Coast Down questions

[MikeLoos]

Just finished doing the Coast Downs for my new iBike Pro (wireless). I think I did everything correct , including the subsequent 4-mile ride, but had some questions that I was hoping for some guidance with:

1) I did 8 Coast Downs. When I look at the Aero and Fric numbers, they vary quite a bit (Aero from 0.314 to 0.772, Fric from 0.73 to 6.45). Is that normal? The final values are Aero=0.329 and Fric=4.772 and neither the iBike or the software reported any issues. I attached a screenshot of all the data in case the actual combinations matter.

2) To do the coast downs, I ended up driving to a particular spot that I felt would be ideal. But when I got out there, I realized that while I remembered all that I needed to do to calibrate the iBike properly, I forgot to check the pressure on my tires. They were generally ok, but not what I typically ride on. Does that matter? Do I need to go do the coast downs all over?

Thanks in advance for any help...

Mike

[Site_Admin]

Just finished doing the Coast Downs for my new iBike Pro (wireless). I think I did everything correct , including the subsequent 4-mile ride, but had some questions that I was hoping for some guidance with:

1) I did 8 Coast Downs. When I look at the Aero and Fric numbers, they vary quite a bit (Aero from 0.314 to 0.772, Fric from 0.73 to 6.45). Is that normal? The final values are Aero=0.329 and Fric=4.772 and neither the iBike or the software reported any issues. I attached a screenshot of all the data in case the actual combinations matter.

2) To do the coast downs, I ended up driving to a particular spot that I felt would be ideal. But when I got out there, I realized that while I remembered all that I needed to do to calibrate the iBike properly, I forgot to check the pressure on my tires. They were generally ok, but not what I typically ride on. Does that matter? Do I need to go do the coast downs all over?

Thanks in advance for any help...

Mike

First, great job that you sent me the image. But I also need to know the rest of the story. E-mail me off-list, or call me directly, and we'll figure it out - the low fric value doesn't look good, and should probably be removed, and the high Aero value should probably be removed as well. That will affect the averages, but they'll be more accurate. Also, I need to know what your average slope read after the o & b, and what the wind scaling factor read. Ideally, we want wind scaling to be around .8 to 1.2, the closer to 1.0, the better. Again, e-mail me or provide me with a number off-list so I can reach you, and we'll take a look. DO NOT delete the data from the ibike, whatsoever. We may have to re-do the CD's and o & b otherwise.

[Ratman]

I recently traded up to an iAero and put in considerable effort to get "perfect" circumstances under which to do coastdowns and a 4-mile ride (here in New Jersey, it's hard to find roads within 20 miles of my house that don't have heavy traffic, potholes and chipseal, stoplights and dangerous intersections, etc.). After having done a meticulous job of zeroing the wind offset and tilt calibration (tilt was exactly the negative of its 180 degree twin), I did 12 coastdowns under very gentle wind conditions (each checked out "OK" by coasting down immediately after and noting watts dropping below the absolute value of 20), I completed a five mile/"four-mile" out and back ride (with no mishaps). When I got home to download the data, I noticed the tilt needed a noticeable software correction (with the attention I paid to the tilt calibration and the rock-solid nature of the mount, I have no idea how that could have happened).

So, the questions is: If you see that your 4-mile ride needs tilt correction, does that mean that the profile will not yield accurate power data? Or, after iBike 2 tilt correction, can we assume that the profile will still allow power measurement to the theoretical limits of the iAero device? If I really want things "right," do I need to redo the coastdowns and 4-mile ride? I'm not sure how I could improve on the technique. If it makes any difference, I'm primarily interested in "on-the-road" power accuracy -- having to correct large errors in ride files on the PC at home would be disappointing.

[travispape]

I recently traded up to an iAero and put in considerable effort to get "perfect" circumstances under which to do coastdowns and a 4-mile ride (here in New Jersey, it's hard to find roads within 20 miles of my house that don't have heavy traffic, potholes and chipseal, stoplights and dangerous intersections, etc.). After having done a meticulous job of zeroing the wind offset and tilt calibration (tilt was exactly the negative of its 180 degree twin), I did 12 coastdowns under very gentle wind conditions (each checked out "OK" by coasting down immediately after and noting watts dropping below the absolute value of 20), I completed a five mile/"four-mile" out and back ride (with no mishaps). When I got home to download the data, I noticed the tilt needed a noticeable software correction (with the attention I paid to the tilt calibration and the rock-solid nature of the mount, I have no idea how that could have happened).

So, the questions is: If you see that your 4-mile ride needs tilt correction, does that mean that the profile will not yield accurate power data? Or, after iBike 2 tilt correction, can we assume that the profile will still allow power measurement to the theoretical limits of the iAero device? If I really want things "right," do I need to redo the coastdowns and 4-mile ride? I'm not sure how I could improve on the technique. If it makes any difference, I'm primarily interested in "on-the-road" power accuracy -- having to correct large errors in ride files on the PC at home would be disappointing.

You probably don't need to worry about it, it is likely that it worked just like it is supposed to. Depending on how big you are, your ride position, your stem length, the stiffness of your bike's front-end, etc. you should expect to see varing amounts of riding tilt. I'm about 175 on a Specialized Tarmac and I typically see about -0.5% or -0.6% of riding tilt. Rich is a couple dozen pounds lighter than me and his riding tilt is about half of mine on his bikes. John is tall and has a longer stem and typically sees a riding tilt of more than -1.0%. One of the main purposes of the calibration ride is to measure that riding tilt so that it can automatically be removed from subsequent rides.

Note that even if you do see a large riding tilt that that doesn't mean your on-the-road accuracy is bad! The reason the fric coef is not named "rolling" is because it accounts for more than just rolling resistance. The riding tilt is accounted for in the fric number and therefore you can get accurate on-the-road power number despite a large riding tilt. Again, no need to worry.

If you want me to look at your calibration set in detail, email me at travispape at yahoo and I'll take a look.

Travis