Ok so I finally got some coast downs...

[Derath]

And a 4 mile ride. How can I tell if it is any good? This is my first foray into power so I don't know what at all to expect.

-D

[rruff]

As it states in the manual, you can put it on the "coast" screen and see what your power is while coasting. Ideally it should be close to zero.

Also, look at how much scatter you got in the CDs... wildly varying values aren't a good sign. Another check is to look at your CdA and Crr values and see if they are close to reality.

[Derath]

As it states in the manual, you can put it on the "coast" screen and see what your power is while coasting. Ideally it should be close to zero.

Also, look at how much scatter you got in the CDs... wildly varying values aren't a good sign. Another check is to look at your CdA and Crr values and see if they are close to reality.

I had tested my coast downs on the coast screen and they looked good.

My CD's were all pretty close except one.

I have no idea if my Cda and Crr values are close to reality

Cda .3774
Crr .00292

[rruff]

Crr looks a bit low. This is mostly tire rolling resistance. If you have the best racing tires and latex tubes, you can get ~.004. More typical values with good tires and butyl tubes are in the .005-.006 range. Some tires will be higher than this though.

It is difficult to get an accurate Crr out of the coast downs unless you do a lot of them and have very good conditions. iBike tries to extract CdA and Crr from the coast-downs by applying a curve fit, but you can end up with Crr being off in one direction and CdA will be off a little in the other to compensate. Frankly I'd I think it would be good to have the ability to tweak the Crr manually, and have iBike adjust the CdA to appropriately. I've been playing around with doing my own CD analysis. If you really want to get those #s right it is best to do the CdA at high speed and Crr at low-speed.

[Derath]

Hmm,

Dunno if it matters but my fric number was 1.45

The reason I was curious is that my first real ride with the ibike after doing my CD's and 4 mile ride seemed to have better power numbers than I expected. Not that I know what to expect. I just know that currently I am in lousy shape.

I am running Continental Ultra race 700x23 at 115psi.

[rruff]

If I had to guess I'd put your tires close to .006 on Crr.

The Fric number combines the tilt correction and the Crr since they are physically identical. What is your tilt adjustment? It is possible that this is off. For instance if your tilt is off by .1% this is identical to a .001 change in Crr. If you do some rides and notice that your slope is off in "analyse tilt and power" then you will know...

[Derath]

If I had to guess I'd put your tires close to .006 on Crr.

The Fric number combines the tilt correction and the Crr since they are physically identical. What is your tilt adjustment? It is possible that this is off. For instance if your tilt is off by .1% this is identical to a .001 change in Crr. If you do some rides and notice that your slope is off in "analyse tilt and power" then you will know...

I highly doubt my tilt is off. I have checked it numerous times and it checks out. The mount is rock solid. and during rides the grade is pretty much in sync with my Garmin, given that the garmin lags a little it isn't always exact but pretty darn close. And close to exact on long steady grades.

[edit- some stuff to add. didnt have time earlier]

For my coast downs I found a stretch of road that had been recently paved. Low traffic. I didn't get buzzed by any cars during any of the CD's. It was very slightly uphill (registered +.5%). Every time I did a CD I made note of my start and stop and they pretty much were right on.

Attached it the tilt calc from my 4 mile ride.

-D

4 mile ride tilt calc.jpg (209.44 KiB) Viewed 7341 times

[rruff]

Go to the advanced tab in the "analyse tilt and power" page. There you can see if your tilt matches the elevation. If your tilt is typically good (errors of more than .1% are big enough to do something about), I'd recommend increasing your Crr to .006 and reducing your CdA to ~.36. This is "unauthorized" of course (the VC guys might have other ideas), but I have some experience with this stuff.

[Derath]

funny I was editing my reply while you added yours. See above

[rruff]

funny I was editing my reply while you added yours. See above

Your tilt looks good... only .05% off. So this is with a tilt correction of zero? Most people seem to be getting around -.5% for a tilt correction (difference between riding tilt and the unladen tilt cal). If it stays this good on subsequent rides then it is fine. Did you do a tilt cal before the CDs and check it afterwards to see if it stayed the same?

[Derath]

funny I was editing my reply while you added yours. See above

Your tilt looks good... only .05% off. So this is with a tilt correction of zero? Most people seem to be getting around -.5% for a tilt correction (difference between riding tilt and the unladen tilt cal). If it stays this good on subsequent rides then it is fine. Did you do a tilt cal before the CDs and check it afterwards to see if it stayed the same?

Yes I checked my tilt before my CD's I did not immediately afterwards. But I did check it before my next ride and it was fine.

Now I can't exactly remember, but I know the wind had started to pick up a little when I was doing my CD's Not bad, but any wind would have been a tailwind while I was coasting. I guess maybe that could have skewed my numbers maybe?

[rruff]

If your tailwind was ever higher than 8mph, then it could have affected your results. The iBike doesn't register negative wind.

[travispape]

If your tailwind was ever higher than 8mph, then it could have affected your results. The iBike doesn't register negative wind.

Exactly! A tail wind during coastdows can mess up the fric & Crr measurement. It is more of an issue in the winter, but a bad wind offset can also mess this up.

Travis

[Derath]

Ok,

I got another coastdown and ride this weekend. On a local MUP so a bit more sheltered and controlled.

I did a tilt calc and wind offset immediately prior to the ride. I got 7 coastdowns and then my 4 mile ride.

I got the following numbers
Aero 0.362
Fric 5.958
Cda .380
Crr .0040

Seem better?