Low CdA?

[chrus101]

Hello
Could someone please check my data. I have set Crr to 0.005. My weight is 80 kg and height 1.78 m. The CdA seems low at 0.278.
Only use coast down data from 10:15.
Thanks for help,
Chris A

[Velocomp]

Is this a TT bike or are you using aero bars?

What was your riding position?

[chrus101]

Hello

Giant TCR C0 road bike with Shimano clincher CL 7850 wheels (not deep section) and continental attack force (22 mm) front and GP4000 (23 mm) rear tyres. Rode in the hoods for cal ride & coast downs.
The road was pretty smooth so in the software is set Crr to 0.005.

Thanks for your help,
Chris A

[Russ]

Man,
You are riding on 'Black Chilli' !

From the Continental site:
"With the new Black Chili Compound rolling resistance is reduced by 26%"

And it is in BOTH of your tires!
One of the vendor sites had reviews that claimed lower CRR in cold weather too

(I noticed this because I am thinking about getting the Attack/Force pair)

Russ

[rruff]

One of the vendor sites had reviews that claimed lower CRR in cold weather too

AFAIK, all tires experience an increase in Crr at low temperatures. Can you find that site?

[Russ]

rruff asked about sites I mentioned...
I am looking as I write so lets see how far I get...

hard to back track web browsing but I found one for sure, personal review:
http://www.amazon.com/Continental-Grand ... B002LB6AGE
Scroll down to bottom review, ex racer comments about how well they roll, including in the cold.

Good review from cyclingnews, cold weather performance not mentioned:
http://autobus.cyclingnews.com/tech.php ... i_gp_force

I noticed a couple of complaints about rapid wear and flat susceptibility especially with locked wheel slids as was mentioned happened to one in the cyclingnews review, others claim good wear for a race tire set. Maybe the 4000 is a better rear wheel choice? Same Black Chilli (black 4000 and 4000s only).

Glad you asked, here is a bicycling forum link where this pair is discussed and an entry near the bottom links to document poor CRR! (factory test) which seems odd (given their claim) since the test was at continental and it is their tire:
http://forums.bicycling.com/eve/forums/ ... 7281083802
The link at the bottom is this:
http://www.rouesartisanales.com/article-1503651.html
Where in they state testing done at continental, bottom right table for clinchers (click on it) has these tires
near the bottom. I must say that the cyclingnews review (probably most trustworthy?) claimed they rolled well which would seem to contradict this data.

I was having fun with the Black Chili comment but anyway I did, on my first search find several more comments claiming good cold weather rolling resistance. Continentals writeup about nano particles and all makes sense but then it is a sales pitch

I have been riding gatorskins which don't have Black Chili and I can tell, especially when it gets cold

Can you tell I am snowed in today?

Russ

[lorduintah]

Russ -

I have the Conti 4000S (Chili) tires on my bike - three sets of wheels. They are doing quite well. I also read about the Force pair as being more susceptible to flats. The only flat I have had was due to a pinched tube at the valve stem feedthrough. That was a direct result of my pump holding on to the stem too robustly and the force to pull it off did a number on the tube. The table of Crr values - the Conti's listed are 3000 series - in fact the whole table is old - Michelins are at Pro 3.

Here is some Crr data from 2008: http://www.biketechreview.com/tires/AFM_tire_crr.htm
Tom

[rruff]

Disregard any tests or comments of Attack/Force pre-Black Chili performance... the new tread compound apparently makes a huge difference. It's the real deal... great stuff. Longer lasting than any tire I've used (the GP4000S that is). Since the Attack/Force is using the same casing and tread compound, they should perform the same as the GP4000S.

Back to the OPs question, I think that .005 is still a good Crr to use... maybe a bit lower if he's using ultra light tubes, but in the ballpark anyway. And yes, your expected CdA would be around .35 on a road bike depending on details of your position and clothing.

[chrus101]

Hi,

Most of the roads are reasonably smooth here in Adelaide, at least compared to what I was used to in New Zealand. The aggregate used is a lot finer. Still get rough areas though where the roads need resurfacing.

Maybe the marketing people at Assos would like to claim that their clothes are giving me a low CdA

Will have another go next week. Will start by trying to find a longer road for the calibration ride. Had a slight tail wind on the coast downs last time, I assume that is accounted for. Coast downs last ca 20-25 s according to my rough estimate...counting 1001, 1002, 1003, etc as I coast down.

Cheers, Chris

[Russ]

lorduintah and rruff,
Very helpful posts! And that new CRR data is what I have been trying to find!
double thanks,
Russ

[chrus101]

Hi,

Btw, tubes are just 80-90 g rubber ones, not those 50-60 g lightweights.

Second that Russ, interesting links.

