Broken Steerer Tube

meb said:

The steerer tube on one of my folding bikes snapped just below the
threads (305 BCD high tensile steel fork, 1 inch quill, 30 inch stem).
I am assuming long stem created a large enough moment that lead to the
failure.

I am presuming the normal repair is replace the fork, but thought I'd
check before proceeding (somewhere I have a 16 inch springer fork and a
16 inch Ballistic suspension fork lying around). Or is there another
approach I should consider?

Also, this failure signals a red flag on another bike. I picked up a
quill stem extension that raises road bike handlebars about a foot.
Could this result in similar failure on a road bike (its a smaller
moment arm, but the handlebars would be much more heavily loaded on a
road bike)?

Quill stems with internal expanding plugs or wedge fixing can distort a
steerer tube (it's about their only practical disadvantage). Is it
possible that yours was doing so just where the threads ended? Even if
not, this is a classic case of a stress-riser failure; *something* was
overloading the metal there.

Steerer tube threads can be die-cut - if the die is not square at the start
it can cut deeper on one side than the other (sometimes it will do this
even if started square). That deeper cut will be a weak point. I'd
carefully examine the next fork you get, inserting the quill to see where
the expander sits, and looking at the tread depth.

Did you fall?

On Wed, 21 Nov 2007 05:02:51 +1100, meb

may have said:

The steerer tube on one of my folding bikes snapped just below the
threads (305 BCD high tensile steel fork, 1 inch quill, 30 inch stem).
I am assuming long stem created a large enough moment that lead to the
failure.

A Dahon, by any chance?

Quoted message said:

I am presuming the normal repair is replace the fork,

Pretty much the only economical and reasonably safe repair, for that
matter.

Quoted message said:

but thought I'd
check before proceeding (somewhere I have a 16 inch springer fork and a
16 inch Ballistic suspension fork lying around). Or is there another
approach I should consider?

Aside from "if a real bike on a rack on the back is a viable option,
skip the folder"? Don't get me wrong, I've got a Dahon folder myself,
and I take it along on business travel as the gopher bike when I'm
packed too tightly to shoehorn the regular bike into the van. I like
it a lot for that usage...but I treat it as "not suitable for heavy
stress usage".

Quoted message said:

Also, this failure signals a red flag on another bike. I picked up a
quill stem extension that raises road bike handlebars about a foot.
Could this result in similar failure on a road bike (its a smaller
moment arm, but the handlebars would be much more heavily loaded on a
road bike)?

In my opinion, the hazard there is likely to be far smaller for
several reasons, but unless you take a hard critical look at the
steerer tube involvd, it's really hard to say if there's likely to be
a failure.

By any chance, was the folder's quill wedge coincidentally just above
the break? Of the two broken steerers I've seen, one failed in
exactly that manner, and I've always suspected that the shallow
insertion of the quill (and excessive length of the threaded section)
had as much to do with the failure as anything else. Fortunately for
the rider of that one, the bike was not moving at high speed when the
tube broke through. It had been "feeling funny" for a mile or so
before that.

--
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Typoes are not a bug, they're a feature.
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quote='_'On Wed said:

The steerer tube on one of my folding bikes snapped just below the
threads (305 BCD high tensile steel fork, 1 inch quill, 30 inch stem).
I am assuming long stem created a large enough moment that lead to the
failure.

I am presuming the normal repair is replace the fork, but thought I'd
check before proceeding (somewhere I have a 16 inch springer fork and a
16 inch Ballistic suspension fork lying around). Or is there another
approach I should consider?

Also, this failure signals a red flag on another bike. I picked up a
quill stem extension that raises road bike handlebars about a foot.
Could this result in similar failure on a road bike (its a smaller
moment arm, but the handlebars would be much more heavily loaded on a
road bike)?

Quill stems with internal expanding plugs or wedge fixing can distort a
steerer tube (it's about their only practical disadvantage). Is it
possible that yours was doing so just where the threads ended? Even if
not, this is a classic case of a stress-riser failure; *something* was
overloading the metal there.

Steerer tube threads can be die-cut - if the die is not square at the start
it can cut deeper on one side than the other (sometimes it will do this
even if started square). That deeper cut will be a weak point. I'd
carefully examine the next fork you get, inserting the quill to see where
the expander sits, and looking at the tread depth.

Did you fall?[/QUOTE]

It appears the expansion point was at the bottom of the threads.

No fall-fortunately I had slowed down to about 3 mph for a red light when it broke.

meb said:

The steerer tube on one of my folding bikes snapped just below the
threads (305 BCD high tensile steel fork, 1 inch quill, 30 inch stem).
I am assuming long stem created a large enough moment that lead to the
failure.

Depending upon the type of fork and sizing, etc., you might prefer to
get the steerer tube replaced (roughly $75 at an affordable steel
frame builder near to you ...) It may be hard to find a similar fork,
if the bike manufacturer does not make after-market forks ...

- Don Gillies
San Diego, CA, USA

Werehatrack said:

On Wed, 21 Nov 2007 05:02:51 +1100, meb

may have said:

The steerer tube on one of my folding bikes snapped just below the
threads (305 BCD high tensile steel fork, 1 inch quill, 30 inch stem).
I am assuming long stem created a large enough moment that lead to the
failure.

A Dahon, by any chance?

Quoted message said:

I am presuming the normal repair is replace the fork,

Werehatrack said:

Pretty much the only economical and reasonably safe repair, for that
matter.

Quoted message said:

but thought I'd
check before proceeding (somewhere I have a 16 inch springer fork and a
16 inch Ballistic suspension fork lying around). Or is there another
approach I should consider?

Aside from "if a real bike on a rack on the back is a viable option,
skip the folder"? Don't get me wrong, I've got a Dahon folder myself,
and I take it along on business travel as the gopher bike when I'm
packed too tightly to shoehorn the regular bike into the van. I like
it a lot for that usage...but I treat it as "not suitable for heavy
stress usage".

One of my aluminum folders had the frame break last month so I'm starting to see this issue.

This one never quite lived up the folder function. The small tires and low gear ratio (29-44 gear inches). I use it as a spare commuter, or late night commuter (if I fall on this low and back positioned bike it should be less serious crash), or an occaisional folder bike when I may or may not have a motorized vehicle available. In the interim I've pushed this bike fairly hard, but will be converting this to a folding recumbent-I've already got the parts.

Great handling bike, and with the rear suspension very comfortable.
So I've pushed this, especially since the low position and rearward position should make a safe landing if I fall late at night.

Werehatrack said:

Quoted message said:

Also, this failure signals a red flag on another bike. I picked up a
quill stem extension that raises road bike handlebars about a foot.
Could this result in similar failure on a road bike (its a smaller
moment arm, but the handlebars would be much more heavily loaded on a
road bike)?

In my opinion, the hazard there is likely to be far smaller for
several reasons, but unless you take a hard critical look at the
steerer tube involvd, it's really hard to say if there's likely to be
a failure.

By any chance, was the folder's quill wedge coincidentally just above
the break? Of the two broken steerers I've seen, one failed in
exactly that manner, and I've always suspected that the shallow
insertion of the quill (and excessive length of the threaded section)
had as much to do with the failure as anything else. Fortunately for
the rider of that one, the bike was not moving at high speed when the
tube broke through. It had been "feeling funny" for a mile or so
before that.

--
My email address is antispammed; pull WEEDS if replying via e-mail.
Typoes are not a bug, they're a feature.
Words processed in a facility that contains nuts.