Vent Holes

I am sometimes asked, why are there tiny holes drilled in certain parts of a bicycle frame, like the ones shown on the left?

These are vent holes. During the brazing process the air inside the tube expands as it is heated. The vent hole allows the air to escape as it warms up and allows for air to enter as it cools.

If the tube is totally enclosed, on cooling the air contracts sucking the molten brass inside the tube leaving a pin hole that is almost impossible to fill.

Worse still pressure can build up in an un-vented tube and hot brass can blow back in your face. Anyone not knowing this will soon learn the importance of vent holes after picking little globules of brass embedded in their face or finds little brass balls hanging like tiny Christmas Tree decorations from eyebrows, mustache, or other facial hair

Vent holes are only needed when a tube is closed both ends like the example shown above. The top tube is closed at both ends and is usually vented with holes into the seat tube and head tube. (When the bike is assembled these holes are hidden.)

Seatstays are enclosed with a fork dropout one end and the seatstay cap at the top. The front fork blades are also  enclosed both ends between the fork crown and the fork tip or dropout.

Other tubes like the seat tube, down tube and the chainstays are open inside the bottom bracket shell. These tubes are not totally enclosed so do not need any additional vent holes; neither does the brake bridge because it has a brake bolt hole.

On some of my custom frames you won't see holes in the chainstay bridge like the one in the picture. They are hidden inside the bridge tube by drilling holes sideways through the left and right chainstay tube, before the bridge tube was put into place. Only one hole is needed for venting but often two holes are drilled for better drainage of moisture later.

The vent hole in the seatstays on my frames is on the inside up near the seat lug. You might have to turn the bike upside down to see it.

On my front fork blades I drilled one vent hole in each fork blade near the bottom, but after the fork was fully brazed and had cooled I went back and filled it by brazing a piece of wire in the hole. The heat generated in doing this was so small and the air space inside the fork blade was big enough that it did not cause a problem. This whole process only took a minute to complete.

I did this for two reasons. Front fork blades are highly stressed, so filling the hole eliminated a potential weakness at that point. Also rust needs oxygen, and with the fork blades completely enclosed and airtight, no corrosion inside is possible, even years down the road.

A small and probably unnecessary precaution, but one that took such a small amount of time, I always figured, why not?

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Selling the Benefit

Go to any seminar, or read a book on selling, (Or marketing as people prefer to call It.) and you will learn that you always “Sell the Benefit” to the consumer.

In other words, “How will the consumer’s life be made better” if he buys whatever it is you are selling. In the case of a bicycle, how will it improve his performance?

One can build or manufacture just about anything then put up some wonderful sounding argument stating why it is of benefit to the user. Most of these statements cannot be proved or disproved.

Even when these theories are disproved, nobody really cares least of all the company who has made a lot of money, and everyone just moves on to whatever the next trend is.

In the late 1960s Cinelli built a frame that was absolutely devoid of all brazed-on fittings, stating that braze-ons weakened the frame. Gear levers, cable guides, etc. all had to be clamped on to the frame. (Picture top left.)

Some years later people realized that the clamps held moisture and started rust spots, and the clamps sometimes caused stress risers and tubes often broke adjacent to the clamp.

For a while every other framebuilder followed suit, because it saved a tremendous amount of time. (Which was of course the real reason.) Cinelli had stumbled on an incredibly simple way to cut labor costs, then actually sold the idea to the consumer as a benefit.

At the time Cinelli charged double what anyone else did for a frame. The psychology was, it costs more, and therefore it must be better. Also, if it costs more you win the one-upmanship game. A psychology that is still being played out in today’s high-end bicycle market.

Weight saving is always an easy sale to the bicycle enthusiast. Push weight saving to its limits and in the case of a frame, it becomes flexible. Then you sell the idea that a flexible frame is an actual benefit to the rider. The big question here is, “How much flex?” Aluminum for example makes a very strong and lightweight frame. However, it has little or no flexing qualities.

Back when I built frames, they were made by brazing a high tensile steel tube into a lugged joint. In the case of Columbus, the tubes were heat treated and were like a very strong steel spring. When the framebuilder heated the tubes to braze the joint it actually softened the tubes, thereby losing a tiny amount of the strength, and spring qualities.

Remember Cinelli’s argument that braze-ons weakened the frame. Actually there was a grain of truth in that statement. However, brazing the lugged joint and attaching braze-ons is part of the frame building process. The tubes are actually designed to withstand losing some of the strength during the building process. Brazed correctly, the end product is still far stronger than it need be.

This is why steel tubes are butted, (Greater wall thickness at each end.) so there is still adequate strength left after the joint is made. The trick is to use just enough heat to get the job done, but not heating the tube a greater distance from the lug or braze-on than necessary, thus retaining as much of the tube’s inherent strength as possible.

Because a frame is like a very stiff steel spring, when the rider makes a sudden effort as when he jumps in a sprint, the frame gives or flexes slightly. This is desirable, but the operative word here is “Slightly." It is like the difference between an athlete jumping from a concrete track or floor, and one jumping from a Tartan track surface or a floor made from wooden boards.

There is an old Briticism, (A saying from the UK.) that “Bull shit baffles brains.” So whenever you are reading the sales pitch for the latest and greatest high tech wonder. (Not just bicycles, but any consumer product.) Keep an open mind.

They are selling the benefit. Your life will somehow be better for owning this product. Turn that idea around and ask, “What is the benefit to the manufacturer?” Is this product really better than the old one, or has the manufacturer found a cheaper way to make it?

Or has the manufacturer simply come up with something "New and Improved," that serves no real purpose other than to make the old one obsolete.

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Three bike maintenance hacks

Here’s one you can use at home or even on the road should a tumble put your handlebar stem out of alignment. The only tool you will need is an Allen wrench to loosen and re-tighten the stem.

Turn your front wheel at an angle, then closing one eye and sighting from above, sight the front and rear of the stem to center on the front tire. (See picture above.)

Stand straight and with one hand on the nose of the saddle, and the other on handlebars. Lean the bike towards you, rather than try to position your head above the bike.

In other words, move the bike to line up with your eye, rather than the other way around. Make sure the brake cables are tucked away under the stem, so they don’t interfere with your line of vision. (See picture)

Do you have trouble in centering side-pull brakes? Here’s a simple little trick that I have always used.

First, tighten the brake. Don’t worry too much about centering the brake pads at this stage.

With a flat punch, (An old bolt or ¼ inch socket extension works well.) and a small hammer, tap on the top of the brake spring as shown below.

Tap on the right or left side, moving the pads in the direction they need to go to center.

What you are doing, is not bending the spring, a light tap with a hammer will not do any harm. By tapping on the spring, you nudge the brake bolt into the center position without loosening it. 

Make sure your brake cable housing is not too long, or too short, or it will constantly push or pull your brake pads off center. 

 

If you need to remove the Power Link that joins your chain. (Right.) and don’t have the specific tool for the job, you can manage quite well using two thin screwdrivers, placed one on either side of the Power Link.

Hold the screwdrivers with one hand and then squeeze them together using a pair of pliers. (See Picture below. Left.)

This will force the two halves of the link towards each other and the two sides fall apart.

No tools are needed to re-join the Power Link. Making sure the link is in the top part of the chain, above the sprockets and chainwheel. Place the pins through the wide part of the slot.

Apply downward (Forward.) pressure on the right-hand pedal to hold the link in place. Then apply the rear brake and push down hard on the pedal with your foot, and the link will snap into place.

Check to make sure the side plates on both sides of the link are in the groves machined into the pins. (See picture above right.)

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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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