Cheers, Chris

[coachboyd]

I took a look at the coastdowns and they look good. When I adjust the crr to .0033 it makes all the aero values for the coastdowns very similar. .0033 seems kind of low for a crr, but I also noticed that your riding tilt was -.6 (kind of low as well). So I saved your profile, changed the riding tilt to -.4 to adjust the crr. That gave me a cda of .351 and a crr of .0053 (which is right in line with what I would expect). I have attached the profile on here that you can use.

Let me know if you have any questions.

[Velocomp]

I looked at your Cal Ride file.

You're riding on some pretty good tires, which means your actual Crr is likely less than the iBike's default value of 0.0055.

In fact, if you look at the Cal Ride file with the Profiles/Download CoastDown and Cal Ride... option, and set Crr to "Crr adjusted" (meaning that the iBike measures and adjusts both CdA AND Crr values) your measured Crr was 0.0024.

Note that the iBike's measured number from your coastdowns is considerably lower than the assumed number of 0.0055.

Is the measured Crr number correct? Crr is somewhat difficult to measure, but the biketechreview site shows a measured Crr of your tire to be 0.00307. In fact, it appears likely that your bike's Crr is lower than the assumed value of 0.0055.

When the assumed Crr of 0.0055 is higher than the actual Crr, the watts caused by rolling resistance will be higher than actual. So, to get total watts into balance, the CdA must be forced DOWN to compensate accordingly. That is why your CdA is low when Crr is 0.0055

All of this is a long way of saying that, using a fixed value of 0.0031 for Crr in the Analyze Profile window, your CdA rises to .361, completely normal for a road bike.

This is an example where, with good care in calibrations, and with good tires on your bike, you should override the iBike's "average" settings to get even better results.

To create this profile for your iBike, open up your Cal Ride in the Profiles menu. Then, select "Crr Fixed" in the middle window, then enter 0.0031 as the determined value. You'll get a measured CdA of .361. Load this profile into your iBike.

coastdown.jpg (154.47 KiB) Viewed 15355 times

[Russ]

The biketechreview link pdf header information says this:
"Tacx Rollers (79 mm PVC - 26 cm spacing) with Hinged Front Fork Mount"

Now this is great for apples to apples comparison between tires and between tests, also was at 65-75F.
but
The discussions of curvature of the load surface for trainers increasing CRR but offset by totally smooth surface makes me wonder just how transferable the CRR numbers are to the road. And of course seems the 'black chili' offsets the presumably lower temperature affect, at least somewhat. I presume Chris has summertime conditions where he is so maybe the temperature for him is even higher.

I noticed they have a 'bumpy data' test result pdf at the bottom of the link. They did test (pre chili?) GP4000 in that mix. Interesting how much the CRR went up with 1000hz vibration condition.

Any thoughts?
Russ

[chrus101]

Hi all,

Thanks for your efforts Coachboyd, John & Russ.

John, I also noticed that when "Crr adjusted" was used that a sensible CdA was generated as well as a low Crr. I had discounted this option due to the low Crr. I had thought the lowest reasonable value would be 0.004 when riding on a smooth concrete surface. I wonder how the suggested Crr 0.004-0.0055 values were obtained? Agree with Russ when he questions transferability of absolute Crr values determined on a tacx trainer to road surfaces. However, my coastdowns were done on a smooth road so maybe not too unlike the tacx trainer surface?

Coachboyd: As soon as I get an opportunity I will do a longer calib ride to check the tilt. If -0.6 is a bit low but is reproduced in the longer ride I can think of two reasons for this:
i. poor bike setup, too much weight forward. Coincidentally, I am booked into see some fitting guru in a couple of months...long waiting list.
ii. I did about 20 year of weight training before taking up this cycling lark so my upper body is a bit bigger than the average cyclist, therefore more weight over the front wheel than normal


Cheers, Chris

[chrus101]

Hi,

Russ: Prob ca. 30 degC (85 F) when I did my calib ride/coast downs.

Coachboyd: Just noticed in my earlier thread "Newbie calibration question" that the first time I did a calib ride (ca. 4 months ago) I got a tilt of -0.3. This was before I changed my bike setup: moved seat forward, altered cleats and installed a longer stem. What sort of riding tilt would you expect? Maybe this is a way of checking at least part of your bike fit/positioning:?


Cheers, Chris

[chrus101]

Hi,

Found this in the BikeTechReview forum.
http://biketechreview.com/forum/viewtop ... f=1&t=2552

kraig wrote:
1) On rough chip seal roads multiply Al’s reported Crr values by 1.3
2) On regular roads multiply Al’s reported Crr values by 1.1


Al's data is that which is referred to in the biketechreview link referred to by lorduintah and John.

Cheers, Chris