Kalloy Uno Ultralight.

[1415chris]

Have you checked them before installation?
I don't believe it could be caused by 6Nm only. My guess is this bolt was like that since it was new.

[pdlpsher1]

I'm thinking of getting Ti bolts but I believe steel has a higher tendency to scretch out versus Ti, which would most likely just snap?

Actually it's the opposite.

Ti’s Real Plus: Elongation and Tensile Strength
So titanium gets two second-place marks as compared to steel and aluminum in the first two properties we examined. But when we look at property No. 3, elongation, titanium is miles ahead of either material. This is the property that tells you how far something will bend before it breaks, a kind of safety factor for framebuilders.

Elongation numbers for titanium are often 20 to 30 percent. For comparison, typical steels can be 10 to 15 percent – the higher strength steels go down as low as 6 percent. Aluminum typically runs in the 6 to 12 percent range. Higher strength aluminums again creep into the low range of single digits, with warning bells ringing loudly. Things without much elongation are said to be brittle. Brittle frame failure is not a good thing.


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[youngs_modulus]

Either these bolts were like this before you installed your stem or something else (something very strange) is wrong.

The stock bolts are most likely 304 stainless (the same alloy spokes are made from). 304 stainless fasteners have rolled threads and are substantially work hardened. 1/2 hard 304 has a yield strength of about 73,000 PSI, or about 415 megapascals. If we assume that an M5 bolt like yours has an effective cross-sectional diameter of 4 mm, then the area is about 12.57 mm^2. Since a megapascal is equal to 1 N/mm^2, you'd have to put that bolt under 5215 N of tension to reach yield. That's 531 kg of force, or nearly 1200 pounds of force.

You didn't tension the bolt that much by applying 6 NM of torque. And I'd expect the threads on your stem are fine.

It's not even clear to me that the bolt is stretched from the photo...it's hard to say whether it's an optical illusion or not. If it is, then the thread pitch should be different partway down the bolt's length. If you thread the bolt into non-critical female threads, does it get a lot harder to turn once you it the stretched part?

[youngs_modulus]

I'm thinking of getting Ti bolts but I believe steel has a higher tendency to scretch out versus Ti, which would most likely just snap?

Actually it's the opposite.

Ti’s Real Plus: Elongation and tensile strength
<snip>

You obviously cut and pasted this from somewhere; it would be good form to say where.

Besides, your quote is misleading for fasteners. While titanium has good elongation numbers, 304 stainless steel has even better ones precisely because it work hardens. It gets stronger as it yields. Your quote specifies 30% elongation for Ti, but unworked stainless has 50% elongation. 1/2 hard 304 exhibits single-digit elongation, but it's a gross generalization to say that stainless has worse elongation than Ti.

[hanakuso1]

Here's another picture. The thread pitch and diameter is definitely different. I didn't screw them into anything else, but I tried to match them up like in the picture below, they do not match up.

I don't remember seeing if it was like this before or not, but I wasn't focused on them either.

FYI, I originally installed this stem 2-3 weeks ago.

[pdlpsher1]

I'm thinking of getting Ti bolts but I believe steel has a higher tendency to scretch out versus Ti, which would most likely just snap?

Actually it's the opposite.

Ti’s Real Plus: Elongation and tensile strength
<snip>

You obviously cut and pasted this from somewhere; it would be good form to say where.

Besides, your quote is misleading for fasteners. While titanium has good elongation numbers, 304 stainless steel has even better ones precisely because it work hardens. It gets stronger as it yields. Your quote specifies 30% elongation for Ti, but unworked stainless has 50% elongation. 1/2 hard 304 exhibits single-digit elongation, but it's a gross generalization to say that stainless has worse elongation than Ti.

Yes I took it from a frame builder's website. But I still agree with him that I'd rather have a material that yields rather than break catastrophically whether it's a frame or a fastener.


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[youngs_modulus]

Again, it would be good form to credit the framebuilder who wrote it.

The idea of ductile failure vs. brittle failure is often a false dichotomy. It's not unusual for the choice to be between ductile yielding and no failure at all. If failure is going to happen, you want it to be ductile, of course. But this is all beside the point: all of the fasteners we're discussing here typically undergo ductile failure.

Speaking of which: if the OP put enough tension into his stem clamp bolt to cause it to neck down like that, he'd likely have either stripped the aluminum female threads or have sheared off the bolt head in torsion. At this point, I'm doubling down on the fastener having been that way prior to installation.

I have one of these stems. I believe they're made of 7050 aluminum. 7050 yields at about 70 KSI, while work-hardened 304 yields at about the same point. The fastener has a lot more cross-sectional area than the aluminum threads do. Making the bolt yield like that would require threading it into something other than a Kalloy Uno stem. But it's totally weird that you got that bolt with your stem.

[Marin]

put that bolt under 5215 N of tension to reach yield. That's 531 kg

You can easily get that by pushing forward on the drops, massive leverage. 20kg "of Force" is probably enough.

