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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/07/2019 :  07:27:17  Show Profile  Reply with Quote
I'm close to getting to the point of putting the stays on the kid CF bike. I think I know how I'm going to do this, but I figured I'd solicit opinions on the join practice/process for putting the chainstays on the back, since that's sort of an important spot in the frame.

Design is like this:



Dimensions are (in that design, and tube stock on-hand) 50mm main tube OD, 25mm OD chainstay material. (Main tube is 2mm wall, chainstay 1.5mm wall.) Overlap area on main tube as well as CF plate transitions is 5cm, will be a bit more once a fillet is made to smooth the cut end and that's wrapped too.


What would you use for wrapping the join assuming they are cut for join mostly like this:



My plan is to do a primary, mash the fabric to the bulging profile layer of 3-4 layers of 6 oz. 3k, then make some fillets of epoxy and microbubbles to smooth the joins to a more regular, ovoid circumference, then a couple wraps of 11 oz uni over that, with a final layer of 6 oz braided sleeve over that. All layers to be compressed by reverse electrical tape and stretch wrap over that.) Does that sound right to you guys with more experience?

Am I correct in assuming that, if I'm wrapping circumferentially, most/all of the stress taken by the wrapping material will be tending toward trying so stretch it against its length, so my main wrap being Unidirectional will yield the strongest join?

--Sub question, assuming total rider + bike load of <180 pounds, would you stick with the 25mm 1.5 wall or go to 28mm 1.5 wall? According to this generic beam deflection calculator (https://www.easycalculation.com/mechanical/deflection-round-tube-beams.php), .98 inch, 0.0591" wall carbon fiber tube over a span of 6 inches (chainstay tube clear span in my design) supporting 100 pounds would deflect 0.15 inches, whereas the 28mm would deflect 0.10) Seems like the 25mm is plenty. It's also a little lighter, cheaper, and I've got more of it on hand. (Also, since I've got plenty of sleeve and uni around, should either prove to be flexy, I can easily reinforce it.)


--SpiderMonkey

Balor
recumbent guru

Russia
697 Posts

Posted - 01/07/2019 :  12:41:28  Show Profile  Reply with Quote
Thinner tubes will flex more, but will also *tolerate* larger deflection amounts more. In some cases you want some flex for vibration isolation, and stays are exactly where you want to.
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Balor
recumbent guru

Russia
697 Posts

Posted - 01/07/2019 :  13:01:20  Show Profile  Reply with Quote
By the way, I've got myself a cracked carbon frame and will be doing a "Marco Ruga REV MBB", so I"m watching all this with great interest!
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Jeroen s
Starting Member

Netherlands
9 Posts

Posted - 01/08/2019 :  16:11:23  Show Profile  Reply with Quote
Hi spider monkey


I have build 3 carbon fiber recumbents, and am willing to share. What i would do is use two layers of carbon sleeve streching about 2 inches longer than the chainstays and about 2 inches behind the joint on the free end of the chainstay. It is very easy to pull a round tube from a round hole, but its difficult to pull an irregular shaped curved tube tru a round hole. This way u get 2 flat wide pieces of carbon extending on the main tube and u increase the join area about 3x. Wet them out and put around the seatstay tubes, then wrap with tape. Make sure u got some sort of jig to hold things in place, because things will start to move and shift where u dont want it. Been there done that, and it's not funny.

Then u got the tubes joined in the right position, to the boom. Next step would be to close of the rear opening of the boom with xps foam and then a layer of carbon cloth. Otherwise in time, it will crack. Then smooth things out in 1 or 2 stages with a epoxy bondo/filler material. Then wrap things with carbon. Make sure u dont just wrap in the same direction. Also lay cloth from the rear wrapping both stays and the boom. A figure 8 or similar from the rear might also work.


The forces u can try with your mock up and a simple tape wrap u can see what happens and wich way things move. Put weight on it from above, and also try what happens under chaintension, pulling the dropout forward. This chaintension is a much bigger force then the weight of the rider. U need multiple directions in the fibers. There is weight pushing down, tension from the chain, but also forces trying to spread both stays, forces from braking, from something on a rear rack.

