Cda of a Chim ? Does anyone know
Discussion
You just need to do the 2 coast down tests and a couple of calculations as described in the manual.
i.e. time taken to coast from 60mph down to 55mph and also 15mph to 10mph. you can then visit Race Technology's website which has a calculator on it somewhere.
I use 0.74 cda and 0.019 rolling resistance - seems to give good results.
Contact me via my profile if you want - i've developed an excel spreadsheet which calculates the power and torque at the flywheel. I don't use Race Technology's website - i download the data in text format and plug it into my spreadsheet.
i.e. time taken to coast from 60mph down to 55mph and also 15mph to 10mph. you can then visit Race Technology's website which has a calculator on it somewhere.
I use 0.74 cda and 0.019 rolling resistance - seems to give good results.
Contact me via my profile if you want - i've developed an excel spreadsheet which calculates the power and torque at the flywheel. I don't use Race Technology's website - i download the data in text format and plug it into my spreadsheet.
jsr. sorry if I may have missed something along the way but I have a similar query.
I also recently acquired the AP22 and immediately set off to do the coast down tests. Problem is how do you measure the speed accurately at 10 and 15mph, the data logger seems to use 10mph increments and the speedo is hopelessly inaccurate at those speeds. Not so bad for the 60 to 55mph bit. The rest of the calculations are straight forward enough i just can't get a reliable figure for the low speed rolling resistance test. Other challenge is finding a really flat piece of road (or is it acceptable to average opposite direction runs (assuming no significant wind speed as well))?
My rough calculations based on a good bit of guestimation gave a Cd of 0.70 and a RR of 0.020 would those figures give reasonable resluts?
>> Edited by tahiti450 on Tuesday 12th October 17:29
tahiti450,
it is difficult to get an accurate result.
The unit is accurate to within 3% straight from the box (except for power), so i set the unit to record continuous acceleration data and then did the coast down exercises. Once the data has been downloaded, it can be used to find the two times.
I suppose repeating the process a few times would yield more accurate figures, but i wouldn't think it is worth it as you have to guess the other parameters such as the roll/tilt factor.
I reckon your 0.70 should be ok - it must surely depend on the type of road surface, wind conditions, how flat the road is etc.
it is difficult to get an accurate result.
The unit is accurate to within 3% straight from the box (except for power), so i set the unit to record continuous acceleration data and then did the coast down exercises. Once the data has been downloaded, it can be used to find the two times.
I suppose repeating the process a few times would yield more accurate figures, but i wouldn't think it is worth it as you have to guess the other parameters such as the roll/tilt factor.
I reckon your 0.70 should be ok - it must surely depend on the type of road surface, wind conditions, how flat the road is etc.
thanks jsr:
Yes i totally agree, I was quite surprised that my results were as close to yours as they were, given the very hit & miss approach. Having said that using the downloaded data and then extracting the info from the unit makes much more sense. I should be able to do that now as i now have a RS 232 to USB converter, whereas before i didnt so couldnt doanload.
Yes i totally agree, I was quite surprised that my results were as close to yours as they were, given the very hit & miss approach. Having said that using the downloaded data and then extracting the info from the unit makes much more sense. I should be able to do that now as i now have a RS 232 to USB converter, whereas before i didnt so couldnt doanload.
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