another Q.: pinch-binder on butted tube

[tho...@kokopedli.com]

Making a primitive "ritchey break-away"
I want a pinch-bolt
the tube is butted ( painted gold section, "nitanium" )
should i cut it back past the thinner section ( and the transition section) ?

Or maybe put 2 pinch-bolts, one at the end, another further towards the bottom bracket where the tube is full thick ?
I have significant length of the black tube ( inside ) to the Bottom Bracket...

I like the security of longer tube insertion
I wonder about the thin-butted-weakness of the gold tube...
Specifically that the tube flexs inwards on the thin section, but is a tighter fit at the full thickness section.
The thin section is about 1" long, the transition is similar, I would have about 1.7" insertion if I cut back all the transition zone.

( I do aim to put in a failsafe bolt/pin )

 

Any thoughts?   recommendations ?

noMadic   Thomas

[Jim Adney]

One thing is clear: Pinch bolts work best when the tube to tube fit is close. I
would definitely NOT put a pinch bolt over the thin section. Trim away the
thin section completely, leave a bit of the transition there to help lead the
tubes together and put your pinch bolt, or two pinch bolts over the closest
fitting section. (Two bolts, straddling a single slot, spaced maybe an inch
apart. Drill a small round hole at the end of the slot.)

I'm not sure what a thru bolt can add to this, other than weaken the whole
assembly. Yes, it's a problem.

I'm not sure what you can safely do to prevent this joint from slowly working
its way apart. Perhaps add a thin smear of valve grinding compound under
the pinch bolt area to give the parts some grip. Remove all paint from the
grip area.

Jim

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Madison, Wisconsin, USA
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[tho...@kokopedli.com]

Big thank you, Jim

someone else sent a p.m. suggesting a shim

I'm now thinking 2 pinch-bolts on the same slot, as suggested, after trimming (the diameters are actually very very close between the butted sections, the front section was cut close enough to the B.B. that it is also butt-thick, which sustains clamping more effectively )
and also one more pinch-bolt perpindicular to the other 2, a seat-post binder straddling the connection between the front and the rear sections of frame... anti-slip insurance, thank you for sparking that idea, which is much better than a through-bolt/pin.

list blessings come through again, I'm am grateful!
Thanks to any I might have forgotten to thank last time as well as Jim and the P.M.er

noMadic Thomas

[Duane Draper]

To the group:
I get nervous when 'very very close' is used as a measurement. If it's not 'close enough', your frame separates and you end up with a large dental bill. It seems we should be more precise than 'very very close'. There are numbers and formulas to describe these things.

It feels like, at a craft-level, we're at an inflection point. "Very very close" has become what people use to describe what they, not having the experience to measure and calculate, feel is probably close enough. I believe these forums are (should be) safe spaces where we can elevate our framebuilding and bike fab game and there are no dumb questions. I'm not an engineer but I work very hard to be a student of what I pursue. For the past 15+ years, I have pursued framebuilding. I have talked 1:1 with dozens of builders, machinists, fabricators and built dozens of frames etc. My conclusion from all of those conversations is we nee to be more precise with our language.

There are published tolerances for clamping force. All of this is physics. We have math to calculate what is 'very very close' and what isn't close enough. At some point, it comes down to judgement and judgement comes from experience.

More clamping surface area reduces the clamping force for each clamp OR increases the max clamping force. If you apply 100lbs of force to something that only handles 50lbs of force, then you have a problem. We're not exploring space here; this is well trodden ground but, in this case, the OP hasn't provided any numeric values that we can evaluate.

In this case, I'm not even sure what we're talking about since the parts have been cut and the path forward is set. I worry this is an outreach to validate decisions (and cuts) the fabricator has already made without the requisite experience or knowledge. I suppose, that is how we learn. Hopefully, without a trip to the dentist.

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[Jim Adney]

The more I think about this, the less I like the idea of a pinch bolt here. The
slot that this requires puts a stress riser exactly where you don't want one
and pinch connections seem too weak for a high stress location like this.

Could you consider making two rings that could be brazed to the two sides of
the joint, with maybe 6 holes in each one, for a number of bolts, like the
single gap-straddling bolt you suggested, to hold the parts together? The
two tubes could still slip inside one another until the flanges met, face to
face. Then the bolts could be added and tightened.

