are CdA and Crr co-related?

[donnybike]

I plan to do coast downs on the paved road.
First set of coast downs will be done with regular procedure.
This may prodece cda=0.356 and crr= 0.0040.

And second set of coast downs will be performed with fixed crr 0.0080.
In this case, what value of cda can I expect?
Will cda with 0.0080 be the same with the former, more and less?

[Russ]

Hi donnybike,

I have observed, as to be expected, a variety of CRR's on different pavement. This however is only directly seen as variations in CDA on the Aero or Newton+ with cda on and coasting, usually downhill.

With the above in mind, I suggest only trying to establish CDA, ideally, on a smooth pave road. In Isaac, you set the CRR to fixed and choose your best estimate for you tires, using the provided estimator should be fine. Then if you want to do more coastdowns on rougher pavement, in Isaac, change the fixed CRR value upwards until you get the same CDA. If the CRR is out of line when you achieve the CDA match, either your CDA, original choice of CRR or variable conditions have impacted your attempt.

These are my observations over the years, it is a bit tough to workout but on lucky days it should work. I hope it is easier with OS2 and the Newton over Gen III. I have not really tinkered with this since Gen III. I have, however observed the same CDA's reported real time coasting on my Newton+ except they seem more predictable and repeatable than in the past.

I would like to see Velocomp add a feature in OS2 FW to save the CDA measurements that are taken during the ridetime coastings for post ride use and analysis. It is a real pain to try to remember/record on paper the readings out on the road. To be clear, I am speaking of the iBike with out a direct force power meter.

Regards,
Russ

[rruff]

Will cda with 0.0080 be the same with the former, more and less?

CdA will be quite a bit lower.

[Russ]

I wonder if I misunderstood the question!

What in the world would change the CDA? I presume with the question that donnybike would be riding on rough terrain or the like where the CRR would be higher. If nothing else on the bike changes, then the CDA should stay the same. Otherwise .008 is incorrect.
It sounded like donnybike wanted to perform some coast downs on much rougher and harder rolling ground or pavement with the higher .008 as a starting point in the calibration analysis. No?

Now given rruf''s answer then I can only figure that you would be changing the CRR from the other value to see the relation or maybe to correct an earlier wrong profile?

Anyway rruf I would agree in those cases but not if my original presumption is valid.

Regards all,
Russ

[racerfern]

I plan to do coast downs on the paved road. First set of coast downs will be done with regular procedure. This may prodece cda=0.356 and crr= 0.0040.

And second set of coast downs will be performed with fixed crr 0.0080. In this case, what value of cda can I expect? Will cda with 0.0080 be the same with the former, more and less?

IF you are doing the coastdowns on the same piece of road in as close as possible to the same conditions, the second set of coastdowns will yield lower CdA numbers. Your total drag (Cd) is the combination of CdA and Crr. If you increase Crr artificially you will have to have lower CdA to achieve the same total drag.

It's relatively easy to determine total drag slowing you down, it's quite another thing to separate CdA from Crr.

[donnybike]

Thanks, everyone!

I try to set a practical guideline of profiles for MTB.

IBike on MTB will be used to measure power on the paved road as well as dirt road.
Therefore, I should have two profiles accounted for each case.

I found out that the Fast Start is not designed for setting MTB up, especially in relation to tire type and tire grade. So, I guess that it is best to do coast downs to determine a unique CdA and Crr in case of MTB.

Therefore, I consider to follow the setup procedures below;
(******suspension locked ******)

For training on the paved road,
Fast Start to enter the rider information
Cal Ride to determine riding tilt and wind scaling factor
perform coast downs on the paved road to determine CdA and Crr


For riding on dirt road,
Fast Start (done already)
Cal Ride (done already)
perform coast downs on the PAVED ROAD to determine CdA, but with fixed Crr 0.0080 (Crr is fixed under Est Fric in Racer Sequence. Crr 0.0080 is chosen to mimic the dirt road)
As a result, the profile for dirt road consists of the determined CdA and prefixed Crr.


We may perform another coast downs on actual dirt road to find out realistic CdA and Crr. However, it may be very impractical. That is why I choose the procedure above.

Is it acceptable?Any comments?

