Simulating stopping of a heavily loaded trailer

[candice.pimenta9E4XT]

Hi,
I want to simulate how a trailer with weight (content) loaded high above the bed will react upon an emergency stopping at 90km/h.
I want to see the lifting effect it has on the bed, as well as structural stresses on the bed and the trailer content.
Using this calculator, I get a stopping distance of roughly 50m. I read that the stopping distance is weight independent, as more weight induces more friction on the tires surface, hence more powerful braking.

I then use the formula a = (Vf²-Vi²)/2d where a is the deceleration, Vf is 0m.s⁻¹, Vi is 25 m.s⁻¹ (90km/h) and d is 50m. Which gives me

a = -6.25m.s⁻²
Then to calculate the force (braking force) I need to apply to the bed (I don't model the wheels), I use the following formula: Fb = m*a.

In Fusion360, here are my static simulation settings:
    ● structural load of value a lot greater to Fb (to be sure to see the expected results) to all the 6 faces of the bed in the reverse direction of motion
    ● full constraint on the road
    ● suppressed the contact between the road and the trailer bed to allow the back of it to lift (possibly)

I don't understand the results as I'm expecting:
    ● the load to bend in the opposite direction of the force because of its inertia
    ● the back the trailer bed to lift because of the distance between the bed experiencing the force and the load (the same effect as would a car heavily loaded on its back experience when brutally accelerating)

Am I doing the force calculations right or am I missing some things ?
Can it be done in Static simulation or should I go for Event simulation ?
Also, I don't model the inertial forces of the {trailer + content} set going at 90km/h, should I (or is it already taken in account when calculating the deceleration), if yes then how ?

The case study is actually a simplification of my actual problem, where I want to build an enclosed trailer with a roof which should be able to withstand the weight of 4 persons, which gives me quite a heavy roof. You will find attached the current design.

[TheCADWhisperer]

@candice.pimenta9E4XT 

Are you a student?

Is this a school project?

[candice.pimenta9E4XT]

Hi ! Thanks for the reply. Well, yes I'm a student but yes and no it's a school project. I'm a student in an Architecture School and I'm doing a gap year, during the one I 'm working as a draughtswoman. But we are also encouraged to fulfill some personal projects in relation to architecture. So I decided to study the feasibility of a custom-made tiny house that we would build someday with my boyfriend, thus giving me a good project for a gap year, motivating, and useful for my later educational path, as I want next year to choose the "Structural engineering" specialty.
I guess your reply shows I have knowledge weakness, and indeed, the last time I studied mechanics was in high-school 4 years ago 🙂

[TheCADWhisperer]

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@candice.pimenta9E4XT wrote:

I guess your reply shows I have knowledge weakness,…

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@candice.pimenta9E4XT 
You can do this with Autodesk Inventor Professional - Dynamic Simulation.

The reason I asked if you were a student is because Inventor is not cheap unless you are a student, in which case you can get it free for educational use.  

Inventor runs on Windows OS though.

[candice.pimenta9E4XT]

Ok, so here is what I understood:
Kinematic simulation apply motions directly contrarly to dynamic simulation which uses the forces to compute the motions and subsequent forces. So you advise me to make a dynamic simulation on Inventor, then stop at a moment, and make a static analysis at that moment ?

[MattPerez314]

@candice.pimenta9E4XT this seems like a really good project! Kudos for pushing yourself on something like this!

One issue with Simulation in general is that you have to make assumptions, normally based on education/experience/research.  And even then you have to interpret the results to know if you are seeing what you expect.  

With your setup you are assuming that the load mounted high above will be transmitted to the trailer in a certain way.  This is largely dependent on the construction of the trailer and its attachment points.  If you assume a rigid body for the living area then you will likely see reaction forces on the trailer rather than a deformation like you are expecting to see.  

So with your project in general you are looking to get the mass effect in the trailer during braking.

