Oct 15

Oct 15, 2005

Finally some drag coefficient numbers!

Here is a really great video of the CP1 with canopy bubble going around the Glenmore track

I need to know if the drag coefficient of CP1 is good enough to accomplish my goals, and finding out exactly what that is, is much more difficult than you would think.

If I were in outerspace, the smallest amount of input power would eventually propel me to a speed very close to lightspeed. However, back on earth we have some physics we have to deal with. Two forces to overcome; the first and primary is the force required to push the vehicle through our dense air mass (called aerodynamic drag), and the second is the force that wants to stop the wheels from rolling across the ground (called rolling resistance).

During a test a few months ago, I determined what the rolling resistance (Crr) of the tires were on the pavement in front of my house. The test is called a slow speed coast down test and is basically the average of a bunch of out and back coasts over a set distance at slow speeds. The amount that the bike slows down over this set distance can be used to calculate the rolling resistance of the wheels and tires. Using one calculator I have, I got a value of .0038 and using another calculator, I got a value of .0049. A component of this calculation is an estimation of the drag of the vehicle, and since I was running the tests on the CP1 frame-only, I could only take a rough guess at what the CdA is - thus part of the reason for the variation in the two Crr values.

We were able to get out to the Glenmore running track on Thanksgiving day to run some high speed tests around the track to see what the CdA is, and also to do some slow speed coast down tests to measure what the Crr is on the soft running track.

The Crr (rolling resistance) tests showed a Crr of .0086 which show just how slow this soft rubbery track is.

The CdA (aerodynamic drag) test was done the following way:

1. I maintained an average power output of 100 watts for exactly 10 loops around the track, marking the start and stop of the 10 loops on my SRM watts meter computer.

2. Then, I did another 10 loops around the track, but this time, maintained 140 watts of output power for the entire 10 loops.

3. Then I downloaded all of the data into my computer and used the SRM software to retrieve the exact average speed and average wattage output for each of the two runs.

4. Using the PDGUSC11 spread sheet, I input the Crr value of .0086 along with the average watts and average speed for the 100 watt loops, and it returns a resulting CdA (aerodynamic drag) value. Then I repeat this for the 140 watt runs.

I repeated the above 4 steps for 5 different configurations: No canopy bubble, with canopy bubble, with and without canopy bubble and a taped together fairing shell, and finally, a half canopy bubble (wind shield - type).

The results were at first a bit confusing, then upon discussing them with John Tetz, made perfect sense. The resulting CdA values got consistently better as the streamliner got more aerodynamic (ie: canopy bubble, tape, etc). However, all of the 140 watt runs were LOWER CdA than the 100 watt runs. And this was consistent throughout all of the 5 configurations. At first this was very confusing because the CdA can't change as you start going faster -well, not as much as I was measuring anyhow.

The reason, as it turned out, was because the Crr was changing as the CP1 starting to move faster. The faster I went, the more the tires dug into that soft rubber track, and the resulting rolling resistance went sky high.

Here are the results using .0086 as the Crr rolling resistance (temperature, pressure, weight were accounted for and there was very little wind)

Configuration Low speed runs High speed runs
Ave watts Ave Speed (kph) CdA Ave watts Ave Speed (kph) CdA
no canopy bubble, no tape
on fairing shells
108.4 32.22 .64 145.5 38.01 .71
no canopy bubble with tape
on fairing shells and covering
seams and holes
106.3 32.25 .6 146.5 38.94 .65
half-canopy bubble (wind shield style),
with tape on fairing shells and covering
seams and holes
102.4 32.26 .5 139.5 38.77 .55
full canopy bubble, no tape
on fairing shells
101.1 32.52 .45 133.5 38.81 .48
full canopy bubble, with tape
on fairing shells and covering
seams and holes
100.3 32.97 .39 138.9 39.39 .5

So - to confirm this, I was able to meet John Mackay and Ben out at the Race City Speedway paved road course track to repeat the test. The day was calm to start off with, but quickly got windy. I don't think the wind effected the results because there was no difference between any of the loops around the track during varying wind conditions (from a mild breeze to 25 km gusts).

Using a Crr of .0049 on a paved road, I was getting the following data:

Configuration Ave Watts Ave Speed (kph) CdA
no canopy bubble, with tape on fairing shells 85.6 39.43 .35
no canopy bubble, with tape on fairing shells (repeat of first config as a wind control) 85.4 39.06 .36
full canopy bubble, with with tape on fairing shells 64.1 34.16 .28

Since the CdA values are all better than the ones at the Glenmore track, I could attribute that to the fact that the .0086 Crr value I was using for the Glenmore calculations did NOT consider the increase in Crr as the bike went into the corners.


Ben is making good progress on the new fairing plug. It should be glossy smooth and should be worth a further increase in aerodynamic efficiency - meaning an even lower CdA.


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