A simple round and round wrap will not work.

If u got some steel frames around, or can find some junk ones, experiment with them. Try to force a rear fork together for example. That way u get a feel for how strong the bike would need to be.

A 25 mm tube for chainstay, i would trust with a kids weight of say 100 pounds, but not 180. U need a high but narrow tube with a thicker wall in critical spots.

With your dropout holding plates inserted in the chainstays, they would impact the thin wall carbon in a very small spot creating a load peak on a small spot wich is the opposite of what u want with carbon. Go for smooth, flowing lines, and stepped increase or decrease in wall thickness, tube diameter.


I try to get some sort of "mechanical" insurance. For example i made my dropout plates taper towards the end of the forks. Even if the glue bond to the alu would completly fail, it would be impossible for them to work their way out of the forkblade. A 50 mm plate just does not fit true a 40 mm hole.

Things are looking good, what i see from your building so far.


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Balor
recumbent guru

Russia
697 Posts

Posted - 01/09/2019 :  04:12:05  Show Profile  Reply with Quote
In theory, using 3d printed 'lugs' to wrap around in carbon later will be 'self-jigging'.
But you must be really sure that your real geometry matches your CAD geometry (so preferably making everything this way).
https://www.instructables.com/id/Custom-3D-Printed-Carbon-Fiber-Bike-Frame/

Plus, make sure your 3d printed tolerances are correct - material is prone to shrinking and 3d printes might require some calibration.
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Balor
recumbent guru

Russia
697 Posts

Posted - 01/09/2019 :  04:17:08  Show Profile  Reply with Quote
By the way, using carbon sleeves and 3d printed 'tube lugs' to joint pieces of tubes around seems like a good idea for a carbon mesh seat like you've made. Just plan, print and assemble like Lego, than wrap the joints. Using sleeves, you can print and than wrap pretty complex shapes I presume (like streamlined tubes) - provided your print area is big enough.
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/09/2019 :  07:33:02  Show Profile  Reply with Quote
Thanks. Lots of good advice and ideas here. A few responses for clarity and maybe further discussion.

25mm tube: I chose this based on comparative analysis using generic bending calculators versus steel tubing. I guess I was going from a thread where Warren notes that 1", 0.35 wall steel tubing would be good for chainstays (on an adult bike) (http://recumbents.com/forums/topic.asp?TOPIC_ID=6765&whichpage=2) Using a round tube beam deflection calculator (https://www.easycalculation.com/mechanical/deflection-round-tube-beams.php), that would theoretically deflect .35 inches with a 50 pound weight at 20 inches, whereas 1 inch, 1.5mm wall CF theoretically deflects less than that: 0.25 inches. The 28mm 1.5 wall calculates to a 0.18" deflection. So that's my basis for the tubing choice there. I also consulted the Tom Porter builds here (http://recumbents.com/wisil/porter/tomporter.htm) which feature 1" steel, 0.35" wall and 7/8" 0.35" wall (ovalized, to an inch high I suppose) which held him at 200 lbs. This photo seems to show that the stays didn't get much of an assist from seat stays (http://recumbents.com/wisil/porter/images2004/porterRWD2.jpg) and this later build seems to show similar tubing (http://recumbents.com/wisil/porter/porterlwb2010/image057.jpg) in a format that's pretty similar to my design. (I see he's got a main frame gusset at the bend. I should probably think about dropping one of those onto my design as well.)

In reality, there will be only about 6 inches of this tubing hanging free. The front 2 inches will be under the CF wrap to the main tube and the rear will be wrapped to the CF transition plates. To put the choice into that context as I did in the original, 0.0.35" wall, 1" steel calculates to a deflection of 0.02156

Oh, those plates won't be inserted into the CF stay tubes (and therefore shouldn't generate the point load you mention). My plan there is to cut the tubes to match the flat angle, sand the edges of the plat to round, and then CF wrap.

