Yesterday, while testing my bike to compete in a local triathlon that was scheduled for today, my Cobra S stem broke while riding at low speed (thank God) on a flat surface, this caused me to fell forward with the handlebar on my hands and hurt my stomach area with the front tube, as well as my hand when it reached the ground. Needless to say the injury prevented me to compete, but thankfully, I went for that test ride, because if that were to happen today during the tri at high speed, I could have ended seriously injured or worst. My mechanic told me that this could have happened because long term exposure to sweat on the stem, however, I find that hard to believe, since the stem was 2 years old.


Could this have been a defective product? Has this happened to someone else? Or is it wise to change your stem every year or something like that? I got really scared about this, and I am glad that it happened yesterday and not today at full speed. But now I am seriously concerned and afraid of why this happened. Any ideas?


Here are the photos of how the stem broke:

buy a good quality stem, install it with the proper torque settings and be glad you're ok.

Yikes, that is scary! Glad your ok

I didn't know AL or Carbon tensile strength was affected by sweat since neither are affected by water or salt. I think you should find a different mechanic.

When i read the title, i was expecting a horrendous accident. It's good you didn't get seriously hurt. With that kind of break, I would go to a mechanic and have them check the steerer tube now also since carbon steerers are known to break. I could be out of my mind for thinking that, but I would just do it as a precaution, have it checked out.

Bit much torque on the top cap?

Doubtful. If he did, the steerer wouldn't turn very well.

The stem was a Profile Design Cobra S, I thought that was a quality stem. Which ones do you recommend?

There were no changes on the stem adjustments recently, and I've ride the bike constantly the last weeks while training with no issues. I just don't know why this happened.

Based on where it broke I'd have to guess that the stem was clamped on too tightly.

don't try to figure it out.

just get a new quality stem and install it correctly.

there are lots of good ones available.

your confidence will come back as time goes by.

They are good stems. I have two of them with never a problem. I have no idea what caused your failure. You should notify the manufacturer.
As far as recommendations, I have taken a real liking to the Zipp Service Course SL stems (also bars and seatposts). The Cobras have been lying around in the garage for a while.

Glad the outcome wasn't any worse. Wow!

The -6 or -17 degree?

-17, damn!

Lol, caused by sweat. Your mechanic's 2nd job must be comedian.

Why bother with -17 when there's plenty of spacer to remove first?

Scary! Looks like the stem might have been clamped with too much force... How tight do you tighten the stem bolts? Glad you're ok!

Probably aesthetics.

I was asking Clipped_in. I did notice how massive the OPs stack was though on a Tri bike, interesting...

Thankfully, all the spacers survived intact.

That actually does look like corrosion which happens to aluminum. I've seen handlebars with similar fracturing. If the break was more clean it would possibly be overtightening, but the flaking indicates corrosion. This is coming from a chemical engineer with a PhD.

So, like galvanic corrosion that makes a carbon seatpost stick inside of an aluminum frame?

If so, maybe the mechanic meant that sweat was acting as the electrolyte between the aluminum stem and the carbon steerer.

Doesn't even have to be galvanic. Sweat can straight up corrode aluminum

Guess there is a thread on roadbikereview about sweat corrosion, from people who like to use trainers a lot. According to wikipedia, 7075 AL is more suscetible to corrosion than 6061/6069.
- Though some claim if it was anodized and painted properly, sweat should not be corroding the AL.

What do you think it is then? Just google it, not the first time it has happened

Just stop. You have no idea what you are talking about. Here's some galvanic corrosion between plated(carbon steel) and stainless climbing hardware

You sure reply quick. And google searching carbon/AL galvanic corrosion, all I see is worry from lightning strikes. And I've read threads discussing possibility of it, but I've yet to see any evidence of AL and carbon bike parts having galvanic corrosion.

Whatever, better luck to the OP. He will probably get a carbon stem now. Sucks his stem broke.

I see, no reply to the actual content

I'm another who is skeptical of the sweat theory.

I tend to first think of improper maintenance/repair in cases like this. One possibility that comes to mind is that at some point a mechanic/owner attempted to adjust steering bearing preload by torquing the cap screw. They felt some play in the steering head and started wrenching down hard on the cap screw which does nothing except compress the stem clamp area unless the clamp screws are first loosened. I wonder if this might cause a localized fracture near the top as the cap tries to compress the stem clamp which, over time, would start a crack which would propagate down the stem clamp area. And as the crack progresses, it would be over-stressing the other side as well which could cause a corresponding crack. It's just a theory, but I think it has some merit if there is any possibility the bike has been worked on by someone who doesn't understand how threadless stems work.

If not this, then I tend to think next of crash damage, particularly a very hard hit on the handlebars.

Final possibility is simply a defect in the product. I'd definitely get in contact with the mfg and I suspect they'll want to see the part. They'll probably be happy to provide a new replacement. Unless this stem has a bad reputation, I'd chalk it off as a one-off failure.

Glad you're Okay.

- Mark