If you get to the point where you’re selling these I want first in line 
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Test ride was a success, no rims breaking or wheels exploding. 17 km (10.5 mi), light snow, little slippy under braking and accelerating, avg. speed 25 km/h (15 mi/h), 22.4 Wh/km (36.0 Wh/mi).
Generally pretty smooth ride considering the riding circumstances, back tires not necessarily at a more optimum higher pressure for efficiency, but it was more a first field test for the wheels and rims.
The wheelbarrow tires are quite unbalanced, resulting in oscillations/vibrations at higher speeds (uncomfortable at long rides), but considering their original use-case and the maximum speed of couple km/h at that use-case, I’m not super-surprised that they aren’t necessarily suited for high speed operation. Maybe see if I can source a better quality outer tire, but same size inner tube.
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Popping off again.
So, after yesterdays successful test ride. I planned on possibly riding my board to the office on Monday even if it was snowing (12km/7mi in one direction), so I decided to check and up the pressures in the rear tires, as they had held alright on the test ride.
Well, I had pumped them up to guessing around 1.0-1.5 bar from the earlier low 0.5< bar, in preparation for tomorrow and returned to my PC and put on my headphones. Well that was a good thing as about 5 minutes later there was a quite loud pop/boom sound and I assumed immediately that the inner tube had popped from the higher pressure.
But it turned out that it had just blown the lip off the rim… again. And the inner tube was bulging out from the tire looking like the angry stay puft marshmallow man’s angry puffed up as****e from Ghostbusters. It was so swollen outwards and inwards that the valve stem was completely hidden/swallowed up by the expanded tube, so I had no way to release the pressure in the tube. I was also pretty careful where I positioned myself, as I wasn’t excited about getting high pressure air blown to possibly my face, eyes, ears or something flinging off into said areas. So I grabbed proper earmuffs and protective glasses before doing anything more near the thing.
That was a good choice, as I started to carefully wiggle the expanded donut of possible death of the rim, the inner tube let go, but as I was wearing my ear and eye protection it was just a pretty mild pop sound only and some air whizzing.
But here is the aftermath.
I’ll be beefing up the lip even more next week for the next iterations, as I get more filament to print with.
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So in after-analysis both for the very first prototype and this one, the lip has started to tear cleanly along the sharp angle of the lip, so this is clearly a stress point, so as the first measure I’m gonna be rounding it off more to remove the one clear stress line on it.
Old:
New:
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Increasing infill percent might help with the binding a bit there too, didn’t see what you had yours set at
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I was printing at 50% and I think 4 or 5 layer solids on top, bot and sides, but I think the bigger culprit was simply the huge stress line caused by the single big angle change of lip, but we’ll see how the next iteration do.
I now know that the old/current ones survive some riding with the tires at lowish pressures, so I would focus on beefing the strength of the lip and then do a pure pressure test and wrap the tire in some textile/blankets to suppress the thud and stop any expediently escaping rim parts from flying away in case it does let go again.
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Yeah, makes sense. I would think that the more gradual slope would help it stay together better
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@surfnacho check this out. I think you were asking about the outer edge but this is still a little relevant to the question you had about your rim a while back
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Thanks Robin. I think this is slightly different though. I think in this case it is more about the strength of the wheel, where in our case it’s more about the tire.
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I think we were previously talking about the magnitude of the hotizontal forces pushing out against that lip, which may or may not induce a cut on a sharp outter edge. I got the gist that we weren’t clear on the magnitude of that force, this I think puts it into perspective nicely.
Youre right, different, but totally awesome to see failure with such a textbook method
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Well I mean if you think about it, the possible forces there can be on the rim from the perspective of the inner tube. It wants to expand inward, outward and towards the sides.
Inwards it’s radially pushing to crush the rim, but depending on the spoke count you can have a pretty short distance between the unsupported sections between the spokes, so in that direction it’s pretty easy. Plus you have an arch which is very strong geometry in this instance.
