3-d printed frame idea

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Aug 6, 2022
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Hey guys. I am barely (almost) finished my iZip restoration (couple of small pieces to print to mount the charging port and master switch ect, and paint the artillery shell glove box) and am already scheming my next project since I have a 36v/350w Bafang hub motor sitting here.

I would like to do a stretch low-rider trike (something along the lines of Atomic Zombies design, see the pic) but rather than weld, 3-d print (carbon-fiber PETG or maybe nylon) connectors and use 3/4" and 1" EMP as the frame rails. The drop-outs, head-tube, bottom bracket ect would be salvaged out of "junk" bikes and I can design brackets around them. The brackets would be split and assembly would be with B7000 (or epoxy) plus thru-bolted. Going with a design something like the picture below lets me use 2 regular 16" kids bikes back 1/2 for the trike assembly with independant rear wheels (no need for a diff or anything)

I'm thinking of designing the brackets so they fit both inside and around the tubes where aplicable for butt joints with about a 1" solid plastic "buffer" between the tubes and at least 12mm sleeve outside the tube, Any change in diameter bewtween the salvaged pieces and the EMT would be accomodated internally in the printed brackets. I'm printing on 220x220 bed so can get pretty decent sized gussets built into the brackets.

Can anybody poke any holes in the idea (so far ?)
 

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No sarcasm at all: I'm a big fan of hearing about and seeing people use 3d printing in applications that should definitely not use 3d printing.

Nylon is a good choice. I'd stay away from CFPETG for this application, personally, it might be a little too brittle and layer adhesion is lower than you'd want. I don't have a lot of experience in bike mechanics, but the way you're describing it, you're going to be joining tube ends that are normally welded? They're going to be under a lot of stress, and you'll want something that can bend a bit with stress before shattering. So yeah, nylon, maybe PA6-GF if your printer can handle it.

Do you have any preliminary models/designs for the connectors?
The brackets would be split
What do you mean by that? Since I don't follow exactly what you're planning, does that mean each bracket will be multiple pieces? If possible, I'd think you'd want your connectors to be a single piece. But maybe there's something i'm missing.

Looking forward to seeing what you come up with.

P.S., a couple other examples of 3d printed things that shouldn't be 3d printed:
 
I'm sure you are about to get some people telling you this is a terrible idea but I'm not that. It's possible although it will be very difficult and there may be easier ways to so some aspects. Realistically buying a welder and learning to weld will be easier, I use 3D printed parts for tons of parts on my bikes and for many things that others would say wouldn't be possible but they only work because a lot of engineering goes into them. Material choice, printing settings, reinforcements and design will all be critical.

Personally I wouldn't use PETG, I have never found a single application that PETG was actually good at compared to other materials and have been disappointed every time I give it another go. The problem is the good materials require a heated chamber to get good results out of but with one you can print ABS and Nylon and with those to anything is possible. Their toughness is what you need to make anything that needs to take any shock loading. Carbon fiber nylon may be a good option as pure nylon is generally not stiff enough but the carbon fiber does weaken it in some ways. It will be expensive and carbon fiber nylons are not all the same.

Reinforcement plates, fiber reinforcement, well positioned bolts, heavy use of truss reinforcement will all be needed. 3D prints can be very strong but they lack specific stength, that is to say they do poorly when you need a lot of strength in a small area which is exactly what the joints of the tubes will require.

The other issue you should be aware of is fatigue properties, which all polymers have pretty poor ones, meaning it may be strong enough at first but if you have a point of high stress it will weaken over time in ways that metals do not.

Also I would use larger than 1" tubing unless it's going to be a ton of trusses, the larger the tubing the more area you have to distrubute force into the plastic which goes back to the specific strength thing.
 
This is what I mean by "split" brackets (thats the bottom bracket of the colorfab bike on thingverse). It's kind-of a cross between printed brackets and maker clamp, both for ease of assembly as well as post-processing. I live in a condo and my "shop" is a spare bedroom, so welding (I already know how) is out of the question. This whole idea was inspired by some of the printed bike designs found on google. Printer is a Neptune 3 Pro with an all-metal hot end upgrade so some of the higher temp polemers arent an issue. 3D Printed Bike by ColorFabb
 

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Have you considered finding a space where you could weld? The all metal hot end is a good start although without a heated chamber parts that large will be very difficult to print from any higher temp polymers, carbon fiber filled options will help but it's still not ideal. Much of the design will come down to what the frame parts you are starting with look like, the more of them you can keep you could potentially distribute the load better. You should also consider some hybrid or alternative methods to some sections, for instance you could use a bolted steel plate to strengthen a part.
 
I am not nearly the bike mechanic of most the folks here. I do have a weird memory though, I unfortunately remember nearly every bit of useless data I have ever been exposed to. Comes in handy occasionally, generally it just gets you weird looks.

If you are referring the OC Chopper bikes of the late 90's early Naughties, I would hope you also saw how poorly they performed. M eldest was mebbe 8-10 years old when her and the neighbor boy got bikes for xmas. Kiddo got a Electric dirt bike that lasted about 4 seconds after she got on one that had IC instead... and I think a Huffy to go on rides with the Nanny.

The neighbor boy got an OC Chopper which I have to say, looked pretty cool.

Then then head unit popped free of the frame. back to the store, manufacturing defect...

8 replacements later the store suggested mebbe the boy was too heavy (he weighed in at like 80 pounds soaking wet) no, it was just a steaming pile of dung, also with the way it was set up and VERY heavy rims/tires it put all the weight of the wheel on the outside edge, essentially the very worst place to have it if you are peddling. Awesome for a fly wheel, which is what got I think replacement #6 toasted when he took a corner at a brisk walking speed and the forks twisted up.

If that is the chopper you are referencing, I don't recommend it based on my sum total knowledge of watching one neighbor trying really hard to love his ride.
 
There are examples of home builders who 3D printed tubing connections as a way to "jig up" the frame and then doing a composite wrapped overlay to ensure the strength.
The concept is similar to a bamboo bicycle frame build where they use water activated orthopedic cast wrap material.
JB Weld also has a few sizes and types of fiber wrap materials to select from.
 
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