Bicycle Rain Cape

Function and Wind Resistance Notes

 
How much wind drag does wearing this rain cape create?   
Answer: about two extra teaspoons of cream in my coffee per hour.
biking with a rain cape
Rain Cape Pros:
For commuter bicycling in rainy weather, I find a rain cape combined with having full wrap fenders is very practical.  The cape does not become a sweat box like rain coat or rain clothing becomes.  Under the rain cape, I can wear normal clothing and mostly keep my feet and pant legs dry and clean.  The rain cape is easy to take on and off, so that I can just remove the cape at a store or at work and be ready for normal pedestrian activity.  The rain cape can also be practical, if awkward, for just walking in the rain.   The cape is loose enough that I can move an arm out for signaling my turning intentions.

This bright yellow looks eye catching and I like having bright clothing and reflective clothing at night for bike riding.  Even with very bright clothing I have had numerous occasions when I have stopped at a 4 way stop and the cross traffic auto drivers still do not see me (especially people making the very common rolling right turn).  I have no doubt that, even though I stopped and waited my proper turn at the 4 way stop and took a position in the center of the lane so I was more visible, for those drivers who did not see me, they must think that I just leapt out madly in front of them and they probably wish I would follow the rules of the road.  

Rain Cape Cons:
I find that the cape somewhat restricts my ability to look backward over my shoulder if I am holding the frabic streatched tight.  
Also, when going over 20 mph down a steep hill, I have felt some vibration turbulence that made the bike feel less steady than without the cape, probably because the way I hold the cape over the handle bars will transmit wind turbulence directly to the steering.
 
In order to estimate the wind drag of this rain cape, I performed a coasting test measuring average speed and maximum speed coasting down a 3.6 % paved slope on a day with no wind, slight rain, temperature of 47 degrees F, and humidity 90%.  I coasted the same section once with the rain cape as shown and once with fleece jacket, moderately snug fitting.

Using a power calculating spread sheet I found on the web years ago (based on Bicycling Science by David Gorden Wilson), I could enter the data and adjust the coefficient of aerodynamic drag parameter to match the two results so that for the known slope the calculation gave a zero effort  power input for the known speeds.  In the case of the performance without the rain cape I estimated a coefficient of drag at 1.2, and this projected into a number of 1.4 to 1.5 for the rain cape performance.  This calculation implies an extra energy effort of 43 to 64 kcal per hour is needed to pedal at 12.4 mph (20 kph) with the rain cape on as opposed to a fleece jacket over a sweater all zipped up snug.  These calculations also included the known 3 watt resistance of my shimano hub generator.

I made this cape from goretex bought at a cheap price as a remnant sale.  



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