iBike Stats/Metrics

[mullerrj]

Can anyone tell me what "Aero" and "Fric" metric means in the iBike Stats window when reviewing a downloaded file? Example:
Aero: 0.289; Fric: 4.38
CdA: 0.55 m^2
Crr: -0.0022
205 lbs; 2/15/2009 2:22 PM
46 deg F; 1018 mbar

I realize these are dimensionless units just like Cd is, but how are they used? I can see that Cd (Coefficient of Drag) x A (frontal area, m^2)= CdA but how is "Aero" or "Fric" used?

Crr values are posted for different surfaces. Are "Aero" and "Fric" values posted somewhere? i.e. Aero<0.29 hoods, Aero<0.2 drops, Aero<0.15 aero bars, etc. Same w/ Fric. i.e. Fric>4 good, Fric>5 bad, Fric>6 ugly.

Bottom line, what does Aero: 0.289 and Fric: 4.38 mean? Hope I explained this properly. Thanks Rob

[rruff]

Aero is your wind scaling factor x CdA. Fric is a similar metric that relates to your weight and Crr and your riding tilt.

Your numbers for CdA and Crr are not "real". I'd suggest redoing the calibration.

[mullerrj]

ok, I'll re-do my cal tomorrow: setup, coast-downs and cal ride.

Out of curiosity, what is "not real" about the numbers I posted in the example above..the CdA or the Crr, or both?

Thanks Rob

[Velocomp]

CdA is your aerodynamic drag and Crr is your frictional drag. These drag coefficients should be positive in value; when they are positive then they are resistive forces. Your Crr is a negative value.

CdA should be in the range of 0.3 to 0.5 and Crr from 0.004 to 0.008. Either your tilt cal or wind offset was incorrect when you did your coast downs and cal ride; that is the cause of your high CdA and low Crr.

[mullerrj]

Thanks John..I think my tilt cal and wind offset were fine..it's my cal ride I think that screwed things up. I remember a couple gusts of wind along the 4 mile route and was wondering if that would have screwed things up, and apparently it did. I chose my cal route specifically for low/no traffic, and out of the wind..but apparently a gust or two snuck out of nowhere. In fact, not a single car passed me on my 2 mile out/back. I suppose the best thing for me to do is start earlier in the day..when the winds are calm. Except, I don't want to start too early cause it's 30 deg F out there right now.

Still learning. Thanks for all your help. Rob

[rruff]

The reason why the Aero and Fric values exist is because these are what are determined by your CDs. The ride afterwards determines your riding tilt and the wind scaling factor, so you can get the CdA and Crr values.

Both your Aero and Fric numbers seem low, so I'd recommend being very careful with your CDs. Choose a flatish road with good pavement and little wind. Do a lot of runs. Make sure tilt and wind offset are done ahead of time. The wind offset should be done when the unit is at riding temperature.

It also seems strange that your wind scaling factor is ~.5. Has anyone else seen such a small value for this? Generally it is ~1... or greater if there is any blockage. Is your unit located behind something that might effect the airlflow?

[mullerrj]

rruff..thanks so much for your reply. Just still all new to me and I'm trying to learn.

I redid my cal ride today..cold 40F..breezy conditions. Actually, I did two of them (Cal rides 1/2 hr. apart). The first time I tried to download the data the iBike Software said there was no data. So, I went back out and did another Cal ride. When I downloaded the data the second time..I saw both Cal rides. Not sure what that was all about.

Here are my Cal Ride A data (done on a relatively flat mile loop in my neighborhood, no cars, no turns..all oval, 2 laps out, 2 laps back):
Aero 0.257
Wind Scale 0.621
CdA 0.414
Fric 8.881
Tilt -.3%
Crr 0.00674

Here are my Cal Ride B data (done on a flat route, few cars passed me, slowed down almost to a stop at an intersection
Aero 0.293
Wind Scale 0.817
CdA 0.358
Fric 9.484
Tilt -.6%
Crr 0.0044

Not sure which Cal Ride to upload to the iBike..which one would you choose? Also, not sure why the Tilt was different, used the same marks for front/back wheel in my garage.

[R Mc]

I know very little about all of this, BUT other folks will post AND one thing that stood out to me is your use of a loop to do the cal. ride.

I suppose that that COULD work, but it seems like you'd get weird wind results when the software tries to compare the 2 miles out and 2 miles back.

As for the tilt--that's an indication of actual on-the-bike riding tilt with you on the bike.

