Imposed sliding pivot displacement

Simulation
1

Hello

I'm on SW2020 to do some design on a cargo bike frame.
I do a simulation or I load the frame to see how it behaves but I can't find the right imposed movements. Here's what the template looks like:

I want to create an imposed displacement that feels like a sliding pivot at the bottom of the tube on the far left. I added a hole in it to create a reference geometry but not possible to create a sliding pivot.

The only method I've found is to make an assembly and create a fork + a bracket from which I create an axle connector and a plane support.


This method works but I wonder if it would not be possible to achieve the same result without going through an assembly that wastes a lot of time.

What do you think?
Thanks in advance,
Natha


modele_simplifie.zip
2

Hello @garnier.natha 

First of all, you need to know what you want to simulate and why!

In your case, a cargo bike, we can assume that what you are likely to be interested in are the deformations as a function of a load.
In your case, as it's not a complicated simulation, I don't see the point of working on one half of the bike, especially if it's a two-wheeler
You will see with a whole model everything that is twisting the frame, which is useful when you are not a beginner.

Then you have to position the load correctly! In any case, it will not be located where you place it. Your load should normally be in the bottom of the box on a flat surface that is placed or fixed somewhere on the tubes. This means that you are no longer in the case of a one-off load but a distributed load.

In addition, the bottom plate as well as the side, front and rear plates will act as bracing. A simple 3 mm hardboard plate or a 1 mm sheet metal, well fixed with pop rivets or self-drilling screws, is stronger and lighter than metal braces
Depending on whether you have a two-wheeled or three-wheeled cargo bike, it changes everything. It looks like it's a two-wheeler if I look at your pictures.

Sliding pivot load is not appropriate.
I am quite willing that in the schematic representations of cinematic description, which are imposed on you in school, we speak of pivotal linkage.
In your case, you will not be able to set up translations that would be arbitrary and that would not conform to reality.

If you want something more realistic, you would have to make a part to be pobelleted afterwards with a plane support (representing the ground), the junction between this part which simulates the wheel does so with a virtual axle using an "axle connector". The part in the second image is perfect for this, but only if you replace the axle with an "axle connector"
In addition , you have the weight of the delivery guy and the support on the handlebars which are different depending on whether he pulls or presses on the handlebars when he leans forward to climb a slightly steep hill. If in the bends the delivery person is the weight against the direction of the load for various reasons the forces will not be the same either (see the remote loads in this case).

This means that you have several kinds of loads, so several simulations to do from a single model

In short: give us an image of the cargo bike as you want to build it and above all don't skip half of the elements to be taken into account.

Kind regards

PS: I often say that doing a bad simulation is harmful because you think you have reassuring elements on the RDM when the safety coefficient can very well be less than 1 or barely higher than 1.
 

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3

Thank you @Zozo_mp for that comprehensive answer.
I should have clarified the context of my study a little more.

The frame I showed in the picture is a simplified model so that I can correctly set the parameters of the simulation. The frame on which I work is more complex with many more tubes, different sections, with sheet metal, supports...
I started working in simulation with the complex model but  I had more than 5 minutes of calculation. Which is not ideal when you change settings often. That's why I created this simplified model and therefore the forces are not placed consistently, the frame is not well sized...

I am looking to carry out my study in the conditions I specified above because I have a prototype of my cargo bike that I loaded in this way (in fixed pivot at the level of the rear wheel axle and placed on the head tube at the front) and whose arrow I measured under the head tube. My study seeks to reproduce the same conditions in simulation to verify that I come across the same result and therefore validate the simulation.

Once the simulation has been validated, I intend to do several studies with different loading cases as you recommend.

Regarding the part to be pobellinated with an "axle connector" this is what I did in an assembly and it worked.
But I would prefer to do this study by staying in the .sldprt and not going through an assembly. Do you think there would be a possibility in this direction?
 

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4

Good evening

from a simulation point of view, there is no significant difference between a PART and an ASM.

For static simulation, in general, we put contacts that are integral or non-penetrating, which is more or less equivalent to transforming the ASM into a single volume. Except for phantom parts that are attached with axis connectors for example.

You say that it takes more than 5 minutes, which probably means that you have not simplified your model enough and/or that your mesh is too fine, which is often the case with tubes.

This can be seen when we do a mesh analysis in the results.
If you post your complete ASM with the side and bottom sheets especially, but without any screws, washers, or nuts that are useless. 
Use the Pack and Go to transmit the ASM with all the PART files. I could, or one of our colleagues, give you some information on how to best carry out your simulation.

Kind regards

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5

Good evening

I preferred to do the simulation in a PART because I need to do less manipulation every time I make a modification to my frame. But if that's the only solution, I'd work in an assembly.

