3D printed eMTB barrels

[thunkar]

I really didn’t know whether to create a new topic or directly go to the “Weird theories and ideas” one…but bear with me, I think this could be interesting.

I built a mountainboard recently and I have been abusing the sh*t out of it. As per usual, my budget was constrained (actually dismantled my longboard and took the electronics out) and got a pair of cheapo flipsky eMTB trucks with 8" wheels that cannot be beaten for the price…but that’s about the best thing you can say about them. I actually had to hammer one of the motor mounts in, just to give you an idea of the quality of the build and tolerances.

BUT, the worst thing by far were the springs. As my skills improved, I could basically pop them out on command by just leaning a little bit too far when strapped to the board, which as you can probably guess is fantastically safe and confidence inducing when riding hard through the middle of nowhere next to a cliff. At first I tried the built in mechanism to tighten them, and 3D printed some inserts to prevent the retaining screw from backing off:

After a few weeks, I was able to pop them again, probably at the WORST possible moment:

Fortunately I was very lucky and jumped off the bindings unharmed. This is when I decided to take matters into my own hands and get busy. I had a roll of TPU and time to experiment:

The first attempt was using 60% gyroid infill an no walls (in order to evaluate the compression of the barrel), and it turned out to be way too stiff. Next up, I went with 25% for good measure + walls…too soft this time. Also noticed that walls would crumple and remain weird after being compressed for extended periods of time, so this is my current setup:

The retainers are also TPU, at 100% infill to make them as stiff as possible. Then the barrels are 40% gyroid with two “crumple zones” that I set up in prusaslic3r to have no walls.

Tested them today on a short run to the gym on the street…so far so good! I think I actually like them more than the springs. The best part is that I can create basically infinite combinations of crumple zones, infill density and type, so I hope I can come up with a more or less universal “equivalent” to the most popular commercial barrels given a TPU brand. Until then…I’ll keep testing!

And please, if I post another near death experience related to this issue, MAKE ME buy some decent new channel trucks.

[jack.luis]

This is such a dangerous idea , you shouldnt do this imo

[thunkar]

Mmm, I’m actually working my way up and testing a lot in controlled areas. The barrels work on compression, not tension, which I concur would be dangerous in case of a snap. What do you find dangerous about the idea? Genuinely curious, I think the trucks are actually more dangerous as-is.

[frankthedragon]

Someone here made actual dampers for those trucks, elastomere dampers on Amazon I think. Could be worth looking into as a less jank solution.

[Fosterqc]

can you explain why not print a mold to cast urethane bushings?

[PixelatedPolyeurthan]

@Fosterqc

can you explain why not print a mold to cast urethane bushings?

This would be way more reliable imo and not hard at all.

I’m an fascinated by this though, even if I agree with @jack.luis .

What TPU did you use?

[thunkar]

Speed of prototyping, ease of access to the material. Also, I don’t doubt that casting PU on molds work, but the idea of on-demand infinitely customizable barrels seemed worth exploring.

I honestly thought this would raise more enthusiasm than alarms…what would you say would be the failure modes of what is basically a rubber rod? I honestly don’t see it ripping by compression alone, and the layer adhesion is superb when printed at 235°C. Also, what testing would you think is best to ensure it works?

[Creavenger]

If you print them solid or with a lot of wall perimeters and with the right settings I think it could work for testing, TPU is very tough. Make sure you dry the material well though. You can do this in a good oven at 70-80 degrees for 1 hour. Dont use a cheap oven since their temperature regulating isnt great they might peak above 100 during heating up and cause your spool to melt.

EDIT: looking closer at your pictures it definitely seems you still have some moisture in the tpu. If my tpu is dried well I often get better surface finishes than on pla or petg.

