Inventing wheels.. with built-in suspension =p

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A_3
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Re: Inventing wheels.. with built-in suspension =p

Post by A_3 »

As yours design is struggling with same issue, spoke failure, from the beginning of project, maybe you must redesign your project and consider use of more flexible material. It looks like that the weak point is the root/end of spoke with a small cross-section and thickness, amount of spokes does not solve the problem. All spokes are facing the same direction, so what holds the wheel in balance and prevents extreme bending to one side, loss of form and catastrophic failure?
g5303.png
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Consider the following:
1-solid, not hollow and curved, flat surface which is in contact with asphalt with the radius in the end,
2-thicker spokes which will reduce they number but it will still be flexible and stronger,
3 -two separate rows of spokes (with gap between) one going clockwise and other going counter clockwise to prevent excessive deflection/bending, (or join spokes to a "net", merge them with each other. Let them be same width as part (no gap between spokes)),
4 -radius between spokes and parts that they connect (2 on the beginning of spoke front and the back side and 2 on the end of spoke-front and back side) to strengthen them and prevent breakage at that point.
g4190.png
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DeepSOIC
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Re: Inventing wheels.. with built-in suspension =p

Post by DeepSOIC »

Wow, what a great post!

I've fitted an arc to your picture, and calculated the maximum deflection your proposal will allow. It would take less than 2 mm of pulling down the hub to completely straighten the spokes I've highlighted, assuming a wheel similar in size to the one I just destroyed. My design allowed for 8 mm, which is still pretty low. But 2mm is completely hopeless, IMO.
A_3_max_deflection.png
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But your suggestion sparked a few ideas, especially the one of stacking two sets of spokes to prevent turning of the hub relative to the tire because of suspension deflection. I'm not yet sure of how I'm going to achieve that from 3d-printing point of view, most likely I will have to split the wheel into two, and glue the pieces together after printing.
Last edited by DeepSOIC on Mon Aug 29, 2016 8:30 pm, edited 1 time in total.
Reason: forgot to attach picture
ezzieyguywuf
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Re: Inventing wheels.. with built-in suspension =p

Post by ezzieyguywuf »

Maybe you could sandwhich a 'suspension wheel' between two 'structural wheels' (or vice versa) such that the flexible suspension wheel(s) does no experience the maximum deflection that causes failure. In other words, the rigid 'structural wheels' provide a hard stop
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DeepSOIC
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Re: Inventing wheels.. with built-in suspension =p

Post by DeepSOIC »

ezzieyguywuf wrote:Maybe you could sandwhich a 'suspension wheel' between two 'structural wheels' (or vice versa) such that the flexible suspension wheel(s) does no experience the maximum deflection that causes failure. In other words, the rigid 'structural wheels' provide a hard stop
That might make a good extra protection. But I believe the wheel must work without this extra protection, at least as long as I'm not abusing the scooter by jumping.
A_3
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Re: Inventing wheels.. with built-in suspension =p

Post by A_3 »

The previous model is for presentation only, wheel is bigger (15-30mm) than original whell of your scooter.
Consider the idea proposed by ezzieyguywuf, it sounds pretty interesting, and you will have chance for lot of gluing :)
Anyway, here is something closer to your wheel.
Notice: axb is 2.5x9mm, spoke radius: 33.5mm and 31mm. number of spokes is now 2x13.
I forgot to transfer the STL file to USB, so I did not attached it.
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ezzieyguywuf
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Re: Inventing wheels.. with built-in suspension =p

Post by ezzieyguywuf »

The more I think about it, the more I like my idea (lol).

When I think about it, the main purpose of your shocks or suspension built into the wheel should be to hold the weight of the entire load (scooter plus you) in a 'steady state' condition. i.e. it needs to provide enough spring force to hold you up.

The reason you've seen the failure is that you've experienced spiked loads due to an uneven surface. A car has a shock-absorber to help mitigate this, and the suspension on a car has a 'hard stop' as well, as the coils will 'bottom out' before they exceed the maximum deflection allowed by the material properties of the steel.

So, your system doesn't have a shock absorber (oh well) but you can build in a hard-stop somehow. This way, you can still use your suspension when it is feasible, but revert back to a rough ride when you have to.
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DeepSOIC
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Re: Inventing wheels.. with built-in suspension =p

Post by DeepSOIC »

I still have no complete understanding on why did the wheel fail. A_3 points out that I have weak points where spokes are attached to solid structures. This was indeed the case with the first design. But second design was made with the aim to aliminate the impact of this place by removing the likelihood of low radius bending there, which can cause fatigue.

So I have no clue basically, where did the initial failure happen. But once a couple spokes are gone, it's quite obvious that the rest will follow like an avalanche. And because I was riding, the hub crashed into the rim and shaved off all the spokes, so that I have no chance of finding the initial failure point.

So, I think I will print a thin section of the last design, and put it to a controlled destruction. I a manner that will not shave off all the spokes, so that I have a chance to analyze the reasons.

So far this is the plan. The rest will depend on the outcome.
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DeepSOIC
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Re: Inventing wheels.. with built-in suspension =p

Post by DeepSOIC »

For reference. Diameter of original scooter wheel is 178 mm. I can print maximum 160 mm (printable area 160*185 mm).
ulrich1a
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Re: Inventing wheels.. with built-in suspension =p

Post by ulrich1a »

DeepSOIC wrote:I still have no complete understanding on why did the wheel fail.
I think, you have just experienced what engineers discovered in the 19th century at railway wagon axles. They failed due to repeatedly applied loads. See also here: https://en.wikipedia.org/wiki/Fatigue_% ... _S-N_curve

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DeepSOIC
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Re: Inventing wheels.. with built-in suspension =p

Post by DeepSOIC »

Fatigue may indeed be the culprit. I have made the radii of spokes to the very limit of plastic deformation, to maximize stored energy density. As it may be obvious, I don't have much room for more spokes, which means if I need to reduce the strain to avoid fatigue by more than two times, then I'm screwed :| and there is no way apart from using different material. Which will not be 3d-printable, most likely.
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