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Friday
Aug182006

Shimmy Re-Visited


Speed wobbles or shimmy occur on bicycles and motorcycles because the front wheel is free to turn about its steering axis and at the same time the whole bike can move from side to side along a horizontal axis with the pivot point being the front and rear wheels being in contact with the road. Think of the motion of climbing a hill out of the saddle and you are swaying the bike from side to side.

So if the bike is swaying from side to side and the front wheel is turning from left to right at the same time the front wheel is in Nutation. Rotation is an object spinning around a fixed axis; Nutation means the axis (or axel in the case of a wheel) is also moving as the object is spinning. Think of an orbiting planet; the Earth spins but its axis also moves as it orbits the Sun

To demonstrate to motion of a front wheel in a shimmy hold a bicycle wheel by the axel in your outstretched hands (not spinning) and move your hands in the motion of pedaling a miniature bike. If the wheel was spinning while you were doing this the wheel would be in nutation. If you spin the wheel you will notice that the axel is difficult to move because the gyroscopic action of the spinning wheel is preventing nutation.

Now get someone to tap the side of the wheel as it is spinning; it will nutate (wobble) briefly but quickly return to spinning straight as the gyroscopic action dampens the nutation. So nutation is a constant and natural occurrence as a bicycle is being ridden caused by the movement of the rider pedaling the bike, side winds, bumps in the road, etc.

We do not normally notice this because the nutation is constantly dampened out, by gyroscopic action; the bike’s trail which provides a caster action keeping the front wheel straight, and the damping effect of the rider’s hands on the handlebars. However at a critical bike speed, the front wheel nutation frequency matches the bike + rider natural frequency amplifying or sustaining the nutation. And you have shimmy.

During a high speed shimmy the front wheel is not just fluttering back and forth about its steering axis but is also moving side to side in the horizontal plane shaking the head tube violently from side to side; the rider’s weight on the saddle provides an anchor point, the rear wheel on the road provides another making a pivot point for the front end of the bike to move from. Adding a pannier or saddle bag behind this pivot point will increase the likelihood of shimmy and cause the bike to shimmy at lower speeds because it gives an added sling-shot effect, especially if the load is loose and free to move.

It is a well know fact that tall riders on large frames are more likely to experience shimmy. I believe this is because the seat tube slopes backward and as the frame gets taller and the rider’s weight is more directly over the center of the rear wheel. This provides a near vertical pivot line between the riders mass on the saddle and the rear wheel on the road for the bike to shake and weave. With a smaller frame the rider’s weight is more forward with a less than vertical pivot line, making it less prone to shimmy.

Pressing your knee against the top tube will often stop a shimmy; in doing so you have dampened the shaking top tube through the muscles and tissues in you leg without actually connecting the leg to the top tube. This is also a clue that the rider needs to be holding the handlebars lightly so that you are damping the nutation rather than being connected to it by grasping the handlebars tightly.

In extreme cases if the rider is gripping the handlebars tightly the body starts to shake along with the head tube and handlebars. It becomes difficult to loosen your grip on the bars with your body shaking violently and because the shaking mass now includes your body it is much larger, higher and more fluid making the situation much worse and a crash may ensue.

This is more likely to happen with motorcycles and it has often been observed that a rider will be thrown from the bike, the bike will then stop shaking (because there is no longer a rider in harmony with the machine’s vibrations) and the bike with continue on for a while on its own before it hits something or looses momentum and falls.

An article on Wikipedia stated that frame flex has nothing to do with shimmy, but I am not so sure. Frame flex may not be the cause of shimmy but I believe it can sustain it. If the seat is not moving because the rider’s mass provides an anchor. And the rear wheel is not moving sideways at its point of contact with the road; but at the same time the head tube is shaking side to side, something has to be flexing and twisting, either the frame or the rear wheel.

Frames I built do not shimmy as a rule; so what did I do different? My bikes had a little more trail so possibly the damping effect of the extra trail helped. But I believe another factor is that all my California built frames had Columbus SP (heavier gauge) chainstays, making the rear triangle much stiffer and less likely to flex.

There could possibly be flex in the rear wheel if you consider that the upper spokes are under tension and the lower spokes are not; making the wheel likely to flex at the bottom. Also a dished wheel has unequal tension on the drive and non-drive sides. So if you have a bike that is prone to shimmy maybe think about switching to a stronger more tightly built wheels.

