Vallie Components - bicycle part and accessory design

Recently, while visiting the Brooklyn Machine Works shop, I had a look at a variety of Laser cut dropouts. I can't get into detail about the the designs I saw there, because I don't have pictures to illustrate them, and there was just TOO much awesomeness going on there to be described in one post.

Laser Cutting is the general technology used to churn out dropout after dropout for the majority of today's track and BMX frames. Obvious exceptions to this are the forged and machined examples found on some high end bicycles. For the most part however, the single thickness dropouts found on your track bike, were cut with a CNC driven laser.

Plasma cutting is another viable option, and may have been used in the past. However, the width of beam is slightly larger than with a laser, and therefore accuracy is less. With the ever decreasing price of laser cutting technology, its almost easy for designers to get carried away.

(BMW actually doesn't do any laser cutting in house. They do their designs in AutoCAD and contract another metal shop to do that)

I'm not going to name names, but here we have excessive laser cutting. Its as though the designer wanted some swoopy crescents to dress up his dropouts. A napkin was marked up with a fine tipped sharpie, and sent overseas. The factory reviewed said napkin, said "Sure thing chief!" and away they went with the CAM program. Never mind that the actual crescent shapes have no flow, and subsequently do nothing to create an air of classiness. Now, I don't personally know how much this will affect the strength of the design. I just get this vibe that its a bad idea.

A city ridden bicycle is subject to stresses many times greater than the riders weight when road imperfections are encountered. The axle pushes UP on the dropout with ridiculous amounts of force. These stresses occur at high frequency, every ride. These are the sort of forces that (over time) lead to dropout weldment failures, or similarly seat stay/seat tube junction failures.

A stress riser is essentially where localized forces can overcome the shear strength of a material. And nothing localizes forces more than sharp angles. The stress concentration factor goes up exponentially as the radius of a cut approaches zero. The idea of adding 8 tiny-radius-acute-angle cuts to a small piece of metal that is undergoing high frequency shear forces from the axle just seems like its ASKING to test the elastic modulus of the steel being used.

Just looking at that picture is bothering me enough that I have to model it. I MUST know just how much these cuts affect the strength of the part!

Here I'm starting by importing the photo of the unnamed bicycle dropout into SolidWorks, rotating it and scaling it correctly. Then I just draw the existing dropout shape in a 2D sketch:


Now, lets suppress the photo and extrude the basic dropout. Shall we assume it's 5mm thick?

Looks decent.

Okay, here I'm just copying the random swoopy crescent-shaped cuts in our unnamed dropout.

Looking closely, there is only one straight line.

The absolute randomness of these cuts really makes my brow furrow.

...and the final cutout:

Looks horrible doesn't it?
Anyways, on to the testing.