Not as elegant as the fancy couplers made for this, but just as effective.

--

[Duane Draper]

Sorry for my previous comment, It probably wasn't as helpful as it could have been. I'm working on a project that is driving me crazy with small tolerances and I'm realizing that 0.1mm makes a difference. No excuses.

The part that worries me about this is the clamp on the downtube is highly likely to be in the thin part of the tube. If you look at the Ritchey bikes there is a beefy stub at the BB that the downtube fits tightly into. My suspicion is Ritchey also clamps onto the butted end of the downtube - maybe even a proprietary tube. It's designed to support the load. Same with the sleeve at the seat cluster on the Breakaway bikes I have seen.

In your case, you're clamping the thin middle section of the black bike downtube into the thin middle section of the gold bike. As Jim pointed out, the slotting required will cause it to fail fairly quickly even though Specialized used relatively beefy tubes for these.

Thx

-----Original Message-----
From: frameb...@googlegroups.com <frameb...@googlegroups.com> On Behalf Of Jim Adney
Sent: Thursday, April 16, 2026 9:04 AM
To: frameb...@googlegroups.com
Subject: Re: [Frame] another Q.: pinch-binder on butted tube

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[John]

The first two frames I built (about ten years ago) were both breakaway style frames with a pinch bolt and a sleeve. The one pictured below probably has about 10,000 miles on it, including 6,000 fully loaded touring miles. I turned down the inner piece on a lathe to get the fit that I wanted and the connection has been rock solid. The inner piece is also a tube brazed inside another tube before being turned down, so it's not super thin at the area where it's turned down (but I forgot to measure exactly how thick it is).

rosy, in providence

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[Jon Norstog]

John, that looks pretty good to me. If you have a lathe and time on your hands ... Thomas! Good luck. 

jn

"Thursday"

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[tho...@kokopedli.com]

Thank you, Rosy, very much !

    I have 4 questions about that bike frame you pictured, if you can, and find the time to, I will appreciate and benefit from answers to any/all of them.
1.   How thick is the wall of unpainted/hidden-when-together tube, I know you said you forgot to measure it, but maybe you have some idea?  approximation?    
2.   How thick is the wall of the tube that clamps around it...    same caveat...   any approximation ??
3.   How long is the stub, just the part that is hidden when the frame is "together"?   ( Looks to be 6 or 7 cm ?? )
4.   Most important to me question, is that single pinch-bolt the only thing you tighten/attach to hold the frame together ( other than the seat-post ) ?

noMadic   Thomas
still learning
still with a lot to learn    =)

[tho...@kokopedli.com]

as often, I remember right after I click send  ...    ={

the diameter of the downtube might be useful to know also, is that a 1.125"  or a 1.25" downtube?
    ( I am assuming it is not butted in any place where the clamping is happening )

noMadic  Thomas

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[John]

1) 1.6mm thickness on the inner tube at the bottom bracket. It's two tubes sleeved and brazed together to make one thick tube, and then turned down to a precise OD for a sliding fit (eh, i think it's a sliding fit, by technical definition, but not sure).

2) Downtube where it clamps (with the pinch bolt) is .9 mm thick. I believe it's just a standard .9/.6/.9 down tube.

3) Stub is 60mm long.

4) Yes, only the pinch bolt.

5) Down tube is 31.8 OD (1.25").

Regarding the fit -- The downtube is 31.8 OD, .9 wall thickness, so nominally 30mm ID. I measured, and it's 30.0 (measuring with my vernier calipers). The piece I turned down on the lathe is only a tiny bit smaller, and also measures 30.0 with my vernier calipers. I'd guess it's less than a thousandth of an inch smaller, or .02mm, but I don't have the tools to precisely measure these. It's a close enough fit that it needs to be twisted as it's inserted in order to overcome friction (it's also greased, but that's for corrosion).

rosy

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[tho...@kokopedli.com]

Oh my, big wave of gratitude!!    <3

noMadic   Thomas
who aims to update this list after I complete this project, which might be a while ( or tomorrow!   who knows  ;-}    )

[Cliff McLeroy]

