Trailers with 2 axles are tandem axle trailers. The federal tandem axle weight limit on the highway is 34,000lbs. Sometimes sliding your tandems is necessary in order to get down to the legal weight limit.
States have varying limits on tandem axle weight which can be found in the front of commercial vehicle maps.
At a good trucking company they will teach you about sliding your tandems in order to shift the weight to get legal. At other companies you will have to figure it out unless you ask someone to show you. That is what you should do.
The diagram give you a visual but actually sliding is an art.
Each hole on the plate represents a shift of a certain amount of weight. The only problem is that the amount that each hole represents will vary from trailer brand to brand. Usually though it is either 250 or 500 lbs shifted for each hole.
If the trailer axles are too heavy you may need to slide the tandem further to the back to shift weight forward on the tractor axles.
If the trailer tractor axles are too heavy and the tail of the trailer is light, you can slide the tandems forward which shifts the weight back on the trailer axles.
Say you need to shift 1,000 lbs off of the trailer axles because you are over axle weight. If possible do this on even surface, level ground out of the way of others. NOT on the scales.
Step 1. Assume either 250 or 500lbs is represented by each hole.
Step 2. Count either 2 or 4 holes back from current position and mark it. That is where you want to slide to.
Step 3. Lock trailer brakes. (Should already be locked)
Step 4. Unlock tractor brakes.
Step 5. Pull forward to the length that you think represent the distance of the number of holes you are trying to slide.
Step 6. Lock the tractor brake, and go see how accurate you were.
Step 7. Repeat the process as necessary, but you may need to do it in the opposite direction if you went too far. This means you would be backing to slide in the opposite direction.
Sometimes you might try to slide your tandems and wheels turn because the brakes are not holding. In this case you may need to adjust your brakes.
To help keep the wheel from rolling you can put a large rock, 2 x 4 or wheel chock at the front or back of the wheel on both sides of the same axle. Remember if you are trying to slide forward you need the wheel block at the back of the wheel.
Also, make sure you are on level ground if possible.
When it is freezing cold and your tandems won't slide, tap the plate with a hammer to break if free.
If you are on snow or ice, you will need to find a curb or parking stop to use for leverage.
You might also need to place a block, 2 x 4, large rock or other object behind or in front of the tandems
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