Coast Downs and Stuff...

[dbrouse]

Good evening...I finally had the opportunity to do my coast downs and hopefully I have some info that is accurate. Here are my results:

Run Aero Fric
CD1 .200 9.46
CD2 .167 9.46
CD3 .250 9.46
CD4 .297 9.46
CD5 .297 9.46
CD6 .278 9.46

Below the coast downs is another section with the following data:
Aero: 0.275
Wind Scaling: 0.661
CdA: 0.416
Fric: 9.46
Riding Tilt: 0.1
Crr: 0.0095
Weight: 250
Wheel Circ: 2096

The only issue that *I* am aware of is my weight...which should be 247 (this is me+gear+bike).

(Other that the weight issue) Do these numbers look realistic/accurate? What do I need to do now? Do you need any more info from me?

Thanks in advance!

Delane
http://www.shutuplegs.com

[MdC]

My view:
I'd ignore the first and second coastdowns. The friction numbers look to be outliers.
Perhaps with some more information you'll get some more definitive responses.

What tyres and pressures are you running and how would you describe the road surface (smooth tarmac, chip seal, rough chip seal...) ? The Crr value looks pretty high, I'm a bit lighter than you (77kg) and get a Crr of 0.0055 for Conti 4000S @ 115psi.

Next, where is the wind port positioned on the bike and is there anything that may disprupt the airflow? Looking at the out and back ride, it seems the wind was positive to wheel speed for the whole ride. Usually you'd get one half positive (into the wind) and another negative (with the wind). To my mind the wind offset before the Cal ride did not zero correctly. This would be the biggest error source if not done correctly. I'd suggest redoing it just to be sure.

One final thing, I get a much better result doing a longer out and back ride of around 5 - 6 miles. I'd recommend doing that if possible as well. You just need to ensure the conditions are stable and that wind does not change direction or magnitude.
Good luck.

[dbrouse]

Sorry for the delay in my response, somehow I overlooked your reply.

Continental Ultra Race (700x25c) @ 100psi

the road surface is good old fashioned Virginia chip seal

There's nothing disrupting the airflow.


Let me ask another question...if I change anything (tires, wheels, stem) do I need to do a new set of coast downs and a new out and back ride?

How about if I lose weight? I've lost 6lb over the past 3 weeks...

Thanks in advance!

Delane

[MdC]

I'd say you should redo your calibration if you change your wheels, stem (particularly if the unit is stem mounted) or saddle height. As far as weight goes, dropping 3 kgs is most likely to affect the Crr readings so it would be worth doing the calibration for that.
Try another calibration and compare it to the previous version. At least you can determine if the old one was in error.

A few times I have wondered if my calibration is out, I'm riding hard but not getting the numbers. I redo the calibration, get the same result as before and realise I am just not working as hard as I think I am.
The laws of physics are a harsh mistress.

[dbrouse]

Cool...thanks for the feedback. Coach Boyd heled out and has provided me with what he considers a good profile. I think I'm done with them until the weather get's good and I can get rid of the tights/shoe covers/jacket/balaclava/etc.

Coastdown/calibration is the only thing I don't like about the iBike...mainly because I don't understand the science behind the results.

See ya on the road!

Delane