C
Chris Zacho "Th
Guest
I have, or rather had, a quill type road stem with a removable face
(Kalloy UNO). Here's what this stem looks like:
http://www.chucksbikes.com/st002.htm
This is identical to similar stems made by Profile Design and "Zoom" (A Performance brand name). I
have owned it barely four years, when it failed last week. The top bolt pulled out (stripped).
This did not occur while tightening the bolt, It failed during use. It was not over tightened. Nor
had it been frequently loosened and re-tightened. I am not an overly heavy rider, I weigh 175 lbs
soaking wet. I am not a particularly strong rider, but a tourist/recreational/endurance cyclist. And
while I do like hills, I conquer them with low gears, not by standing and torqueing my bike into a
pretzel. In any case, the failure occurred while riding casually (13 MPH) on level pavement. My
handlebars just started twisting downwards, as the retaining cap came loose. I was able to finish
the short ride safely, by riding carefully, bearing as little weight on the bars as possible.
When I got home, I saw the top bolt had pulled out, stripping the hole. The fit of the bottom bolt
was also sloppier than I felt it should be, like the hole was too big. Although this may have
resulted from having to ride the bike with the top bolt kaput. The bolts were tight when I left, and
there were the remains if five or six threads on the bolt. This was the same depth as the bottom
bolt. Further examination of the stem and how it failed led me to believe that this _may_ have been
due to a flaw in the design. Here is my reasoning:
1. Unlike the MTB stems these are based upon, they only have two bolts, not four. Each bolt,
therefore has to bear twice the load.
2. They are parallel to the ground, not angled upwards, so the retaining cap must bear the entire
load of the rider.
3. The road rider is also leaned further forwards than on an MTB, so there is more weight for the
stem to bear in the first place.
As a result, this greater weight bourne by the handlebars is transmitted to the stem via the bottom
portion of this cap. Actually, through the 5-6 aluminum threads the steel bolt was holding on to.
This "force vector" would appear to be trying to "peel" the cap off the stem, in a downwards and
forewards direction. In addition, every jolt from the road would force the steel bolts into the
bottom of the threaded holes, gradually elongating them.
By this action, the top bolt _would_ give away first, since it is bearing the greatest strain (both
down and out, instead of just mostly down, like the bottom bolt). This is what eventually happened.
Fortunately, it did not happen during a panic stop or on a steep decent.
Anyway, this is my "barnyard engineering theory". If anyone here really IS an actual engineer, or
has had the proper training in engineering or forensic science, and can confirm, or correct this
theory, their input is welcome.
More importantly, if anybody else has, or knows someone who has experienced failures like this on
similar stems, I would definitely like to hear from them.
"May you have the wind at your back. And a really low gear for the hills!"
Chris Zacho ~ "Your Friendly Neighborhood Wheelman"
Chris'Z Corner http://www.geocities.com/czcorner
(Kalloy UNO). Here's what this stem looks like:
http://www.chucksbikes.com/st002.htm
This is identical to similar stems made by Profile Design and "Zoom" (A Performance brand name). I
have owned it barely four years, when it failed last week. The top bolt pulled out (stripped).
This did not occur while tightening the bolt, It failed during use. It was not over tightened. Nor
had it been frequently loosened and re-tightened. I am not an overly heavy rider, I weigh 175 lbs
soaking wet. I am not a particularly strong rider, but a tourist/recreational/endurance cyclist. And
while I do like hills, I conquer them with low gears, not by standing and torqueing my bike into a
pretzel. In any case, the failure occurred while riding casually (13 MPH) on level pavement. My
handlebars just started twisting downwards, as the retaining cap came loose. I was able to finish
the short ride safely, by riding carefully, bearing as little weight on the bars as possible.
When I got home, I saw the top bolt had pulled out, stripping the hole. The fit of the bottom bolt
was also sloppier than I felt it should be, like the hole was too big. Although this may have
resulted from having to ride the bike with the top bolt kaput. The bolts were tight when I left, and
there were the remains if five or six threads on the bolt. This was the same depth as the bottom
bolt. Further examination of the stem and how it failed led me to believe that this _may_ have been
due to a flaw in the design. Here is my reasoning:
1. Unlike the MTB stems these are based upon, they only have two bolts, not four. Each bolt,
therefore has to bear twice the load.
2. They are parallel to the ground, not angled upwards, so the retaining cap must bear the entire
load of the rider.
3. The road rider is also leaned further forwards than on an MTB, so there is more weight for the
stem to bear in the first place.
As a result, this greater weight bourne by the handlebars is transmitted to the stem via the bottom
portion of this cap. Actually, through the 5-6 aluminum threads the steel bolt was holding on to.
This "force vector" would appear to be trying to "peel" the cap off the stem, in a downwards and
forewards direction. In addition, every jolt from the road would force the steel bolts into the
bottom of the threaded holes, gradually elongating them.
By this action, the top bolt _would_ give away first, since it is bearing the greatest strain (both
down and out, instead of just mostly down, like the bottom bolt). This is what eventually happened.
Fortunately, it did not happen during a panic stop or on a steep decent.
Anyway, this is my "barnyard engineering theory". If anyone here really IS an actual engineer, or
has had the proper training in engineering or forensic science, and can confirm, or correct this
theory, their input is welcome.
More importantly, if anybody else has, or knows someone who has experienced failures like this on
similar stems, I would definitely like to hear from them.
"May you have the wind at your back. And a really low gear for the hills!"
Chris Zacho ~ "Your Friendly Neighborhood Wheelman"
Chris'Z Corner http://www.geocities.com/czcorner