The trick to narrow metal fenders at stays/blades without changing the radius is to "form" (= "dent") them for clearance. The fancy guys who know what they're doing have radiused wood blocks and stuff to do this artfully. I have chunks of cut-off seatposts, steerer tubes and other chunks of round-profile metal, and a mallet. I mark the spot, rest the fender edge on one open side of my bench vise, which has a plastic jaw cover installed to cushion the blow, and...take out any aggressions I might have. Little love taps go a long way, usually not a lot of wailing needed.
Sometimes that needs to be done both on the sides and top, especially at the seatstay bridge, to get the fender high enough over the tire, while also getting the necessary side clearance. Forming the sides can/does cut down on side clearance for the tire, but not more than just squeezing the fender would. I usually gently pound the rounded/rolled bottom edge flatter where the fender is formed, which can get you an extra mm or so per side.
Another method is to potato-chip section the fender, basically cut out circular sections from the side. The advantage is it doesn't limit side tire clearance at all, and it's also a little easier to control. You have more control over cutting than Hulk Smash With Mallet. Better control of forming comes from using the radiused wood blocks. But potato-chipping isn't reversible, and you are also potentially weakening the fender somewhat, depending on the size of the removed potato chips. A dent can always be pounded back out. Not necessarily pretty, but possible.
I generally have a couple pair of old, battered, pounded & chopped metal fenders around that are sometimes used as first-try installs. Once I get the holes drilled and dents properly placed, I'll live with them for a little bit, then use them as a guide for installing a clean set.
I think I have a couple of photos from a chainstay chipper job I did on my Weigle. That removed enough metal that I'll probably add a reinforcement tang at the bridge. This particular Weigle is an '81, and wasn't designed with easy fender installation in mind, so Things Had To Be Done. Just denting the fender sides at the chainstay really limited tire width. You can see some scratching on the left chainstay from the first installation, when I just dented the sides, and didn't put any protective tape underneath. Protective tape underneath is a good idea.
And you definitely want to smash down the rolled edge at the cuts, and file off any rough edges so there's no chance of finger-cutting.
I could have (should have?) radiused the cuts a little tighter, so the top/bottom edges were closer to the stay tops/bottoms. But it was the first time I tried this, and I ain't Peter Weigle. That extra space does leave some room for water to splash outside a little bit, and doesn't look as elegant as, well, as if Weigle-sensei did it himself.
FWIW, one of my New Year's Resolutions was to stop worrying about perfect fender lines. In my shop, "close" counts for horseshoes, grenades _and_ fender line.


Paul Brodek
Hillsdale, NJ USA