a new kind of bike failure

Chalo Colina writes:

I was applying heavy braking to my bike today, when the downtube
buckled with a distinct 'crack'. The tire met the downtube, and i
was tossed over the bars. Both my arms were broken, so please
forgive my abbreviated typing method.

This is the most common failure mode of better bicycles (ones with
durable forks) under excessive frontal load, like running into a
whoop-de-doo that just about fits the wheel radius. I have had such
failures on trails with my "road" bicycle and of course when running
into an obstacle such as a car that suddenly stops ahead in traffic.

With your weight (that I recall only as more than average), wrinkling
a downtube at the end of the butting is probable on a wheel locking
event. Usually there is enough clearance for the tire to not touch
the downtube, which is a certain endo event. In any case the fork
should not bend because it would otherwise fail in fatigue from
average braking if it were not stronger than the frame. My bicycle
frame has an oversized down- and toptube for that reason. I ride
primarily in mountains on roads and trails where I brake hard on
descents.

Since I have not yet contacted the manufacturer of the frame to
discuss the warranty or anything else, I'll withhold the identity of
the makes involved except to say that the frame and parts were
relatively new and had been used for only a few hundred miles, and
that I trusted them pretty well until the time of the failure.

It was a touring frame with a single crown suspension fork [steel
steerer] and a disc front brake.

I have never seen a failure of this sort before. The downtube is
creased perpendicularly to the tube's length maybe 25cm back from
the lower headset race, and arched upwards.

From the description, I think the frame tube seems to be too light for
you. The wrinkle is no doubt the end of the butting. There is no
other reason that it would wrinkle there instead of at the head tube
where the greatest bending moment acts.

Get well and back on the bicycle soon. All the best,

Jobst Brandt

Bruce Graham wrote:

I'm very sorry to hear this Chalo. You have mentioned that you are a
very large man so I am wondering if the initial problem was the fork
which is under extra stress with a disk brake.

obviously the braking force was more than the system could bear.

Is the fork bent
backwards below the steerer? If that failed first, the tire would hit
the downtube somewhere in the middle of the downtube and buckle it. This
is only an uninformed question, not claiming to be an explanation.

i have bent many a fork under braking, but oddly in this case the fork
seems ok on cursory examination.

chalo

Mark Hickey wrote:

Was the down tube butted? 25mm back from the head tube junction
sounds like where the butt would normally occur (forming a bit of a
stress riser).

it was about 25cm or 10in from the miter. I don't know whether the
tube is butted but it's a safe bet. The buckled portion is in the
part where the tire made contact, though it seemed that the 'crack'
came first, before the bike stopped. the paint is barely smudged at
all by the tire.

I've seen failures like that, but only when something solid was hit.

me too, but never from just braking, and in those cases the fork was
always kinked way back. my fork looks ok to me though I've not
measured it closely.

Now I understand one of the reasons tandems have a lateral tube (runs
between the top and down tubes, back to the rear bottom bracket).

not a bad idea for me either.

chalo

Richard Tack wrote:

Is it a steel or alloy bike?

aluminum. i've had a lugged steel bike bend from less braking, but
not buckle.

Can you get some photos up?

i want to talk about warranty and replacement options with the frame
mfgr. first. I'll do them the courtesy of not parading their broken
wares for now, since i don't think this is a hazard that others must
worry too much about.

chalo

(Chalo) wrote in
om:

Mark Hickey wrote:

Was the down tube butted? 25mm back from the head tube junction
sounds like where the butt would normally occur (forming a bit of a
stress riser).

it was about 25cm or 10in from the miter. I don't know whether the
tube is butted but it's a safe bet. The buckled portion is in the
part where the tire made contact, though it seemed that the 'crack'
came first, before the bike stopped. the paint is barely smudged at
all by the tire.

I've seen failures like that, but only when something solid was hit.

me too, but never from just braking, and in those cases the fork was
always kinked way back. my fork looks ok to me though I've not
measured it closely.

Now I understand one of the reasons tandems have a lateral tube (runs
between the top and down tubes, back to the rear bottom bracket).

not a bad idea for me either.

chalo

Have you removed the fork from the frame? I wonder if the "crack" you
heard was the steerer tube failing where it attaches to the crown?

