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Weight limits

Pony

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Apr 18, 2014
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364
Location
SE Queensland
I was going to post the question in the overload of the day thread, but then figured I'd just start a new one.
Having a few beers with some mates and started talking about what we call GVM's (Gross Vehicle Mass) here in Australia and how they relate to the real world capacity of the truck.
Now we were talking real trucks, air brakes, 11R 22.5 tyres etc, but the conversation can go to smaller vehicles as well.
I'll stick with with the details of my new truck as it's fresh in my mind, but question can apply to any vehicle.
Truck is bogie drive, single steer, flat bed, 400hp.
Australian?, Queensland for sure, road limit mass for 3 axle truck as above is 22,500kg.
GVM for my truck is 24,000kg.
Front axle 6,500kg.
Rear axle group 21,600kg.
Rear suspension group 18,000kg.
295/80 22.5 steers and 11R 22.5 drives.
Truck is speed limited as required to 100kph.
The argument we were having was around the 'safety factor built into the mass limits. My theory was that speed is a factor in determining these limits.
My peanut brain says that a truck weight 22,500kg going 100kph hitting a bump in the road produces 'x' amount of stress on all components. However same truck hitting same bump at 120kph will produce 'x+?' stress. Therefore same/same at 80kph will produce 'x-?'.
Therefore if my truck were speed limited to 80kph would the GVM be, say 28,000kg?
Following on, speed limited to 40kph would the GVM be 35,000/40,000kg?
Again, my mind says speed would increase stress exponentially.
Obviously there comes a point where static load is simply to high and permanent deformation of components occurs.
Getting back to the Overload thread, my idea is that it mostly reverts back to speed, more specifically the ability to stop the overloaded vehicle safely. So if the speed were taken out of the equation, at what point does it become unsafe?
Most of the photos in that thread are of pick ups towing overloaded trailers, steer tyres very light, limited braking, lethal accident waiting to happen at highway speeds.
How about on your own property at walking pace, I'm assuming that none of the pictured pick ups/trailers had permanent deformation from the loads, therefore would that be ok?
Long rambling post I realise, but just wondering what safety factor is built into components?
 

Truck Shop

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First off-22.5 or 24.5 have a speed rating {L} in most cases, 75 mph. It's not really about speed it's the GVW rating on combined suspension components and braking distance
at 60 mph. Plus parking brakes have to hold on a 20% incline and can roll back no more that three inches. On class eight trucks there are so many calculations in design such
as frame material, axle material, suspension material and designed weight it can safely carry, steering components, wheel weight rating, drive axle diameter and bearing loads.
Class eight trucks are designed as trucks not a large pickup that doubles as a car.
 

Pony

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Apr 18, 2014
Messages
364
Location
SE Queensland
It's not really about speed it's the GVW rating on combined suspension components and braking distance at 60 mph.
I'm not trying to take you out of context but I suppose this was my point, if braking distance is 'x' metres/yards at 60mph then if you're only going 50mph then obviously its going to stop in a shorter distance. Therefore all other things being equal GVM should be higher?
 

Truck Shop

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No-for the simple fact no one drives the limit.
I don't have the Kenworth calculation on braking anymore. But from memory if you have a 350 hp diesel pulling a 80,000 lb load up a 6% grade for 5 miles in the proper gear,
at 20 mph it will take almost 3000 kinetic brake HP to keep it under control at the same speed 20 mph down the other side. The heat created will boil a bath tub full of water
in three seconds.
 

Pony

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Apr 18, 2014
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Location
SE Queensland
Yeah I suppose when I said real world earlier, that was a bad choice of words. We're talking hypothetically about a real world senario.
How about if you were driving my (or any truck), would there be a relationship between speed and what you would consider a "safe" overload.
And I'm happy to take it to extremes, eg 40mph limit 10% over and then walking pace 50% over. Again I don't know, I just made those figures up for an example.
And for the record, no, I don't support or encourage overloading, as stated was simply a discussion over a few beers.
 

AzIron

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Az
Realistically on flat ground your theory works what your taking out of account on the simplist part weather suspension tires and other components can handle it is at slower speeds there is less room left between you and the car in front of you so yea it takes less distance but the people on the road will leave you less space

Get on any grade witch is all you have running 11 western and without bigger brakes it ain't happening

There are milk trucks here grossing 120 on enough axels they do 55 down the freeway creating a small hazard I cat imagine there stopping distance being shorter at 50 to 55 than a regular load at 60 to 65 I bet the distance increases
 

Truck Shop

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AzIron-There probably is nothing worse to stop than a live load {food grade trailer, especially a milk transport} Those trailers don't have the compartment baffles a
fuel tanker would have. For one reason-bacteria. those trailers have to be easy to clean. When a milk transport comes to a stop it will rock/surge back and forth for
over a minute.

