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 Separable frame go or no go?
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/28/2019 :  11:38:37  Show Profile  Reply with Quote
It occurs to me that most of the separable frame bikes I've looked at with a ferrule/sleeve join are FWD.

(The volae voyager is an exception, but it's got this nifty centering post / twist stopper? built into the tube.)


My bike's RWD, and so I'm wondering if I should re-consider the separability goal. I'm unfamiliar with the magnitude of the torque torsion on a SWB, RWD, but I've seen this mentioned as a reason not to have separability in RWD 'bents.

My design idea is to have a 46mm interior ferrule bonded into the rear part of the bike, and it slips into the front part of the main tube before the bend up around the front. It would be secured on the front/pinch side with 2x m6 bolts into metal threaded nuts that are secured by tow wrapping around the tube and fabric. There would be a slanted M6 secured across the joint on the surface to help with centering the assembly and (hopefully) to help avoid twisting. (Performer did this, but on a FWD frame.)



Does this seem like a decent idea or something guaranteed to fail? Maybe double the exterior, cross-join retention screws?

I suppose my worse case scenario if it doesn't hold well is that I just glue the front as well and the bike's 130g heavier, and un-splittable. The bike is for a kid with legs as thick as my (skinny) arms, so there's that.



--SpiderMonkey

Edited by - SpiderMonkey on 01/28/2019 14:01:50

Balor
recumbent guru

Russia
697 Posts

Posted - 01/28/2019 :  12:08:52  Show Profile  Reply with Quote
Mmm... most RWD frames already have a separable, telescoping section - the boom.
And boom slippage is a frequent problem on a CHR, btw... otherwise, I'm not sure.

Edited by - Balor on 01/28/2019 12:09:50
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/28/2019 :  13:53:38  Show Profile  Reply with Quote
Yup, many do, and it's a problem on some, but not others. I had carbon sleeve slippage problems on steerer tubes before assembly paste was a thing we used. None of my aluminum boom bikes have ever had boom slipping issues. My NoCom's boom is rectangular, so it couldn't rotate, but it could slip. Never had an issue there either. It's in the same spot I clamped it into 14 years ago I think.

--SpiderMonkey
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alevand
human power expert

USA
3425 Posts

Posted - 01/28/2019 :  17:09:13  Show Profile  Visit alevand's Homepage  Reply with Quote
My trike has a flanges and four bolts that separates behind the seat. There is no reason the your boom and camp wont work.

C:
Tony Levand
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/28/2019 :  17:23:09  Show Profile  Reply with Quote
Thanks. The more I think about it, the more I think if there *is* any torsional slippage in the mid section, it could be addressed after the fact extra work.

If I have an M6 across the seam as planned as a centering finder/stabliizer, and if torsion at that seam put sheer stress on that bolt (if I have imagine this right), the single M6 should be able to resist something like 2000 pounds of shear force? I think my CF lamination of the tube & nut would break loose before that, but either way I think friction at the clamp + an M6 across the join would hold against the power than anyone in this family puts out (unless my wife keeps lifting heavier at the body pump classes, maybe). Maybe not against some of the racers out there who tear up chainrings...

--SpiderMonkey
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strayray
Warren

USA
6 Posts

Posted - 01/29/2019 :  06:50:35  Show Profile  Reply with Quote
Could you mold teeth with a herring-bone pattern into your tube to clamp into with a matching plate?
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SpiderMonkey
recumbent enthusiast

USA
471 Posts

Posted - 01/29/2019 :  11:53:35  Show Profile  Reply with Quote
I could, but when I thought about trying to machine-in some kind of anti-torsion bite or shape, it kept coming back to me that some kind of tube/receptacle on the exterior to supplement the friction was probably easiest and strongest. I thought for a while maybe tube on top of the tube (like half a 22mm tube that bayonet mounted into a molded-in receptacle), but then twisting to get the pieces together/apart would be more limited. With an M6 across the join, I could get twist then a pretty strong shear-proof bond, *I think*. I think there are plenty of ways to cope with any potential slip/twisting by putting stuff on the outside of the tubes later, if necessary.

--SpiderMonkey
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