Today I've got something different for you.

It's not an esk8, and still it's a cool motorized structure which we'll analyze.

"Slide Inline Frames" (by @arianetrek) replace your regular roller-blades as a MUCH faster alternative. Naturally we don't want these to break, that's where the stress analysis comes in.


@arianetrek approached me a while ago with this design and asked if I might do a quick stress analysis.

After bunch of delays where life gets in the way, finishing my studies, moving back overseas, COVID and quarantine, I can finally deliver.


Today's post will be about FEM analysis of large (complicated?) assemblies, enjoy!


What are we talking about?

This is an inline skate assembly made for 3x110mm wheels, when the middle wheel is motorized!

Side view
Side view

The frame is made out of machined AA7075-T6 pieces bolted together.

This aluminum is aircraft grade with the following (typical) properties:

Sy=503 MPa
Su=572 MPa



I had a boss once who said that in the future all engineering programs will have a giant button on the screen which reads "Solve!". Naturally all we'll have to do is just press it.


Until that happens we still need to consider what goes into the model.

You'll find very quickly that using everything in the assembly is just not possible, the software cringe and returns an error, especially with the automatic/fully embedded FE packages.

Luckily using everything isn't needed and we can safely eliminate some of the more complicated parts.


The wheels, for example in our case, are commercially made for skating. Clearly there won't be any issues with them.

The same goes for bearings, the motor and the standard diameter axles.


When clearing all of this we end up with much simpler, and manageable, assembly:


When removing parts just make sure nothing important gets deleted, i.e. you're not removing something of interest to the analysis.

Boundary conditions

This is a simple fixing of the upper attachment points where the thing slides into the shoes:


Transferring the loads

We're still using our standard 120kg rider, who can be on one skate only, equally divided between 3 wheels. Adding the same as the side load - gives us 40kg vertical and 40kg side load on each wheel.

The problem is of course: we don't have any wheels in the final (simplified) model, so how can we make this work?

The solution is transferring the loads into the axles using moments (also called couples), in two simple steps:


    1. The loads are moved to the axle as is.
    2. Bending (side) moment is added in the following way:
      M=40kg*(110/2mm) Using the side load and the radius of the 110mm wheel.

In this way we're correctly applying the loads in the simplified model.


Loads per wheel. How do we transfer those into the simplified model?
Loads per wheel. How do we transfer those into the simplified model?
Correctly applying the loads
Correctly applying the loads


Transferred loads, now the solution can proceed

Final look before hitting solve

Make sure the materials are properly defined, the loads are applied (unfortunately the moments are not shown properly in Fusion360), and the mesh is generated.


Hit run.


It's a good idea to look at displacements first, and see if the model works.

The way to do it is to exaggerate the scaling and examine whether the parts are working together as they should. You'll get these awesome and totally distorted pictures which make no sense graphically, but reveal a lot about the model.

Scaled up displacements
Scaled up displacements

The pieces are held together, we can clearly see the moment working, and the actual displacements (look at the numbers) are totally acceptable.

It's important to switch back to actual displacement scaling, otherwise an occasional spectator might freak out...


Let's look at the stresses

It's usual to see peak stresses at the boundary conditions (BC).

The stresses at BC aren't the real levels, these are numerical values that tend to increase as we decrease the mesh sizeĀ  (smaller elements). Look at the following comparison:

Coarser mesh, lower stresses
Coarser mesh, lower stresses
Finer mesh, the stresses go up
Finer mesh, the stresses go up

The point here is not get excited by the elevated stresses at the BC, certainly don't drive your design by it.

The levels aren't changed much far from the BC (when playing with the mesh), these are the real stress levels:


The levels are much lower than the Yield stress. Using 148MPa as a typical value we get a safety factor of more than 3 to yield, which also takes care of the fatigue concerns.


I'm very excited to try these out once they are available, nice job @arianetrek!



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Have something that needs analysis or have other ideas?

Leave a comment and let me know, thank you!


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2 Replies to “How to do a simple FEM Analysis – The case of Slide Inline Frames”

  1. Awesome analysis, thanks again for your help and interest! Now I can safely carry on with the design, looking forward to the future road tests too!

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