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Dimpling/crimping 853 chainstays

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Dimpling/crimping 853 chainstays

Old 01-21-20, 07:29 AM
  #26  
guy153
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Originally Posted by Kuromori View Post
Mandrel butting involves cold drawing a tube, but with a butted mandrel inside. If you want to cold draw other tubes, you use the same machine with the appropriate dies and without a butted mandrel. The butting process is just a modification of cold drawing. don't know what Reynolds finds economical to do in house versus contracting out
Yes although a steel works will usually be making much longer tubes, about the length of a lorry if not longer. You need to cut them up to appropriate lengths for a bike (about 600mm) before double-butting, which also means you only need quite a small machine to do that on. We can see them doing this in-house in that video. I don't think there's much more to Reynolds these days than that one fairly small workshop.
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Old 01-21-20, 07:56 AM
  #27  
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Drawing tubes the length of a lorry is not relevant at all to the question of if Reynolds can and does cold draw tubes (thus requiring annealing). Their literature implies that yes, they do, at least for other industries. I realize that Reynolds is a shadow of their former selves and no longer the massive company it once was. I do not know the current inner workings of Reynolds. On the other hand, it isn't that unreasonable for a tubing specialist to have the machines to cold draw tubes to spec, since that's what butting is, nor is it unusual for a company that deals with heat treated parts to own an in house furnace.

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Old 01-21-20, 08:38 AM
  #28  
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Originally Posted by Kuromori View Post
Drawing tubes the length of a lorry is not relevant at all to the question of if Reynolds can and does cold draw tubes (thus requiring annealing)
I meant that drawing long lengths like that requires much bigger machines (and ovens for the annealing) and is therefore only economical if you have high sales volumes. Reynolds bike tubes are a pretty niche product these days so it makes sense that they would only do the butting (and shaping of stays, fork blades, etc) plus the heat treatment of the heat treated tubes.
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Old 01-21-20, 03:55 PM
  #29  
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I guess I fail to see the relevance of who does the annealing, whether it's Reynolds itself, or a supplier of Reynolds, and am not really sure why it is important to speculate which processes are currently done in house and which are currently done by suppliers. I wouldn't find it surprising if Reynolds did more cold drawing in-house, as it would give them more flexibility in meeting custom orders and market demands when these tubes are clearly not off the shelf. I also wouldn't find it surprising if Reynolds tried to minimize operations they could contract out. However, they are clearly they are capable of cold drawing tubes.

The point is in terms of material properties, that as delivered, 631(853 alloy) is cold workable according to Reynolds, but 853(631) is especially susceptible to work hardening according to Reynolds, and thus it is probably delivered in a near annealed state with only minimal cold work done, in contrast with 853 that is delivered heat treated. 853(631) doesn't like to be overly cold worked even in an annealed state. It's just another data point to consider when discussing the cold working of 853/631.
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Old 01-26-20, 11:05 PM
  #30  
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Coming in late (sorry) but since it's not my bike, I advise to "just do it!"
I was just talking to a framebuilder (who I won't name because this may have been meant for my ears only) who says he has often crimped titanium chainstays for more tire clearance on built frames. Some of you may know, 3/2.5 tubing was developed for hydraulic lines in aircraft. Something about the pilgering process that reduces the diameter and wall thickness from the pierced billet, combined with Ti's hexagonal-close-packed crystalline structure (cf. steel's cubic structure) gives it anisotropic properties. Great for bending, like to run hydraulic tubes thru a wing, but terrible to try to make fluted chainstays. Therefore a lot of builders that want fluted stays get their chainstays in the annealed state, much weaker than the more-common CWSR (cold-worked, stress-relieved).

Anyway, long digression, hope you don't mind (I just think it's interesting) but now I'm finally getting to the relevant part: On CWSR stays, that you can't crimp without them cracking, this builder I was talking to just crimps them, lets them crack, then welds the crack shut! Says he's done that years ago and those bikes never had a problem afterward.

I think that might be pretty analogous to crimping 853. If it cracks, no prob, just braze the crack with brass ("bronze"), which I think will be strong enough, and should simultaneously stress-relieve and probably get some strength back from the air-hardening.

Obviously, "I think" isn't very convincing, and it's not my bike so I don't expect you to take all that risk on my say-so. Just sayin', food for thought?

I like to crimp with a fairly sharp radius at the bottom of the indent. I think this is actually better (stronger or better fatigue life) than some indents I see out there with a large radius at the bottom of the indent, typical on many Chinese made bikes I've noticed like Rawland, Crust et al. I won't go into why I think that's preferable, not at length anyway, but suffice it to say this shape has been well-tested for many decades. I have seen Barra aluminum frames from before WWII with that sharp crease down the middle of the indent. Here's a pic.
Barra frames do break sometimes, but not there, down in the bottom of the chainstay indent. Or think of the sort of leaf-shaped indents in older Columbus SL/SP c-stays, before ROR became fashionable. Let me know if you don't know what I mean and I can come up with some photos. If ever one of those cracked down in the bottom of the indent, it's news to me.

