Creating the perfect fork blade

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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Fractal Geometry and Frame Design

Benoit MandelbrotFractal is a word coined by mathematician Benoit Mandelbrot (Left.) who published his findings as recent as the 1970s.

Up until that point, throughout history geometry was all about straight lines, triangles, pyramids, circles and cylinders, etc.

This geometry applied to everything man-made, buildings, bridges and other engineering projects, and of course bicycle frames fit right into this category, being made up of straight lines and triangles. Up until Mandelbrot’s findings, everything in nature outside of that which is man-made could not be explained by geometry and mathematics.

Mandelbrot changed all that when he discovered that shapes and forms in nature. For example, cloud formations, trees, mountains, river flow and even blood vessels in our bodies, were a series of repeating irregular shapes, which could be recreated and proven mathematically.

Like many great minds, Benoit Mandelbrot was at first scoffed at by other experts, but now with improvements in computing his theories are being proved mathematically. Now taken seriously, fractals are being studied and used in the medical and the environmental fields.

One of the places we also see fractal geometry in practice is in special effects for movies. Not only are images of nature being created, like landscapes, plants, trees, and even human and animal forms, but fire and explosions can be created and animated on a computer. 

This subject has always fascinated me, because on the one hand you have the bicycle which is traditional man-made geometry, and the task of the frame builder is to match that to the human body, so the two become one, and the bicycle becomes an extension of its rider.

The human body, like all forms in nature, appears to be chaotic and infinite in its makeup. Yet it was possible for me to build a series of production frames, the Fuso for example, in a range of sizes that would fit just about anybody.

From as far back as the late 1960s I found I could fit someone to a frame “Intuitively.”  I did not let this be widely known for fear of being labeled a crack-pot. I was basing my estimation of frame size primarily on a person’s height.

Long after I left the bike business, and therefore the effect of the “Crackpot” label had diminished, I wrote an article here in February 2006, stating that frame size could be estimated around a person’s height.

I came to this conclusion, not so much by what I could do in sizing a person, but more by what I could not do. A person who is six feet tall would normally fit on a frame around 58 or 59 centimeters (Measured center to top.)

However, it is quite a common occurrence to find a person six feet tall (183 cm.) with a 30 inch (76 cm.) inside leg measurement. You cannot put a person like that on a 51 centimeter frame as his inseam would suggest then build a long top tube to accommodate his long body.

I would simply drop the frame size down to a 56 or 57 centimeter because of the short legs, and leave the top tube as standard for that size frame. (55cm. or 55.5 respectively.) This same frame would also suit a person 5’10” tall, (178 cm.) with an inseam around 33 inches.(84 cm.) The difference being the taller guy with short legs would have his saddle lower and possibly use a longer stem.

I knew this was so, but never knew why. It all became clear to me on watching a PBS Nova episode back in 2010 on Benoit Mandelbroc and his discovery fractal geometry. The program mentioned a group of environmentalists were studying rainforests. They cut down a large tree, then measured and documented the dimensions of all its branches, overall height etc.

They then found that a seemingly random pattern of trees of all sizes growing throughout the rest of the rainforest followed the same pattern as the branches of the one tree they had documented; both in the position of their branches, and their position in the forest relative to other trees.

Watching this, it occurred to me that if you took a large group of humans all the same height, you could fit them all to the same size bicycle frame. (Within a centimeter or so.) This is why this theory works, although on the surface it appears that my group all the same height are each different in every other way, they are no different than the trees in the rainforest. They all follow the rule of fractal geometry that can be plotted mathematically.

Fractals are once again in the news with this recent article, which prompted me to re-visit my previous piece written back in December 2010.

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Going around corners

Roll a wheel or for that matter any round flat object on a flat surface and it will roll in a circle. Even something as small as a coin. It will continue rolling in ever decreasing circles until it finally falls and settles in one spot. This is a demonstration of gyroscopic action, and the way it works.

That is, a spinning wheel will remain upright as long as it keeps spinning. When it loses momentum and starts to fall it will turn in the direction it is falling, which is why it rolls in a circle.

This law of physics gives a bicycle a simple built-in self-steering capability. You can demonstrate this to yourself by holding a wheel in both hands by the spindle and spinning it. The first thing you will notice is that the wheel wants to stay upright in the same plane, demonstrating the first law mentioned in the paragraph above.

If you forcibly move the top of the wheel to the left or right as it is spinning it will also turn in the direction you are leaning it. Just as a rolling coin will turn in the direction it is falling. As you lean a bicycle into a corner it will steer itself around the corner.

Let’s not forget the rear wheel. Although it is in a fixed position and cannot turn within the frame, it is still spinning and leaning therefore assisting in steering the bike as a whole around the corner. 

Because the steering tube on a road bike is angled forward, usually at an angle of 73 degrees, when the steering is turned, the fork blade that is on the inside of the turn drops and the other side raises. Therefore, the front and rear hubs are not in the same plane. (See top picture.)

If the head angle of a bicycle was vertical (90 degrees.) when you turned the handlebars to round a corner, the front and rear hubs would remain in the same plane. 

Going through a turn the front wheel is leaning slightly more than the rear wheel. This adds to the stability of the bike because the front wheel is outside the centerline of the frame. 

Because the front wheel is leaning slightly more than the rear wheel, it is turning at a slightly tighter turning radius, creating over steer. This is a good thing, centrifugal forces are pushing the bike wide on the corner, over steer is counteracting this.

Again, the law of physics states that a moving object will travel in a straight line until an opposing force causes it to change direction. These centrifugal we speak of are nothing more than momentum causing the bike and rider to continue straight while attempting to turn left or right.

