Coast Down issues?

[benlane]

I have had Boyd help me out with my various profiles.
I have arrived at a profile on my race bike with some race wheels that Boyd and I think delivers good accurate results:

These are
Aero 0.383
Fric 7.095
CDA 0.335
CRR 0.0057

However, these are quite different to what I get if I calibrate completely from scratch. Initially it looked like this

Aero 0.294
Fric 4.645
CDA 0.294
CRR 0.0047


Today, I decided to get some coastdowns done with my best race wheels, some Reynolds Carbon tubs (infllated to 120 psi)

I did around 20 coastdowns (got quite a few bad coastdowns). I re-set tilt and wind offset before beginning.

Aero 0.275
Fric 3.279


If I re-apply this profile to past rides then I get ridiculously low power values.

I have tried the CD calibration ride 3 times on Gen III now on different bike set up's and unless Boyd tweaks the results I get really low power values. I know we all want higher power numbers but I am talking silly low numbers changing a chaingang ride at 26mph from 170 watts to 140 watts.

Help anyone?

[MdC]

Are you doing your calibrations on the same bit of road, in the same direction, with similar weather, in the same riding position?
Are you weighing yourself, kit and bike each time?
Perhaps there is something in the method you use?

One more question, more for the Velocomp fellas. Would the calibration results be more accurate if we had to speed up to say 18-20mph, where the wind resistance would become more signifcant?

[benlane]

Mdc
Yes, same road, same protocol, same position.

1) input weight
2) tilt calibration
3) Wind offset
4) Begin coastdowns.

I don't know why it is, the road is pretty smooth tarmac, sheltered on both sides by a hedge row. I only attempt them when there is minimal wind.

[lorduintah]

What tilt correction are you getting? Is the wind scaling reproducible? These are other factors that should be inspected. Is the orifice for the iBike free of any interference by your hands or arms?

Tom

[mds]

I agree. Redo the calibration ride and check wind scaling and riding tilt and then redo coast downs.

Looking at your first set of numbers Aero 0.294, Fric 4.645, CDA 0.294, CRR 0.0047, note that Aero equals CdA. This indicates that wind scaling is 1. On the other hand Boyd's number have a wind scaling of about 1.08 (Aero = CdA * Wind scaling). A similar comparison of Fric and Crr indicates that your Riding tilt is likely suspect.

So redo a couple of calibration rides and see if you can get reproducible wind scaling and riding tilt values. Then redo your cost down.

Mike

[benlane]

Thanks for the comments folks.

I might over the next couple of weeks try and do some new cd's and calibration rides for my race machine with both choices of wheels and see if they come close to reproducing Boyd's tweaked profile for me.

[wellmt]

You know I've had similar issues. 4 lots of doing CDs/4 miles rides with different results each time. Using the 4 results I reckon I've tweaked a profile to be correct, I'm going to test it tonight or tomorrow so we will see. The feature of the iBike software that allows you to apply a profile after the ride is really useful for comparisons.

[benlane]

Ok - I did another set of coastdowns and a calibration ride, following exact same protocol as below, same bike set up, again a fairly different result

Aero 0.289
Fric 9.695
CDA 0.241
CRR 0.0058

Only difference was, as the weather has warmed up a little I had lost a thin long sleeve jacket and was in shorts, knee warmers and a short sleeved jersey.

I don't understand in laymans terms why the friction would be so different on the exact same stretch of road?
Is it wrong to imagine that repeating the process a few times should produce fairly similar results?

The difference between last nights results and Boyds tweaked profile is that the above produces about 9 watts less for a 190 - 200 watt ride.



These are Boyds tweaked version based on the ibike defined details below

Aero 0.383
Fric 7.095
CDA 0.335
CRR 0.0057

However, these are quite different to what I get if I calibrate completely from scratch. Initially it looked like this

Aero 0.294
Fric 4.645
CDA 0.294
CRR 0.0047

[rruff]

I don't understand in laymans terms why the friction would be so different on the exact same stretch of road?

I don't understand how your riding tilt changes so much between Cal rides, or why you are getting "bad coast". When you are done, are the coastdowns pretty similar, or do they vary widely?

Is it wrong to imagine that repeating the process a few times should produce fairly similar results?

If conditions are good and you are doing everything correctly, then the results should be very similar each time.

[benlane]

I do follow the dame protocol every time.

1) Input weight
2) Tilt calibration
3) Wind offset
4) complete coastdowns

I have done 3 different calibration rides on the same stretch of road on still days. The calibration rides always follow the series of coastdowns.

I must have done about 75 cd's with different wheel set ups.

I have sent Boyd my latest profile files so hopefully something may come to light.

[rruff]

When you do a series of CDs what is the scatter in the values?

Do your wind scaling and riding tilt vary much between Cal rides?

Do you check the wind offset when you are done?