Oh here are some pics of a chainstay denting tool I made a couple years ago. Not very sophisticated -- the one @Andrew R Stewart showed off earlier in the thread is much nicer. But I think it at least shows what I mean about the crease down the middle.

Anyway, if it cracks there while indenting, you should be able to float some brass down the crease, which will look very normal after painting -- it won't look like a repair.

Mark B in Seattle

Last edited by bulgie; 01-27-20 at 02:11 AM.
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Old 01-27-20, 07:10 AM
  #31  
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brazing a crack in steel is a far less satisfactory repair than welding a crack in Ti. Sure, you could weld it, but you can't re-heat treat it.
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Old 01-27-20, 06:40 PM
  #32  
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Originally Posted by unterhausen View Post
brazing a crack in steel is a far less satisfactory repair than welding a crack in Ti. Sure, you could weld it, but you can't re-heat treat it.
Hmm, seems to me heat-treating has several meanings, and in the case of air-hardening steel, heating it above the transformation temperature and letting it self-quench and cool by conduction/convection in air does result in what many would call heat-treating.

And if it subsequently gets ridden lots and never breaks there, any harm done is pretty theoretical.

I would do it on my own frame for sure, but then repairs are cheap for me, costing only the price of a new pair of chainstays -- since I count my time as worthless for my own bikes (no comments from the peanut gallery!). Even the chainstays could be free if I had a junk bike around I could chop them out of. I can imagine that cheap, tho I can't recall ever recycling used chainstays that way.
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Old 01-27-20, 10:46 PM
  #33  
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I just didn't want your idea to be left here unchallenged, because I don't think it's a particularly good one. Even a longitudinal crack in a chainstay is going to see forces that will probably break brazing. Butt welds just aren't a good application of brazing.

When I look at procedures for air hardening steels, they don't look anything like what happens when a tube is brazed. I'm curious if anyone ever looked at the hardness after brazing.
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Old 01-28-20, 12:30 AM
  #34  
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Originally Posted by unterhausen View Post
Even a longitudinal crack in a chainstay is going to see forces that will probably break brazing.
Could be, could be. My gut feeling that it will last forever is informed by 20+ years of custom frame building, but it's still just a gut feeling.
In favor of it lasting: the bottom of the indent is closer to the neutral axis for twisting loads, and closer to the neutral plane for sideways bending loads (chainstays see both). Stress flows to the parts of the stay farther from the neutral. I bet in one of those false-color representations of FEA analysis, the bottom of the crease will be blue or whatever color indicates low stress. That's why I like a relatively sharp (small radius) crease at the bottom of the indent -- stress avoids that area. Again, gut feeling (grain of salt recommended!), but frames made with that sharp crease have been around for a long time and don't break there.
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Old 01-28-20, 01:24 PM
  #35  
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I don't think that arguing from authority is a good thing here, because I win and it's not instructive.

But anyway, my opinion has been formed by seeing bikes with cracks at the bottom of dents that were caused by hard riding. I'm trying to imagine why it wouldn't take just as much load as any other part of the chain stay.
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Old 01-28-20, 02:54 PM
  #36  
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Bulgie's argument is that the center of the stay will not see as much elongation or compression, which is pretty much the entire the basis of using tubular structures, thus a sharp crease there won't see the stress/strain as a sharp crease near edge would. Note that this doesn't apply to compressive loads along the drive side chainstay, only the bending and torsional loads. Thus a sharp crease near the center will see ~0% strain under torsion and bending, and a sharp crease near the circumference will see some arbitrary value X% strain under torsion and bending.

That being said, I don't see any particular reason to chance it. The wall thickness on Barras are a mystery. Letting a tube crack then welding it is never following best practices. The crease gets closer to the circumference near the end of the flute. If I did do this on 853 and it didn't crack, I'd torch it and harden it again in the dented state.

Also, I've thought about the heat-treatment process some more. 631 tubes might not be annealed, but stress relieved (essentially a high/soft temper) after drawing like other non-air-hardening steels are, which would mean getting the temperature high but not past critical could offer the ductility of 631.
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Old 01-28-20, 04:01 PM
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Originally Posted by unterhausen View Post
I don't think that arguing from authority is a good thing here, because I win and it's not instructive.
Sounds like you think we're doing science here. The OP's questions made me think he was entertaining ideas along the lines of kluges or non-standard field repairs.

Whether a bike will fit the width of tire you want to use is one of the bigger parameters affecting the bike's utility or excellence for the stated purpose. OP has stated that he's willing to try something even if there's a chance it may break down the road. I understand the feeling; rather than ride a bike with a too-small rear tire, I'd be inclined to either try something dodgy or get rid of that frame and get something more suitable.

He asked folks here for advice and I gave him some, with a few words about my experience level because I assumed he would be curious what I'm basing that on. To call that arguing from authority seems a bit high-falutin' for this discussion. Still, congratulations on your win.
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Old 01-28-20, 06:25 PM
  #38  
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It's fine if you post your solutions. I just felt like I should not let them go unchallenged for fear it would look like everyone agreed with you.
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