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 Torsion in the CF monotube
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SpiderMonkey
recumbent guru

USA
725 Posts

Posted - 11/21/2019 :  11:40:25  Show Profile  Reply with Quote
I've been considering torsion issues with the bikes since that was the primary fault with the DF before I reinforced it. I'm wondering if I'm adequately addressing it with the monotube CF recumbents I'm doing.

This, basically:

https://i.imgur.com/5YHxFSml.jpg

As far as I know, my main tubes (50mm x 46mm) are mostly uni along the axis with a few 90 degree thrown in. (I had mis-read, or misremembered correspondence with the seller and though there were some 45 degree plies in there, but best now to assume not.)

The middle section of the bike, from near the lower, first bend in the front to about 10cm from the stays, has a thick ferrule for joining the bike. That thing was originally 45mm x 41mm, but it got 2.5 layers of 45-degree braid as I bulked it up to fit the ID of the main tube. The sliding boom/bb insert is similar, with 2 layers of 45 degree braid over the same tube.

The joins themselves have carbon heading in various directions, with heavier unidirectional where I expected the strongest pedaling forces (like the top of the foremost bend). So there are 0, 90, 45 degree plies at the stay joins, the tube bends, and the head tube.

I'm wondering if I've done enough here, or if I should sleeve other sections (like the angle between seat and head tube) to resist torsion/wind-up. Unfortunately, this one will be too small for me to test myself, so I'll be working a bit blind.

I have done a test of sitting on the thing, and I can't feel it flex in the slightest vertically, probably owing to the fact that in the center of the frame where my butt goes, it's essentially 50mm OD (or bigger in wrapped spots) and 41mm ID, so 4.5mm wall, because of that ferrule.

Warren, when you made your stick/Z it looks like you got a layer of Kevlar sleeve on there that was 45 degree sleeve? It looks like the carbon you did over that was straight twill? Do you know if your main tube had angled plies in it?

I'm not too worried about Version 1 of this bike right away, since the kid going on it doesn't have a ton of power, but she'll grow and get stronger, and I've got 2 more of these in the build queue for us adults, so I'd like to get it as right as possible.

In prior threads here, I have seen Tom O. say that the monotube mostly gets bending stress, which agrees with what happens to the seat post on a DF, according to what I've seen, and it seems like the two are analogous.

I'll admit I'd not considered torsion much since most of the flex my puny muscles can generate on any of my recumbents seems to be pushing the BB downward at the end of the boom.

--SpiderMonkey

warren
human power expert

USA
6560 Posts

Posted - 11/21/2019 :  13:24:58  Show Profile  Visit warren's Homepage  Reply with Quote
Seth - I think your kid bike should be plenty stiff. It's time for a final sanding and to spray some shiny clear coat on it!

On my stick bike I started with a 1.75" CF tube that Mark Anderson gave me. It was probably the same thickness and composition as the ones you are using. It didn't seem thick enough to me though so I decided to reinforce it. I used the Kevlar sleeve (45 degree) and then wrapped a one or two layers of CF (90 degree) over it. I was pretty noodley as a stick bike and was tough to ride. Then I chopped up the stick bike to make it into a highracer. Who knows how many layers I ended up adding to section in the extra lengths and bends. I'm pretty sure it was all added at 90 degree to conserve CF. It could be a bit stiffer as I can see the boom moving as bit when I crank up hills, but my boom is pretty long. I agree with Thom. IMO if you build it up enough so that the frame isn't bending when you bounce on it it should be fine in torsion as well. Hopefully you can find some CF sleeve to make it easier to lay up your 45 degree layers, but don't sweat it if you end up with all 90 degree layers.
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SpiderMonkey
recumbent guru

USA
725 Posts

Posted - 11/21/2019 :  13:47:04  Show Profile  Reply with Quote
Thanks Warren. That makes me feel better. Next after finishing off Jen's upright is doing seat fabric for kid, putting her on bike, then finalizing steering and inventing a flipit for her. She has grown a lot in the past year, so she'll fit the bike better when it's ready.

I had thought your tube on the Zbike was thicker. My tubes are 2 inches OD. The second pairs I bought are a little thicker in the walls than the kid one, which will help with overall stiffness for sure.

I get 45 degree sleeve from composite envisions. It's not too bad, and they have it from 0.5 inch up to 3 or 4 inches. Just ordered some more today. It definitely makes sleeving the tubes easier. Wetting it is a bit trickier since I can't pre-wet it and hope to get it into place, but I wet the tube first, then put it on dry then over-wet from the outside and wrap with a lot of green cling tape and poke a million holes in it. That stuff really squeezes out the epoxy. I get a much denser and less lumpy final product than with electrical tape alone. It's also cheaper and easier to handle. I didn't think to use it directly on the epoxy (had used it to press in foam before) until one day when I was unwrapping it from foam and it put so much pressure on my fingers it hurt. Hmmmm.

I also get some biaxial 45/45 fabric from them that's pretty good -- much easier than cutting regular 0 90 weave on the bias- though it's not as compliant. The more I learn, the more I'm using better quality uni and biaxial 45 and leaving the plainer weaves for surface finish.

Sleeve: https://compositeenvisions.com/composite-reinforcement-fabrics/carbon-fiber-reinforcements/woven-sleeves/

Biaxial:https://compositeenvisions.com/hexcel-himax-carbon-fiber-fabric-biaxial-45-45-degree-12k-50-127cm-8-85oz-300gsm-hyosung-h2550-fiber/

Good Uni:https://compositeenvisions.com/carbon-fiber-fabric-12k-5-7oz-193gsm-uni-directional-hexcel-im2-intermediate-modulus/




--SpiderMonkey

Edited by - SpiderMonkey on 11/21/2019 13:48:20
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Jeroen s
Starting Member

Netherlands
34 Posts

Posted - 11/29/2019 :  09:29:44  Show Profile  Reply with Quote
The main torsion in a monotube recumbent will be the result of the power on the pedals and the resulting chaintension. Pushing on the left pedal results in a pulling force on the chain, so both add up. On the right pedal, the pushing force is somewhat counteracted by the pulling.

Pushing the left pedal will result in a force on the BB that is to the right and down.Idealy the fiber orientation should result in a direct pulling of the fibres in that orientation.

I have had good results increasing torsional stiffness by using about 1,5 inch wide strips of very thin Carbon UD fibre starting on the left side of the boom well behind the headset and then very gently making a 1/4 wrap along the boom,ending up on the top of the BB.

I have made my own tubes using carbon sleeve. Like socks, they fit around a wide range of diameters. If u get a sock where your tube is at the small end of the range the fibre orientation will vary, and get more lenght wise.

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SpiderMonkey
recumbent guru

USA
725 Posts

Posted - 11/29/2019 :  16:18:49  Show Profile  Reply with Quote
I'll probably be fairly good with my layup as is, then. The extensible boom tubes are already covered in 1 or 2x sleeve with 45 degree angle, and the boom portion of the main tubee from the headset to about halfway up has a lot of fiber in the twisting direction.

My wife was riding her steel stick bike on the trainer today and I was watching how much she bends and twists that frame as she rides. When you're looking for it, it's seems excessive, but we've got thousands of miles on those bikes and they ride great.

--SpiderMonkey
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