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Bicycle Wheels vs Wagon Wheels

October 21, 2010
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I was thinking last night about bicycle wheels… probably triggered by an excellent bike ride I did yesterday through the West Hills here in Portland, Oregon.  I was mentally riffing on a metaphor regarding positive thinking, which my girlfriend has been teaching me about.  She tells me that if I think positive thoughts I will draw positive events.  Whereas negative thoughts bring negative results.  It’s a lot like when I was teaching my daughter to ride a bicycle.  She kept running into the parked cars and bushes in the parking lot where we were practicing until I remembered the key to steering a bicycle from my own youth: If you look at what you are trying to avoid, you run right into it.  You have to look at the open spaces where you want to go rather than at the objects you want to avoid.

The frame of this bicycle is being pulled aloft by the spokes and compression of the rim.

I continued thinking about the balance of energy forces that keep us upright on a bicycle, leading me to perhaps the most critical element: the wheel.  Those slender metal spokes in a bicycle wheel operate in tension.   They carry the hub of the wheel through the air.  When on a bike, the rider’s weight causes the spokes above the hub to carry more tension than the ones below.  But all the spokes are in tension at all times.  The rim on the other hand, is in compression.  The circular form resists the inward pull of the spokes and transfers the tension load to the earth.

A heavy wagon wheel with compressively loaded spokes. Image via James Marvin Phelps and Flickr

This is exactly the opposite of how a wooden wagon wheel works.  In  a wagon wheel, the load is carried through compression instead of tension.  The spokes under the hub must be strong enough to support the weight of the load above. Those above the hub carry little or no load whatsoever.  Just a bit in controlling the x & y location of the hub but nothing in the z-axis.  The upper rim is largely unloaded in this instance.  It merely directs the spokes through the correct rotation while each successive portion below the hub is compressed into the ground beneath the loaded spokes.

What is interesting here is how the different materials are used in completely opposite ways within the same apparent structure and goal.   Both wheels are holding their respective vehicles aloft.  Both provide a circular surface on which to move the vehicle.   The selection of material and technique for wheel construction is largely based on availability and weight considerations.  Tensionable steel was not readily available in the wild west.  Wooden spokes are larger, heavier, and create more drag than steel or carbon fiber.

In architecture we tend to utilize heavy, compressive elements more than lighter, tensioned ones.  Buckminster Fuller pointed us in a different direction with geodesic domes and “tensegral” structures.  We see modern extensions of this in the highly engineered structures of Calatrava, suspension bridges, and tent buildings such as done by Arup Engineering.   The green lesson to be learned here is that, materials wise, it may be better to pull a building up rather than pushing it.

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3 Comments leave one →
  1. Peter Stubbs permalink
    October 22, 2010 9:37 am

    Fuller . . . Grr.

    Fuller stole “tensegrity” lock, stock & barrel from Kenneth Snelson and had the gall to patent the Geodesic Dome in the ’50s, thirty years after its invention in Germany. Good P.R. man, good popularizer, solid engineer, defiler of the English language, mediocre designer, terrible architect, thief.

    Not that I have a strong opinion. As for Calatrava, I’m more inclined to see his work as an extension of the work of Eiffel and Maillart, but that’s me.

    Keep in mind, the embodied energy in steel is massive, and wood is a renewable resource. In that sense the bicycle wheel is less green than the wagon wheel, especially if they are built to respond to equivalent forces. Green is rarely clear.

    • October 22, 2010 1:57 pm

      Yes indeed, green is rarely clear (somewhat like precedents!)

      I would say the point is not that one is more sustainable than the other, but that both wagon and bicycle wheels are extremely well optimized for the tasks they fulfill. As a result, both are highly successful. Both are also elegant and beautiful because of the efficiency in their structure and care in craftsmanship. I appreciated how one prof at the UO put it, “Design it well, that will make it sustainable”.

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