I'd planned to close the end of the boom with a bonded on .75mm layer of carbon fiber plate I made, then end covered/wrapped with more CF wet layup. I figured I'd do this after the chainstays were bonded on.

Good to hear I should vary my dirctionals when laying up over the chainstays. I get some of that built in if I use my braided sleeve, since its fibers run at 45 degrees to the axis of the braid.

If I understand correctly, you're suggesting I sleeve each tube before / during attachment. I get the idea with the much greater area with that flattish (potato chip) hanging off the front. It would mean not tacking things down my usual way. I've got sleeve fat enough to encircle the front of the stays + the whole main tube itself, which was part of my layup plan, though it doesn't accomplish all of the same as you suggestion -- though laid up together with smaller sleeves coming off the stays toward the end of the boom, it would still be single-layer-ish. I'm not confident in my ability to keep things straight on the jig if I don't have the stays tacked well with JB quick, which would prevent doing it the way I think you're suggesting.

Also good to hear about chain tension forces relative to force of rider weight, though I wonder about the magnitude for scrawny spinners like my kid (or even me). I wonder what I can leg press. Not much, I'll bet. (Marathoner, not a real cyclist.)

Mechanical backup also a good idea. It's been my practice in what I've made so far to always have some pretty good notches in the aluminum so that if the adhesion breaks for any reason, the metal can't pull out (not all at once, anyway) because it's got at least a couple mm of mechanical tooth stopping it up. A taper is an even better idea.

Alas, it'll be a week or so before I start doing the joining, because it's taking me some time to re-work my jig's main tube holders to be perfectly straight. Little experience + no talent + really basic tools = time.


--SpiderMonkey
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warren
human power expert

USA
6423 Posts

Posted - 01/09/2019 :  08:21:05  Show Profile  Visit warren's Homepage  Reply with Quote
I'd jig the stays to the frame tube and use JB weld or whatever to tack them in place. Rough up the entire area.
Then mix up a batch of dragons teeth (chopped CF filler) and fillet the stays to the frame tube.
Then before that has cured, cover the joins and several inches of overlap with 5 or so layers of CF. Add a peel ply or plastic wrap.
wrap it all in some soft foam that will push into all of the crevices. Wrap it all in electrical tape to compress it. I have done this many times and it works well but will require filing and sanding. I'm sure there are better ways.
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/09/2019 :  09:25:34  Show Profile  Reply with Quote
Thanks Warren. Knowing myself (and my fears of the unknown in this first build) I imagine I'll end up somewhere between your method and the one Jeroien outlined above. I very much appreciate the help here on the board. This build is sort of a big leap of faith for me and I want it to go pretty well since it's for my kiddo. (She came downstairs the other day and saw tubes on the jig and thought it was really cool, so that's excellent encouragement too.)

--SpiderMonkey
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Jeroen s
Starting Member

Netherlands
9 Posts

Posted - 01/09/2019 :  12:31:43  Show Profile  Reply with Quote
The method Warren discribes, i also used, that works to. With both methods its a 2 stage job and u need to sand in between to get things flat. A rough wood file is a nice tool for that job. Use the method u feel comfortable with. There are many roads to a good result.

The big sleeve that would fit around 3 tubes is probably thick, resulting in a layer thickness comparable to 2-5 layers of the thinner sleeve. Carbon sleeve is mostly very loose woven, so be carefull handling that. With gloved hands u can stroke it and it will form tighter round the tubes. Like finger traps.


I don't quite understand the way u want to fasten the plates to the tubes. If the movement to the inside of the tube is only restricted by the two 1,5 mm edges that would be a bit flimsy to me. There also that dragons teeth filler might work to fill the hollow end, and as glue to hold the dropout in place. Let that harden in a jig. Then use final sleeves.