Edit1: 2 bolts, so let's say 40 kg

Edit2: You're looking for an event that would require you to brace the front wheel against the torque, and that would potentially make your stem slip.

[Marin]

Here's another picture. The thread pitch and diameter is definitely different.

Both bolts from 1 side of the stem?

What do the other 2 look like?

[youngs_modulus]

put that bolt under 5215 N of tension to reach yield. That's 531 kg

You can easily get that by pushing forward on the drops, massive leverage. 20kg "of Force" is probably enough.

Edit1: 2 bolts, so let's say 40 kg

Edit2: You're looking for an event that would require you to brace the front wheel against the torque, and that would potentially make your stem slip.

"Potentially make your stem slip?" The stem slipping on the steerer is a mechanical fuse that prevents exactly this sort of thing. Besides, if what you're describing was remotely common, we'd see yielded stem bolts all the time, or everyone would use M6 bolts instead of M5s.

OP, what caused you to remove your stem bolts three weeks after you installed your stem? If your bolts yielded like that while you were using your bike, your bars would have instantly become loose. Did that happen?

[Marin]

Hmm, the only event I've experienced that would make *my* stem slip were crashes.



Ok, looked at the numbers a bit more carefully: assuming 22cm/4cm clamping width you'd need 2000N to load the stem with 11000N, 5500 for each bolt. Sound like a lot.

[hanakuso1]

Here's another picture. The thread pitch and diameter is definitely different.

Both bolts from 1 side of the stem?

What do the other 2 look like?

It's the bolts that connect to the steerer tube/fork, not the handlebars

[youngs_modulus]

That makes this doubly strange. It's hard to imagine a stem/steerer clamp bolt yielding this way in use. It seems very unlikely that you overtightened these on installation. If these bolts were torqued properly on installation but yielded at some point after, I'd expect you to have noticed your stem slipping on your steerer. What prompted you to take your new stem off, anyway?

Where did you get the stem? Fairwheel sells these stems, and knowing Jason, they'd catch yielded bolts like this before the stem left their shop. Did you buy your stem new from a shop? Was it maybe-probably-new from Ebay? It's possible that someone accidentally swapped a set of yielded bolts into your stem, though that seems unlikely. Do the bolts that yielded match the handlebar clamp bolts exactly? (Your yielded bolts match the intact ones on my copy of this stem, FWIW).

Most likely, your stem is fine to use, but it's hard to say definitively without expecting it. Try cleaning the affected female threads and then look to see whether you can discern any yielded aluminum. If yes, the stem is best thrown away. That said, the load on threads decreases asymptotically from the nearest to the bolt head. In other words, if 80% of the threads are fine, then they'll take the load. But I hesitate to say "oh, ride it, it's fine" for such a critical component.

On the off chance that those bolts yielded while clamping your steerer, then it would be worth inspecting your steerer as well. Depending on the layup and/or wall thickness, bolt loads high enough to induce yielding are in the same ballpark as those required to score or crush the steerer.

[hanakuso1]

That makes this doubly strange. It's hard to imagine a stem/steerer clamp bolt yielding this way in use. It seems very unlikely that you overtightened these on installation. If these bolts were torqued properly on installation but yielded at some point after, I'd expect you to have noticed your stem slipping on your steerer. What prompted you to take your new stem off, anyway?

Where did you get the stem? Fairwheel sells these stems, and knowing Jason, they'd catch yielded bolts like this before the stem left their shop. Did you buy your stem new from a shop? Was it maybe-probably-new from Ebay? It's possible that someone accidentally swapped a set of yielded bolts into your stem, though that seems unlikely. Do the bolts that yielded match the handlebar clamp bolts exactly? (Your yielded bolts match the intact ones on my copy of this stem, FWIW).

Most likely, your stem is fine to use, but it's hard to say definitively without expecting it. Try cleaning the affected female threads and then look to see whether you can discern any yielded aluminum. If yes, the stem is best thrown away. That said, the load on threads decreases asymptotically from the nearest to the bolt head. In other words, if 80% of the threads are fine, then they'll take the load. But I hesitate to say "oh, ride it, it's fine" for such a critical component.

On the off chance that those bolts yielded while clamping your steerer, then it would be worth inspecting your steerer as well. Depending on the layup and/or wall thickness, bolt loads high enough to induce yielding are in the same ballpark as those required to score or crush the steerer.

Thanks for the replies.

I took the stem off to cut down the steerer tube. The stem wasn't slipping or anything. I bought the stem new off a seller on ebay that seems to carry a bunch. All 6 bolts look the same and the 4 that connect to my handlebar look perfectly fine.

My steerer looks fine. Lucky the steerer is some kind of alloy and not carbon.

I ended up buying the same stem yesterday. I'll probably play around with the current "damaged" bolts/stem to see if I can replicate the yielding.

[Marin]

Really weird. I have like 6 of these, and another one incoming. I'll be checking the bolts on it!