Calculating Coefficient of Drag from FTP

[mullerrj]

Is there a quick and dirty way of telling what your coefficent of Drag (Cd) is from an FTP output? Similarly, can you calculate what your estimated speed should be for a given FTP? I say quick and dirty because I don't want to have to calculate this information from analytical software (if I don't have to) where you have to input bike + rider weight, air density, surface friction, wind speed, etc. Lets just say for simplicity an average rider weight of 170#, avg. bike weight of 17#, no wind, smooth flat road, etc. And, the FTP was determined by a down/back time trial that lasted 20 minutes (FTP=95% of max sustained avg. power for the effort). I'm asking because I'd like to know for an estimated 20 min. avg. power of 350 watts what my average speed should have been. If my average speed for this effort was 27.2 mph, is my Cd pretty good? Thanks Rob

 

[RChung]

Is there a quick and dirty way of telling what your coefficent of Drag (Cd) is from an FTP output?

Here's the quick and dirty answer: No.

 

[daveryanwyoming]

Here's the quick and dirty answer: No.

I expected an even quicker and dirtier answer, something like "Hmmmm"

To the OP, you can't tell much from what you've presented but just using an average speed, average power approach (really rough BTW) and assuming sea level air density on a windless day:

12.2 m/s (allows a bit for start and turnaround but doesn't take into account any acceleration directly)
and 85 kg total weight on fairly smooth asphalt road (Crr=0.005) would put your CdA around 0.27 if your effort required 350 watts.

Yeah, that's a lot of assumptions and rough first order modeling but based on that your position and or kit could use some work. A CdA of 0.27 is nothing to write home about if you're riding aero bars.

Surf over to the Google Wattage lists, go through the free five minute process of joining the group and make use of the excellent CdA testing spreadsheets and protocols RChung, Alex and others have put together. Finding an appropriate testing venue can be tricky and some methods are more dependent on windless days than others but they're really not too tough to do and the spreadsheets make the results easy to process.

Alternatively try the caveman approach and just do roll downs on an appropriate downhill followed by flats or an uphill. Try out different positions until you find the fastest combo, not very scientific but you should be able to tune your position a bit.

Good luck,
-Dave

 

[mullerrj]

I expected an even quicker and dirtier answer, something like "Hmmmm"

To the OP, you can't tell much from what you've presented but just using an average speed, average power approach (really rough BTW) and assuming sea level air density on a windless day:

12.2 m/s (allows a bit for start and turnaround but doesn't take into account any acceleration directly)
and 85 kg total weight on fairly smooth asphalt road (Crr=0.005) would put your CdA around 0.27 if your effort required 350 watts.

Yeah, that's a lot of assumptions and rough first order modeling but based on that your position and or kit could use some work. A CdA of 0.27 is nothing to write home about if you're riding aero bars.

Surf over to the Google Wattage lists, go through the free five minute process of joining the group and make use of the excellent CdA testing spreadsheets and protocols RChung, Alex and others have put together. Finding an appropriate testing venue can be tricky and some methods are more dependent on windless days than others but they're really not too tough to do and the spreadsheets make the results easy to process.

Alternatively try the caveman approach and just do roll downs on an appropriate downhill followed by flats or an uphill. Try out different positions until you find the fastest combo, not very scientific but you should be able to tune your position a bit.

Good luck,
-Dave

Thanks Dave..I think I kinda knew the answer before I asked..but thought I'd try anyway. Yes, I have Alex Simmon's spreadsheet. I should actually head to the track one of these days to get a better albeit more accurate CdA. Thanks again. Rob

 

[RChung]

I expected an even quicker and dirtier answer, something like "Hmmmm"

I considered it. Something a bit more definitive seemed appropriate.