What is the maximum possible, load a typical 18 wheeler can carry

I know that Federal and State truck size and weight policies limit a 5 axle tractor trailer to 80,000 lbs GVW, 34,000 lbs tandem axle and 20,000 lb single axle (though GVW limits work out to a 12,000 lb steering axle to meet the GVW limit). However, I am interested to know how much weight a 5-axle 18 wheeler can potentially weigh above the 80,000 lb limit and still operate safely? I am unsure what the limiting factor(s) would be, that is, which part of the truck is the 'weak link', so to speak: a. tire to wheel bead - will the tire assembly blow out before, say, the suspension system, trailer frame or drive shaft fails? b. I have read information about extremely heavy loads traveling over bridges and the additional strain of the bridge approach grade resulted in the drive shaft twisting off. Is this the most vulnerable (or weakest link)? c. I've also seen photos of grain haulers on the side of the road with the trailer buckled due to failure of the main structural beams - I would suspect this could happen if the truck hits a rough section of road and vehicle bounce places a large dynamic load on the trailer....but how much load was the trailer carrying? d. I am aware that 4-spring suspension systems can 'roll over' under certain circumstances due to lateral and vertical loading - though air bag suspensions are likely much more prevalent on long haul trucks. Work trucks, like aggregate, sand and dirt haulers, bulk cement haulers etc. often have a type of inverted spring suspension on the trailer - though this might be a certain companies design (like the Mack Camel Back suspensions). e. Only rarely do I see walking beam suspensions on a tractor-semi trailer - these I suspect could carry much more weight before failing. I live in Texas along IH 35 and see lots of trucks from Canada and Mexico - I have read that Mexican trucks are built stronger than US trucks due to the rougher roads in Mexico and that the curb weights are much higher.....I'm unsure about Canadian trucks designed to operate in the US. f. I have read that modern diesel engines are powerful enough to pull much greater loads than the current legal limits. I am unsure if brakes and other safety components are designed with a type of safety factor in consideration of overloaded weights ---- certainly they must be designed in consideration of operation in mountainous terrain (Colorado) vs level terrain - much of West Texas. g. Three axle straight trucks might also have the potential to carry much greater loads than the legal limits - again, I'm unsure what the weak link would be. In any case, I would appreciate any comments you'd have on this subject. Thank very much, Mike Murphy

 

Lucar -

Thank you for responding. Yes, I've seen B-train doubles in Canada - and I know that Mexico and Canada can run truck configurations (like the U.S. Rocky Mountain doubles, Turnpike Doubles, triple trailer units etc.) which are much longer and heavier than Texas currently allows.

The ISTEA Freeze that the Feds passed in the early '90s prevented any states from allowing these Longer Combination Vehicles (LCVs) to operate unless the state had legislation already on the books permitting these longer heavier loads.

I have also visited Michigan several times and have seen the 'caterpillar' rigs that are permitted there.

However, my question relates specifically to a 5-axle 18 wheeler that is typical in the U.S.


thanks again,

Mike

 

Thank you all very much for the replies.

I am a researcher at the University of Texas - Center for Transportation Research. The belief of many DOT folks is that trucks (can) run 100% overloaded......160,000 lbs GVW on a typical 5-axle truck.

However, I can't believe this could be possible without over-stressing one or more of the truck components beyond what it was designed to carry ------- truck suspension, tire, brake, and chassis manufacturers surely can't afford to design for a 100% overload and remain competitive??

Pavements are designed considering that the relationship between load and consumption (or damage) increases based on a 4th power relationship....this means, double the load, the pavement damage increases 2 x 2 x 2 x 2 = 16 times. The design factors for bridges are different and are related to both axle load and spacing as was mentioned.

I suspect that damage to a truck from carrying heavier loads also has a power relationship...thus if someone carries 100,000 lbs on a 5-axle truck it's going to wear out faster than if operated at 80,000 lbs. If it is carrying 120,000 lbs or 160,000 lbs the truck will wear out even faster or you might have a failure of a major component that puts you on the side of the road....


So what I'm trying to figure out is, can a regular U.S. 18 wheeler routinely operate at 90,000 lbs....100,000 lbs....120,000 lbs.....even higher? Can a tandem axle rated at 38,000 lbs operate routinely at 46,000 lbs (ready mix trucks in Texas do...but they have off-road suspensions designed to carry heavier loads and remain stable).

I've been to the PACCAR research center in Mt. Vernon, Washington State and Ford's Research Center and Rockwell's axle research center in Troy, Michigan (though a long time ago).......Ford and PACCAR can test full size trucks on shaker tables that are programmed to simulate the vibration a truck undergoes when driving along rough roads.......perhaps the higher loads caused by the combination of a truck's design and stiffer suspension components to provide stability from over-turning and steering control on rougher roads is the spec the designer uses for design? Rockwell instruments their axles and then drives over actual roads to find out how the loadings from rough or smooth loads impact their designs.

I realize that 80,000 lbs is the maximum legal limit in most U.S. states and that European trucks haul much higher loads - however, European truck manufacturers know the allowable weight limits are higher and the trucks are designed to carry the heavier loads.....with more axles......and they operate with at least 70% super singles for fuel economy.....there are lot's of differences. I've also seen trucks with huge loads in India, China, Malaysia and other countries......but again, these are typically Mercedes or MAN or other European trucks purchased for operation in India, China etc....therefore, the manufacturer might offer different design specs for chassis, brakes etc. which allows higher loads.

I've seen Mexican trucks built in Mexico which are designed much stronger than U.S. trucks simply because they're road system is much rougher on trucks....the curb weight of a Mexican truck can be 20,000 lbs higher than a similar U.S. truck.

However, the question I am asking is how much can a typical U.S. manufactured 18 wheeler carry as the maximum load and what is the 'weakest link'? This question might be considered controversial since it implies (some) truckers are running overloaded.


Again, I will appreciate any additional comments you have.

Mike