V4 calibration results - Coastdowns

[wellmt]

Following my earlier thread, I've done a cal ride and coast downs. Interesting results. I've fitted a new 1cm longer stem (hence the need for the recal). I think the longer stem should lower my CDA slightly as my shoulders etc will be lower. I ride with a bend in my elbows on the hoods typically.

So now I've got two choices - Use fixed a CRR, say at .0051 or use adjusted CRR whatever that is?.

The thing is, the adjusted CRR seems to give a more realistic CDA, I think. Back in the days of V3, I've done numerous coastdowns/cal rides on this same bike and I've almost always had a CDA of .333 to .365 (and a CRR .0003 to .00051 max and this is hardly smooth, tarmac back roads). Look at the CDAs below, they are lower to much lower!

Fixed CRR

crr-fixed.JPG (130.3 KiB) Viewed 23475 times

Adjusted CRR

crr-adjusted.JPG (131.97 KiB) Viewed 23473 times

So which is the right one to use? I'd guess "adjusted" but I don't know....otherwise does this data look OK?

If I used estimated values for everything, what would my CDA be calculated at, 5' 8", 154lbs, riding on hoods?

Sorry for all the questions, but I can't find any reference to the adjusted option anywhere, and I need my iBike to be right for an event at the weekend.

[lorduintah]

Looks to me like a Crr of 0.0051 is too high and 0.0025 is low. As coach Boyd has mentioned, Crr values are reflected in the road surface as well as wheels, etc. 0.0025 is almost approaching a track surface...

What kind of surface did you do the CDs on? What type of wheels do you have and tires/tubes (23mm??) ? (Some of this also relates to a low, medium or high-end bike.)

Did you run the rides with 1 sec. or 5 sec. recording? I have yet to see definitive statements on doing the calibrations with one or the other, but it seems many use 1 sec.

What values are close to the pre 4.0 FW calibration? Especially if a Crr and wind scaling are in the neighborhood and also your tilt calibration.

If they are around, John or Coach and others might be able to make better comments before you need to make a choice.

I would not be surprised to find out that the values you have are going to yield radically different results - even if you just average the Crr and recalculated with a fixed Crr. The magnitude of FRIC values are not radically different...

Good luck -

Tom

[wellmt]

Thanks Tom. The bike is a high end carbon F&F, Super Record, pretty high end Campagnolo wheels. 23mm Vittoria 320tpi clinchers, 110 psi. The bike will roll what feels like forever on a flat surface. I would describe the road surfaces as rough tarmac rather than the nice smooth stuff (I used to do my coast-downs on a lovely smooth piece of tarmac but because the crr I used to get was so low, I figured a more realistic road that I ride on would be better). I always use 1 second recording.

I've never really managed to get any consistency really. Always got a range. Wind scaling usually in the .72 to .82 range so that's OK, crr like I said was usually low. Tilt is now a bit more than normal but it's a new longer stem (and the iBike mount has changed position slightly).

My thoughts is that perhaps the coast-down road is too rough or undulated too much? (it was a different road from normal, I did these straight after the cal ride so I kept the same tilt and wind scaling)

Again for comparision...I wonder what the CDA would have been if I had used estimated values?

(I guess the adjusted value in the screenshots is ibike calculated crr, the same as pre-V4?)

[wellmt]

I think with V4, I can now do the coast downs again to double check them. John did a pretty good explanation here: http://www.ibikeforum.com/viewtopic.php ... 1078#p5884

It's pretty windy here today, so hopefully tomorrow will work, then I can verify that the CDA is right or wrong. I'll try some different roads for the Coast downs.

I can be fairly sure the wind scaling is OK as I did the cal ride twice (I decided to do it again because I got passed by a couple of cars) and there was .002 between the scaling readings!

I found this article on Bikeradar too, regarding CDAs: http://www.bikeradar.com/news/article/h ... aero-19273

[rruff]

23mm Vittoria 320tpi clinchers, 110 psi.

Those are excellent tires. On a "normal" road I'd expect a Crr of ~.0038 to .0045 depending on tubes. Your CdA values look pretty consistent when you peg the Crr, so my advice would be to choose a more appropriate Crr than .0051, and run with that. You can always check the cal when you are out on a ride by coasting, and of course you can redo the cal and wind scaling check anytime you like.

[wellmt]

Cool thanks. I had a quick go with that, setting the CRR to .0038 and the CDA was calculated at 0.291. I've kept that profile, and I'll go and do some testing and ride the event tomorrow with it. I've got to use something.

For interest, I did some more coastdowns today (two lots in fact) on a smooth road using the CRR set to .0038. I'm using the cal ride data from the posting before. It's a gusty day so far from ideal, but the results are completely strange.If I use today's coast down data with fixed CRR of .0038, I'd be getting a CDA in the .224 to .233 range which is obviously completely bogus.

[wellmt]

So here are the results of the Sportive I did earlier today. Power looks a bit low to me? What do others think? I did a reasonable amount of sitting in trains but plenty of solo riding too.

Capture.JPG (33.23 KiB) Viewed 23242 times

I did a very similar event in April this year and the average was 198 watts, and I'm certainly fitter and stronger now. I'm not saying it's definitely wrong today, but it sure is a lot lower.

