"Steer Tube" Alum. Weld Repair

Not strictly e-bike related, but I could use some input from frame-builders here. For reference this is a giant expressway.

I ended up cracking my folding bike's "steer tube", which by this term I mean it's the telescoping/folding tube section which press-fits/glues onto the folding hinge. Please see the photo:


This is what it looked like before. You can see they put some sort of splines in, and then pressed it on with glue.



So I dremeled out the crack and drilled out the end of it:


Then I welded it with E4043 and a TIG machine on AC at about 65-70 amps. I don't think I got ideal penetration on the weld, but if you can tell by the photos please let me know if I should re-do it.


I then sandblasted the glue off inside and out, and plan to press it back on using 3M's DP420 epoxy to hold it in place. Please also let me know if you think a reinforcing ring of aluminum might be an order, as I could produce that as well. Thank you!

dequinox said:

I don't think I got ideal penetration on the weld, but if you can tell by the photos please let me know if I should re-do it.

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if you can still see the crack, on either side, the weld didn't do it's job yet.

since the part is small, you may also want to figure out a way to re-heat-treat it for whatever alloy it is made out of, if you can find out.

Please also let me know if you think a reinforcing ring of aluminum might be an order, as I could produce that as well.

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personally, i'd either keep the hose clamp, or some other steel-band clamp around it. otherwise the forces against it are likely just going to repeat the failure, eventually. the glue or epoxy is not there to keep the stress that caused the crack off the tube, it's really there to keep the tube from slipping against the splines from rotational forces and to keep the steerer from being pulled upward off the joint. (since that's what glue in these types of joints is good at--resisting shear forces)

a ring, machined to an interference fit, that could be slid down the tube over the joint and pressed into place at the base of the tube, may also work--but whatever it is made of, as it expands from the leverage forces against the tube/joint while riding/etc., it will then begin being able to be stressed and eventually stress crack just like the tube did.

a clamp you can retighten periodically (even if you don't "feel" any play) may be a more permanent solution.

What's the dimensions looks like an upside down steerer as in the tops been knurled instead of the bottom, if' it's 1 1/8th top and 1.5 bottom then there's replacements available on the net made out of cromo but the knurling will need to be applied easy enough on a lathe.

As-welded yield strength for structural aluminums is assumed to be about 25% of its heat treated strength, for design purposes. Even if its actual yield strength is half the pre-weld strength, that’s still not confidence-inspiring (considering that the original part broke).

I would spend more effort trying to get a replacement part, or manufacturing a new one myself, rather than trying to repair the failed one. I don’t think anything good can come of that.

You can buy the folding steerer tubes for anywhere from $20-60 on amazon, ebay or aliexpress. They come in 1" and 1 1.8" quill mounts, or 1 1/8" threadless.

I've been trying to replicate the quality of the steerer in my Downtube Nova (they want $100 shipped for a replacement), but haven't seen anything that seems as good. The last thing I want is the handlebars coming off when I ride.

Hmm, sounds like this might be a misguided undertaking. I was 90% sure I wanted to redo the weld anyway. I've seen some other evidence of cracking in the part as it is, so we may be headed for a new part. I'd like to upgrade it, so we might be in for engineering a new piece. CF layup, anyone?

:lol: Anyhow, for now I think a tidbit of glue and a nice solid clamp/collar is the way to go. I just need to get one of the right size. I've got nothing against a steel one to keep the cantilever stresses to a minimum.

dequinox said:

I've seen some other evidence of cracking in the part as it is, so we may be headed for a new part.

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you should've said that to start with, because that means the part is going to fail unpredictably at any moment at one of those other cracks. and there may be even more cracks you don't know about yet.

since the stresses that cause this are during riding, it means the handlebars are going to come off in your hands, and you're going to crash. the crash is likely to be one that dumps you on your face sliding forwards as the wheel turns sideways with nothing to keep it from doing so, so i hope you don't ride this bike faster than walking speeds....

