CDA estimate hopelessly out

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DaveTriska
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Joined: Thu Jun 22, 2017 2:31 pm

CDA estimate hopelessly out

Post by DaveTriska »

I had a couple of goes doing calibration rides to use the CDA feature on a Newton. Knowing what my cda actually is, it was hopelessly out despite the newton being clear of obstructions.

When coming to the calibration check, the DFPM was 100s of watts under looking at the graph and the CDA was given as 0.32 as opposed to my known cda of 0.179

Broken wind sensor?
Velocomp
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Re: CDA estimate hopelessly out

Post by Velocomp »

Are you riding a TT bike?

A CdA of 0.179 is the lowest I have ever seen, on any cyclist, anywhere. How was this determined?

Please post a ride file with your Newton and DFPM measurement.
John Hamann
DaveTriska
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Re: CDA estimate hopelessly out

Post by DaveTriska »

hi,

in a wind tunnel. I assure you it is correct; my coach has a lower one at 0.175 so not the lowest I've ever seen. i am just an obsessive TT rider ;-)

As I was saving the file I ran the estimate again - now saying 0.100 ????

Normally I'm not such a technical dunce but I seem to be doing something terribly wrong - mounting <20 and 10 deg in the axis, clear of obstructions.

Rider error???

Thanks in advance - looks a very cool tool i am looking forward to drag testing
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DaveTriska
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Re: CDA estimate hopelessly out

Post by DaveTriska »

Oh and yes a TT bike, full aero kit etc.

Not too fussed if it isnt BANG on the money for accuracy purposes as I am more interested in differences but the discrepancy led me to think either i am doing something wrong or the sensor is broken
Velocomp
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Re: CDA estimate hopelessly out

Post by Velocomp »

Wow, it is going to be fun to work with you..."aero man" :-)

1) You've got dropouts in your speed sensor signal. Please put your magnet-less speed sensor on your front hub

2) Please post photos showing how your PP is attached. I'm concerned that, if you have it mounted on your aero bars, that your aero bars are flexing.

Once we get your PP rigidly mounted and your speed signal going, you should get great results.

Instead of sending a cal ride file, do a short ride after your cal ride, and post that.
John Hamann
DaveTriska
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Re: CDA estimate hopelessly out

Post by DaveTriska »

Thanks that's really helpful. Dunno why the speed sensor was dropping - its on the front wheel. Will have a fiddle

I'll try and get a ride in today with the cal file then the normal ride after
Velocomp
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Re: CDA estimate hopelessly out

Post by Velocomp »

What kind of speed sensor are you using? We've seen dropout problems with Wahoo sensors.

Don't forget the photos...
John Hamann
DaveTriska
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Joined: Thu Jun 22, 2017 2:31 pm

Re: CDA estimate hopelessly out

Post by DaveTriska »

Ah.... That'll be it, it's a wahoo speed sensor :-(

This is the mounted Newton. Rare as hens teeth in the UK shops now and unable to get the TT mount kit here, had to make one using a 3d printed garmin 'male' attachment and a normal TT mount reversed to go to the outside of the bars.

Are Wahoo sensors just a no-no then?
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DaveTriska
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Re: CDA estimate hopelessly out

Post by DaveTriska »

This is the arm and bar position, I was worried about the noisy air in between the bars if the newton was mounted there, hence the side mounting.

Alas can't get RWS for love nor money here.
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Velocomp
Velocomp CEO
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Re: CDA estimate hopelessly out

Post by Velocomp »

I've sent you a private email regarding this.
John Hamann
DaveTriska
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Re: CDA estimate hopelessly out

Post by DaveTriska »

For the benefit of the forum users, John is being more than a legend and deserves a shout out for it
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