Quote:
Originally Posted by Grant-53
It may seem odd but based on the bowling balls I might try sanding the leading edge of my spokes. Any suggestions on the grit number that would be best?
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I found some info in my fluids text.It's quirky!
Spokes very in drag coefficient,from Cd 6.0,to Cd 0.30,depending on Reynolds number.
The drag minimum occurs when the Rn is at least 1,500,000.
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I measured the spokes in my mountain bike,3/32nds-inch.
If you plug this into the standard Reynolds number formula and solve for critical velocity,it's beyond the speed of light,given a smooth spoke.
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Looking at artificial roughness,and critical roughness we're given:
Critical Roughness (Epsilon) = 15 X (kinematic viscosity/ [Square root of the Shear Stress / air density (rho)].
the equation is simplified to :
Epsilon = 26 X [( kinematic viscosity/ Velocity) X Reynolds number raised to the 1/4th power ].
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If we plug in Rn= 1,500,000 and Velocity =, say 15-mph (22 ft/sec) and solve for Epsilon,we get 0.0005-inch as the critical roughness.****************
I'd sure like some independent verification on this!
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I went into my Technical Aerodynamics text and pulled out the following:
*A 0.375" round spoke has 0.0054 pounds drag per foot at 15-mph
*A 4:1 elliptical spoke of 0.375" has 0.00414 pounds drag per foot at 15-mph
*A 4:1 streamline spoke of 0.375" has 0.00108 pounds drag/ft at 15-mph
The streamlined spoke has 80% less drag than the round spoke.
Seems like the spokes should be smooth and streamlined.And minimum in number.
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In a calm,the spokes would be in train behind the tire/rim,embedded within the rims turbulence.
In a crosswind,the air would be hitting the spokes at a spectra of angles,complicating streamlining.
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I would be tempted to fully-enclose the wheels as with a solar racer,and create shutters which were spring loaded to open instantaneously in dangerous crosswinds and forget about trying to streamline the wheel assembly itself altogether.