If you continue in Fusion 360 with this what I would say is to use a Remote Force(or forces) to represent your mass above the trailer.  You just need a selectable point and then your connection points on the trailer.  This will assume that your point is connected to your trailer and will let you forget about the structure for a minute.

The next thing I would look at is your constraints.  I would likely fix the "tongue" of the trailer and be really mindful of your frame connection points.  There is a remove rigid body constraints in the sim setup that can help if you want it to add external dampers to your under constrained system.  In this case you are actually removing some critical bits of the system like the suspension, trailer braking systems etc.  In reality there is a combination of force from the tow vehicle through the frame during braking and depending on the gain of the trailer brake system some internal moment/rotation. 

I think in general this might be a pretty tricky problem with multiple steps you will need to figure out.  Be mindful of contact sets in Fusion as the default auto applied contact is bonded, which might be fine for all your welded frame connections, but you might find some of the options with friction like the separation contact type might be helpful for some of the things you want to find.

I don't think I would try an event simulation in Fusion for something like this.  This is likely better handled by an external option like Inventor, but I think you could gain some insight into the problem with Fusion.  They dynamic sim would come into play if you were looking at things like a reduction in force during the braking distance, but you are looking at max fixed values so Static "should" work as long as the materials are inside of their yield.

[candice.pimenta9E4XT]

Thank you a lot for your reply 😊

I didn't precise, but the tiny-house frame is actually independent from the trailer (French road regulations, else you need a complete homologation of the whole). So it 'll be basically U-bolted to the trailer frame.

So here is the new setup I tried. I highlighted my questions for them to be more visible. As you said I must focus on max forces experienced during braking. So I took the best braking deceleration for a modern usual car: 11 m/s² (if you think this assumption is too high or too low, please tell me)

I don't model the trailer, only the tiny house frame.

It's composed of the I beams of the base, the ones of the roof, and the wall tubes.

I calculate the momentum applied by the velocity (90km/h=25m/s) for each of them.

                I beams base:

                                the long ones:

                                                m:14.181kg

                                                n: 5

                                                total momentum: m*n*v  = 3185.5 N

                                                I then select all of these beams and apply a structural load of 3185.5 N directed like the velocity, shared among all the faces (is it right, or should I select only some of them ? ) of these beams

                ...and so on

Once the momentum applied on each part, I want to calculate the max force applied by the deceleration:

                The trailer weights 540kg, and the total I can carry is 3500kg. So I take an overall max mass of the trailer content of:

                m = 3500-540 = 2960 kg

                F = m*a = 2960*-11 = -32560 N

                I apply this force only on the top and bottom faces of the I beams of the base, since the force will be carried from the wheels and the tongue to the trailer frame, then to the tiny house frame coupled by U-bolts

I then add gravity.

I add a road made of 1 m of stone at the bottom of the tiny house frame and constrain its bottom.

I specify a fine mesh on the wall tubes as most of the stress will be in that region

Here are the forces:

Here are the results:

 

 

 

The weakness points seem quite localised and I understand their position. Three things however I can’t grasp:

1) why is the deflection of the left (the back) upper I beam this bigger than with others ? Indeed, the displacement relative to the long I beams is almost twice more important

 

2) what is this huge deflection on the base first I beam? How to explain this force just at that point ?

 

3) My design is symmetrical toward the x axis, so are my loads. So why is it that the simulation results aren’t ?

Thank you again for the time you're dedicating to help me !

[MattPerez314]

Great work @candice.pimenta9E4XT , i don't have the time at the moment to really pick through it but I will try in the morning.  1 thing. the beam that is deforming a good bit is likely because of the triangle at the front allowing for more deflection in that region.  I imagine if you have an extra horizontal beam near the triangle that it will remove that.  Best to look at the actual deformation scale though.  If it is 10x that might be a gross overexaggeration.

I tow a 5000lb RV (2300kg) and its stopping distance/force isn't bad but the vehicle has good brakes as does the trailer.  I don't know how that relates to your number off hand though.