I also have kids. Playing around with existing frames or forks helps to get a feel how stiff steel frame parts are. Normal human power is enough to flex things, so u get a good feel for how stiff is stiff enough and test the final product before the kid gets on it. What i have seen on the crazyguy site and here, i have confidence that u will end up with something safe.

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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/09/2019 :  14:34:28  Show Profile  Reply with Quote
Thanks. The tube to plate join I plan is that the plate will be on the outside of the tube, meeting it at a flat fishmouth(?) cut into the outside of that tube (parallel to the axis of the bike) and that area, about 50mm or slightly more, will be wrapped in CF. The little piece of tube protruding outside the plate in this sketch will be on the floor of the garage, and the mating surfaces will just be flat outside, tube inside, CF wrap.



The plate, then, has 2 6mm holes in it, and the aluminum dropout is threaded M6, so two hex-head M6s attach the dropout to the inside of the plate. I stole the aluminum-inside-CF-plate idea/method from the back end of our NoComs, where it has been faultless (unlike many other aspects of that machine ;0 )

--SpiderMonkey

Edited by - SpiderMonkey on 01/09/2019 14:35:40
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warren
human power expert

USA
6423 Posts

Posted - 01/10/2019 :  19:33:29  Show Profile  Visit warren's Homepage  Reply with Quote
Cool 3D CAD!

Why is the CF dropout mounting plate so long?

Edited by - warren on 01/10/2019 19:34:45
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/12/2019 :  18:04:17  Show Profile  Reply with Quote
That 3d is just the 3d-printer app that comes free on windows 10. It's really not for this, but it works OK since I can make cylinders and rectangular solids are place them in 3d with fraction of a mm precision.

The purple plate is actually a bit longer in real life. The CF plate has about 65mm of overlap with the aluminum dropout (about 50mm between the screws) and then about 20mm free-ish, then 50mm-ish overlap for bonding with the chainstay tube ends. Overlap below:



How much will depend on final angles, my precision with the mitering, whether/how much I fillet and cover the end of the tube, etc. I wanted to build in a little wiggle room, plus have clearance for a 50mm tire + fender, as well as keep my chainstay tubing use to 25mm per side, since that lets me use a single one of these pricey tubes. Works out all very conveniently.

I bought a 3-pack of cheap harbor freight V-blocks for drilling tubing ($9 each) and cut one up, drilled it up to make a more reliable tube holder for the top of the jig struts. Manufactured a couple quick aluminum brackets to help clamp them down and keep them from moving.





Filed out the interior and epoxied in a scuffed-up M6 nut.





--SpiderMonkey

Edited by - SpiderMonkey on 01/12/2019 18:05:31
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/12/2019 :  19:42:12  Show Profile  Reply with Quote
[quote]Originally posted by SpiderMonkey

That 3d is just the 3d-printer app ("3d builder") that comes free on windows 10. It's really not so suitable for this, but it works OK since I can make cylinders and rectangular solids are place them in 3d with fraction of a mm precision. I have to then take all the dimensions away on scratch paper, but that's not too tough. Only thing I really wish I could do is get a miter pattern straight out of it.

The purple plate is actually a bit longer in real life. The CF plate has about 65mm of overlap with the aluminum dropout (about 50mm between the screws) and then about 20mm free-ish, then 50mm-ish overlap for bonding with the chainstay tube ends. Overlap below:



How much will depend on final angles, my precision with the mitering, whether/how much I fillet and cover the end of the tube, etc. I wanted to build in a little wiggle room, plus have clearance for a 50mm tire + fender, as well as keep my chainstay tubing use to 25mm per side, since that lets me use a single one of these pricey tubes. Works out all very conveniently.

I bought a 3-pack of cheap harbor freight V-blocks for drilling tubing ($9 each) and cut one up, drilled it up to make a more reliable tube holder for the top of the jig struts. Manufactured a couple quick aluminum brackets to help clamp them down and keep them from moving.





Filed out the interior and epoxied in a scuffed-up M6 nut.





--SpiderMonkey
[/quote

--SpiderMonkey
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