Bicycle Aerodynamics

Index

• Results (Table)
• Discussion
• Links to Other Bicycle Aerodynamics Pages

Results

As a first test, I went out on a calm day and climbed Whiskey Hill a bunch of times. I chose Whiskey Hill because it's straight, low traffic, and short enough that I could climb it multiple times. For each run, I'd accelerate to around 30mph, start coasting at the mailbox at 550, and coast until the bottom of the hill. My cyclocomputer recorded the max speed hit.

I carefully calibrated my gradiometer before measuring whiskey hill, and hope that each sample was within 0.5% grade. From the grade, I computed the elevation. This seemed more accurate than my altimeter. Here's the data file, with distance (in miles), grade (in percent), and altitude (in feet):

0.00    1.0%    350.0 ft  650 mailbox
0.01    1.5     350.5
0.02    1.25    351.3
0.03    3.0     352.0
0.04    1.75    353.6
0.05    3.5     354.5
0.06    4.0     356.3
0.07    4.5     358.4
0.08    4.5     360.8
0.09    5.5     363.2
0.10    5.75    366.1
0.11    6.5     369.1
0.12    7.0     372.6
0.13    6.5     376.3
0.14    6.5     379.7
0.15    7.0     383.1
0.16    6.5     386.8
0.17    7.0     390.3
0.18    6.5     394.0
0.19    6.0     397.4
0.20    6.5     400.6   560 mailbox
0.21    5.5     404.0
0.22    5.5     406.9
0.23    4.5     409.8
0.24    3.75    412.2
0.25    2.75    414.2
0.26    1.75    415.6
0.265   2.0     416.1   550 mailbox

I believe my grade measurements to be within about 0.5%, which would make a difference of up to about 8% in the final drag numbers. Winds also affect the drag -- I have to take many samples to average out the effects due to wind.

It's interesting to note that Jim Martin (see links below) lists 8 pounds of drag at 30mph as "typical", and 6 pounds of drag at 30mph as "excellent". Thus, even with aerobars, I'm still little better than what he considers "typical". Perhaps I should remove the rear rack and try it again... :-)

Discussion

Aerobars are (in my opinion) the most effective single thing you can do to improve flatland speed. In my tests, the aerobars reduced drag by about 1.4 pounds at 30mph, compared to a comfortable (non-crouching) position on the drop bars. The next best improvement would be aero wheels, which (according to Jim Martin) can reduce drag by about 0.4 pounds. I'll have to acquire some aero wheels someday, in the interest of science... :-)

Links to Other Aerodynamics Pages

• Jim Martin's analysis of the aerodynamics of cycling
• Cycling Science, Fall 96: Considerations for Data Quality and Uncertainty in Testing of Bicycle Aerodynamics