Changing aero/cda

[GWPOS]

I finally was able to check my aero calibration by going down hill in the coast screen at apprx. 30mph. I got a negative -35-50 avg. doing so. If I change my aero numbers my cda raises to .400. Does that sound right?

[lorduintah]

How does your overall wind speed average and bike speed average look - for a ride the retraces the out and back? They should be nearly the same averages if your wind scaling in on.

If not, it is possible that a wind scaling change would get your Aero coefficients back in line. There is a lot of interplay with these parameters.

Tom

[GWPOS]

Its kind a scary, but the wind cal came out to zero after an out and back ride with the original calibration. Coachboyd already looked at the original file but since then I noticed very low or hard to reach power numbers while descending so I decided to do the cal check procedure going downhill at ~30mph just coasting and I get the negative 35-50 watts reading. It was even off by more but I adjusted it a little already. I can attach a ride file but that may or may not help. I have a lot of rolling hills and when I am going hard I sometimes see almost nothing (100-150 watts) when going on the downhill sections. This applies even if the grade is not very steep. I almost have to blow myself up to do an endurance- tempo pace so I thought since I had a little time the other day to try and fix this issue by adjusting the aero. I sure hope this makes sense. Thanks..

Clint

[lorduintah]

Those values of 35 to 50 are not way out, but do indicate your aero can be adjusted more.

Don't forget you can check your slope on the downhill by toggling the display - this can give you an idea of how much downhill really means. Gravity WILL reduce your watts displayed on a downhill. Going uphill on the same stretch how many watts can you generate?

You can also select just a downhill section on the plots and see what gravity is for watts vs. standard power. It might help you regain some sanity to your gut busting.

Tom

[GWPOS]

Thanks, for the response back. I realize that gravity really affects my power on downhills, but I also know that if I am doing a zone 2 effort while climbing for instance my HR will be around 140-150 depending on the day and my training cycle etc. Now when I turn around and go back down in order to get my power numbers into zone 2 my HR goes to 165 and higher. I went ahead and tweaked my aero and will see how it looks tonight. Afterwards I will post my ride file and hopefully someone can look at it and see if it looks right. Thanks...
Aero=0.276
Wind Scaling=.690
CDA=0.400
Fric=7.137
Tilt=-0.3
CRR=0.0056

Clint

[GWPOS]

Ok so I am getting closer to zero watts while in the coast down screen, but now some ghost watts are appearing while in the normal riding screen...around 30-50 watts while coasting downhill. So now I get a little confused on what to do. I have attached a ride file that has some of the first part removed because of the wattage spikes caused by going into the setup/coast screen.

iBike_03_25_2010_1734_32_Miles.csv

(1.89 MiB) Downloaded 333 times

[Velocomp]

I like the term "ghost watts"!

In a well-calibrated the presence of ghost watts on a downhill are a consequence of very minor deviations in the rider's position. Remember that opposing wind varies as the cube of the bike speed, so the least change in ride position can show up as ghost watts.

The good news is this: 30-50W causes an insignificant amount of change in the overall results of the bike ride. So, they are nothing to worry about.

[GWPOS]

So, then does my ride file look ok? That is all I was trying to make sure of after tweaking my aero. Thanks

Clint

[rruff]

If your wind scaling was good, then the thing you should have changed was CdA... is that what happened? How much did you change it? Do you zero the wind offset before these tests?

An occasional blip of 50W when coasting at 30mph is quite acceptable IMO. If your CdA is .4, then the power required to go that speed on the flat would be ~800W... so 50W is a small percentage. The tilt can also be off slightly especially when transitioning from uphill to downhill riding, and this can easily cause a 50W+ error at high speeds.

When riding downhill with gusty winds at ~35mph it isn't uncommon for me to see fluctuations from 100-400W when the real power is ~250W. These sorts of conditions are the most challenging for the iBike.

[GWPOS]

Thanks, and yes my wind scaling came out even after an out and back ride, and I always zero the wind before going on my rides. I actually put a hold on the wind scaling and adjusted the aero which adjusts the cda at the same time, so I guess I did it right. Again thanks and will do some rides and see how it looks....

Clint