See other failures under 000.html.

Cannondale frame buckle under braking

The owner reports fitting a disk brake, riding about 50km, then fitting new brake pads and going for a ``hard braking'' test. The owner weighs about 175kg and has broken many components in normal riding, so he wore a full-face motorycycle helmet for the first ride. In sustained hard braking down a hill, he reports a ``crack'' sound, then it felt like the wheel jammed. The bike flipped and the rider broke both arms. In looking at the damage he suspects the frame buckled, allowing the front tire to move back and jam on the frame.

Although the combination of rider weight, suspension fork, and disk brake seems like it might be ``overload'', it is worth noting that

This was not a fatigue failure.
The frame was purchased new and was nearly new at the time of collapse. No rider weight limit is listed.
The rider bought the largest frame size, which is presumably also used by heavy riders.
Although most riders are below 175kg, the frame is a touring frame and it is ordinary for riders to carry 30-40kg of gear. 140kg riders are more common.
The suspension fork was likely compressed to the bottom stop during braking -- the rider can compress it most of the way while standing next to the bike simply by pressing down firmly on the handlebars. Thus, the forks were not an unusally long lever.
Although disk brakes provide strong and repeatable braking with modest hand force, rim brakes can exert just as high peak forces. With either kind of brake, hard braking causes either wheel lockup or skid, depending on traction.

Some suggestions/observations from other folks:

The down tube damage may have been for some reason other than braking load, including pre-accident damage and post-accident damage.
The paint in the dent was not obviously damaged by a hard object. The paint is also not obviouslly damaged by the dent, so the paint is both flexible and fairly strong.
Hold a straight edge against the close-in side picture. Note the non-buckled side of the frame has a distinct curve, indicating the tube is bent. It is unlikely the tube was manufactured that way.
The front brake was serviced immediately before the accident. Service might have caused the damage, but service makes it more likely any pre-existing damage would have been noticed during service.
The pavement was relatively clean, it is not obvious what would have been thrown up to cause such damage.
The bike and rider wound up tangled together after the accident, which might have caused frame damage afterwards. It is not obvious what part of the rider or clothing would have caused such a dent.
The ``crack'' sound and lack of other evidence of broken or damaged equipment suggests frame damage during braking. It is possible the frame was struck by street debris to make the noise, but note the street was fairly clean.
The top tube was not closely examined for damage (a mistake). The full side view picture shows the rear brake cable close to the bottom edge of the tube at the ends, but in the center, the wall behind is visible. That suggests the top tube is slightly bent. However, that may be ``thinner in the middle, thicker at the ends.'' A straight edge along the top of the tube suggests slight bending, but less than suggested by the cable.
If the frame bent during braking, the combination of varying decelleration and rearward motion of the tire contact patch may have contributed to the rider being thrown.
If the frame bent during braking, the tire might have contacted the down tube but the tire might have been stopped when that happened. Thus, there might be no skid mark on the down tube.
The suspension may have bottomed. The impulse may have raised frame tube forces. At the same time, the impulse may have increased traction, thus increasing the braking force and further increasing frame tube forces.
The Cannondale frame is made of thin-walled large-diameter tubes, different than typical bike frame tubing. Thus, the failure mode may be unfamiliar. An empty aluminum soda can was crushed using a vise applying an off-axis load. The can has an even larger diamter and even thinner wall. Minor loading caused the can to emit a ``crack'' sound and buckle. The non-loaded side of the can did not wrinkle or show other damage evidence, even under further crushing. (This experiment should be repeated with slight initial damage to the can.) Note the frame tube is tapered so it has further variations along its length.