What's With These Trucks?

GWS · Oct 4, 2019

So, I have been seeing these trucks hauling dirt or gravel. When they are hauling a second trailer there is a large 20+ foot gap between trailers They are joined by IMO a very lengthy trailer hitch.
Why are they so far apart?
State law or some special way of loading or unloading the trailers?

Flatfish · Oct 4, 2019

Truck and transfer (If memory serves).

The rear trailer fits inside the front. So both can be dumped.

Lilhigbee · Oct 4, 2019

The reach is long enough they can jack-knife the trailer, dump the truck over the reach and drive away without having to unhook the trailer.

The Heretic · Oct 4, 2019

Part of it is weight limits, axle loads and distance between axles per weight limit law.

Heyjoe · Oct 5, 2019

Dump truck question

I see a lot of HD dump trucks hauling dump trailers. Why is there such a long tongue on the pulled trailer? Looks wierd but I suspect there is a good...

www.turbodieselregister.com

Thread pulled from another forum.

Of note:

"The long hitch is to keep the trailer tires/axles a certain distance from the truck tires/axles. This is to meet the "Bridge Law". In order to haul the maximum gross weight of 80,000 lbs, you need a required amount of distance from the truck to the trailer. This keeps the total load from crossing the little bridges that you see on old two lane roads. First the truck weight is on the bridge, then the trailer. The same thing applies on spread axle trailers, if you have at least a 10 foot spread between the trailer axles, you can carry 20,000 lbs per axle, instead of 34,000lbs for the two in close tandem. Some states allow an extra 6,000 or 86,000 gross for this feature, some states do not.

"

41mag · Oct 5, 2019

I recently watched a project where several of these dump rigs were adding fill to an erratic parking lot. In this case the design of the tongue length and the skill of the drivers made it possible to dump their gravel in specific selected spots independent of the other trailer load. It was pretty amazing, as I'd never observed such a close-order drill. The maneuverability of load placement speeded up and enhanced the efforts of the bulldozer further spreading the load as needed. "Jack-knifing" in this case was most beneficial to the time needed to place the load accurately.

jbett98 · Oct 5, 2019

Dick Bielenber · Oct 5, 2019

Heyjoe said:
Dump truck question

I see a lot of HD dump trucks hauling dump trailers. Why is there such a long tongue on the pulled trailer? Looks wierd but I suspect there is a good...

www.turbodieselregister.com

Thread pulled from another forum.

Of note:

"The long hitch is to keep the trailer tires/axles a certain distance from the truck tires/axles. This is to meet the "Bridge Law". In order to haul the maximum gross weight of 80,000 lbs, you need a required amount of distance from the truck to the trailer. This keeps the total load from crossing the little bridges that you see on old two lane roads. First the truck weight is on the bridge, then the trailer. The same thing applies on spread axle trailers, if you have at least a 10 foot spread between the trailer axles, you can carry 20,000 lbs per axle, instead of 34,000lbs for the two in close tandem. Some states allow an extra 6,000 or 86,000 gross for this feature, some states do not. "

I doubt that "bridge law" has anything to do with little bridges and everything to do with the engineering term "bridge" meaning a span between supports. Bridge Law in long trailer trucking is the distance between the 5th wheel and trailer's third axle. A third axle has no wheels but is the point where the two axles with wheels pivot up and down.

The Heretic · Oct 5, 2019

Federal Bridge Formula

The Truckers Place is a Class A Truck stop for American Truck Drivers & Owner Operators.

www.thetruckersplace.com

The spacing and distances are significant to meet various weight limits, and yes, the small bridges are an issue. I see this every day, both because of where I travel (my commute takes me past various construction sites and small bridges the dump trucks from gravel pits transport loads) and because of my job which has to do with configuring these trucks.

Certaindeaf · Oct 5, 2019

What about them trucks with a set a wheels tucked up under amid ships.? kicked in da balls?

The Heretic · Oct 5, 2019

Certaindeaf said:
What about them trucks with a set a wheels tucked up under amid ships.? kicked in da balls?

Those are called 'pusher' (in front of a drive axle) and 'tag' axles (tagging along behind). Again, it is about weight limits and number of axles. When carrying a heavy load the axle can be dropped down to add another load axle and raised up when not carrying a heavy load.

Heavy vehicles (large trucks/tractors) do the most damage to roads (studs come next) so there are a lot of restrictions on weight/etc. Once you get past a certain yield strength the roadway is damaged which == $$$

There are a lot of variations for different applications. Twin steer, pusher/tag axles, tandem axles, tandem drive axles, set back front axles, sliding 5th wheel kingpins, different sized wheels/tires and so on. The other day I saw what was probably a trailer for carrying light loads as it had reduced size wheels/tires more akin to a class 4-5 truck.

