Sanity check on 3d-printed/composite frame

[BalorNG]

I'm mostly a software engineer by trade, but I have a cycling hobby and been learning CAD and composites on my free time lately.
Her is the result of my latest efforts:

Gyroid infill:

Actual 3d printed parts:

Basically, my idea is that by 3d printing (carbon-filled petg, pretty tough plastic with good adhesion to epoxy) you can play with part shape, internal 30mm (1.5mm thickness, mostly 0 deg layers, a few 90 deg) carbon tube will provide bending stiffness, it will be wrapped with a thick carbon sleeve (about 1.5mm wall thickness) for torsional strength, making a kind of 'tube in a tube sandwing'. The bends with interrupted tube will be wrapped with more carbon.

The tail section is 60 by 40 oval with outer layer (1cm thick) printed in shore 95A TPU, inner layer an other petg (I've read that graded core stiffness is good idea for toughness) with 3mm carbon rods as both locating pins and to give some additional bending stiffness epoxied in.

The idea is that it should be not too heavy, relatively easy to make, and the tail section should work as some suspension.

The rear wheel, btw, is asymmetric cantilevered, there are precedents of that:

https://www.bikeradar.com/reviews/bikes/urban-bikes/burrows-ratracer-sl-review/

Any tips on layup or constructive criticism before I commit to cover it (and myself in the process, grumble-grumble) with carbon and epoxy?

 

[moon161]

1. It looks awesome all built up.

2. A bit late to chime in, but ±45° wrap will stiffen wrt twist and deflection, and 0° wrap will stiffen wrt deflection only.

I think twist may adversely affect rideability and comfort because your seat will be fixed to it and rock you back and forth. You'll feel it most at the extreme point (your head).

If you mount the seat at two points, it's gonna twist along with the frame.

 

[DB-TACG]

Any updates on how this is working out?

 

[BalorNG]

Well, I've decided to add another experimental feature - seat support "wings" doubling as frame sliders.

Seat mockup:

Mocking up the 'wings':

I'll have them 'trimmed' once glued of course.

Will take a few days I presume...

 

[BalorNG]

https://youtu.be/sVGFOTxVhlc

Well, at least I would test it this way... It does feel 'funny', especially because it turns out my front triangle is also a bit off (not by design but happenstance) and in the SAME direction, make the bike even less balanced, but the feeling quickly disappears after you simply learn to compensate for it by leaning the bike a bit. I think going with (about half of rear) asymmetric front as well in *different* direction should do the trick.

 

[DB-TACG]

Keep at it. The looks dont matter, especially when its the first. Making it work is important.

You could eliminate the tie rod ends and the bolts and laminate an adhesive mount stud or nut on the top of that wing part. It would save weight and balance the downward force on the wing part.

https://www.mcmaster.com/adhesive-mount-studs/

 

[BalorNG]

Good idea, didn't think about it! Makes a lot sense.
But I'll add a sort a sort of sacrficial cap that will double as support of cantilevered load - there are actualy 3 nuts epoxied into the 'wing' to provide attachment points.

By the way, I've experimented with forged composite technique using 3d printed mold and it turned out pretty well:

Now I'll need to make an other one... btw this is mostly chopped basalt that I have a large bag of, but it pretty good! Something inbetween carbon, GF and aramid (very abrasive resistant hence a bit of bitch to sand). Not light tho.

 

Never built a bike, only repaired them, but what I know about frames is 'the stiffer the better'.
Overall stiffness is only as good as the weakest link, and that 45° bend arrangement looks like a weak link that could defeat the best carbon fibre. The designs shown here also increase bending moments (and therefore stress at bends and joints) by providing much longer moment arms.

(The problem with reinventing the wheel is the high likelihood that the new wheel will be less round than the old wheel.)

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[BalorNG]

Yea, I'll certainly be fixing interrupts in the interior reinforcement in my next design, but this one seems to work pretty well so far.

The 'stiffer is better' applies to torsional and lateral rigidity though - vertical is fine or full suspension bikes will be unrideable Also, there is an entire school of thought dedicated to 'planing' and my own experiments with an other recumbent with extra flexy frame suggests this is partly true, but this certainly a DYNAMIC frame property coupled to individual pedalling ergonomics and biomechanics, so hard to ascertain even in a controlled experiment... since this bike is FWD and drivetrain is short, steel and triangulated, does not bob - I'd say this is pretty efficent.

I agree that a recumbent monotube design with never be as light AND as stiff as a 'diamond frame bike', but no diamond frame bike will offer same combination of speed and comfort - you must either completely sacrifice speed, or completely sacrifice comfort to equal something like my midracer, and a decent lowracer will handily beat any TT bike while also being much more comfortable - at the cost of being even less practical to be fair. The downsides of a recumbent form-factor lie in low-speed/low-traction controllability usually, and how intolerant you are towards 'legs up' position (hemodynamics).

Now, whether this particular frame will stand the test of time - dunno. There are most certain weak spots...

 

Ride height counts for something when surrounded by 2-ton fossil fuel enthusiasts who are for all intents and purposes out to kill you.

Good to see you installed winter tires though

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[BalorNG]

It does decrease your chances of survival, but even by withholding from cycling altogether your ultimate chances are still zero Ok, maybe if your name is Elon Musc or Vladimir Putin, there is a remote possibility of living 'technically forever' now, but I do not find prospect particularly enviable even...
I don't cycle in heavy traffic, and on more or less open roads you will either been seen anyway, or you might as well drive a bus and it will make no difference to a distracted driver. This is not for 'commuting', I walk-commute anyway. This is for fun