Whippet Crank Arms

I have quite a few photos of the Whippet sitting in the Ottawa Museum of Science and Technology. It’s under plastic, sitting up in a mezzanine, and not on display. I do not, however, have one photo that shows an entire crank arm. So, here is the lower section, held to the BB axle with a tapered cotter pin.

The top of the crank arm featured an adjustable slot for different crank lengths. Isn’t that perfect for a cyclist that has one leg longer than the other?

The starting point was a bar of 954 bronze that cost me $100 several years ago. 954 bronze has a higher tensile strength than 4130; perfect for strong crank arms.

There was a bit of extra material, so that got milled off.

Machinist blue for layout, as usual. Critical holes have been drilled/bored. Now it’s just a matter of getting the shape right.

A facemill was used to make a very slight taper from top to bottom.

The sides were still parallel, so it was easy to hold the arm in the mill vise and slot for the pedals.

A carbide insert in a Boring Head was used to mill a flat for the washer and pedal nut to tighten against.

See the arc scribed in the (lower) machinist blue? We need to machine a radius very close to that line.

A large endmill is used to do that. See how one crank arm is held in each side of the vise? This keeps the vise jaw even and square.

Now the sides are milled at a slight angle. See how the crank end is C-clamped to a right angle plate? This stops vibration and tool “chatter”.

The crank arms are taking shape, but the top and bottom surfaces would be better with a slight radius, I think.

This is my porting tool. You can see the electric motor in the background, and the hand grinder up close with a carbide burr. A cable connects the two, and a foot pedal (not shown..) controls the speed. The speed is not that high. This (great) tool is now being used to radius the top and bottom.

A hand file is used in the corners for blending.

Back into the rotary table for machining the radius around the BB axle. I don’t know if Whippets really had bronze crank arms, but why not? The late 1800′s were a time of great bicycle (and tricycle) experimentation, and bronze was used in other cycle parts, such as hubs. I enjoyed making these crank arms, and they polished up quite well.