We could be riding in this.
New York snow does not melt in one day like Santa Fe snow.
Our friend Sandy, President of Cambridge Valley Cycling, sent the following experience with his classic steel bicycle, as he is thinking about a re-furbish project. With our season about to begin, this is a good time to revisit the interesting bicycle shimmy phenomenon, and to remind us, as Sandy says, it is about the fun.
“…bring back to life 1985 Scapin “custom” San Martino (bike model) from measurements made there (Italy) … on my first fast downhill it went into a death wobble that put the fear of all the gods in me. ..thought I would make it hill climber as a fixie. (single cog on the back with no freewheel, like a track bicycle, pedal forward and pedal backwards) No go said my trusted professional. Too heavy, too much flex So, back to my Felt bicycle with a 36 tooth cog on the back and just suffer more. I do gravel grinders either Cannondale or Trek Single Track. Hey it’s for fun, right!”
Was wondering if your Scapin was Columbus steel similar to the above photo? Umberto Scapin’s company, before it ended in the hands of the typical conglomerate had a pretty good reputation. He kept the company small and I’m pretty sure it was his sons who eventually sold the brand. Starting with the bike boom, 1975, Americans developed a love affair with European crafted cycles, French, British, Italian, Swedish, Belgium. Tremendous demand for small hand-production supply. Eventually all the classic builders succumbed to the big money, Faliero Masi was the last of the classic builders to sell his brand.
When I was learning to build frames I managed a shop and had access to several European brands, usually crashed machines which I disassembled and studied. There really fine crafted miters, worked lugs, and superb brazing, and some not so fine. I even found highly prestigious name bikes that were out of alignment, which in itself is not as critical as it seems. Classic builders did not use frame fixtures or jigs until the bike boom demanded mass production. When the Americans started building frames many Europeans, especially the Italians, scoffed at our over-insistence on exact frame tolerances and building preferences. We were definitely considered the annal retentive builders 🙂 I have a humorous story about the time I called the Cinelli factory trying to order a couple of their fully sloped crown forks that I will share sometime. It is neat that American builders became the world steel frame building leaders in design, innovation, and craftsmanship by the nineties, just as the industry was moving away from steel. Win a few, lose a lot, pop used to say.
And by the way, the bike fitting craze is also a bit overrated, again the American focus on minute scientific detail in all things. Even Ben Serotta, the inventor of the bike fitting machine, recently said we rely too much on the many fitting theories. The human body adapts quite well to a range of bike size measurements. Recently he was again convinced of body adaptation when he and his adult daughter on vacation rented a tandem, not his size, and within a few miles they adjusted and had a great trip.
Anyway, back to the Scapin shimmy. It has to do with physics harmonics. (…an object forced into resonance vibrations at one of its natural frequencies, vibrates in a manner such that a wave pattern is formed within the object) The math I do not understand, but bike wobble I do. The head tube twists right and the top tube twists left… very, very fast. Happened to me once coming down Mt. Lemon in Tucson on a small carbon race frame. The phenomenon has been studied over the years by everybody in the industry. Below is an article by Jobst B., an engineer who wrote the Bicycle Wheel. Shimmy can happen with any of the steel frames, Reynolds, Ishiwata, now Keisei, Falk, Columbus, True Temper, Tange, and surprisingly so called vibration-dampening carbon fiber, much to my chagrin and mortification one day.
And with respect to your trusted professional, who may or may not be a master age rider, your Scapin, if you enjoy the ride characteristics, can and should be re-furbished and adapted into anything you wish, hill climber to gravel bike, single or multi-speed. I won a couple state championships on old era circa 70’s designed flexible steel bikes. I loved the frame whip or snap in a sprint as my leg muscles synched perfectly with the frame flex. At our pedaling age no one needs to care about bike weight unless competing for the big bucks.
Subject: Shimmy or Speed Wobble
From: Jobst Brandt
Date: June 25, 2004, revised February 25, 2005
Shimmy, a spontaneous steering oscillation of the front wheel, usually occurs at a predictable speed when riding no-hands. The likelihood of shimmy is greatest when the only rider-to-bicycle contact is at the saddle and pedals. This position gives the least damping by hands, arms, and legs. When shimmy occurs on descents, with hands on the bars, it is highly disconcerting because the most common rider response, of gripping the bars firmly, only increases it.
Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.
Bicycle shimmy is the lateral oscillation of the head tube about the road contact point of the front wheel and depends largely on frame geometry and the elasticity of the top and down tubes. It is driven by gyroscopic forces of the front wheel, making it largely speed dependent. It cannot be fixed by adjustments because it is inherent to the geometry and elasticity of the bicycle frame. The longer the frame and the higher the saddle, the greater the tendency to shimmy, other things being equal. Weight distribution also has no effect on shimmy although where that weight contacts the frame does. Bicycle shimmy is unchanged when riding no-hands, whether leaning forward or backward.
Shimmy requires a spring and a mass about which to oscillate and these are furnished by the frame and seated rider. Unloading the saddle (without standing up) will stop shimmy. Pedaling or rough road will also reduce the tendency to shimmy. In contrast, coasting no-hands downhill on a smooth road at more than 20mph with the cranks vertical seems to be the most shimmy prone condition.
When coasting no-hands, laying one leg against the top tube is the most common way to inhibit shimmy and also one of the most common ways to coast no-hands. Compliant tread of knobby tires usually have sufficient squirming damping to suppress shimmy. Weight of the handlebar and its extension from of the steering axis also affects shimmy.
Shimmy is caused by the gyroscopic force of the front wheel whose tilt is roughly at right angles to the steering axis, making the wheel steer to the left when it leans to the left. This steering action twists the toptube and downtube, storing energy that both limits travel and causes a return swing. Trail (caster) of the fork acts on the wheel to limit these excursions and return them toward center.
To feel the gyroscopic forces involved in bicycle shimmy, take a front wheel, holding it by its axle in both hands, and give it a spin. Manually steering it from side to side generates strong tilting forces always at right angles to the input. These forces sustain shimmy and are the motions one uses to make quick steering maneuvers while riding no-hands, shifting the hips laterally while firmly seated. The same effect as when wheeling a bicycle while holding it only by the saddle.
Shimmy that concerns riders the most occurs with hands firmly on the bars and it is rider generated by muscular effect whose natural response is the same as the shimmy frequency, about that of Human shivering. Descending in cold weather can be difficult for this reason. The rider’s “death grip” only enhances the incidence of shimmy in this situation. Loosely holding the bars between thumb and forefinger is a way of avoiding shimmy when cold.