Creating the perfect fork blade
Mon, May 18, 2020 
When a fork blade comes from the tube manufacturer’s factory, it is straight, the framebuilder bends it to a curve that suits his requirements.
An un-raked road fork blade is oval at the top. The oval section runs parallel for about a third of its length.
Then the cross section becomes round and starts to taper gradually to its smallest diameter section at the bottom end.
The fork blade is bent cold on a curved form that is sometimes made from hard wood. I used one I made myself from two heavy-duty steel fork blades, bent in the desired curve, and brazed together side by side. This made a natural groove between the two blades where the blade would sit as I was bending.
I would slip a short piece of tube over the thin end of the form and the blade I was bending. This acted as a collar to hold it in place. Then I'd start bending, first by pushing down by hand. The thin end of the blade bends easily, and I would finish off by squeezing it in a vise.
Bicycle tubing is hardened, and it will spring back after bending. Because of this, the form needs to be a greater curve than the finished fork blade will be.
A fork blade is several inches longer than it needs to be. The framebuilder chooses where he will put the bend, and where he will cut to length. For example, if I were making a criterium frame and wanted a very stiff fork, I would cut from the bottom, thin end. The desired fork rake (Offset.) also has to be considered. The amount of bend, and where the tube is cut will determine this.
If I were building a touring frame, and wanted a flexible fork for a more comfortable ride and more offset, I would cut from the top end and leave the blade thin at the bottom end. The framebuilder creates the perfect fork blade, by selecting the best place to bend the blade, the amount of bend and by choosing how much to cut from either end.
It is rather like a furniture maker choosing where to cut from a piece of wood to achieve the best end product. Once I arrived at the perfect fork blade, it was then an easy matter to repeat the process again and again. On this John Howard frame below, for example.
One exception to this process was the Reynolds 753 fork blades. 753 was heat treated to a degree that the material could not be bent after. These were bent at the factory, then heat treated, and the framebuilder then cut the blade to the required length.
The cut alone determined the fork rake, and I had no control over the placement or shape of bend. You will notice on this 753 Fuso Lux frame (Pictured below.) that the fork bend is a different shape than the ones bent by me.
On the red 753 frame there appears to be more rake, but this is not the case, the amount of offset is the same. The 753 blade has a tighter bend near the bottom, whereas the one’s I bent by hand, have a gradual curve that begins about half way.
Chainstays and seatstays are also tapered and the same selective cutting to length is employed. In this case, where the cut is made depends a great deal on the size of frame and its end use.
The perfect fork blade is stiff enough to allow precise handling, but with some flex to absorb road shocks. It also looks pleasing to the eye. I have a theory that when something is designed correctly from a functional standpoint, it has a natural aesthetic beauty. This is true of a boat, a bridge, a building, and even a bicycle frame.
The modern trend of building straight forks of course saves the framebuilder a great deal of time and effort. If this look has become acceptable, why should today’s builder go through all the time consuming process I have described here?
The straight blade is angled forward so the same fork rake or offset is achieved and handling would be the same. I can’t comment on the shock absorption qualities because I have never built a frame with a straight fork.
In my view, a great deal is lost aesthetically, so where does that leave my theory about function being linked to aesthetics? On the other hand, is it simply that beauty is in the eye of the beholder?
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Reader Comments (7)
My humble opinion is that the rationale behind straight fork blades is strictly labour-saving by taking one step out of the building process. i have never ridden a bike with straight-bladed forks, but i'd guess that the ride would be rather harsh. That plus -to my eyes- straight forks are butt-ugly.
i had an old Carlton that had forks that sported about a 3" rake. The steering was a bit sluggish but the bike rode like a comfy sofa, although it did take some getting used to to see those forks flex on a bumpy road!
Vertical compliance in forks (and frames) is an interesting subject. Aesthetics aside vertical flex is no issue at all until it starts messing with steering geometry.
Aesthetically your blades are what I think of as 'correct' and they still look good to me.
I like the Bates fork, it looks like its holding the wheel in its teeth.
Dave, I always liked the gradual, large radius forks you made. They do look elegant.
I own bikes with steel forks, both curved and straight. Frankly, I can't tell any difference in ride quality. I'm inclined to think the diameter and cross section of the fork blade along its length play more of a role in ride quality.
Could it be that in nature a straight line doesn't exist?
Perhaps that is part of the aesthetics you speak of. If a curved fork looks "natural" to us, then why does a curved top tube, or down tube, or seat tube, or seat stay or a curvy chain stay look "unnatural"? Why the difference?
It is real, as many don't like curvy tubes on bikes. Or double, triple curves on forks. Maybe this has something to do with the simplicity of bikes. We look at them as not having unnecessary designs, but enjoy fancy lugs and pantographs and parts to hang on frames, but at the heart is the frame.
Maybe there is a point where design goes beyond function. And we can see it in certain frames. Problem is those designs still allow function to exist, whereas function doesn't depend on design to exist. Function will always be there, and design will always be up to us, the builder.
Is a bike frame an expression of function or design?
A good frame has to be an expression of the designer, the builder, to be functional. That's why there are so many possibilities. Perhaps endless...
I have ridden on sharply curved, long gradual curved and straight bladed forks. They all worked well for their intended purpose, but aesthetically the long gradual curve just looks right to me. The sharp abrupt curve looks very “old fashioned” to me and I like the way it looks on a pre 70’s bike. Straight blades are functionally fine but they are unattractive to me.
Very informative post. Small details matter. I had a fork made for a "gravel" bike and despite all the CAD work that goes into things these days, I was disappointed in the clearance under the fork crown.
This was because the crown was perpendicular to the steerer tube, so the rake was achieved by angling the fork tubes forward. This results in the front of the crown being significantly closer to the tire than if the crown itself was angled so that the tire clearance was closest in the center, not the leading edge.
I lost about 5mm of potential clearance, and given this was a bike designed to run larger tires, we agreed that the fork would be remade