Fork Blade Bender/Mandrel

I, too, am a fan of fork blades with a sharp low bend and read John Clay’s article in Bicycle Quarterly (Spring 2009) with interest. My friends at Circle A Cycles are building a frame  for me and I asked if they could make a fork with blades like the ones that John Clay’s bender produces.

Chris asked if I could draw the new bender and mandrel that would be required, so I studied the Clay bender and produced this drawing.

I spent a lot of time thinking about pivot and pulley placement and how they affect the mechanical advantage of the bender. It turns out that the ideal location(s) are off the mandrel, but knowing them can help find the best realistic location for the pivot. Additionally, in placing these two points, you need to be careful not to have the pulley run off the end of the fork blade before you’ve reached the proper rake.

With that said, I think that this bender’s pivot placement will be an improvement on the John Clay bender.

When choosing pivot and pulley placement, the idea is to maximize angular movement for the bending arm for the desired rake. That is to say, mechanical advantage comes not only from the length of the bending arm, but also from locating the pivot/pulley such that the arm moves through a greater angle than the blade.

For example, if by bending the blade so that the tip is effectively at a 70 degree angle, the bending arm moves through 105 degrees, you have a mechanical advantage of 1.5, just due the greater angular motion of the bending arm. This would make a 2 foot arm equivalent to a 3 foot arm.

It’s also possible to have a mechanical advantage less than 1 due to poor pivot/pulley placement. This would occur if in bending the blade through a given angle, the bending arm moved through a lesser angle. This would effectively make a bending arm behave as if it were shorter than it actually is.

Once Chris has this set up, I’ll post with a report as well as any modifications we make.


2 Responses to “Fork Blade Bender/Mandrel”

  1. 1 simon November 12, 2011 at 3:40 pm

    so did this get made?

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