Here's another image of one of those braking induced fork failures that crop up from time to time. This was sent to me by a reader. The entire story of how the accident occured is mentioned on this blog.
Because of the lack of telescopic front suspensions like those nice mountain bikes have, rigid forks take the full brunt of a combination of two forces. One is the braking force that acts longitudinally backward to direction of motion but this has a component along the axis of the fork as well. The other is the force due to braking load transfer towards the front of the bike. This force acts inline with the fork axis. In essence, the two forces add together. I'm fairly certain that the quantity of this directed force along the fork is strongly dependent on the wheelbase of the bike and the rake angle of the fork. Lower wheelbases equate to more load transfer. Higher rake angles must also promote higher fork forces.
Undersized, thin walled tubes, such as forks, do not act kindly to hard braking forces. An example of this kind of buckling along with its physics was provided sometime back on my blog.
Thanks for reading. Have an enjoyable weekend and exercise safely.
* * *