[thunkar]

Regular Sainsmart TPU, pretty common stuff, but quite sturdy if printed correctly. I’m starting to sound like a broken record, but please, let me know what you guys consider a safety hazard. I genuinely want to test this concept and I’m a 100% down to destructive test them until everyone is happy… meaning that either it works or I get a decent set of channel trucks, this time for real

[thunkar]

I’m a fairly light guy (68kg), but even at 60% infill I couldn’t get the board to turn even when hanging off the bindings, so solid is out of the question unless I reduce the diameter a lot (currently working with 30mm) TPU is a lot more dense than people think!

[thunkar]

I keep my filament on a humidity controlled case, but I’ll recheck anyway. Maybe what you are seeing in the pictures is the sections with 0 walls? I get some stringing in the infill due to the speed of printing and high temperature, but layer adhesion is very good.

[Creavenger]

Depends on the shore hardness, Ninjaflex for example is a lot lower like Shore A 85.

I was thinking to print it solid to prevent the whole barrel from buckling too much and creating a fold. I think this would be likely to happen if you do a turn and the barrel walls are too thin. if you get a lower shore tpu and print it solid it still gives you the flex you need without buckling too much and folding. Then you pretty much have an equivalent to a casted urethane barrel.

Ah the humidity controlled case is good, but it probably is faster if you extract all the moisture by an oven or dryer first and after that put it in the case. Ive had tpu coming out of a sealed bag and having moisture already in the filament. Drying it in an oven before printing makes a night and day difference for me. You can try to see if it makes a difference for you.

[Fosterqc]

@thunkar

but the idea of on-demand infinitely customizable barrels seemed worth exploring.

I fail to see how that idea is not achieved with printed molds and cast urethane.

[thunkar]

Testing different hardness require changing material if you cast urethane, however in the printed barrel I just change the infill percentage. In pretty much the same way, I can even set different sections of the barrel to have different slicing settings, affecting the overall behavior easily.

[glyphiks]

Send it. I would.

Would be interesting to record the lifespan

[thunkar]

I’ll keep this post updated! Also working on different alternatives, I REALLY want to squeeze as much life as I can from these trucks.

I just find it so frustrating that in the eMTB space almost everything is incompatible. If I want new trucks, I would also need to invest in motor mounts, and I’m basically locked in a “ecosystem” that may or may not be even compatible with the next generation. Sucks big time, so I’m gonna DIY as much as possible.

[glyphiks]

I support this ethos.

Keen to see your progress dude

[jack.luis]

@thunkar

the idea of on-demand infinitely customizable barrels seemed worth exploring.

@thunkar

testing a lot in controlled areas.

In this case go for it, I love science!

Sorry if my comment came off very negative, I just know that 3d printed parts dont belong in the drivetrain and I dont want you to get hurt.

But as an experiment, hell yes, go for it @thunkar !!

I really like the idea of making PU molds too.

[thunkar]

No worries man, always open to criticism, and most certainly when it is safety related.

I will keep this updated and try to rig my GoPro to get you guys some footage of them working. Also thought about getting some barrels squished for a few days using some woodworking clamps and see if deformations are permanent…

[thunkar]

After a weekend of testing, I got some good news and some bad news.

The bad

You guys were mostly right. The barrels worked incredibly well for 5-8km, suprisingly so. I could lean and carve with full confidence, vibration dampening was awesome and the feeling of the truck overall got 5x better. BUT:

Durability is shit. As you can see, they quickly lose their shape and even though still work fine when compressed, when centered the trucks become scary sloppy. The only way forward for this would be to print them solid and either reduce the diameter (more risk of buckling) or increase the size of the inner hole.

The good

The retainers I designed worked pretty well. So much that I decided to modify them for the old springs to try and fix the popping issue and came up with this:

They are way longer than the original ones, but due to the concave shape of the bottom retainer, the trucks still maintain the full compression range.

For reference, this is the state of the old retainers, and the main reason why I tried this in the first place:

The new ones are printed using 100% infill and press fitted into the adjustment screw, avoiding the dissasembly risk I had before. I’m gonna be trying them today and will publish the STL if there’s interest!