building an alloy cafe racer fender

I took some time to document building an aluminum fender for a bike I'm working on.  I make quite a few of these parts but usually don't have time to record the process.  I start with a piece of 14ga 3003 aluminum, cut to about 20" and 4-5" wide, depending on the style I'm going for.

The fender is a combination of two curves, the main curve to match the diameter of the wheel, and a tighter curve to match the profile of the tire.  To get this shape the center of the fender needs to be stretched and the sides shrunk.  Since the shrinking is minor, I'll use a dolly and soft slapper to do it, rather than a tuck shrinker which is for more dramatic shrinking.

I start by drawing a center line and two other lines each about 3/4" from the edges, the center line is for reference and the two other lines are so I have a guide to where shrinking and stretching part ways, I could do this by eye now, but doing the lines is habit and probably not bad practice.  In theory, the lines off the edge mark metal which isn't either stretched or shrunk, so technically it's not moved.  That's a good way of thinking about it.

I start off by hammering the center portion into a shot bag, and then going right to the dolly to bring the sides in with the soft slapper.  Right away the fender starts to take shape, it's a pretty basic form so the only challenge really is to keep things even.  I use the wheel to smooth out the hammer and shrink areas, then do a check on the tire.  One more round of hammering the center and lightly shrinking the edges gets the curves where they should be.  Another go in the wheel, and a second pass with a large radius anvil from side to side smooths the fender out.  I use a ruler to make sure the edges are spaced evenly, some light working against the dolly brings them in where needed.  The final check is done against the tire and the fender is ready for sanding and polishing.

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