Next question: Is it possible that I can use the second profile in the iBike with the cal ride data I have already saved (since wind scaling appears to be good) and use default estimated values for CRR and CDA, just for a comparsion? I've cleared the iBike memory since the cal ride. I'm thinking I could download the saved iBike profile above to profile2 on the unit and then use the Est CDA and Est CRR functions to override the values I obtain during coast downs? (not sure whether I should use the iBike default of .005 for crr or .0038).

I could then do a back to back ride on the same route at the same speed with each profile on the unit and see how the figures compare.

Thanks

[R Mc]

the crr is an obvious target: .038 is very low.

In tools, if you play with the tweak crr function, see what happens if you change that to a more realistic .055 or .06 . . .

[wellmt]

I'd get 168/172 watts average.

The problem is that if I increase the CRR in my profile, the CDA is decreased which is clearly unrealistic, since .290 must already be on the low side for someone riding on the hoods...

[rruff]

For interest, I did some more coastdowns today (two lots in fact) on a smooth road using the CRR set to .0038. I'm using the cal ride data from the posting before. It's a gusty day so far from ideal, but the results are completely strange.If I use today's coast down data with fixed CRR of .0038, I'd be getting a CDA in the .224 to .233 range which is obviously completely bogus.

Sure seems like it. The thing that would be off is your wind scaling, and possibly the riding tilt. Try redoing your cal ride.

[wellmt]

I don't know if I can be bothered. No idea when I might see another non-windy day. It shouldn't be this hard to get it right. To top it off I've just accidentally dropped the unit and it bounced down the driveway (looks OK though apart from a couple of scuffs fortunately).

Edit: Now you say that, I did two cal rides back to back. The first cal ride was done after a tilt and offset done at home. It gave me almost exactly the same wind scaling as above (like .002 difference) but the tilt was completely different it was positive 2 (If I recall correctly). The second cal ride was done and the coast downs straight after that, that was what was attached in post #1 above. If I use those tilt values of the first cal ride I'd get a a CDA of .392! I don't understand how the tilt can be so different since I'd have got a bad tilt during the pre-cal tilt if it was not done properly. I'll double check that the new stem/headset is not loose or something and try again...

I'm seriously tempted to borrow a Powertap and get an iPro to iAero u/g. Surely with a Powertap I'll be able configure the unit with the most accurate figures using Calibration method 4? (I know I'll need to do a 4 mile cal ride again). Then I can run the iBike day to day with a non-powertap rear wheel but be confident that the figures are pretty much spot-on. Would that statement be true?

When I look at calibration method 4 it says use "Once the Calibration ride is completed, use the Profiles/Download Coast Down and Cal Ride…step in iBike 3 to determine the CdA and Crr values that best match your iAero to your DFPM.". How does that work? Do you just enter figures until the results match or does it give you a list?

Sorry everyone I know I'm hogging the forum...

[rruff]

I'm seriously tempted to borrow a Powertap and get an iPro to iAero u/g. Surely with a Powertap I'll be able configure the unit with the most accurate figures using Calibration method 4? (I know I'll need to do a 4 mile cal ride again). Then I can run the iBike day to day with a non-powertap rear wheel but be confident that the figures are pretty much spot-on. Would that statement be true?

It is easy to check the calibration of the iBike (easier than the calibration of the PT). This is a little dated since we've had some upgrades, but it still works: http://www.ibikeforum.com/viewtopic.php?f=6&t=867

You can also just do your CDs and cal ride again. I wouldn't worry too much about the wind... if the iBike couldn't be calibrated in the wind then it wouldn't work in the wind either. The biggest problem is that you will get more variability, and this makes extracting both CdA and Crr difficult. So just peg your Crr at ~.0045, and go for it... do 10 runs and see what you get.

The change in tilt sounds like your mount could be an issue... you'll get erratic values if it moves.

[wellmt]

Thanks for your time rruff. I'll check everything and give it one more go, when it's not blowing a gale.

[rruff]

I tried to do some CDs today with a fair amount of wind and the results were very poor. Not sure why... I've done it before on windy days and it was ok.

If you have your wind scaling correct, and have loaded a good value for Crr, then checking your calibration is as easy as coasting... just put your unit on the "coast" screen and coast a few times. If it is always high, then CdA is too high, and if it's always low CdA is too low.

[lorduintah]

Interesting for me was the latest set of CDs I ran - I kept getting bad CDs - not due to tilt. I had enough of a tailwind that something was not converging. I turned around and ran the CDs in the opposite direction without issue and managed to get acceptable values - a little different than those in a near dead calm, but acceptable.

Tom

[rruff]

My wife and I were both doing CDs yesterday and had the same results... very low numbers for both Crr *and* CdA... which means that the iBike thought there was more headwind and/or upslope than there really was. It wasn't super windy, but it was a variable head/cross wind. The tilt and offset stayed good on both. I'm pretty sure that I've done them in the same conditions before and there were no issues... but I guess the winds can play havoc with the CDs sometimes.