Seems like you might want to have someone video tape that being removed
and inspected.
Bill Brannon

"David L. Johnson" wrote:

Did the front brake tend to chatter a lot? Had you had some impact
earlier on?

no and no. i had just replaced the stock disc pads with sintered
metal ones, and I was burning them in. The brake was perfectly
well-behaved.

The bike had never been crashed and was my newest and most pristine
bike by far-- not even a scratch.

chalo

(Chalo) wrote in message . com...
i was applying heavy braking to my bike today, when the downtube
buckled with a distinct 'crack'. the tire met the downtube, and i was
tossed over the bars. both my arms were broken, so please forgive my
abbreviated typing method.

Holy crap. Sorry to hear about that. Way to suck it up though, keeping
on typing to r.b.t. the same day! You didn't call yourself Bluto for
nothing.


since i have not yet contacted the manufacturer of the frame to
discuss the warranty or anything else, i'll withold the identity of
the makes involved except to say that the frame and parts were
relatively new and had been used for only a few hundred miles, and
that I trusted them pretty well until the time of the failure.

it was a touring frame with a single crown suspension fork [steel
steerer] and a disc front brake.

i have never seen a failure of this sort before. the downtube is
creased perpendicularly to the tube's length maybe 25cm back from the
lower headset race, and arched upwards.

25cm or 25mm? I think people claiming it was just behind the butt were
thinking you said 25mm. A foot down is a different story. The wheel
impacting it at that point sounds more like the proximate cause. That
would explain the header as well, which shouldn't have occurred if all
that happened was that the downtube buckled on its own. What shape is
the steerer in? Surely it would have taken a permanent set if this had
happened.Unless there is also a secondary bend in the downtube, nearer
the headtube

I don't look at suspension forks, but by single crown, do you mean
unicrown, like y-shaped welded steel? Also it seems to me a
suspension fork would be more likely to be able to bend enough to have
the wheel whack the downtube, even without the steerer or anything
other than the downtube taking a permanent set. Besides the
tube-within-a-tube construction, at the limit of its travel, which it
presumably would have been under hard braking, the wheel might be
uncomfortably close to the frame already.

Since you are usually so careful to get super-duty stuff, and this
doesn't sound like your Bohemian, it does sound like some regular
production aluminum job. That would explain the cracking noise, which
is hard to imagine with steel.

"Chalo" wrote
i was applying heavy braking to my bike today, when the downtube
buckled with a distinct 'crack'. the tire met the downtube, and i was
tossed over the bars. both my arms were broken, so please forgive my
abbreviated typing method.

since i have not yet contacted the manufacturer of the frame to
discuss the warranty or anything else, i'll withold the identity of
the makes involved except to say that the frame and parts were
relatively new and had been used for only a few hundred miles, and
that I trusted them pretty well until the time of the failure.

it was a touring frame with a single crown suspension fork [steel
steerer] and a disc front brake.

i have never seen a failure of this sort before. the downtube is
creased perpendicularly to the tube's length maybe 25cm back from the
lower headset race, and arched upwards.

I've seen one similar, but that was on a low end Huffy, with 2 elliptical
holes in the downtube to route the cables through. It failed at the top
hole.

Pete

Richard Tack wrote in message news:88d8d.3998$cd1.3594@trnddc03...
Chalo wrote:
i was applying heavy braking to my bike today, when the downtube
buckled with a distinct 'crack'. the tire met the downtube, and i was
tossed over the bars. both my arms were broken, so please forgive my
abbreviated typing method.



I bought a cheap bike and Meijer once, and slammed it into a curb,
and had the same thing happen to me, I did a complete flip over the
handle bars and landed almost square on my feet, not a scratch.

It was new so I returned it.

Zog The Undeniable wrote:

Is the fork bent backwards?

fork is not visibly bent.

Could the wheel have contacted the downtube
and locked up?

tire definitely touched downtube; it sounded like the tube failed
first. that is, the crack of the tube buckling was the first sound i
heard, right before being tossed to the ground.

chalo