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Dictionary of wheel terms


Aero wheel An aero wheel is designed to be aerodynamically efficient, e.g. low drag, in the air conditions it will be ridden in.  This is not to be confused with wheels made with deep rim sections which are rarely aerodynamic or low drag.  Deep rim section wheels never create lift or positive drive forces.
Chord ratio  
Drag See low drag
Drag coefficient  
Fluid dynamics  
Frontal area  
Low Drag A primary goal of any wheel design is to lower drag.  Drag is the largest source of energy loss in a wheel and on a bicycle.  Drag is the resisting force on the body or parts or of a body moving through a medium like air.  The more efficient the shape, the smaller the size and the fewer count of body parts the lower the drag force.  Drag is generally desribed by section (see frontal area), by shape (or drag coefficient), and by medium (see Reynolds number).  Typically drag is expressed by the multiplication of frontal area times drag coefficient.  

A 20% increase in frontal section (the size of the hole punched in the wind) is similar to a 20% loss in shape efficiency.   Contrary to common perceptions, deep section rims that improve shape efficiency by lengthening the chord ratio and improving the drag coefficient by 5% but which add 20% to the rim width (or frontal area) are a detrimental net increase in drag.  This negative effect is without even considering the detrimental losses from variable wind direction and the increased apparent frontal area.  Also see air density, fluid dynamics, and viscosity.
Variable wind direction  


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