TTSX · Oct 5, 2019

It almost sounds like he's describing triple dumps? I've heard about them but never seen, like a transfer backup up to another transfer.

The Heretic · Oct 5, 2019

MechaNik said:
It almost sounds like he's describing triple dumps? I've heard about them but never seen, like a transfer backup up to another transfer.

Certaindeaf · Oct 5, 2019

The Heretic said:
Those are called 'pusher' (in front of a drive axle) and 'tag' axles (tagging along behind). Again, it is about weight limits and number of axles. When carrying a heavy load the axle can be dropped down to add another load axle and raised up when not carrying a heavy load.

Heavy vehicles (large trucks/tractors) do the most damage to roads (studs come next) so there are a lot of restrictions on weight/etc. Once you get past a certain yield strength the roadway is damaged which == $$$

There are a lot of variations for different applications. Twin steer, pusher/tag axles, tandem axles, tandem drive axles, set back front axles, sliding 5th wheel kingpins, different sized wheels/tires and so on. The other day I saw what was probably a trailer for carrying light loads as it had reduced size wheels/tires more akin to a class 4-5 truck.

I knew that.

Not really

TTSX · Oct 5, 2019

The Heretic said:

Yes that's what I work on daily, but add yet another trailer behind that. Getting into Aussie road train territory soon

Ura-Ki · Oct 5, 2019

Various states have weight limits based on the amounts of weights that can be carried by an axle AND by tire size, the bigger the tire, the higher the weight limit! Say you have a typical 18 wheeler, those are limited to 80,000 pounds with "Standard" 11R22.5 size tires, and can go to 96,000 pounds with the larger 12R24.5 size. With that, they also have a set number of tires that must be on the ground per amount of weight, AND for articulated rigs ( Semi Tractor/trailer) a Bridge length not to exceed 40 Feet between axle centers. The greater the weight,( Above 80,000/96,000) the closer the axle centers need to be to make Bridge. Bridge isn't about crossing any road bridge, its the set limits of space between axle centers afforded to carry a certain amount of weight on articulated rigs! Strait trucks have different requirements, requiring multiple axles between the drive(s) and steer axle in order to make bridge limits for the amount of weight the truck is registered for! Your Typical 3 axle 10 wheel dump truck is limited to 66,000 pounds single unit, and up to 96,000 with trailer and the required number of axles! Certain trucks Like ready mix trucks are required to have tag axles and or drop axles based on the close spread ( Focused Load) of their payload vs their empty load, so they require more tires close aboard to make bridge. Once a semi rig is desired to go above 96,000 pounds, they require an extra axle on both the tractor and trailer and can go as high as 105,500 pounds, partially for the weight, partially for the extra tires and Brakes those axles provide!
Then you run into the Multi combo rigs, doubles and triples, which are used to maximize weight limits when you have loads that don't normally make weight per unit! It sounds complicated, but once you know the limits per axle, and the tire deal, it all makes sense!
So, the dump truck combos in the O.P. are set up for a specific weight limit/spread to legally make both bridge and max weight available! That it makes it easy to discharge payload is a bonus, and the different types of combos are designed for different types of work/payload discharge. Rigs like the "Slam Bam" are typically used where there is no room to back a trailer and turn, or a very narrow space to discharge the payload, the typical Pup trailer can be more useful in open job sights as long as there is maneuvering room!
End dump, Side dump, and belly dump all fit into the Semi Tractor/Trailer category, and other then weight limits and bridge, there are few complications!

Heyjoe · Oct 5, 2019

Dick Bielenber said:
I doubt that "bridge law" has anything to do with little bridges and everything to do with the engineering term "bridge" meaning a span between supports. Bridge Law in long trailer trucking is the distance between the 5th wheel and trailer's third axle. A third axle has no wheels but is the point where the two axles with wheels pivot up and down.

You may be right, I'm no expert.

Just passing along what looked to me like pertinent information.

StevenPetersen · Oct 6, 2019

The long drawbar is to be long enough to make bridge. To be legal for 105,500, your bridge(distance from center of front axle to center of rear axle) needs to be at least 69 feet with 8 axles, or 78 feet with 7 axles. I used to drive a dump and pup setup in Oregon. 4 axle truck(3 plus drop axle) and 4 axle pup trailer. The trailer had a telescoping drawbar, you kept it sucked in when empty and extended while loaded. Also you only lower the drop axle while loaded, saving tire wear.