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Tandem Cycling A bicycle built for two. Want to find out more about this wonderful world of tandems? Check out this forum to talk with other tandem enthusiasts. Captains and stokers welcome!

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Old 07-02-12, 08:27 AM   #1
merlinextraligh
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Inspect your stoker stem

This apparently happened on our ride Saturday, and at the most inopportune place (top of Hogpen Gap with a 7 mile descent to get home.)






We let the bike fall over on its side at a rest stop. It just fell in the grass, at a complete stop. When we picked it up my stoker said her bars were loose. I tightened them till they felt snug, and we rode on.

Next morning, she told me they were still loose, and inspection revealed the above.

I'm confident they weren't overtorqued. I used a torque wrenc when they were first installed, used carbon assembly paste to lower the need for tightening, and when I tightened them Saturday, I used just a mini tool lightly till snug, not more. Additionally, enough torque to break the stem itself would have likely snapped the ti bolts, or crushed the cf post before trashing the stem.

So the mystery is what happened? Just the bike tiipping over shouldn't have done that. My working hypothesis is that a crash 2 years ago started the problem, and it when unnoticed until it go to the point of failure, and dropping it was the straw that broke the camel's back.

However, there's also the unexplained little gouge on the top of the stem as well, which I never saw before.
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Old 07-02-12, 09:27 AM   #2
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As I stated in another thread last month, with aluminum it's only a matter of time before it fails under fatigue loading.

If you can post high quality pictures of the fracture surfaces I may be able to discern some clues.
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Old 07-02-12, 11:32 AM   #3
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I'll do that when I get home tonight.

I have determined from old pics that the little ding on top of the stem has been there for at least 2 months.
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Old 07-02-12, 01:25 PM   #4
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Is that actually a stoker stem, or is it a standard stem that was made to fit the seatpost? I ask because there is no gap at all on the clamp (usually there would be a gap present).

Anyway, It just goes to show that you never know what is going to fail.
Personally, I would have been real nervous about clamping a stoker stem on a CF seatpost. Never would have expected the alloy stem would be the one to fail though.
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Old 07-02-12, 01:33 PM   #5
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Wow. I've heard of carbon seatposts shearing off, but never a stem clamp blow out like that. Considering that a carbon post and Ti bolts were in use, I'd never assume the torque would be too high. Go figure.
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Old 07-02-12, 01:46 PM   #6
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Is that actually a stoker stem, or is it a standard stem that was made to fit the seatpost? I ask because there is no gap at all on the clamp (usually there would be a gap present).

Anyway, It just goes to show that you never know what is going to fail.
Personally, I would have been real nervous about clamping a stoker stem on a CF seatpost. Never would have expected the alloy stem would be the one to fail though.
It's a regular Deda Newton stem. It fits the seat tube. There was a gap in the clamp before the stem broke. The bolts have been removed and to inspect and then screwed back down for the trip home, so the pic doesn't reflect the precise position when it failed.

As for clamping on to a carbon seat post, I was a bit suspect too, but that's the post Co-Motion spec'd and I've tried to be careful about the torque.

Also, it would not surprise me if that seat post had an aluminum core, wrapped in carbon, but I don't know that.
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Old 07-02-12, 02:26 PM   #7
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I'd bet that one boss broke first and then all the stress went to the remaining boss. If you look really closely at the fracture zone, you might see "beach marks" that would indicate a fatigue failure. Once a minute fracture starts, each flex cycle advances the crack and leaves a mark like high tide on a beach. Once this propagates to the point that the remaining material can no longer withstand the tensile force, it will fracture through as a brittle fracture. This part of the fracture looks more like crystals.

I had an aluminum rim flange fail in this way. Fortunately, I was pumping up the tire when it failed (I wasn't on the bike). Failure analysis can be interesting and instructive.
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Old 07-02-12, 03:03 PM   #8
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Based on the white powder between the stem and seatpost, I say corrosion stress crack. Corrosion began on account of two widely different dissimilar materials.

See it all the time in aerospace stuff.

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Old 07-02-12, 03:53 PM   #9
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Based on the white powder between the stem and seatpost, I say corrosion stress crack. Corrosion began on account of two widely different dissimilar materials.

See it all the time in aerospace stuff.

PK

Very good point. This is why Calfee lines their seattube/seatpost interface so that alloy posts do not interact with the carbon frame, though other mfr like Santana does not spec any special sleave with the Tamer Carbon posts used with Santana's stoker stem or the AL frames. Dunno.
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Old 07-02-12, 06:55 PM   #10
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I'd bet that one boss broke first and then all the stress went to the remaining boss. If you look really closely at the fracture zone, you might see "beach marks" that would indicate a fatigue failure. Once a minute fracture starts, each flex cycle advances the crack and leaves a mark like high tide on a beach. Once this propagates to the point that the remaining material can no longer withstand the tensile force, it will fracture through as a brittle fracture. This part of the fracture looks more like crystals.
Without magnification, it's real iffy to see the beach marks, but the fatigue crack may be oxidized dark due to extended exposure to the air. The brittle fracture surface will be shiny silver.
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Old 07-02-12, 08:13 PM   #11
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Closeups of the failure:







( could have done better with the macro shots, but I don't have a ring flash, and I was too lazy to pull out a tripod and studio lights.)
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Old 07-02-12, 11:15 PM   #12
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In the 2nd photo, there is an area in the upper right which has a different appearance, possibly shiny from the surfaces rubbing together over time. I wonder if that might be the initiation point?