One other thing to go with this thread-Manufacture liability.
 

suladas

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I think what you're getting at is say you have a truck that is rated for 22500kg for the roads, manufacturer rating is 24000kg but if you only moved it at 10 mph would it hurt it to load to 28000kg taking out liability, legal limits, etc what is safe for the truck to not be damaged? It would handle much more load then what it is rated for in a situation like that. You don't see it as much, but you still see the odd dump truck on a site with dirt piled in every nook of the box insanely overloaded to proof roll a pad. Or hauling dirt where trucks won't go on a actual road and they are loaded until there is absolutely no room left. Within reason at that slow of speeds, short of hitting some massive holes it's not going to hurt anything. The amount of stress the truck will take at 60 mph hitting a pot hole fully loaded is exponentially greater then a truck even 25% overloaded hitting one at walking speed.
 

Truck Shop

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Except frames crack, axle housings crack and springs break. Get the same crap every year from farmers over loading during wheat harvest. Loader till it breaks.
The welding shop near me does a monstrous business after harvest. There is no reason to over load a vehicle period not one reason is viable. And if it's on site
hauling what is the rush. If the contractor can't bid a job to get done in a reasonable time frame he needs another line of work. Been at this 48 years, owned/
operated also.
 

Pony

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Apr 18, 2014
Messages
364
Location
SE Queensland
The above 2 posts is what I was getting at. Ignoring legalities, liabilities, etc, what safety factor is engineered the truck before you start breaking stuff.
I have been told (correctly or not, maybe crane op can help) that lifting chains/slings have a capacity double of the WLL (working load limit)?
I didn't know if that same sort of capacity was engineered into trucks.
I realise now that there is no simple answer.
 
Last edited:

Tones

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Ubique
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Ex land clearing contractor, part-time retired
Yep Pony ,the safe weight limit(SWL) is half the breaking strain. That's for chains and ropes including SWR
Now weight limits. My truck is the same axle configuration as yours and same tyre sizes and spring suspension. The GVM should be 22.5 tons but according to the paperwork from QLD DOT I can go to 26.4 tons. I guess if I got hauled over by the Mermaids they could only fine me on axle limits.
 

Kiwi-truckwit

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New Zealand
Over here, a twin steer, tandem drive truck has a maximum mandated GVM of 26 ton. The Mercedes Actros I used to drive had a factory gvm of 33 ton.

For off-road transport for a short distance I think I had it loaded to around 36 ton.
 

AzIron

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Dont g err t me started on my days hauling silage and runng manure trucks

Now here is one thing that's almost always overlooked is bearing heat about 5 months a year here it can be a major contributor to blow outs weight is friction so hot hubs transfers thru the rims to tires I have seen tires increase about 15 psi due to heat of a hub especially aluminum rims

A driver for a local contractor was telling me on there 65 ton lowboy they had to get choosy about hub oil and real choosey about tires and with a gross load the trailer was designed for if over 95 degrees absolutely not more than 50mph or they would eat seal and separate tires
 

Truck Shop

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The real problem is fatigue, and how many cycles before failure. Average truck frame {class eight-OTR} is grade 80. Trucks for vocational use in some cases grade 100.
Here's one for thought-The king pin on semi trailers was the same material grade for over 80 years under SAE. Not until 8 years ago did SAE change the specifications
to a higher grade material.

But what ever-load it up till you get caught or your pocket book says no more.
 

Nige

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G..G..G..Granville.........!! Fetch your cloth.
I can recall many years ago some trailers we used for transporting our scraper fleet (D9G/H & 631/641). In all conversations they were referred to as "100 ton trailers" and nobody could figure out why they had so many tyre failure problems, especially if one of them happened to be loaded with two D9's. Turns out that nobody read the plates closely enough. Sure the trailers were rated at 100 tonnes BUT at a maximum speed of 5mph...!! Any speed above that and they were rated at 50 tonnes maximum.

As TS said above - "nobody drives the limit"
 

Truck Shop

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100* ambient temp with 14/16 ply 295/75 22.5 tires running at 65 mph will have a psi of 125 to 130 on a set of drives. cold psi 100. Steers inflated cold to 105 psi
can reach 135 psi plus. So when they blow it's a really big boom.
 
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