Wednesday, February 10, 2010

BROKEN SEAT POST



One of the participants in yesterdays Traditional Bike Club group ride had his alloy seat post snap.

He's not a big guy (160 pounds), he wasn't putting any unusual stress on it (17 MPH on a flat city street) and it wasn't over extended when it just let go with a very loud snap.

Fortunately, no disaster ensued, but the outcome could have been ugly under other circumstances.

Any opinions as to what happened and why?

.

11 comments:

Anonymous said...

Because that's what happens in a traditional bike club?

Schrauf said...

Small nicks and repeated overtightening of the clamp can cause stress that eventually results in sudden failure.

Why is the seat post wall so thick? I've never seen one so thick.

John Speare said...

Chad: it looks like it's one of the fancy fluted campy type of posts. the diameter is smaller at the top than it is where it goes into the seat tube -- I wonder if the smaller diameter requires more aluminum for (not quite) adequate strength?

Traditional Bike Club Curmudgeon said...

I don't know that the seat post has-or-has-not been over tightened in the past, but I doubt it.

I, too, was surprised at the thickness.

It is "unusual" in that it tapers to a nipple-like profile above the seat tube and the extra material may be to compensate for the smaller diameter (in which case there is likely to be no reduction of weight, so why bother?).

John Speare said...

Cyrus: aero dude! aero!

Anonymous said...

Hah, looks like a bigger post isn't always a better post!

Jonathan Hawkins said...

That serotta caused it.
just kidding. It broke at the weld?
I broke the top off a control tech post.

saddle position. too far back creates a cantiliver for extreme leverage on the post head.

Anonymous said...

A picture of the rest of it would be helpful.

OlyBikes said...

It is hard to tell from the photo -a close-up with good focus is needed - but it almost looks as if _all_ of the material in the break is: 1. rough 2. light in color. This is weird.

Most aluminum alloy failures I see have two distinct textures and colors at the break. The reason for this is that such failures start with a small crack that gradually expands. This first phase progresses so slowly that the exposed aluminum can oxidize and become darker. Overall, there is enough strength in the material to prevent catastrophic failure, though the crack will slowly progress through the material.

Eventually, in the second phase, a peak load causes catastrophic failure - a gross ripping of the material, which looks rougher in texture (a sudden cascading tear) and lighter in color(because there is no time for the material to oxidize). See: It is hard to tell from the photo -a close-up with good focus is needed - but it almost looks as if _all_ of the material in the break is: 1. rough 2. light in color. This is weird.

Most aluminum alloy failures I see have two distinct textures and colors at the break. The reason for this is that such failures start with a small crack that gradually expands. This first phase progresses so slowly that the exposed aluminum can oxidize and become darker. Overall, there is enough strength in the material to prevent catastrophic failure, though the crack will slowly progress through the material.

Eventually, in the second phase, a peak load causes catastrophic failure - a gross ripping of the material, which looks rougher in texture (a sudden cascading tear) and lighter in color(because there is no time for the material to oxidize). See for pix of this: http://tinyurl.com/yfvfg56

So, again, the pix, as far as I can tell is odd because it looks like one catastrophic failure, like a gorilla ripped it apart, yet the rider is aparently not heavy and not "doing anything" that seems likely to cause the failure.

Nate said...

Yes, I also immediately suspected a gorilla.

RJ said...

Did he fart?