Determining Axle Load On Trailer Assembly

Dear joshya...:

On Jun 10, 12:30=A0pm, snipped-for-privacy@gmail.com wrote: =2E..

A picture would help...

Sum the moments formed by each load or axle, using the "fifth wheel" of each segment as zero for the coordinate system. A moment is another name for a torque, and is a force times a distance, *with a sign* that indicates a direction. All the moments must sum to zero, or the segment would start spinning.

Choose a side of the assembly (say the US driver's side), and use your right hand (say) to assgn signs. If the thumb is located at the fifth wheel, and each force component is aligned with your fingers, the direction you thumb must face makes the sign positive or negative.

If the axles are close together, (or are far from teh fifth wheel) they each can be assumed to provide the same lifting force.

5th LOAD X1 X2 | dL | | dX | 0 =3D - dL * weight_LOAD + dx * force_BOTH_Xles.

Anyway, rather than wave my hands any further...

David A. Smith

Draw a straight line, with wheel positions marked to scale, and the load position CogG marked at the scale position.

Then if the trailer is static, the forces acting clockwise equal the forces acting anti clockwise if their positions are taken into account.

Take as the pivot, any point - say the end of the tail. The load is its weight times its distance from the end. The wheel support is last axle force times its distance from the tail end PLUS the next to last axle force times its distance from the tail end PLUS ... The front axle force times its distance from the tail end.

You have too many unknown forces at the axles to get a definitive answer immediately for axle forces, but you could start by (wrongly) supposing that all four (or however many) axles provide the same support force. You CAN work out that force. Then do the same again, ignoring the force from the two axles furthest from the load. This gives you a low limit and a high limit on each axle, which will get you towards the ball park.

Brian W

further...

This is all good information, and I'll be posting with a link to the diagram we use of the trailer, but the issue I seem to be having is that, as you'll see in the diagram, some of the wheels lie directly under the 5th wheel position so their distance along x is 0, which makes some of these equations a little wonky as it's certainly not ttaking the entire load or no load.

This doesn't seem to work out quite right either as the way a trailer is designed is such that all 4 axles on the back of the main trailer should be taking a *relatively* similar load, the dolly's axles should also be taking a similar load amongst themselves but quite a bit less than the trailer axles, same for the tractors drive axles being less than the dolly and much less than the trailer, and finally the tractor's steer axle should have the lowest of all. If I go about it your way, the steer axle should be taking the most load as it's furthest from the physical load...

Here's a copy of the diagram we use to write down the information.

It has most of the neccesary measurements. The only thing it doesn't show is the kingpin & 5th wheel locations, which are as follows:

On the truck the 5th wheel is 6" in front of the middle drive axle. On the dolly the 5th wheel is 10" behind the first axle.

Just to throw some numbers at this, it was figured using the old inaccurate method that 180,000# load should be putting 31,500# on each trailer axle, 22,500# on each dolly axle, 21,000# on each tractor drive axle, and 15,000# on the steer axle. Now I understand that not all the axles in each group will take exactly the same amount as each one is slightly further away, but a simple force * distance would result in the steer axle, being the farthest from the load, taking the most weight, which cannot be the case as it's designed to be the structurally weakest axle. Thoughts?

You will draw *three* separate free body diagrams.

• One for the truck, with the downward force from the dolly on the fifth wheel on the truck.

• One for the dolly, with the downward force from the trailer on the dolly's fifth wheel.

• Last for the trailer proper. You can assume each axle's springs are uniformly compressed (hence evenly loaded), since the trailer will stay largely horizontal.

Solve the trailer first.

I would not assume the dolly stays fully horizontal, so each axle will be slightly differntially compressed. But that does give you a simplification, that the dolly's structure is non-deformable, so from the pivot point each axle futher away form the truck's fifth wheel will carry porportionally more or less load.

The truck loading will be much more complex.

You might be money ahead to find a transportation professional to run these calcs for you? Consider the penalties if damage is incurred, and if the penalties are all paid by "John Hennes Trucking", because they did not do due diligence...

David A. Smith

(PS. I am NOT such a professional.)

Hmmm...you don't quite get the idea of opposing moments balancing out: think of a long lever with a weight near the far end, so the near end needs a low force to lift the heavy far weight. Anyway, I ran a calculation on your diagram on the back of an envelope, just for the trailer wheels, and the numbers you have for the sample load look close enough: (these are the 'incorrect' numbers as you describe them) for the 180k lb load plus the 123 k lb trailer deadweight, I got 30.8 k lb. average per axle, leaving 129 k lb on the trailer fifth wheel

Brian W

Make that: 180 k lb load and 72 k lb trailer weight....

Brian W

Dear Brian Whatcott:

I wonder if a deformable tralier frame would make the difference? Seems to me it would only add / enhance differential loading to the individual axles. The resultant at the fifth wheel "should not" change...

David A. Smith

Yea I redid things your way and I think I understand what you were saying now. It turns out after some digging and getting the original guy who did the numbers to come in that he was getting accurate numbers, he was just going about getting them in a very round-about way. His method doesn't make a whole lot of sense step by step but in the end he's determining the right values so they still worked out to the same resultants. Thanks for the help.

///

Some double axles have that kind of load leveling built in, with a swinging link on the adjacent leaf spring ends supporting the axles. Going over a bump with one wheel high and the other low, they still both get a more equal share of the load than without the swinging link. You may have come across a 'swingle-tree' that connects two horses hauling a cart: one horse can step forward a little, but the other horse still gets to pull its share. On a fancier scale, I recall visiting an arcraft facility where an airframe was being fatigue tested. The attachment points along the wings were connected to a beam two at a time, and these two beams were connected to a collector beam two at a time,and so on, making a kind of giant swingle tree For a trailer with pneumatic axle springs, load-sharing is a matter of connecting all the bladders together [in principle] so they are guaranteed to provide similar forces, though they may ride high or low over bumps.

Brian W