I have had a stem break while riding, about 15 years ago, on a brand new bike, leaving me holding the handlebars with the bike pointed at a curb and no way to change direction fast enough; i didn't quite break anything in myself, but i landed on the point of my right shoulder, and it tore thru the jacket i was wearing in the subsequent roll and slide (i still use that jacket, if you want a pic of the hole). took weeks for the arm and shoulder to be usable, and i was pretty lucky at that. my helmet kept my head from slamming into the concrete, or i might not be typing this now. (though i don't generally wear helmets now, it probably saved me from injury then).

with this new information, i can only recommend not riding it at all until you replace the part.

don't try "fixing" the existing one and riding with it.

amberwolf said:

you should've said that to start with, ... don't try "fixing" the existing one and riding with it.

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I'm going to examine it more closely, but what I was referring to was the micro-cracking evident in the finish (what I assume to be plating... doesn't really share characteristics of anodizing). See photo below:


At any rate I don't really use this bike that much anymore, and intend to fully repair/replace it before doing anything else with it anyways. I still have my cruiser if I really need a bike. It sucks though, this is my main non-electric bike.

Well this just became a bit of an engineering project. I started looking at woven carbon fiber tube... oops.
Here's the boss on the hinge that my busted part was pressed on:


Next, the design of the new part:


I'll digress for the time being.

How much weight is on this. Awful looking frame structure with the (single) "top" frame tube. The stem with a overly short stud mounted tube when overloaded as it looks to have been, stress concentration and hoop stresses are just too much for a thin walled tube as designed. "Steer" tube is the tube that extends from the fork crown in frame terms. If purchased new, I would check with Giant as it may be more wide spread a issue with the model.

If your determined to attempt a fix, the tube - stud overlap should be at least 1-1/2 times tube diameter. 2 times diameter is even better but at some point you will buckle -tear the thin walled tube at the transition when overloaded.

speedmd said:

the tube - stud overlap should be at least 1-1/2 times tube diameter. 2 times diameter is even better

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based on his measurements in the pic above, the overlap is only about 78%, rather than 150% to 200%.

Whoa folks it's not as if I've made this thing already! :lol:

I realized it was far longer than it needed to be. The original part was only like 6.25" long. I'm just spitballing here, and I've got to get a line out to giant to see what parts they still have.

This now has 150% of the tube diameter... but it almost seems like a waste of CF tube to do this. The metal bits would be steel by the way... not aluminum. That stuff's too finicky!


In all seriousness I'd probably just buy a new part if I can get one. I don't need another major project, I already have two (bike related, that's not counting all the others)!

Quick look through the models more recent pic's show they have since changed this joint. Wonder why? How much weight are we dealing with.

Giant does have a lifetime warranty on frames. I would check a few of the local shops. You may have voided any chance with the tube fix. Maybe not. If I were Giant, I would replace it.

dequinox said:

This now has 150% of the tube diameter... but it almost seems like a waste of CF tube to do this.

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i believe the length vs diameter reference was to the overlap of the tube with the stub coming out of the folding joint.

that's not something you can change unless you machine a new folding joint part, or giant has a new version made "correctly" that is available as a spare part.


stresses are higher in a joint like this the shorter that overlap is, and depending on the load direction and amount, and probably the materials used, less or more overlap is required, to make the tube unlikely to fail at or above the joint.

in this case there's a lot of leverage perpendicular to the joint and tube, rocking back and forth. there are also stresses in turning but they're not nearly as high as the leverage of a rider pushing and pulling on the bars.

the leverage against the joint is where the overlap becomes important. if the overlap is insufficient, the tube gets to bend above the top of the joint, while not bending below it. if the joint is glued and the glue fails, which is probably what happened here, the tube will attempt to stretch in circumference as it is pried by the leverage diagonally to the stub it's sitting on. the shorter that stub the less of the tube that goes over it, and the easier it is to pry it diagonally, stretch the tube, and eventually crack the tube. once the crack starts, complete failure is only a matter of time, dependent on material properties and stresses on it.