[mullerrj]

R Mc- the more I think about it the more I think that a "loop" is probably NOT good..because there is just too much variability with the wind direction and speed..and probably really throws the iBike for a "loop" when it's trying to calibrate. I think I'll stick to the straightaways. It's just a LOT easier to do a mile loop in my neighborhood void of traffic than it is to find a 2 mile straight stretch of road where the cars aren't going 60+ mph. The nearest 2 mile straight stretch of road near my home (with low car traffic) is close to 5-10 miles away. Thanks Rob

[rruff]

R Mc- the more I think about it the more I think that a "loop" is probably NOT good..because there is just too much variability with the wind direction and speed..and probably really throws the iBike for a "loop" when it's trying to calibrate.

That is correct. Ideally you want to do an out and back ride on the same part of the road even... else you can get significant wind variations. If you have a nice loop, simply ride around, then turn around and go the other way until you get back to where you started. If you can ride in the middle of the road then that is best, else do what you can.

As for which "cal" to use, I'd take an average unless you know better. You can always do some more cals when the conditions are better and really zero in on the best values. Your numbers are looking pretty reasonable now.

[Velocomp]

The purpose of the cal ride is to provide an accurate determination of wind scaling and ride tilt factors.

Ride tilt is the change in tilt calibration caused by the rider's presence on the bicycle. Ride tilt is a consequence of the rider's weight flexing the bike components, and/or the rider's weight distribution causing one tire to become more loaded than the other--anything that will disturb the unloaded tilt calibration. Ride tilt is measured over the course of an entire cal ride. An out-and-back ride loop is perfectly satisfactory to get a good measurement of ride tilt.

Wind scaling is another matter.

Wind scaling is the amount of "correction" required to be applied to the wind sensor's raw readings so that its compensated measurements are as close as possible to being correct. For example, in a condition of no ambient wind, if the raw reading of the wind sensor is 15 mph and the bike speed is 20 mph, then the wind scaling factor is 1.33 ( 15 x 1.33 = 20)

Wind scaling is affected by many things, including the location of the iBike on the handlebars/stem, the angle of the iBike relative to the ground, small differences in the calibration of each electronic circuit, wind offset calibration, and more.

Wind scaling is an important number to measure but is also a measure that requires some care to "get right".

With respect to wind scale measurement from the cal ride, the assumption of the cal ride is that the average wind speed for the cal ride is zero. Why is it zero? Because, if the route is truly out-and-back, then whatever ambient wind is experienced on the out portion will be canceled precisely by the corresponding wind in the return direction. Since the ambient wind effects are canceled by the out and back ride, the wind scaling factor is simply the ratio of bike speed to wind speed.

Clearly, riding a loop puts the assumption of zero net ambient wind at a higher level of risk. On a loop the wind at the "3PM" position may not be the exact negative of the wind at the "9PM" position.

So, it's best to do the cal ride on an out-and-back, and not a loop. That said, if an O&B is impossible, then a loop is better than not doing any ride.

[pjboyle]

John,

SUPERB explanation...as always.

PJBoyle

[mullerrj]

Thanks John..now it makes sense to have a Wind Scaling Factor as close to 1 as possible. If it's exactly 1, then there either wasn't any ambient wind during the calibration ride, or the head wind on the out, cancelled the tail wind EXACTLY on the way back. Correct?

Another quick thing if you don't mind: on my 14 mile ride data if it says Speed 14.4 mph avg and Wind 15 mph avg, does that mean during the entire ride I had an average of 0.6 mph (15-14.4 mph) headwind? BTW, I was doing hill repeats..didn't want you to think I ride that slow..hahaha.

Thanks again John for all your help. Cheers Rob

[rruff]

For example, in a condition of no ambient wind, if the raw reading of the wind sensor is 15 mph and the bike speed is 20 mph, then the wind scaling factor is 1.33 ( 15 x 1.33 = 20)

I think the scaling factor is squared (ie really the aerodynamic drag factor). So (20/15)^2 would be 1.78... yes...no?

[rruff]

Thanks John..now it makes sense to have a Wind Scaling Factor as close to 1 as possible. If it's exactly 1, then there either wasn't any ambient wind during the calibration ride, or the head wind on the out, cancelled the tail wind EXACTLY on the way back. Correct?

No. The ride *assumes* that the wind canceled and calculates the scaling factor based on that. iBike uses a forward facing dynamic pressure sensor and a static port (little hole on the bottom of the unit) to determine air velocity. It needs to be calibrated for your particular unit, riding position, and mounting location. That is what the "wind scaling" means... it is the factor applied to your raw pressure measurements that gives the correct air speed.

You can easily do tests to see if the scaling factor is correct without doing the whole calibration. Zero the offset, reset the trip, then do a series of out-back rides on the same stretch of road... and reset the trip again. When you get home look at the "analyze wind" in iBike3 and see what the net wind was... should be close to zero. BTW, be sure to always ride faster than any tailwind while doing this.