Here is the pack & go of my ASM which corresponds to the model of my existing prototype:
https://drop.chapril.org/download/c58fd74616bef073/#NlGwW0V7pn0y8eSbIe2kmg


Kind regards

6

Hello Natha

Sorry I can't help you because you have a version higher than the 2019

You did not include the simulation study with your DSO.
The little I see allows me to tell you that if you want to  do a serious  simulation you have to put the fork even simplified because your caster angle will act on the inclined stem.
In addition, your polypropylene Nida underbody is intended in principle to serve as a core for structural sandwich panels. So in your case you have to remove it because it will not be taken into account by the simulation (because composite materials with glue).
Find out about the compressive strength on a small area placed in the center of the panel. Indeed, even if the compressive strength of the Nidaplast 8HP is 2.6 Mpa (260 Tons/m2), this is in the case of a uniformly distributed load. And sandwiched between treated aluminum or MDF plates, etc...
I don't give much of your six little paws if the Nida is not totally rigid.

Good! I did make  a suggestion in the attached image to make your simulation as your tube assembly is.

Keep us informed;-))) dear Natha

Kind regards

 

 

Kind regards


velo_cargo_2021-04-28_18_36_25-window.jpg
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7

Hello @garnier.natha ,

Does my illustration below conform to your idea?
If so, I submit my point of view on your simulation project...

Hypotheses for a first simulation:
The delivery man and his bike are on a horizontal ground, launched at a constant speed, without any action on the pedals or brakes.
There are essentially 5 forces at play: the weight of the load, the weight of the cyclist, the weight of the bike (distributed over the structure), the actions of the ground on each of the two wheels.
We neglect the action of the air, the rolling resistance of the tyres, the friction in the wheel pivots...
Under these static assumptions, the actions of the ground on the wheels are normal on contact, and pass through the center of the wheels.

Consequences on the model and simulation
The three external efforts:
-    Gravity applied to the structure of the bike
-    The weight of the cyclist, localized force placed at the top of the seat tube
-    The weight of the load, distributed on the bottom plate, or distributed on the 6 support legs.

The two connections "with the ground":
Since the wheels are not considered in the study, the forces representing the ground action apply to the center of the wheels, and must have a vertical direction.
In the simulation application, it is "Imposed Displacements" that model these connections with the environment.
-    At the rear: a "Fixed pivot" type movement, on the cylindrical areas of the wheel bearing;
-    At the front: the model requires two pieces to be adapted. The "Short Prototype Fork" part should be extended to the center of the wheel. The piece "Floor fork support for prototype" is preserved, with its underside parallel to the ground. A "contact between components, without penetration" is defined between these two parts to represent the pivot. The contact of the "Support..." with the ground is an "Imposed Displacement" of the plane support type. In this way the glide is left free, and the action will be normal on the ground.

A simulation later...
The deformation has the appearance below, with a maximum displacement of 12 mm.


Several means of validation: probing the projections of displacements in the connecting areas, checking that the ground actions on each link (front and rear landings) are directed according to Y. By the way, we can check the balance of the whole.
At your disposal to consider braking, cornering, or wheeling situations...
Have a nice day.

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8

 @Zozo_mp
Indeed, the fork is important to simulate the bike in normal operation. In the linked study, I didn't put it because I'm simulating a loading case that I did on my physical prototype. The objective is to verify that the results obtained between the simulation and the measurements on the protophysics are concordant. And during the measurements on the physical prototype, I had removed the fork and put the frame on the head tube and on the axle of the rear wheels. That is why I am doing the study under these conditions.

On the other hand, I agree with you, to simulate the efforts on the bike in operation it must be taken into account. This is what I will do in my next loading cases.

Thank you for the advice regarding the cash float. I removed it from the study and applied the forces directly to the 6 legs.

In any case, thanks to your feedback, I obtained a more than conclusive result. I get the same result in simulation as during my measurement with an error of 1%!
I think we can say that the simulation and the boundary conditions are OK.

(I submitted the study with the results to the PC)


assemblage_proto_volumique_fourche_courte_3.zip
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9

@m.blt Thank you for your feedback and its very telling illustration!

You hit the bull's eye for the assumptions in the case of a load in "normal" operation. This is exactly what I will reproduce in my next studies.

Indeed, the fork must be extended to the center of the wheel in these conditions. The study I linked is a bit special as I indicated above because it does not seek to reproduce the conditions of the bike in normal operation but a case of loading that I actually carried out on my prototype. In order to compare the result between the SW simulation and the real one and therefore to validate or not the calculation method.
My latest results show that everything is fine, I get the same results with an error of only 1%, I'm very happy!

I will now be able to reproduce this simulation on the frameworks that I am currently developing and put in place the conditions that you described very well in your answer:)

I'll give you feedback as soon as I start in other cases of loading, especially in wheelie situations

Have a nice weekend!

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