Spring Chicken

Chicken-ManBy The Metric Maven

Bulldog Edition

It is said that in 1960 Richard Leghorn coined the phrase “information age.” He founded a company that manufactured spy cameras and later worked at the Pentagon. The phrase “information explosion” was also in vogue at the time. In my view there has also been a “non-information explosion” depending on if one is concerned about the veracity of information presented. Klystron sent me a link to an online article where an automotive writer discusses the different types of compression springs one can use in car suspension. The article introduces the reader to “spring rate” which is proportional to the stiffness of a spring:

In simple terms, a spring’s rate is the amount of weight required to compress itself a single inch. It’s a universal measurement, it applies to everything from lowering springs to valve springs, and it’ll look something like this: 500 lbs/in. The bigger the number, the stiffer the spring.

This took me back to my introductory physics class in college where I was introduced to Hooke’s Law. In 1678 Robert Hooke (1635-1703) offered a simple linear mathematical equation that relates the force produced by a spring in terms of its extension or compression (depending on the type of spring). The equation is simple: F = kX. The letter F stands for the force the spring produces, X is the distance you have compressed or stretched the spring. The value k is a number that converts the distance the spring has been compressed or stretched to the amount of force it produces. The value k is called the spring constant, and it is the same as the “spring rate”  offered by the automotive writer. In this case k is in pounds per inch or lbs/in. Indeed, the larger the spring constant k, the stiffer the spring. As I point out in my essay, The Count Only Counts—He Does Not Measure, this relationship was used to produce the first spring mass gauges. Springs often obey this relationship only over a given displacement range, but we will ignore that here and assume we are within the linear range.

The author then points out that the rest of the world is metric and converts the spring constant (rate) over to metric for his readers:


Kilograms are not a force, and so Kg/mm when multiplied by a displacement distance in millimeters produces a mass value and not a force. This is very poor dimensional analysis on the part of this professional automotive writer. When one stands on a bathroom scale in the US, the readout is in pounds of force, but if one flips a switch to metric it instead offers mass in Kilograms. If the scale had a metric readout of force, the value would be in Newtons. If you have a mass of 75 Kg, then your metric weight would be 735 newtons, which is a force value.

A 500 lb/inch spring constant properly converted to metric would instead be 87.8 newtons/mm.

While springs appear rather prosaic they are used ubiquitously in our modern world. Their benefits are enthusiastically portrayed in this 1940s film about the benefits of springs.

Metric springs in the US apparently use non-SI for a spring constant:


click to enlarge

The 60 mm inner diameter spring in the top line of the table above has a metric “spring rate” of 18 kgf/mm or 18 kilogram-force per millimeter. Kilogram force has never been a part of the metric system and is not accepted for use with the modern metric system. A “kilogram-force” is 9.806 newtons, so the spring constant when actually converted to metric is 9.806*18 = 176.5 newtons/mm.

We are a country that thinks it is technologically unmatched, yet everyday I see that most professions never think quantitatively or technically.

Spring-ColorsSome years back, one of the tension springs on my garage door snapped making it inoperable. The previous owner had taped a garage repair business card to the wall and I called the number. The fellow who showed up was friendly and had a large number of springs in his truck. He took one look and checked his truck to see if he had a replacement. The technician looked up from his pickup-bed and asked “is the color white or blue?” It was then that I realized the spring had a section along the middle painted white. He returned with a set of blue and a set of white springs, one of which had paint on one end. The workman indicated that both garage springs needed to be replaced so they would have the same “strength.” This made sense. He took out the broken spring and then the intact one, which he then put over a hook on the back of his truck and pulled. He next pulled on a new blue one, and then a new white one.

I asked why he was doing two colors. “They’re all different” he said, “the colors are meaningless. Every manufacturing company is different—I use feeling.”  I immediately suspected this was not a good idea. The interpretation of force (weight) on an object by humans is logarithmic. It struck me that it would be possible to create a device that would measure the spring constant of each spring so there would be no guessing. When I asked if such a device existed, the technician asserted he did not need it. His human measurement perception indicated white was needed as I recall. He put them into the garage door and after opening and closing it a time or two decided the blue spring was probably better. He installed the blue springs and then pronounced them the best. Indeed, my garage door has been fine over the last few years and works well.

