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A trailer doesn't react to a tractor's backing up until 10 feet of travel

  1. Aug 19, 2016 #1
    I am a trucker. I pull 53' dry van trailers with a big rig truck (a tractor). When a tractor-trailer backs straight backwards (not turning ) it is called straight line backing. Probably most people here know from backing boat trailers or other trailers at some point in their lives that when backing a trailer, if you turn the steering wheel to the left, it will make the rear of your trailer go to the right and if you turn steering wheel to the right, it will make the rear of the trailer go towards the left.

    Both the trucking school i attended and the trucking company i now work for told me that when a tractor-trailer is doing straight line backing, if the truck driver turns the steering wheel to the left or the right, the trailer will not respond by going the opposite direction until the tractor has backed up around 10 feet. To me, this seems counterintuitive and also potentially false. The tractor is connected to the trailer at the fifth wheel of the tractor. The trailer has a cylindrical bolt hanging down from the front of the trailer called a kingpin. The fifth wheel of the tractor has locking jaws that lock around the kingpin. The connection between the fifth wheel and the kingpin is a rigid connection. To me, it seems like since the connection between the fifth wheel and the kingpin is a rigid connection, that should mean that the trailer will react to the driver's turning the steering wheel (during straight line backing) IMMEDIATELY. Maybe the trailer's reacting to the driver's turning the steering wheel might be too subtle to detect until the tractor has backed ten feet, but it seems to me that the trailer's reacting to the driver's turning the steering wheel should happen when the tractor has moved just one foot. The instructors at my truck driving school said that when a trucker is doing straight line backing, the trailer won't react to the driver's turning the steering wheel until the tractor has backed 10 feet b/c the circumference of the wheel on the tractor-trailer is ten feet. I don't see how the circumference of the wheels could determine when the trailer reacts to when the driver turns the steering wheel.

    Are the instructors at my truck driving school and at my trucking company correct that the trailer will not react to the driver's turning the streering wheel until the tractor has backed 10 feet?

    Please explain.
     
  2. jcsd
  3. Aug 19, 2016 #2

    jbriggs444

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    You are correct. The instructors are blowing smoke.

    The track of the trailer will not react significantly for a while because, as you well know, it takes some distance for the cab to move appreciably to the side, thereby changing the trailer's orientation and it then takes some distance for the trailer to move to the side as a result of its new (and still changing) orientation. The circumference of the wheels does not enter in. The same would apply if you replaced the wheels with a set of (incredibly sturdy) roller blades.
     
  4. Aug 19, 2016 #3
    I think the key word in your quote is "significantly". I think that when a trucker is straight line backing, the track of the trailer will already be reacting (in a negligible and undectable amount) to the driver's turning the steering wheel when the tractor has just backed a foot, but the track of the trailer will not react "significantly" when the truck has just backed up a foot.


    I agree. When my instructors first told me that, I didn't understand how the circumference of the wheels could affect when the trailer reacts to the tractor, but I did not necessarily think it was wrong. I had to keep paying attention to other things the instructors were saying, and I thought to myself: "I will think about it later." Then when I did take the time to think about it, I still didn't see how the circumference of the wheels would affect when the trailer reacts to the tractor. Now I think: How does the circumference of the wheels affect when the trailer reacts to the tractor? The answer is: It does not.
     
  5. Aug 19, 2016 #4

    berkeman

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  6. Aug 19, 2016 #5

    RonL

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    A brush guard on front of a big rig :cool: a first time view for me :smile:
     
  7. Aug 19, 2016 #6

    sophiecentaur

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    I have had dealings with instructors, teaching parachute jumping, plumbing and scuba diving etc. etc.. They can be excellent at telling you what to do but that doesn't mean that their knowledge of the Science is enough to give valid reasons. I have heard an amazing amount of pseudo Science from guys with whom I would willingly trust my life. It's two entirely different disciplines.
    The Army teaches 'by numbers' because that wins battles - and a good thing too. "You're not here to think, soldier!!"
     
  8. Aug 19, 2016 #7

    OCR

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    It's called "the ten foot rule"... and, due to "rear trailer swing", it comes into play more so when doing sharp or 90° backups....:oldwink:
    Here's the full video...

    Now!... let's see you back up a set of doubles .... :headbang:

    If parked in a straight line, a set of triples can usually be backed up until the last trailer starts to jacknife, which isn't very far... it's about like trying to push a chain... lol
     
    Last edited: Aug 19, 2016
  9. Aug 19, 2016 #8
    One thing to point out here, hinted at but not explicitly mentioned, is that the 53 foot trailer doesn't react to the orientation of the steering wheels, but to the orientation of the tractor wrt the triailer. The tractor has to be orientated to the left to make the rear end of the trailer move left. How to do so - turn the steering to the right to swing the tractor front to the left.