The rest of the fracture surfaces look like fibrous tearing.
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Old 07-03-12, 04:53 AM   #13
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I'm not sure of the exact angles of the photos. Guessing the straight edge was against the stem.

Carbon fibre against aluminum with sweat, about as perfect of a corrosion environment as can be.

To me it sure has an intergranullar corrosion look about it.

Common to begin at a machined surface area or edge. Possibly the stem boring operation was the machined edge. Also, the stem appears to be machined from billet, which has linear grain boundaries that run in the direction of failure.

This is an excellent reason why regardless of being a single or tandem, carbon fiber against aluminum should be disassembled, cleaned, and put back together with some form of a non conductive (dielectric) lube and or corrosion preventative compound.

Now for the real question. If you are going to part with that, and the associated broken bits, can I pay shipping and get it from you? This would make an excellent classroom teaching aid to visually show the guys I train in composite repair why we take precautions in our industry to minimize this condition.

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Old 07-03-12, 04:59 AM   #14
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Want to add, if the bolts were / are titanium, that could have shown similar corrosion that would have started at the counterbore for the bolt head.

Similar to carbon / aluminum, titanium and aluminum together are just as bad if unprotected.

My suspect for the stem to post joint as the origin is based on the white powder and the jagged edge of the broken tab. It still could have been the bolt hole, regardless it is in my thoughts a corrosion stress or crevice type failure.

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Old 07-03-12, 05:16 AM   #15
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Yikes....

Lots-o-stuff going on there.

Alpha Q seat posts are/were just composite; no aluminum core like the Wound-Up seat posts that Co-Motion first started to spec.

Galvanic corrosion shouldn't have been an issue with the Alpha Q seat post as it was originally clear-coated. However, it looks like the clear coat has chipped away at the stoker stem clamp interface.

I'm wondering if the somewhat plastic Alpha Q seat post shaft didn't invite the original fatigue failure (top) by weakening the clamp before the crash which caused the original fracture and the fall-over delivered the coup-de-gras (bottom)? As others noted, it definitely looks like the top fracture has had time to oxidize...

Yeah, Hogpen Gap is not the place you want to have an equipment issue. Just thankful that it failed in a non-catastropic way vs. coming off in your beloved's hands under a full-strength effort, alone or in a group.

As to using a stoker stem on composite shaft seat posts, we've done it without issue using the older Easton model posts and probably have 10k miles on our Erickson in that configuration. But when I built our Calfee and ran the numbers I came to realize that the Thomson Masterpiece AL seat posts were a better choice.

I'm surprised the fractured stoker stem wasn't making noise and didn't make the stem feel a bit "springy" after the original failure.
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Old 07-03-12, 10:10 AM   #16
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Yikes is right! I hope galvanic corrosion can't do that between CF and aluminum. I guess I have to check two mtn bikes at home with CF seat posts, and also my stoker's stem with CF handlebars and my mtb with CF bars on an aluminum stem. Jeez, what a pain. A small expense for the bars/stem, but a nightmare if a CF seat post can ruin an aluminum frame.
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Old 07-03-12, 10:57 AM   #17
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I'm surprised the fractured stoker stem wasn't making noise and didn't make the stem feel a bit "springy" after the original failure.
In hindsight it might have been. The bike recently developed a bit of noise under hard pedaling while climbing. I attributed this to a BB, and hadn't fixed it.

It's possible it was coming from the stem. I'll see if the noise is still there with new bars.

As for feeling it, my Stoker tends to just ride the bike, and not be particularly tuned into mechanical issues, so something has to be pretty far out of whack before she says something.

And I think I'm going to go with Thomson seat posts as well.
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Old 07-03-12, 11:34 AM   #18
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As for feeling it, my Stoker tends to just ride the bike, and not be particularly tuned into mechanical issues, so something has to be pretty far out of whack before she says something.
Brother, I feel your pain...

Debbie is oblivious to anything being out of sorts on the tandem, her single bike or her car for that matter. I went to top off her tires a couple weeks back and saw that the LiION battery pack for her diNotte tail light had slipped down her seat tube and was sitting against the tire with a nice groove worn into the plastic casing. I asked her if she had noticed any kind of noise or felt anything during her after work ride. "You know, there was this buzzing sound".

It keeps things interesting and keeps me on my toes.
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Old 07-03-12, 12:50 PM   #19
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Brother, I feel your pain...

