

With 35,400 pounds on the drive axles you must move a minimum of 1,400 pounds off the drive axles. To shift weight off the drive axles and onto the trailer tandems, you slide the trailer tandems forward toward the front of the truck.

With 6 inch spacing you're moving 400 pounds per hole so you need to move 4 holes to shift a minimum of 1,400 pounds (400 x 4 = 1,600).

With 4 inch spacing you're moving 250 pounds per hole so you need to move 6 holes to shift a minimum of 1,400 pounds (250 x 6 = 1,500).
Prev
Next
Your trailer has a 6 inch hole spacing in the tandem slider rail and your trailer tandems are 900 pounds overweight. What is the minimum number of holes you will have to slide the tandems to get the tandems within legal limits?
  • 3 holes
  • 1 hole
  • 4 holes
  • 2 holes

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

6 inch hole spacing means you'll move approximately 400 pounds per hole. In order to move a minimum of 900 pounds, you have to move at least 3 holes (400 x 3 = 1200).
Prev
Next
You receive a scale ticket and your drive axles are 32,200 and your trailer tandems are 35,100. Based upon the two most common tandem slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and in which direction?
  • 6 inch spacing - slide tandems back 3 holes
    4 inch spacing - slide tandems back 5 holes
  • 6 inch spacing - slide tandems forward 3 holes
    4 inch spacing - slide tandems forward 5 holes
  • 6 inch spacing - slide tandems forward 2 holes
    4 inch spacing - slide tandems forward 4 holes
  • 6 inch spacing - slide tandems back 2 holes
    4 inch spacing - slide tandems back 4 holes

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

With 35,100 pounds on the tandems you must move a minimum of 1,100 pounds off the trailer tandems. To shift weight off the trailer tandems and onto the drive axles, you slide the trailer tandems back toward the rear of the truck.

With 6 inch spacing you're moving 400 pounds per hole so you need to move 3 holes to shift a minimum of 1,100 pounds (400 x 3 = 1,200).

With 4 inch spacing you're moving 250 pounds per hole so you need to move 5 holes to shift a minimum of 1,100 pounds (250 x 5 = 1,250).
Prev
Next
You receive a scale ticket and your drive axles are 31,700 and your trailer tandems are 35,100. Based upon a trailer with 6 inch slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and what would the final weights be?
  • Slide tandems forward 4 holes - 33,300 tandems - 33,100 drives
  • Slide tandems back 4 holes - 33,300 tandems - 33,100 drives
  • Slide tandems back 3 holes - 33,900 tandems - 32,900 drives
  • Slide tandems forward 3 holes - 33,900 tandems - 32,900 drives

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

With 35,100 pounds on the trailer tandems you must move a minimum of 1,100 pounds off the trailer tandems. To shift weight off the trailer tandems and onto the drive axles, you slide the trailer tandems back toward the rear of the truck.

With 6 inch spacing you're moving 400 pounds per hole so you need to move 3 holes to shift a minimum of 1,100 pounds (400 x 3 = 1,200).

Prev
Next
What affect will sliding the trailer tandems forward have on the weight distribution?
  • You will put more weight on the steer axle and take weight off the tractor's drive axles
  • You will put less weight on the trailer tandems and put more weight on the tractor's drive axles
  • You will put more weight on the trailer tandems and take weight off the tractor's drive axles
  • You will take weight off the tractor's drive axles and put more weight on the steer axle

Quote From The CDL Manual:

By sliding the trailer tandems forward, you will put more weight on the trailer tandems and take weight off the tractor's drive axles.
Prev
Next
What factor has the greatest affect on how much weight is moved per hole in the tandem slider rail?
  • The center of gravity of the cargo
  • The distance between the locking pin holes
  • The distance from the drive axles to the trailer tandems
  • The amount of fuel you have onboard

Quote From The CDL Manual:

The distance between the locking pin holes on the trailer will be the main factor affecting how much weight is moved per hole. This distance varies between the different trailer manufacturers, and the larger the distance between the holes, the more weight will be shifted with each hole.
Prev
Next
You receive a scale ticket and your drive axles are 35,700 and your trailer tandems are 30,600. Based upon a trailer with 4 inch slider rail hole spacings, what is the minimum number of holes you would have to slide the tandems to get the weight legal, and what would the final weights be?
  • Slide tandems back 6 holes - 33,950 drive axles - 32,350 trailer tandems
  • Slide tandems forward 7 holes - 33,950 drive axles - 32,350 trailer tandems
  • Slide tandems back 7 holes - 33,750 drive axles - 32,550 trailer tandems
  • Slide tandems forward 6 holes - 33,750 drive axles - 32,550 trailer tandems

Quote From The CDL Manual:

The two main hole spacings you'll find are 4 inches and 6 inches. You can estimate that you will move 250 pounds per hole for 4 inch spacing, and 400 pounds per hole for 6 inch spacing.

TruckingTruth's Advice:

With 35,700 pounds on the drive axles you must move a minimum of 1,700 pounds off the drive axles. To shift weight off the drive axles and onto the trailer tandems, you slide the trailer tandems forward toward the front of the truck.

With 4 inch spacing you're moving 250 pounds per hole so you need to move 7 holes to shift a minimum of 1,700 pounds (250 x 7 = 1,750).

Prev
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