Analyzing Vibration of Bicycle Frame as a Spring Mass System

In summary: No, you should not constraint the positive z-direction on the nodes corresponding to the link to the wheels because the weight affects the nodes in all directions.
  • #1
chandran
139
1
I have a bicycle frame. I want to analyze this frame as a spring mass system.
What details i should know. I should know the frame's stiffness. ok let us say it as x. Then the centre of gravity of the frame. Let us say it is also known.

But now the frame can be pulled or pushed and then released from any point on it so that it vibrates. (unlike a simple spring mass system where the mass is separate and is pulled through some distance and released to induce vibration).

How can such a system's natural frequency or how dynamics can be done on this?
 
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  • #2
You need to do a search on modal analysis, rap tests and the like. On something that large, you can simply place accelerometers at different locations and hit the frame to induce vibration. The accelerometers record the data. From that data you should be able to discern the various modes' frequencies (up to a point). If you use enough instrumentation you can then discern mode shapes.

This kind of analysis is done quite often. We do modal analysis on blades to discover what their excitations are so we can avoid them in the operating ranges of the engine.
 
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  • #3
fred,
What are the places the accelerometres should be kept? Any website
on this type of analysis
 
  • #4
Usually one has an idea of where the maximum deflections are going to occur (mid spans of beams, etc). These are the places you try to place the accelerometers.

First and foremost, the Society for Experimental Mechanics is a great place for information. http://www.sem.org/PUBS_ArtDownload.asp [Broken]

An introduction to modal analysis:
http://www.sem.org/ArtDownLoad/msma98.pdf [Broken]

Other articles of interest:
http://www.gmi.edu/~drussell/guitars/electric.html
http://www.sem.org/ArtDownLoad/msja00.pdf [Broken]
http://www.sem.org/ArtDownLoad/msjf00.pdf [Broken]

As a side note, I have done a lot of testing simply using a microphone to pick up the local vibrations. Again, this is a single blade, not an entire bike frame. This will not give you the mode shapes, but it will give you the mode frequencies. We then use the models to determine which modes are being excited. This way is a much easier test set up.
 
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  • #5
Hi,
If I wanted to get the natural frequencies of the bicycle frame by means of FE-analysis, should it be performed as constrained or unconstrained ? Indeed, the wheels are not bounded to the ground, so please, could you help me?
Thank you very much
 
  • #6
If you want to use the results for some purpose, the model should be constrained the same way as the "real life" situation.

If you want to correlate the model with test results, it's always a good idea to start with no constraints. That removes one source of uncertainty in the model, because "real world" constraints are never "perfectly rigid".

Assuming the bike is standing on the ground, the wheels are "bounded" to the ground by the weight of the bike, unless the vibration amplitude is so large they lose contact with the ground. But you would probably need to include the flexibility of the tires to get a good correlation with test measurements.
 
  • #7
Ok! Thank you very much!
 
  • #8
However, I am not sure about the assumption that the wheels are "bounded" to the ground by the weight of the bike...
Should I constraint also the positive z-direction on those nodes corresponding to the link to wheels because of the weight? But weight affects to all nodes of the structure...
 

1. What causes vibration in a bicycle frame?

Vibration in a bicycle frame can be caused by a variety of factors, including road surface, tire pressure, and rider weight. The most common cause, however, is the transfer of energy from the rider's pedaling to the frame.

2. How does vibration affect the performance of a bicycle?

Vibration can negatively impact the performance of a bicycle in several ways. It can cause discomfort and fatigue for the rider, leading to a decrease in power output and speed. It can also affect the handling and stability of the bike, making it more difficult to control.

3. Can vibration cause damage to a bicycle frame?

In most cases, vibration alone will not cause significant damage to a bicycle frame. However, if the vibration is severe or prolonged, it can lead to fatigue and potential failure of the frame over time. This is why it is important to regularly inspect and maintain a bicycle's frame.

4. Is there a way to reduce vibration in a bicycle frame?

Yes, there are several ways to reduce vibration in a bicycle frame. One option is to choose a frame material that has natural damping properties, such as steel or titanium. Alternatively, adding components such as a suspension seat post or shock-absorbing handlebars can also help reduce vibration.

5. How can I tell if my bicycle frame is vibrating too much?

If you are experiencing discomfort or difficulty controlling your bike, it is likely that there is too much vibration in your frame. You may also notice excessive noise or rattling while riding. It is important to pay attention to these signs and address the vibration issue to maintain the overall performance and safety of your bicycle.

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