Debbie is oblivious to anything being out of sorts on the tandem, her single bike or her car for that matter. I went to top off her tires a couple weeks back and saw that the LiION battery pack for her diNotte tail light had slipped down her seat tube and was sitting against the tire with a nice groove worn into the plastic casing. I asked her if she had noticed any kind of noise or felt anything during her after work ride. "You know, there was this buzzing sound".

It keeps things interesting and keeps me on my toes.
Some of us are very in tune with subtle symptoms and others are completely surprised when something breaks.
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Old 07-03-12, 03:02 PM   #20
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Yikes is right! I hope galvanic corrosion can't do that between CF and aluminum. I guess I have to check two mtn bikes at home with CF seat posts, and also my stoker's stem with CF handlebars and my mtb with CF bars on an aluminum stem. Jeez, what a pain. A small expense for the bars/stem, but a nightmare if a CF seat post can ruin an aluminum frame.
It's very real how intense the corrosion can become between Carbon and Aluminum. Especially if you add saltwater.

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Old 07-03-12, 03:06 PM   #21
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In hindsight it might have been. The bike recently developed a bit of noise under hard pedaling while climbing. I attributed this to a BB, and hadn't fixed it.

It's possible it was coming from the stem. I'll see if the noise is still there with new bars.

As for feeling it, my Stoker tends to just ride the bike, and not be particularly tuned into mechanical issues, so something has to be pretty far out of whack before she says something.

And I think I'm going to go with Thomson seat posts as well.
Should prove a good choice.

PK
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Old 07-03-12, 03:11 PM   #22
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Brother, I feel your pain...

Debbie is oblivious to anything being out of sorts on the tandem, her single bike or her car for that matter. I went to top off her tires a couple weeks back and saw that the LiION battery pack for her diNotte tail light had slipped down her seat tube and was sitting against the tire with a nice groove worn into the plastic casing. I asked her if she had noticed any kind of noise or felt anything during her after work ride. "You know, there was this buzzing sound".

It keeps things interesting and keeps me on my toes.

Amen!

The girls are strong no doubt, mechanics is not always their strong point. Still love em even if I get a deer in the headlight look. I keep explaining that flat tire we got is on her end of the bike, she needs to fix it. Maybe they are smart since, well you know the rest of the story.

PK
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Old 07-03-12, 03:30 PM   #23
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Yikes is right! I hope galvanic corrosion can't do that between CF and aluminum. I guess I have to check two mtn bikes at home with CF seat posts, and also my stoker's stem with CF handlebars and my mtb with CF bars on an aluminum stem. Jeez, what a pain. A small expense for the bars/stem, but a nightmare if a CF seat post can ruin an aluminum frame.
My carbon Trek has gone through 2 sets of aluminum cable stops. They just melted into cottage cheese through corrosion. Some of that could be attributed to sweat dripping onto the top tube while climbing hard with a tailwind, but that doesn't seem to me enough to account for it. The bike is never really near salt water, though I do live in the coastal PNW. I have an aluminum seatpost on that bike and have seen no corrosion damage on it, however I think Trek has lined the seat tube with epoxy or something. It only happens if aluminum comes in direct contact with carbon. A layer of pure epoxy will completely prevent corrosion. However because of weight and time issues, there's usually not much if any pure epoxy painted over the carbon.
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Old 07-03-12, 11:46 PM   #24
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My carbon Trek has gone through 2 sets of aluminum cable stops. They just melted into cottage cheese through corrosion. Some of that could be attributed to sweat dripping onto the top tube while climbing hard with a tailwind, but that doesn't seem to me enough to account for it. The bike is never really near salt water, though I do live in the coastal PNW. I have an aluminum seatpost on that bike and have seen no corrosion damage on it, however I think Trek has lined the seat tube with epoxy or something. It only happens if aluminum comes in direct contact with carbon. A layer of pure epoxy will completely prevent corrosion. However because of weight and time issues, there's usually not much if any pure epoxy painted over the carbon.
I wipe down my 12 year old carbon Trek after every ride with a damp rag to remove residual sweat. The cable stops are showing some deterioration but are still intact. Trek did replace the fork last year under warranty as the aluminum tips were beginning to disbond from the carbon fork leg.

My understanding Trek uses a fiberglass liner in the seat tube to avoid corrosion. I still had to cut one aluminum seat post out of the frame after it corroded and became stuck, so I now avoid the issue by using a carbon post.
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Old 07-04-12, 11:12 AM   #25
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I wipe down my 12 year old carbon Trek after every ride with a damp rag to remove residual sweat. The cable stops are showing some deterioration but are still intact. Trek did replace the fork last year under warranty as the aluminum tips were beginning to disbond from the carbon fork leg.

My understanding Trek uses a fiberglass liner in the seat tube to avoid corrosion. I still had to cut one aluminum seat post out of the frame after it corroded and became stuck, so I now avoid the issue by using a carbon post.
I bet you were very happy to have discovered that before it failed!

I have heard a lot of stories of stuck seatposts but not many other aluminum-steel problems.
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