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Broken handlebar

September 16, 2005

Fortunately this happened while I was just riding across campus with no one around, and not while crossing Linden street in heavy traffic.

So much for the inherent superiority of steel over aluminum alloys and composite materials.

Categories: Geeky
  1. September 16, 2005 at 20:38 | #1

    Yep. I reckon any thing will break under stress after a while though, even tempered stainless steel. Hope you were not thrown and hurt. I would be afraid to even get on a bike anymore-no doubt I can ride one but might not be able to get off with out falling off if I rode one around the block.

  2. September 17, 2005 at 10:07 | #2

    Thanks for the good wishes – I just coasted to a stop and thought to myself, “Huh.  Never seen one do that before.”  Then I rode home left-handed.

  3. September 18, 2005 at 04:11 | #3

    Makes me think what shoddy workmanship. The bike doesn’t look old?

    And then reminds me of the time the swinging arm on my little C70 moped snapped in the same way. The swinging arm (a bit like bicycle forks) holds the back wheel to the body of this motorised vehicle and it too, for a while, was hanging on by a sliver of metal. Then it completely snapped on one side. I found that if I lifted my body and put my weight on the front of the bike I could keep riding! I was very many miles from home and in the middle of nowhere. So had no choice!

    I don’t know what that part of the bike was made from but it was a rusty old bike and I had just driven about 100 miles on it. Your bike has no such excuses!

  4. September 18, 2005 at 10:36 | #4

    Lucky we were both able to ride home on the crippled bikes!

    A little crude arithmetic explains.  These are pretty wide handlebars – about 12 inches from the edge of the gooseneck clamp (adjacent to the crack) to a point that corresponds to my wrist joint.  The bar itself is an inch across.

    Imagine each side of the bar as a first-class lever with (during braking) the fulcrum at the front edge of the bar where it enters the gooseneck clamp, the load at the back edge of that same joint, and the force applied out at the handgrip under my wrist.  Putting, say, a 30lb forward push on the handgrip during braking is multiplied by a factor of 12:1, resulting in a 360 lb tension on the focal point where the crack appeared.

    Mild steel has a tensile strength usually around 36,000 psi before plastic deformation and 60,000 psi for sudden failure.  As the 360 lbs was focused across a cross-sectional area of about 0.25 square inches (the thin walls of the tube) it is multiplied by four to about 1,300 psi. (Hey, I said these calculations were crude!  It’s just for illustrative purposes.)  Still just 1/28 the force of plastic deformation, seemingly well withing the safe range.

    I’ve had the bicycle for 10 years, and ride vigorously in an urban area.  I go over curbs, apply the brakes, accelerate vigorously in traffic (which puts an opposite force, turning our load point into a fulcrum) and every day on the way from one office to another, I dive over a steep little grassy hill behind a parking garage.

    Say that means fifty applications of force each day, times maybe 200 riding days per year, times ten years, is flexing that bar a half-million times at 1/28 of its plastic deformation force.  Force is concentrated around tiny flaws in the metal and invisible cracks form.  Force is concentrated at each end of the cracks and they begin to expand.

    Note that as the bar angles back from the gooseneck, the crack crossed it exactly perpendicular to a line from the fulcrum point to the handgrip, and we have our culprit.  Not shoddy, particularly, but not the right kind of handlebar for the sort of riding I do.

    Of course all this occurred to me after the bar broke.  I’m replacing it with one made of chrome-vanadium steel; tensile strength in the neighborhood of 120,000 psi.

    One of my favorite books about metal fatigue is the fictional novel, No Highway, by Neville Shute.

  5. Richard
    June 12, 2006 at 03:20 | #5

    glad your ok. I’ve had a similar accident but not so lucky. The handlebar snapped similar to your own but i ended up going straight through a set of red traffic lights on a busy main street. Needless to say i was hit. Minor injuries. At least you and Doris could ride home. I took a trip in an ambulance.

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