JESawyer

Hello, everyone. I have an old Reynolds 531 Gitane frame and fork that I would like to use to get comfortable with brazing again. I'm going to sandblast the frame to make some modifications, including adding braze-ons to the fork blakes for Mafac centerpulls. Tire clearances in the rear are pretty wide. Up front, clearances are tight, constrained by the crown -- vertically, especially, but also horizontally. The crown isn't the most artful thing, pretty crude and thick. Looking at the existing fork as well as a raw example of the crown online, it looks like there's a decent amount of material that could be removed from the lower inside of the crown... but should I? If the brake is being mounted at the tops of the blades instead of through the crown, will that put more stress on the crown or less?

I could also build a new fork. Or I could just accept narrower tires.

Well, I don't have enough posts to attach an image, so check this out with a cut and paste. Thanks.

imgur.com/a/K9z5FYk

bulgie 

The tangs can be thinned by a mm on each side for sure, 2 mm total width improvement. More if you like living dangerously.

The height though -- can you see that the crown is hollow? It's made out of thick sheetmetal, wrapped around and welded. I know that sounds cheap (and it was inexpensive), but it's actually a great crown in terms of strength-to-weight or stiffness-to-weight ratio. Pretty light, lighter than most IC crowns, and I've never seen one break or bend, despite them being very popular for literally decades. I was talking to Jim Merz about them (in case you don't know, top custom FB in the '70s, designer for Specialized in the '80s). He said he too was a fan, likes the efficiency of the design. They do need work to make them look nice though, pretty crude as-delivered.

Here's another view of the raw crown, showing how hollow they are, and where not to file them thinner:


(The weird grey thing on the right with the cooling fins is a flashlight, trying to illuminate my point here...)

Mark B in Seattle

JESawyer

That makes sense. Thank you for the reply!

Andrew R Stewart 

I would not try to change the crown's shape/shorelines after the fork was made. That's for before the blades get attached.

The different widths of the blades VS the stays is not uncommon on the older bikes. One can use other bits of tubing or misc stock to build up a base/foundation for the brake bosses to be attached to. This is not common but can work well if done well.

My suggestion is to consider making a fresh fork (and selling that old one, Gitane forks are not easy to find when one wants one) that has the crown/blade combo that works for your needs, not change what you want brake or tire wise.

BTW is the Gitane fork French spec? If so it has greater resale value although fewer potential buyers then an Eng/Itl/ISO threaded one. Andy

unterhausen

I take Andy's point, but those crowns are thick and crude. Not sure how much extra clearance you're going to get though. I wouldn't mess with the shorelines, just the thickness. If you cut into the blades, call it done and use a different fork.

I built a fork with a similar nervex crown. I thinned it quite a bit, but was a bit afraid to go too far because they are such POS.

GrayJay

Another option for more tire clearance, you could remove the fork-end dropouts and replace them with some longer dropouts that would make the fork overall longer and thus provide more vertical clearance and maybe even bit more horizontal clearance if you go long enough.

I had a steel fork on an older race bike that would only take max 23mm tires without rubbing the crown. As a less invasive (and less effective) fix, just adding a bit of braze to fill-in the top of the fork dropout openings by couple of mm made it so that the hub axle sat low enough in the existing dropout so that I could use slightly larger tires without rubbing.

bulgie 

I did just that not too long ago, on a Nervex crown just like the OP's, a '50s French racing bike being turned into a randonneuse. I thinned the tangs, on a taper so they're full-thickness at the top where they join the rest of the crown, tapering down to pretty thin at the points, which is the clearance choke point. This let me fit a fender, which is all I needed, so I call it a success.

if it's actually the tire that's hitting on the tangs, then your tire is up too high (yes I know you know this already!)

If your fork has excess rake, you can gain a little blade length by straightening the curve. That can be done with a big rubber mallet. Sounds cave-man but it's actually a pretty good way. I do it with the front of the crown laid on the edge of the workbench, both dropouts touching the bench, rake convex side facing up. The steerer will be angled down below the level of the benchtop. Whack the curve with the mallet, increasing the oomph until you get it to unrake the desired amount, then repeat on the other side. Assuming the four points (right and left side of crown + two dropouts) were touching the surface before you started, you'll get a gap at the dropout on the side you unrake first, then that gap will go away and all 4 points should again be touching when you're done.

This works on a cheap-ass workbench like mine with a softwood top that you don't mind making dents in. Put some sacrificial pieces of softwood under the tips and crown if you don't want dents in the surface, or dents in the fork if your benchtop is metal or other harder material. Some paint damage may occur regardless, but I've done this without marring the paint.

Needless to say (?), recheck fork alignment after this procedure.

Mark B

JohnDThompson 

Good point. And the replacement fork could be English thread, with a far broader range of headset options available.