Most high speed shimmies occur while coasting down hill so here are a few things you can do to avoid this phenomenon. Lifting your weight from the saddle without actually standing up will transfer your weight to the pedals which are a much lower and more forward point of contact. Keep one pedal down and most of your weight on that pedal; you can switch pedals as you corner keeping the lower pedal on the outside. This will make the point of contact between you and the bike very low, but also off center of the frame.

Keep your knee lightly against the top tube as I have already mentioned, and hold the bars lightly. Many riders report that a shimmy it gets worse when applying the brakes. Well of course when you squeeze the brakes you automatically grip the handlebars tighter. So practice applying the brakes while loosely holding the bars.

Try switching your hands to the brake hoods and applying the brakes over the top with your fingers. It may take some nerve to do this if your bike is already shaking, but letting go of the bars for a split second may bring you out of the shimmy. Remember it is the connection between you and the bike that is causing the shimmy and the more connected you are the worse it will get.

Sunday
Aug132006

Clueless


Some people selling stuff on ebay do not have a clue. Take this Fuso pictured here; listed as a “one of a kind” custom Fuso. I built just under 3,000 from 1984 to 1993 so hardly one of a kind.

Model year 2000: I retired in 1993.

Size: XL. I’m sorry the seller may be clueless but the people buying my frames are not. Extra Large will not cut it; it’s not a tee shirt. Looks like at least a 66 cm.

Frame Material: Carbon Fiber. When it is obviously lugged steel; I doubt there is one single piece of carbon fiber on the whole bike.

Starting Price: $2,300. I don’t think so. Even my custom ‘dave moulton’ bikes of which I built only 219 in California, do not command even near that yet. The going rate on ebay for a Fuso in nice condition is from $500 to $800. The higher price would be for a Lux model. That is for a complete bike with nice components; frames in good condition (Not needing a repaint.) go for around $240 to $340.

I can understand someone selling a bike and knowing nothing about it, but have they never heard of Google? Type in “Fuso Bicycle” or “Dave Moulton” and you will find my website with all the information you need to know.

If you are selling one of my frames or a complete bike; take lots of pictures. Take close-up pictures of the lugs and seat cluster. Takes pictures of any damage to the paint, etc. Turn the bike over and take a picture of the underside of the bottom bracket shell showing the serial number and the frame size stamped there.

Take at least one good side on shot from the drive side of the bike. But don’t stand above the bike or frame and take the picture from a high angle, or the camera will distort the frame geometry. The head angle will appear steeper than it is and the seat angle shallower.

If the frame is a Fuso Lux for example; take a picture of the Fuso Lux decal on the top tube, because people will ask you, “Is it really a Lux.” Put a link to the "Bicycle" section on my website if you wish. (www.ProdigalChild.net/Bicycle.htm)

When one of my bikes is sold on eBay there is no financial interest in it for me; but I am still interested. These are my “children” and I care whether they have a good home, or if they are being treated right, and are loved.

Friday
Aug112006

Utter Rubbish


My Blog of July 21st on stem length has been shot down, debunked, dismissed as “Utter Rubbish.”

I failed to take into account the Competitive Fit, the Eddy Fit, the French Fit, and also the little known and yet to be accepted, Hissy Fit. I also realized that rude people may have a longer middle finger than most throwing the whole formula out of whack.

If you rushed out and bought a new handlebar stem on the strength of my Blog, I apologize. But look at it this way; your local bike store needs the business.

Let’s face it; every time a bike store closes its doors people become homeless. Forced to get out and actually ride their bikes instead of being able to stand around at the weekends in a friendly, climate controlled atmosphere, and BS about bikes.

This elbow against the tip of the saddle trick is just an “Old wife’s tale” and should be totally ignored. I was simply pointing out that there is often a grain of truth to most folk lore and that the length of forearm is relevant to foot length and therefore to the rest of the body.

Most old wives will also verify that length of the foot is relevant to penis size; which makes perfect sense when you consider that long feet prevent well endowed men from falling on their face.

Monday
Jul312006

Tantrums


Tantrums: That was the nick name I gave to tandems; building them was for me a love/hate thing. The problem building a tandem is in the fact that it is more than twice the work of building two single frames, but they do not command the price of two singles. So I knew I was screwed financially every time someone talked me into building one. I would cuss and swear all through the building of it (Hence tantrums.) and would promise myself I would never build another.