    The fifth wheel is located right above the axle of the tractor, so one doesn't get the same more immediate angling by turning the steering ( as one would get with a hitch trailer). About 10 feet movement backwards is about right for the trailer to be noticeably orientated differently - and the rear might be now only 2 to 3 inches off from the line it was on before - probably about a 2 to 3 degree orientation of the trialer, depending upon the turning radius of the tractor - not really all that much of a reaction. Backing up another ten feet, with the same cranking of the steering, swings the cab out more, and the effect becomes much more noticable.

    Ten feet is about one rotation of the tires, in other words the circumference of the tires. What better way to remember for the new driver how the trailer will react, and organizing his distance, and off-centre to the dock than putting it all in those terms. KISS has a workable outcome here.

    Are we so sure the instructors are not saying that - ie that it takes one rotation of the tire to make, for the first 10 feet, to make a noticable effect, and it is getting misconstrued instead into "radius of the tire".

    Any proof in trucking literature that circumference of the tires affects backing up, is an industry-wide recognizable explanation. I don't see the whole industry completely falling hook line and sinker falling for that one.
     
  10. Aug 19, 2016 #9

    OCR

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    Really ?... interesting....:oldwink:

    Bull_bar_roo_bar_on_b_double.JPG

    https://en.wikipedia.org/wiki/Bullbar

    http://www.herd.com/gallery/default.aspx?cmd=reset [Broken]
     
    Last edited by a moderator: May 8, 2017
  11. Aug 19, 2016 #10
    Try a hitch trialer that also steers! Lots of fun. But you get used to it, so you don't end up bending the hitch and tow bar and other things. Especially at harvest time, when downtime means $1000's per hour or more. Came close though.
     
  12. Aug 19, 2016 #11

    RonL

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    So now I have really embarrassed myself, that first picture is an over the road beauty, your picture made me remember my oilfield days, those pictures nudged my memory and that led me to remember "I actually own a 1966 Ford tandem axle truck with a brush guard" DUH!!! it has been a good weed killer for quite a few years:confused:o0):eek:o_O:nb):smile: used to have a cement mixer over the wheels:oops:
     
    Last edited by a moderator: Apr 19, 2017
  13. Aug 19, 2016 #12

    OCR

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    Yup.....:check:
     
  14. Aug 19, 2016 #13

    OCR

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    This one, right ?

    Ford LTL 9000.jpg

    Lol, just kidding... pretty, though, isn't it....:ok:
     
  15. Aug 19, 2016 #14
    That is true. On the first day that I ever tried alleydocking a tractor-trailer at the truck driving school I attended, I did not realize that the trailer does not react to the orientation of the steering wheels but to the orientation of the tractor with the trailer. I had the truck to myself, and I tried to alleydock into a parking spot for about an hour and a half without getting it into the spot. I mean, I was no closer to the parking spot after 90 minutes than after 5 minutes. Then I realized that the trailer reacts to the orientation of the tractor with the trailer. The way I thought of it is that when alleydocking (or doing any type of backing) , the trailer reacts to how the tractor "pushes" the trailer. Once I realized that, I could alleydock without any problems.




    Wrong. When backing, the trailer will immediately react to whatever way the driver turns the steering wheel. It's just that the reaction is too subtle for most people to notice until the tractor has backed 10 feet.


    First of all, please review my original post on this thread. I never used the word radius in my original post.

    My instructor said that because the tires are 10 feet in circumference, that means that it takes the tractor's backing 10 feet for the trailer to react. I don't think that the whole industry thinks that the tires' circumference is what determines how long the tractor has to back for the trailer to react. That's just what one instructor said.

    Since both my trucking school and the trucking company I now work for both say the same fallacy about the trailer not reacting until the tractor backs 10 feet, I think that the fallacy of the trailer's not reacting for 10 feet might be an industry wide fallacy. The trucking school I attended and the trucking company I now work for are not affiliated with each other, so I doubt one of them got the fallacy from the other. This adds plausibility to the theory that the fallacy is industry wide.
     
  16. Aug 19, 2016 #15

    OCR

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    ..... :thumbup:
     
  17. Aug 19, 2016 #16

    RonL

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    Mine has the gas engine and not nearly as heavy duty, nor a sleeper cab as the one above. :smile:
     
  18. Aug 19, 2016 #17

    berkeman

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    That's how I back my motorcycle trailer. Visualize how my steering wheel movements push my hitch, and visualizing me using my hand to move the hitch to turn the trailer. Works well for me. :smile:

    I can't imagine how to try to back a double trailer. :woot:
     
  19. Aug 20, 2016 #18
    circumference = 2π radius

    The fallacy I was referring to, as your instructor seems to believe, is with the circumference of the tires being a factor.
    Go for the 10 foot rule and you should be good.
     
  20. Aug 20, 2016 #19
    Kudos for you.:woot::partytime:
     
  21. Aug 20, 2016 #20

    DaveC426913

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    Backing a boat trailer, I put my hand on the bottom half of the steering wheel. The trailer's stern will move in the direction I move the wheel. Push steering wheel left, trailer stern goes to left. Push right, goes right.

    Ths would work on a rig just as well.
     
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