I'm a little late to the party here but I've done a couple like this. The latest one uses a 28.6 DT (a columbus cromor .9/.6/.6 top tube) and the outer sleeve is a piece of 31.8 x 1.6 4130 turned down to around 0.9 wall thickness, with about 1.2 where the pinch bolt is brazed on. I did a couple of things to make this a little more secure. I added a tab with a hole to the DT that the pinch bolt goes through. This adds a fail-safe that if this bolt is not torqued correctly, the DT cannot slip out. It adds a minute or two to the assembly/disassembly time, but I think the insurance is worth it. I also added a little thicker-walled ring to the end of the DT and chamfered it. This is mainly to protect the end while it's disassembled, but perhaps stiffens the joint a little. The chamfers on both pieces make for easy insertion. The 0.9 thickness on the DT seems to be perfectly suitable for the clamping force. I can't remember exactly how much over lap I have, but I think it's around 40-50 mm. I don't have a good close up, but you can zoom in on this one. I can take more pics/measurements if it helps.

https://www.flickr.com/photos/cliffsbikes/52013387905/ 

https://www.flickr.com/photos/cliffsbikes/52098515666/

[tho...@kokopedli.com]

Thanks for your sharing and experiments, Cliff
  That is an excellent idea to put a loop on the downtube for the binder bolt to go through
I expect to look into copying that...

2 clarifying questions.

1.   when you say: 

"a piece of 31.8 x 1.6 4130 turned down to around 0.9 wall thickness, with about 1.2 where the pinch bolt is brazed on."

I am unclear what the 1.2 refers to, and would love to know ...

2.   And I am also a bit confused when you say:
"I also added a little thicker-walled ring to the end of the DT and chamfered it...  perhaps stiffens the joint a little. The chamfers on both pieces make for easy insertion. The 0.9 thickness on the DT seems to be perfectly suitable for the clamping force."

I think you mean attached to the front section of frame, the 28.6 downtube.   It sounds like you mean this is brazed inside this tube to stiffen/protect the end, however stating that the 0.9 thickness seems enough makes me wonder just what is meant...


I am thinking of putting a disk inside the end of mine to close the end of the tube and strengthen it.
My stub attached to the B.B. is 0.9mm wall thickness
My longer downtube is 1mm thick    ...    It looks like I might have to braze a short section of .6mm sleeve onto it to get a good tight slip-fit with the stub.

noMadic   Thomas

[Cliff McLeroy]

Thomas,

The 1.2 mm is the wall thickness where the binder is brazed on. I wanted a little more strength and resistance to distortion there. All 3 binders I made myself and made them considerably wider than the off the shelf ones. My intention was to reduce the risk that the binder could pull in and crush the tube where the split is. I'm linking some pictures that should illustrate it better but the little ring goes on the end of the DT of the front triangle and makes the wall thickness 1.6 mm just at the end. The goal was to reduce the chance of the end getting dented when in the suitcase etc., and also provide more room for a chamfer without creating a potential core sampler. My little lathe was invaluable on this project. It allowed me to do some nice little details like this and keep it as strong and light as possible. One other note about the little safety tab. It bottoms out in the slot so that the bolt holes align easily. The DT only over laps about 45 mm and it does not extend down to touch the BB shell. Since this design packs with the right crank/chainring installed, the stub can be quite long without increasing the size of the demounted package. This allowed me to shorten the DT a little, which helps with packing. For your project I would definitely sleeve the connection somehow. 0.6 mm is too big of a gap. If you think about seatposts, even something like a 27.0 post in a frame for a 27.2 is going to slip and flop. You could braze a couple of rings on the black DT. They don't need to go all the way around. You might be able to use the center section from the gold frame, which is likely around 0.6 mm. Either do a full sleeve, split it so it can be clamped tightly to the black tube, drill a few holes along the length to feed the filler in, and braze. Or you could do two rings, one at the end and one underneath where the clamp will be. This would be lighter and easier to do a "shoe shine" with emory cloth to get the fit right. But it might make assembly a little wonky.

-Cliff

This shows the ring on the DT:

https://www.flickr.com/photos/cliffsbikes/55220354976/

https://www.flickr.com/photos/cliffsbikes/55220755465/in/photostream/

This shows the homemade binder, there are 2 more just like it on the ST:
https://www.flickr.com/photos/cliffsbikes/55220754270/in/photostream/

This shows the slight change in wall thickness:

https://www.flickr.com/photos/cliffsbikes/55219452287/in/photostream/