Why is it better to have the tandems slid forward when alleydocking?

I added the direction points to the drawing in the photograph in the OP just to make 100% sure that there is no ambiguity. Here is my edit of the drawing in the photograph in the OP:

I am finally starting to understand this. I am going to walk-thru how I would have to maneuver the tractor-trailer if I backed into the parking spot in the drawing in the above photograph if my tandems were all the way to the rear. For my hypothetical example, to keep it simple, let's assume I have perfect aim, and I don't have to do any pull ups. Also, let's assume that my tractor-trailer is in the position of the red tractor-trailer that I edited onto the drawing, NOT the yellow tractor-trailer. My understanding of the scenario in the drawing in the above photograph is that if the tandems are slid all the way to the rear, I am going to first have to turn the steering wheel a lot farther to the right (clockwise) to jack the trailer towards the parking spot. This will make the front grill of my tractor swing a lot farther to the west than would be necessary if I had the tandems were all the way forward. So this would necessitate more forward space to the west. Then (with the tandems to the rear) I am going to have to turn the steering wheel a lot farther to the left (counter-clockwise) "to get under it" to get the trailer straight with the tractor. This would make it so that I would need more forward space to the North than if the tandems were all the way forward. So if the tandems are all the way back, it makes it necessary to have both more forward space to the west and to the North. Is my understanding correct?

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P.S. I have discussed this issue with at least ten other truckers, both on the message board and in real life. You're the first person who was able to help me understand this. I needed someone to put a bib on me and spoon feed me this information, which you did. Everyone else just let what you wrote in the quoted post above go without saying. You would probably make a good trainer.

 

I know not to try to slide the tandems unless the tractor is straight with the trailer.

 

Lester, you are so good at helping me understand this stuff that I hope you respond to this post as well, even though it is a quote of rockee's post. In fact, I invite everyone to respond to this post, not just rockee. Rockee, of course, I hope you respond to this post as well.

When I do all that in reverse, like I am going to bump the dock, my understanding is that the rear bumper of the trailer would swing to the west (or, in other words, to the left). If the rear bumper swings to the west (left), I don't see how that is a problem. The only way the rear bumper could hit the trailer in the spot to the west of my tractor-trailer is if the nose of the trailer moved so far to the east that the nose of the trailer was east of the tandems. I never move the nose of the trailer east of the rear bumper on an alleydock back that is not a blind-side back..

Is my understanding correct?

I'm confused. I think most truckers know not to jack the trailer to the point that the nose of the trailer is east of the rear bumper on a sight-side back. So why is everyone mentioning tail swing as being a significant disadvantage of doing an alleydock back with the tandems all the way forward?

Am I missing something here?

 

The directions north, east, south, and west make it clear which direction I am talking about. When I was a student & at trucking school, my instructors/trainer would tell me "Pull up", and I did not know if they were instructing me to pull up to the north or the west. It was all a big guessing game.

Yes. I first learned of trailer swing when driving forward ten years ago at trucking school.

I totally understand the fact that when I am making a 45 degree alleydock into a loading dock, if I don't turn hard enough (in other words, if I don't jack it hard enough), the right (passenger side of the trailer) rear of the trailer will hit something period. Why did you say "if I don't turn hard enough, the right rear of the trailer will hit something because of the overhang."

What does the overhang have to do with it? If i don't turn hard enough, the right rear of the trailer will hit something no matter where the tandems are.