I will say that once the tandem frame was built I did get double the satisfaction in seeing the finished product. Time would pass and someone else would come along and ask me to build them a tandem frame; I would say “No” they would whine and beg and eventually I would give in, and the whole process would start over.

Working on a single frame whether it be brazing it or filing the lug work after; you simply hold the frame in one hand, place a wooden block around a tube and clamp it in the vise with the other hand.

With a tandem frame it is so big, heavy and unwieldy that it takes two hands to hold it; you rest one end of it on something while you let go with one hand to put the wooden block in place. Next you carry it to the vise and try to tighten it using your knee. Invariably the wooden block falls off during the process and the result is more tantrums.



There is one tandem frame that I am particularly proud of and may have actually enjoyed building it. It is the track tandem pictured here and at the top of the page and it was ridden in the 1978 World Championships by Roy Swinnerton and Trevor Gadd representing Great Brittain. Seeing two very powerful young riders get up to speeds of 55 mph. on a banked oval track is both satisfying and at the same time very frightening.

I built a few tandems when I first moved to the US in 1979 and I worked for Paris Sport; here is one of them.


Later I went to work for Masi in Southern California and in 1982 started my own business there. On doing so I promised myself I would never build another tandem.

I was asked several times but I always said “You can’t offer me enough money to get me to build one.” And no one ever offered enough that I was even close to being tempted.

Friday
Jul212006

What does the length of your forearm and the price of fish have to do with stem length?


This method of determining handlebar stem length has been around for ever. My cycling experience dates back over 55 years and it was practiced then; actually it is not a bad guide and works for most people. Place your elbow against the nose of your saddle and if your finger tips do not fit behind the handle bars as shown above, then your stem is probably too short. If the bars are more than 2cm. away from the finger tips your stem maybe too long.

When I was racing I used a stem that placed my finger tips one centimeter from the bars; now as a mild concession to my aging body I’m using a stem a centimeter shorter. If you are wondering as I did for many years what the length of a persons forearm has to do with stem length? I will explain.

When I am determining frame size I take into account three body measurements:
1. Inside leg length (Often referred to as inseam.) measured crotch to floor without shoes.
2. Overall height.
3. Shoe size. (Length of foot.)
I do not require body length because I have overall height minus inseam. I do not require arm length because this is relevant to leg length and foot length combined.

Human bodies although all different do generally follow certain rules of nature. We have the same basic design and structure as most other animals on this planet except we walk on our hind legs while most others walk on all four. So it follows a person with long legs will also have long arms; short legs, short arms.

Four legged animals generally walk on their toes (and finger tips) whereas we stand and walk on our heels. So some people have a long body, but short legs and it is not unusual for a person with this build to have longer feet, and also longer arms. The long arms are not out of proportion if you consider the leg length is a combination of inseam plus the length of foot.

When pedaling a bicycle the toe is pointing downward at the bottom of the pedal stroke so the foot becomes an extension of the leg; which is why it has to be taken into consideration when determining frame size. The person with short legs, long feet needs a larger frame than their inseam alone would suggest. The larger frame with its proportionally longer top tube will also accommodate this rider’s longer body and arms

The length of the forearm is proportionate in length to the length of the foot; take one of your shoes and hold it against your forearm and you will see it is the same length as the distance from your elbow to your wrist. In other words the big bones in your forearm are the same length as your foot.

So assuming you are on the right size frame and your seat is set at the correct height; then chances are if you have very long feet then you will have a short inseam and a long body.

Because you have long feet you also have a long forearm and if you do this little elbow against the saddle trick it will show you need a long handlebar stem which will be right for your long body and arms.

A person with very long legs for their height will also have long arms but will have a short body and small feet relative to their height. Small feet mean short forearm and a shorter stem which will be right for their short body. Because this rider has long legs his saddle will be set high making a greater distance from the seat to the bars; this will accommodate his long arms.

There is another method for determining stem length which states: “A rider seated with their hands on the drops of the bars, will have the front hub obscured from view by the handlebars.”

This works in the same way; longer body calls for a longer stem and vice-versa. The only thing is that this method could be affected by the head angle of the frame and the length of fork rake. I prefer the length of forearm method because it is simpler. It works for most people but there is a small percentage that it will not. I always say if you are comfortable and happy with your current position, don’t change it. Go by the old “If it ain’t broke” adage.

And what does this all have to do with the price of fish? Nothing at all, but it got your attention.