locktopus wrote:
Hehe thanks m8 but i had already looked through almost all of those (first one seems to need password...) and they give some general information but nothing that i can use for here. Thanks though :p
maxxevv wrote:
I did something similar some 15 years ago. But it was more to verify my load data/points to some previously published papers on another study so that I may apply the verified methodology for my undegrad thesis paper.
For one thing, because of how simple and optimised a bike frame is, if your details of modelling such as angles, joints and wall thickness are not accurate, you'll not get any results close to what's 'real world', don't mind about performance criteria as you're talking about. In short, "rubbish in, rubbish out"...
i) Get your model as accurate as possible. Including wall thicknesses. (Note different materials have different thicknesses, as that's what out in the 'real world'. If its all the same, then the value of the study diminishes dramatically. You can either alter your models to reflect identical geometry or identical mass to simplify things.
ii) For your reference, you can't look at deflection/stiffness per tube section, but at the overall von mises stress at key points such as below the downtube junction with headtube, the BB intersection and see how it references back to the material's UTS , Yield Strength, fatigue cycle tables.
iii) as for overall deflection, you'll can refer to the Tour Magazine figures, they do publish their testing methodology and see how you can extrapolate your data from there.
And finally ... good luck! It can be hair pulling at best of times.
Edit: Forgot to add -
your load points are not correct. Because load is applied both at the saddle and at the handlebar.
i) Due to saddle offset, load at the seat-tube joint you pointed will consist of a moment and a load.
ii) The will be a vertical load + a moment at the headtube juncture too.
iii) due to the angle and offset rake of the fork, your headtube junction cannot be "fixed"if your rear axle is fixed. This is because of a shear motion induced by the angle.
iv) Also, there should be an effective moment load at the BB axis too. This is due to the twisting forces induced on the frame from your pedaling force. This is assuming you are inducing a fixed side-to-side movement at the 2 wheel axles for your frame loading parameters.
Bear in mind, all these are from memory of work done all those years ago. Not all the details may be correct but generally, the modeling of loads/loading is a whole lot more complicated than what you're inferring here.
1) Maybe i should have said on the begginning, these boundary counditions are not for me to chose. The teacher defined these, and i have to use them...
2) Thanks for the tips on looking for the maximum stress and deflection. =) The maximum stress for the frame will be the yield point stress of the material (so said the teacher), about the maximum deflection he told us to figure out.
davidalone wrote:
Nice stuff there.
Fellow Engineer who works with FEA as well. I work with biomedical stuff though.
Maximum Deflection for a given load FOR a given material and a given tube cross section can be found by using the generalized euler's beam bending theory or beam buckling theories, depending on whether your tube is vertical or horizontal and your direction of force. But this will be for simple beams, mind. you will need to analyze each tube differently. there would be no 'normal' because it would depend on tube cross section, composite layup etc. there would be a 'range' of what bicycle makers would probably do, the ones with less deflection being your all out race bikes and those with more deflection being your enduracnce models. others have pointed that out already. Which is why we have FEA to do all this nasty legwork for us.
Will agree that your loading and boundary conditions seem to be incorrect. maxxevv has covered most of it. However as this is a school project I don't expect it to be a super complicated, so try to start with a simple simulation before you start adding forces. the general rule with FEA is that you start with a simple simulation with lots of general and possible inaccurate assumptions. when that works, refine the model and the associated assumptions.
Im using ABAQUS. As for the rest, i don't think we're supposed to find out the maximum deflection that the tube can bend, but some value that is normal for this industry. It's just that from my research no one seems to do this test because it's basically close to useless, because it neither tests the most important stiffness (tornsional and lateral) and doesn't give a good idea about rider comfort (for which case it seems manufacturers seem to prefer to test with the fork and more of the bicycle elements included than just the frame).
Hence why it's being to hard to find some values for vertical stiffness/compliance/maximum vertical deflection for a test like this. And yes this is not supposed to be super duper realistic, just a start point, but still i need some base values.
From what i could see, some people talk about "vertical compliance" and i've seen values like for vertical stiffness of 200N/mm. But that would mean if i put a load of 100kg (heavy rider) it would bend 5cm which seems a lot if it was just the frame. But then again i think that's a value for the vertical stiffness of the whole bike not just the frame!
This is one of the results i got for an ordinary set of steel (reynolds 531) tubes available from a site that sells bike tubes
For F=120kg*4G=4800N (approx 440kg) which gives max vertical displacement of 0.09mm... isn't that too little??
I actually went ahead and tried 5ton and it just went down 2mm! (didn't reach yield stress (540MPa) anywhere on the frame... (max was 378MPa)
Thanks for your replies fellow engineers! I'll keep looking for some values to use