Regarding Beam sizing for Heavy duty trailer

[kaffy]

Good Morning,

I hope everyone is well. I am currently working on a trailer design project and specifying the beam and axle sizes. Based on my free body diagram, the configuration includes a long beam with a fixed support at one end and three roller supports positioned along its span (FBD as shown in attached file picture #1). Since the tires are not infinitely rigid, I’ve modeled them using nonlinear compression-only springs at the roller support locations (Random spring behavior as shown in attached file picture #2, assuming after some deflection, it will get more and more rigid).

While I understand that variations in the spring stiffness values have minimal impact on the beam sizing, they do significantly influence the reaction loads, which are critical for axle selection. Given that axles are rated for specific load capacities, I need to ensure that the reaction loads are accurately captured to avoid unnecessarily upsizing from a 30k to a 45k axle. Any recommendations on how to get the actual tire behavior / alternate approach to solve those kind of problems?

Thank you

 

[BridgeSmith]

You'll never be able to model all the possible permutations of load distribution between the axles. Relative vertical axle positions will vary widely. I would use ultimate capacity of the springs to determine the required axle capacity.

 

[XR250]

I can tell you what the RV industry likely does. Figure out the tongue weight, subtract it from the loaded weight and then divide by the number of axles. For 7,000lb axles they will make sure the number is at least 6,999 lbs. Not sure they add in the extra trailer axle load if a weight distributing hitch is used.

 

[BridgeSmith]

Not sure they add in the extra trailer axle load if a weight distributing hitch is used.

They should, if the trailer is equipped for one.

 

[BridgeSmith]

I can tell you what the RV industry likely does. Figure out the tongue weight, subtract it from the loaded weight and then divide by the number of axles.

As unnerving as that is, you're probably right. For design, I would multiply by a robust increase factor to account for uneven load distribution between axles. I'd probably estimate it using the relative difference in vertical position of the axles equal to the height of a large speed bump. (Speed bump height / max axle travel from zero load to max load)

 

[kaffy]

You'll never be able to model all the possible permutations of load distribution between the axles. Relative vertical axle positions will vary widely. I would use ultimate capacity of the springs to determine the required axle capacity.

By “ultimate capacity,” do you mean I should model the spring as extremely rigid in order to capture the maximum possible reaction load?? Just to clarify, we are only specifying the axles—not designing them.

 

[BridgeSmith]

By “ultimate capacity,” do you mean I should model the spring as extremely rigid in order to capture the maximum possible reaction load?? Just to clarify, we are only specifying the axles—not designing them.

By ultimate capacity, I was thinking the force needed to push the spring to it's design limit, i.e. it's max deflection/deformation. However, I may have been too hasty in suggesting that, as that's probably overly conservative. Probably a more reasonable required capacity would be obtained using the method I described in my later response (Post #5)

 

[waross]

What to expect on the road; (More "The rubber hits the road" than conventional engineering.)
I pulled RVs cross country commercially for a year.
One of the biggest was 43 ft long and weighed 14,000 lbs with 1800 lbs on the hitch. (Per the documentation)
And this was a bumper pull trailer.
Bad bumps, wavy pavement. My truck weighed about 12,000 lbs and the trailers weighed as much as 14,000 lbs.
Wavy pavement would through the back end of my truck up and down.
The worst was a pavement heave that unexpectedly caused both my front suspension and my rear suspension to bottom out.
That is quite a bit a stress on the hitch and drawbar.
Equalizer hitches.
This is a matter for the driver and the towing vehicle.
In some cases an equalizer hitch is a legal requirement.
In my case, my insurance required an equalizer hitch for trailers above a certain weight. (1000 lbs or 1500 lbs, I forget but it wasn't much.)
Tongue weight is the least of the stress. Imagine the inertia load when a dip in the roadway causes the tongue to accelerate the mass of the trailer about the pivot of the center of the axle support and then suddenly reverses the inertia loads as you come out of the dip.
The axles will survive momentary loads of at least double the rating.
Two axle trailers often encounter short pitches going into driveways and parking lots where one axle is off the ground and the total weight is supported by one axle.