It bothers me that people who support what is left of our infrastructure in the US seem so out of tune with the quantitative aspects of it. It would make a lot of sense to me that if one needs a pair of springs with the same spring constant for each side of a garage door that measuring this value would make sure the springs are the same. At the next level, those who write articles to inform the public are often no better. I see this as part of a cultural problem that promotes an anti-intellectual view in the US. The lack of the metric system appears to be but a symptom of this larger problem.


The Metric Maven has published a new book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.


Potemkin Design

1024px-1928_Model_A_FordBy The Metric Maven

Bulldog Edition

One Autumn afternoon I was driving along back roads in the Rockies with my friend Thern. I had been discussing some aspect of metric and he posed a question: “The Model A had only one metric part on it, what is it?” I was floored there was any metric part on Ford’s Model A. I thought about it a while, gave up, and then asked him to reveal the answer. His reply was that it was the spark plugs. They had 18 mm threads. Thern possessed a vintage Ford Model A repair manual and had taken note of this fact. The reason for metric threads? The Europeans were way ahead of the US in the design of spark plugs. Belgian engineer Étienne Lenoir is credited with the invention of the spark plug in 1860. I looked on some Model A forums, and sure enough, there are discussion threads about why the spark plug threads are metric, and asking if they should be reworked so that “standard” spark plugs can be introduced, and such.

I was quite surprised there was any metric part on a Model A. It is the iconic American automobile. When the first Model A was introduced in 1903 John Shafroth was still making his bid to make the US a metric country. It is reasonable to state that well over 99% of the parts which comprise a Model A are Ye Olde English. Today over 99% of the parts in a US made car are metric. Only the bolt heads on the battery post clamps are not metric (those that are used for the bracket to hold the battery in place are metric). Thern tells me that on modern cars (newer than mine) the battery clamps are now generally metric. Pat Naughtin in his first newsletter (Metrication matters – Number 1 – 2003-06-10) mentioned the fact that people in the US do not realize they are driving metric cars, because that fact is not on display:

I wonder why the USA is the last nation in the world to admit the extent to which they use the metric system of measurements. For example, of the 10 000 parts in a modern car, made in the USA, all of them are measured in millimetres to the nearest tenth of a millimetre. But because the speedometer is labelled with the letters ‘mph’, drivers in the USA are generally convinced that they are driving an ‘English Units’ automobile.

Our road signs are all in medieval units, and so there is nothing that would directly indicate that American cars are well over 99% metric.

In an earlier essay I have examined the intellectual grip that familiarity has over simplicity or change. In another I point out that for over 3000 years the design of a bee hive did not change until the 19th century. In the US, innovation has often been associated with a decrease in profit. J.P. Morgan is reported to have said after he had consolidated the majority of electrical providers into General Electric: “Research is unnecessary if you have no competition.” A changeover to the metric system is simply an unnecessary business expense when viewed this way.

In recent years I’ve been exposed to engineering designs at a number of firms. When they use metric, it is often with millimeters, and nothing else metric. I’ve seen more than one situation where a customer is from outside the US, and appears to assume that because the drawings are in millimeters, that the design is metric. In one situation I unintentionally created a brouhaha when I asked if the fasteners were all metric. It was clear I had unknowingly asked a politically incorrect question. They were not. The customer was not happy as they thought the design was completely metric.

What I have noticed is that companies which state they use metric are often creating Potemkin designs. As long as millimeters and meters are invoked the design is thought to be metric, but in fact the rest of the design is left in Ye Olde English. I’ve seen situations where medieval measure screws are used, and in an attempt to “accommodate” metric fasteners, a Ye Olde English screw size that is as close as possible to a metric one is chosen. No newtons per square meter or liters or grams make an appearance, only millimeters do. It is hard to know what motivates this Olde English death grip, it could be some cultural identification, or simply a resistance to change. The use of metric and Ye Olde English in a design is designated as a PigFish design in this blog.  Rather than become metric, NASA  made the “International” Space Station a combination of medieval and metric. I discuss this in my essay When PigFish Fly.

Whatever the motivation, it demonstrates that left to their own devices US business, education, and all other aspects of our everyday lives will remain as they are—Ye Olde English. Resistance to a government mandate for metrication, is simply equivalent to promoting the continuation of our archaic measures and design methods. As each year goes by without a metric mandate in the US, we become more anachronistic as a nation and as a culture. The only thing Americans seem to be good at these days is talking, and when the subject is the metric system, it is seldom in a positive manner.


The Metric Maven has published a new book titled The Dimensions of The Cosmos. It examines the basic quantities of the world from yocto to Yotta with a mixture of scientific anecdotes and may be purchased here.