[wellmt]

Hang on, isn't this the same as my issue (low CDA, CRR). Is there something funky going on, or are we all just unlucky. I won't know until another suitable day comes around for testing.

[rruff]

You don't actually have to wait. Provided that you already have the correct WS and a good value for Crr, you can zero your wind offset and tilt, put your unit on the "coast" screen and do some coasting down modest grades. Consistently high or low readings (if it is windy include winds from all directions) means that your CdA is off, and you can tweak the CdA to bring it into line.

[Morocco Mole]

Does the coast function still work the same in V4.00 firmware, I noticed on the weekend doing some coast downs that it now just stabilizes at zero when you are no pedaling and no longer fluctuates positive\negative. This is just sitting at the coast screen, not doing the actual coastdown process, any body else seen the same thing with V4 ?

[rruff]

Mine fluctuates as usual, but... I don't have a cadence sensor... don't know if that matters.

[wellmt]

OK I spent the evening doing another cal ride and more coast downs. It's not an ideal day, wind is around 9mph but I gave it a go anyway. There is good and bad news...

Good news: Wind Scaling is close to the result in the first posting. Tilt is exactly the same.
(Anyone who says the iBike isn't consistent is wrong!)

Bad news: CDA is still way too low, even lower than before!
(the iBike is consistently wrong )

So either roads are made of something different here in the UK and CRR .004-005 isn't typical, Campagnolo/Vittoria provide better wheels/tyres to European customers or the iBike just hates me!

As far as I can see my only option is to do numerous tests (slow, fast etc) watching the coast screen and tweak the values manually? Any other suggestions?

A fixed CRR of .0020 gives a CDA of .336 which I would consider realistic, or a least a good starting point. Edit: One other point to consider is that with less tilt the CRR would actually be higher. So a CRR of .0020 at a tilt of -.4 is the same as a CRR of .0040 at a tilt of -.2 (I think?)

Fixed CRR

crr-fixed2.JPG (132.52 KiB) Viewed 22914 times

Adjusted CRR

adjusted2.JPG (133.65 KiB) Viewed 22911 times

[wellmt]

More coast downs today, similar results. Using the coast screen was no help, wild fluctuations in both directions.

I officially give up trying to calibrate this thing without using a DFPM.

[Velocomp]

Please send me your ibcd4m file. jhamann@velocomp.com

[bex]

Was there a resolution to this?
I have the same issues and I think they are related to wind scaling and CRR.
CRR is always around 0.0012
Wind scaling has been 4.74(I think) down to 1.787 the latter coming on a day that pegged the avg grnd wind at .7 on the out/back
I have asked coach Boyd for some help, he modified a previous profile to get me going.
But, I want to be able to do this as per the instructions and just dont seam to be getting there.
I am really frustrated with my inability to do what appears to be such a simple procedure.

Bex

[wellmt]

John's looking at the files in the next day or so.

Do you mean CDA has been down to 1.787 from 4.74 or do you really mean wind scaling? I've never seen windscaling above 1.2 or below .7

[lorduintah]

I have personally found that wind scaling is easily affected by where the iBike is mounted - perhaps you can either describe or better post a picture of your configuration. If your hands interfere or influence the air flow around the port under the iBike you are likely to have a high scaling factor. Either that or you have some debris stuck in the way of the port. In part - this is why the TT adapter was introduced for the TT setup and a "remote" position for air intake.
The iBike, itself should be mounted so that it is relatively parallel to the ground - a high tilt angle will greatly affect the area of the air port exposed to the air flow while riding.

Tom

PS- I moved my iBike from the edge of the bar tape to as close to the stem as I could - the WS went from ~1.4 to 0.95. Both had been mounted pretty much as level as I could get. This removed a significant influence of my hands on the bar - even when in the hoods.

[bex]

Yes, I mean wind scaling.
WS on the modified profile, the one Boyd did for me is 3.979
Aero 1.293, fric 6.506 weight 172 riding tilt -0.3 CDA .325 Crr 0.0055
contrast that with my latest effort
Wind Scaling 1.747 Aero 0.667, CdA 0.382 Fric 4.344 Riding Tilt -0.4 Crr 0.0017
I would do a screen shot but don't know how to
Can someone explain how to do that please?

As for the Ibike position, its on my road bike, attached to the bars, next to the stem with no interference from cables, its rock solid, I glued and taped the head unit. almost fully level/parallel to the ground. I have checked the wind ports and see no debris (good suggestion though, thanks).

Bex

[wellmt]

Wow that must be a difference between bar mounts and stem mounts I guess.

Don't know what Operating System you are running but if it's XP there is a little free screen grabbing program called zapgrab that you can get - just google it. You can also use the Print Screen key under windows or Alt+Printscreen to just capture the active window to the clipboard. Then just paste it into Paint or similar. Vista has Sniping tool built in under Accessories and Mac OSX has something similar.

Just save your captured image and upload using the "upload attachment" tabs under the text of the posting.

Cheers.

[bex]

OK here goes

Clipboard01.jpg (81.28 KiB) Viewed 22667 times

Bex

[bex]

And another

Clipboard02.jpg (96.35 KiB) Viewed 22660 times