Outwards radially we generally don’t care unless it was tearing the outer tire apart from the pressure, but that is less often the problem.
So this then leads to the axial expansion being the most difficult to counter, aka the lip of the rim. The trouble with 3D-printing the rim, especially with the FDM-process means that usually means the layer direction is planar axially, aka the weakest part in a print being the layer adhesion and that is exactly what the inner tube is putting load on axially.
So to counter this you either need to improve layer adhesion (hotter temps, layer thickness) or beef up the surface area , which the inner tube is spreading the axial force upon, which is what I’m trying to do.
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I had a thought but I’m not sure if it’s feasible. If you split your model right where it failed and printed the “lip” portion" separately, but in such a way that the layers are orthogonal to the current direction, you could optimize the stress lines against the print lines. I would print the lip as two halves as arches.
Not sure how you would go about attaching the two sections, although two thoughts come to mind, either high strength adhesive of some sort, which if chosen properly, could easily be the strongest part of the print.
The other option is to extend the lip portion into the bolts, which sees more complex and worrying because of the inherent lever from the point of force applied from the tire to the bolt itself
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Just found this in my recommended in YT
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Excellent weather on Saturday to have a test ride… Thick, wet, slippery, slush…
Got about 1 kilometer away from the front door when I started to feel the wet starting to come through my pants (to clarify, the wetness was cold and coming from outside my pants).
Higher tire pressure was used at 1,5 bar and the rims held up for the short 2 km test ride.
Managed to then blow the other rim next day on Sunday when I was gonna change the backside rim to a better modeled one, well the rim gave when I was trying to pull it off. Got hit on my fingers and some blood was drawn, but nothing serious, sharp edge on the rim peeled a little bit of skin back but otherwise ok.
So below is the bad and good rim side by side and it’s quite clear where the bad/first rim started to fail as I was in the process of starting to switch the bad back rim to the better modeled one when it gave.
As I had moved the bolts inwards I hadn’t noticed that the nut on the back side had became only partially supported/half-closed by the 3d-print. I had corrected this for the second print for the other wheel and had just printed another one to change the first one into. Not quite sure I trust even the better model to be able to take the loading on it to be honest…
Based on this and the fact that my fingers are now hurting, it’s time to go beefy or not go at all. I’m gonna be switching from M4 to M5 bolts. I’ll increase their length from 40mm to 60mm in order to help the bolts take on the axial load better and help support the 3d-printed mechanic.
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This is very interesting. Im wondering if reinforcing the inside of the rim with fiberglass or carbon fibre and epoxy would make any sense, to keep this from happening.
It would just be one layer and be used to distribute better the forces. Maybe carbon fibre makes more sense hera as it is more directional, and you could apply it in perpendicular to the print layer.
Edit: i dont know how to write.
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I’d say anything that helps eliminate the stress on the 3d-print in the non-planar direction of the layers and/or adds strength axially, is gonna significantly strengthen the durability of the rim.
So you mean having a single layer of carbon-/glass-fiber like this? (tape used for demonstration purposes)
Or did you mean on the inner side?
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Yes, like the first picture. In the inside of the rim.
It would have to be very well sanded down so it’s a perfectly smooth surface.
Also, if you look at carbon fiber, you can see it’s composed of several small strands. If you make them perpendicular to the layer direction, you would be optimizing it’s strength.
To apply it, as the rim comes in halves, you would have to apply it to each half separately.
Glass fiber would probably work for this also but I think carbon fiber is more suited for the job.
I don’t think trying this would be very expensive as you would need small quantities of it.
Lastly, you may want to look into the epoxy you use, just in case it melts the plastic or something. I don’t really know if epoxy contains dissolvants.
I’m also working on my own tiny ass rims to fit 125mm pneumies @tipsy showed me. They would just be the ones in the back with kegel core so I can drive them with my BN gear drives.
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Print the rim on its side vertically with supports, then you won’t have the layer lines being the weak point. Then just a little sandpaper to clean it up where supports were
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the bearing seats would be a little shit then
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