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Thread: (PA) Tri-axle dump truck question

  1. #1
    Junior Member
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    (PA) Tri-axle dump truck question

    Obviously, I'm no expert on the subject, but looking over axle weight limitations got me thinking.

    I usually see tri-axles in PA set up with 18-20k fronts, 44-46k rears, and an 18k pusher, often with singles instead of duals. I know the max GVWR is 73,280 lbs, but this is what I don't get - for rear axles I see that if you're less 6' center-center on two axles (tandems) your max axle weight is 18k (or 36k as tandems). If that is the case, why are tri-axles built with such heavy tandem rears? Also, if the pusher is "supposed" to hold as much weight as the tandems, why are they often 18k and singles instead of duals? Finally, does this sort of thing apply to things like tag trailers, where a 20 ton trailer is effectively an 18 ton due to axle spacing?

  2. #2
    Senior Member
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    I know the rules are different out your way, but the basic idea is the same.

    The reason for 46,000 tandems is generally for when the lift axle is up when offroad or onsite, if you guys use lift axles for that third axle like we do. Also a heavier axle setup can take more abuse, etc.

    As for axle spacing we have rules, "bridge law" that limit how much weight can be on a axle combo spread. For example we have a TrailMax tri-axle trailer, model TRD-50T with 17.5 tires. This trailer has a spread of 49" between axle centers or a total of 8'2". The later model TrailMax TRD-54T is the exact same trailer but has a axle spread of 54.5" ctc or a total of 9'1". So with our laws here I can carry 42,000 pounds on the axles, but the TRD-54 can carry 42,500. Not a lot of difference based on our bridge laws, but the trailer GVWR's are a lot different 61,800 for the TRD-50 and 66,600 for the TRD-54, trailer load capacity is 49,000 for the TRD-50 and 53,950 for the TRD-54. These differences would be a advantage back east where the laws are a little looser, whereas with our bridge law we never can reach the full capacity of the trailer.

  3. #3
    Senior Member
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    Granby , Qc, Canada
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    Hi !

    I'm not the best at bridge law but I'll try to recap if I understand properly

    TRD50 carry 42,000 spread 8,2 GVWR 61,800 load cap 49,000
    TRD54 carry 42,500 spread 9,1 GVWR 66,600 load cap 53,900

    Am I right saying that the TRD 54 you could load a machine weigting 53,900
    without any problems or like you said due to bridge law the max load machine = 42,500 ???

    I can see that the trailer have a load difference due the the spread but why is the
    of GVWR is different ( I only see that the spread is different not the number of axles )??

    very confused canucks !!! LOL

  4. #4
    Senior Member
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    Also let me add that both these trailers have 20,000 lb axles, so a total of 60,000 lbs.

    And the trailers can carry their max capacity as long as enough weight is transferred to the tow vehicle via the tongue, and as long as you are not over bridge for the truck and trailer combo and wheelbase. With our TRD-50 I have no problems with 7,000 lbs on the tongue, and probably wouldn't with the TRD-54 and 11,000+ on the tongue.

    Do you guys up north have the same bridge law as we do in the western US? In the eastern US a lot of the states have different laws allowing a lot more weight. Like in "Lars" example they can have 73,000+ on a 4 axle truck, whereas I can only carry 60,500 on my 5 axle truck, and 55,500 on my 4 axle truck because of bridge law.

    Here is a link to our legal weight chart for bridge law.
    http://www.wsdot.wa.gov/NR/rdonlyres...gal_Weight.pdf

  5. #5
    Senior Member
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    Granby , Qc, Canada
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    Hi !

    We do not have the bridge formula like you guy's but yes we have laws for axles and so on .

    You wrote that the Bridge formula is only for the western states !!! are you sure about that ? I thought that the Bridge formula was all over the country !!!

    I have a lot of problem to understand those tables , I will have to read it very slowly
    again and again ( english being my second tongue) .

    thank you for your reply

  6. #6
    Senior Member rino1494's Avatar
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    NEPA
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    If you go off-road like we do, you need those heavy duty rears.

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