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Bicycle Spoke Technology

Bicycle Spoke Technology

Spoked wheels have been around for millenia, and have reached a high degree of refinement in bicycle wheels.  Bicycle wheels must operate for years with no maintanance, support hundreds of pounds of weight, and be as light as possible.  There are a number of possible spoke configurations, but the one that is used on most bikes is the “triple cross” tangential pattern.  If you look at bike wheels, the spokes are attached to the hub tangentially, and each spoke crosses another spoke three times, hence the “triple cross”.  This tangential spoke pattern was invented by renowned bike designer James Starley, and was patented in England in 1874.

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When properly made, each spoke has a certain amount of tension on it when unweighted, pulling the hub towards the rim with a force of about 50 pounds per spoke.  When the wheel is weighted, by weight pressing down on the axle, the hub tries to move toward the ground.  When it is does this, the three spokes directly below the hub become unweighted, and the rim deflects a small amount, about .001 inches.  That is about the thickness of a piece of paper.  The tire around the rim deflects a good deal more than that.  Although the three spokes under the hub are unstessed, the other remaining spokes pull equally on the hub, and prevent it from moving in relation to the rim.  As the wheel is weighted and rolled, the rim is thus constantly flexing at the spot under the hub, and the spokes are constantly being unstressed and stressed as they come under the hub.  Below is an exaggerated view of the wheel under weight.

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These spokes appear to cross more than three other spokes because the spokes on both sides of the wheel are shown.  Each spoke only touches and crosses three spokes on its side of the wheel.  When a rider hits a hard object, such as railroad tracks or a curb, the spokes most likely to break are the spokes that are above the hub and closest to vertical, and they are most likely to break where the spoke head enters the rim hole.  The hub basically shears them off the head of the spoke.

This information has almost no practical value, but isn’t it fascinating! Of course some sites go into the physics of things like spoke strain and hub deflection, such as  Held by Downward Force by John Forester, and Henry P. Gavin’s paper on Bicycle Wheel Spoke Patterns and Spoke Fatigue.


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