Why did the rear bumper get lower when I slid the tandems back to the rear of the trailer?

My trainer taught me that it’s only an issue when you’re blindside backing.

 

Succinct summation.. From man who can produce level land, or make a water way twist different. Dock heights? Elementary...

 

Exactly why I didnt waste the time with him... Ive had an encounter once before and realized it wasnt worth the effort.

 

Don't feed the troll.

 

You're trying to generalize while looking at the problem the wrong way. It's not the slope of the ground that changes the behavior, it's the change in slope.

On a constant slope, because most trailers are set up to be slightly lower at the rear, axles back raises the rear. Up or down slope doesn't change the behavior on a constant slope.

The most common dock set up where this is an issue is where the dock approach steps down from ground level. Especially with older docks, the lower flat area isn't long enough to enable the tractor to be on the same plane as the trailer, so the drives will be higher than the trailer axles. Sliding the axles back decreases the angle of the trailer compared to the ground, and thus raises the tail, often significantly. Some really old (short) and deep docks will have concrete risers poured in the wheel tracks, as the drives are so far up the slope compared to the shorter trailers the docks were designed for that there is no other way to get trailer rear high enough for the dock plate.

"Tandems to the back" policies started when dock locks were first getting installed. This is also roughly concurrent with the introduction of 53' trailers, whose longer rear overhang exacerbated the problem. On docks with steeper trailer angles, the ICC bumper would be so low they could bottom out the travel of the dock lock and damage the bumper as the wedge of the dock lock tried to lift the entire rear of trailer by the center of the bumper. In extreme cases the bumper would be so low it could bottom out on the ground and impact the lock below the wedge and it could damage both the lock and the bumper. It was equivalent to backing into a pole. A quick perusal of dock lock pictures shows they have brought the slope much closer to the ground than the early designs, too avoid the latter problem.

Docks with the opposite configuration, with uphill approaches (pushing drainage away from the docks) work just the opposite if the slope is not constant over the wheelbase of the truck and trailer. Axles back still brings the trailer closest to dock height by flattening the angle, but sliding the axles forward raises the rear in this case. Although most docks of this configuration are newer and thus have a much longer constant slope approach, eliminating the radical changes in bumper height.

 

You haven't specified if this applies to all trailers or only 53 ft. Dry van trailers.

 

I don't even want to mention spread axles. This thread will never die!

 

I like to slip a little ex-lax into their food.

 

Every reply helps him/her get there jollies and the dream of being a big rig trucker someday