We lean into the corner; the wheels steer us in the direction we need to go, and gravity counterbalances the forces that want us to keep us going straight.

At slow speeds this is an instinctive move, higher speeds require more skill. Lean too little and you will go wide and off the road on the outside. Lean too far and the bike will slide out from under you, and you will slide across the road in the direction momentum wanted to take you in the first place.

The design of the bike, in particular the frame will give the bike these desired steering qualities. Head angles, fork rake and wheelbase, even the weight distribution of the rider, all play a role. After that it is the skill of the rider. Done right it is a joy to execute, and a joy to watch others do properly.

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My Design Philosophy Explained

I look at frame specs of all the major bicycle manufactures today, and they all follow each other within certain parameters. Of course the UCI (The governing body of the sport of cycle racing.) lays out certain rules and regulations pertaining to the design of a racing bicycle. However, within these UCI rules there is a pretty wide scope for any individual to do something a little different.

Most take the safe approach and follow what their competitors are doing. It has always been that way, framebuilders do whatever is easiest for them, and bike riders make do with whatever is available. When I got into cycling in the early 1950s, the standard frame of the day was 73 head and 71 seat angle.

Sitting back that far was totally unsuited to my short stature of 5’ 6” (167 cm.) I got into framebuilding trying to build a better frame for myself. I found as soon as I made an effort, I would slide forward onto the tip of the saddle. This was not only extremely uncomfortable, it had the effect of my saddle being too low. The answer seemed simple to me. Make the top tube shorter, and the seat angle steeper, thus moving the saddle forward to where my backside wanted it to be.

Why were seat angles so shallow in the 1950s and before that? It was a throwback to the “Ordinary,” the high-wheeler that was the forerunner of the chain driven bicycle. In 1950 the chain driven bike was only 65 years old. There were still people around that had actually ridden the old high-wheeler.

By the 1960s the parallel angle frame came into vogue. By making the seat and head tube the same angle, the same size top tube could be used over several sizes, tubes could be pre-mitered, and simple frame assembly jigs could be used, thus speeding up production.

First came the 72/72 degree frame, followed a short time later by 73/73 degree angles. The reason being, people were not ready to jump from a 71 seat angle to a 73. 72 parallel was a good compromise. When people found that worked, it was an easier sell to the 73 degree parallel. 73 degrees was a better head angle anyway. That had been established as far back as the 1930s, and is still the standard today, for a road frame.

In the 1970s most Italian builders, and many English builders switched to 73 degree seat with a 75 degree head angle. No one was going back to a 71 seat angle, but having that 2 degree difference in the angle, and with the two tubes getting further away from each other as the frame got taller, was an advantage for the framebuilder. The top tube automatically became longer for the larger frames.

The selling point was, ‘Steeper head angle makes a livelier handling bike.’ It did indeed. Lively to the point of being dangerous for an inexperienced rider. I did not follow this trend, but instead made the top tube shorter. For example a 54 cm. frame (C to T) had a 54 cm. (C to C.) top tube.

A 55 cm. frame had a 54.5 cm. top tube, and a 56 cm. frame had a 55 cm. top tube, and so on. As the seat tube increased by one centimeter, the top tube only increased by half a centimeter.

This simple formula meant that by increasing the handlebar stem length to compensate for the decreasing top tube. It meant the front part of the handlebars was always in the same position directly above the front hub and the point where the tire contacts the road.  This was the case throughout the range of sizes. (See top of page drawing.)

When sprinting out of the saddle, there is always a certain amount of “Throwing” the bike from side to side. If the rider’s weight is directly above the tire’s point if contact, the wheel will remain straight. If the rider’s weight is ahead or behind this point of contact, any sideways movement could translate into the front wheel steering this way and that. I found with this set up, the 73 degree head angle can feel just as lively in a sprint, as the steeper angle, but without the “Squirrely” feel of the steeper bike.

Except for my very smallest size frames, 51 cm. and below. Which had a 72 degree head angle, and 38 mm. fork rake, all other sizes had a 73 degree head angle with 35mm. fork rake. This ensured the same handling characteristics for all sizes.

Above: A small 19" (49 cm.) frame, built in England in 1977. the differance in seat and head tube angles can clearly be seen. However, for a rider of small stature the riding position is more balanced than it would be if the frame were built with a shallower seat angle and a longer top tube.  

The seat angle varied from 76 degrees for the smallest frames, gradually decreasing, 75, 74, to 73 degrees for the largest sizes. This was often a hard sell to a market that had always heard 73 degree seat angles.  

What I had, (And still have today.) is a “Niche” following. I gradually built a network of bike dealers, who once they, or their employees had ridden my bikes, they were sold. It was then an easy sale to their customers, because they truly believed in the product. The proof can also be seen in the number of “Original” owners on my Registry website.

Will my ideas ever become “Mainstream.” I very much doubt it. Frames today either pop out of a mold, or they are welded steel or aluminum. There are no restrictions what-so-ever on angles or tube lengths, but most stick to the tried and safe 73/73. Any slight variation on this I feel is not done to improve handling or ride qualities, but rather to keep a balanced look throughout the range of sizes.  

Large corporations have to sell a lot of product to survive, and you can’t sell a lot of product in a “Niche” market.

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In search of the perfect fork blade

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 grove 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.

If I were building a touring frame, and wanted a flexible fork for a more comfortable ride, 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, 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 a John Howard

On a Fuso

On a Recherché

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 to the required length. You will notice on the 753 Fuso Lux frame (Pictured below.) that the fork bend is a different shape than the ones bent by me.

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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