[GWPOS]

crr should be more like 0.0100-0.0130 range for a mtb. I used it on my mtb for 3 years. Bigger/wider/heavier tires, lower tire pressures etc give you a higher crr than 0.008.

[donnybike]

crr should be more like 0.0100-0.0130 range for a mtb. I used it on my mtb for 3 years. Bigger/wider/heavier tires, lower tire pressures etc give you a higher crr than 0.008.

Thanks,

On dirt road, right?

What was the range of Crr on the Paved Road with your MTB?

[donnybike]

crr should be more like 0.0100-0.0130 range for a mtb. I used it on my mtb for 3 years. Bigger/wider/heavier tires, lower tire pressures etc give you a higher crr than 0.008.

And how did you determine your Crr?
Did you do actual coast downs on a streach of dirt road?

[Russ]

donnybike,

You said:
"perform coast downs on the PAVED ROAD to determine CdA, but with fixed Crr 0.0080 (Crr is fixed under Est Fric in Racer Sequence. Crr 0.0080 is chosen to mimic the dirt road)"

I think you are missing an important point. Racefern said:
"Your total drag (Cd) is the combination of CdA and Crr. If you increase Crr artificially you will have to have lower CdA to achieve the same total drag.

It's relatively easy to determine total drag slowing you down, it's quite another thing to separate CdA from Crr."

This is important, allow me to elaborate a bit. And bye the way, the .004 CRR with MTBK tires is probably artificially low and so would yield a correspondingly high CDA. Now with that said, in my view, the best starting point is to establish your CDA and CRR on the smoothest pavement road you have handy. Think of this as a baseline.

Now unfortunately, the separating of CdA and Crr from each other in the coast down is very challenging and so usually, especially with common road bike tires, typical values are set into the "fixed CRR" option and the CdA falls out fairly close. But taking several sets of coast downs with Crr adjusted, the values begin to settle out reasonably well.

So the problem with what you are planning to do, with the .008 Crr on the smooth road, assuming .008 is higher than is correct, will confuse things a bit more as neither value that you get would be correct.

My main original point, perhaps not clearly stated, is that for a given ride position, kit and tires/pressure your CdA would be the same on and off road. So once you establish your CdA with reasonable accuracy, you should lock that in and try some coast downs off road. There I would try the Crr adjusted to see what comes out and if not very close on the CdA side, then use Crr fixed and change the values until you match the CdA from your road coast downs. This should give you a better shot at identifying your off road Crr value. Or just go with GWPOS values for Crr and ride.

Now a comment back to the .004 Crr. That is an optimistic value for excellent road bike tires on near perfect surface. So I am doubtful that your mtbk tires are that low on the road. Without searching the web, I would bet .006 might be on the great side, possibly a bit lower???

Regards,
Russ

edit: I just reread GWPOS and if his .010 Crr is on the road, then you need to take that into account. Also if you 'saved' your cal ride file then you can reopen it and try the higher Crr values to see what happens to the CdA.

[GWPOS]

I did coast downs etc, but always got weird results from the ibike software, so after playing and using it enough I was able to get it close to what my efforts were on my road bike. I used some online tools and such to compare ride files of 20 minutes or more, which enabled me to get my crr close for that particular fire road. However that being said the terrain varies so much on mtb bike trails and fire roads that getting your crr dialed in is impossible, and then adding in the varriety of different tires, air pressures, and wheels used on mtb's. All you can do is get it pretty close. As for crr on the road with a mtb I never went that route, I just got a road bike and had a seperate profile for it.

[racerfern]

We keep talking about MTB tires but there are so many variations. Aggressive tread for sand/mud, almost slick for hard stuff, narrower or wider, tubeless, tubed, 26 inch, 29 inch...

I think you need to pick a number from the most efficient .006? to the most inefficient .01, maybe even a bit more. Search some websites and pick some numbers.

Next thought, as far as CdA I'm not sure how critical CdA is on "typical" mountain bike terrain. Perhaps you should set CdA and test for Crr with various tires.

[donnybike]

Hi,
Now I got some idea on setting profile up for mtb.
Please look at the title of my post, "Are CdA and Crr co-related?" !
In this regard, "racerfern" gave me a clear cut answer, explaining with total drag.
that was exactly what i was looking for.
it is fine with me to use ibike as a guideline tool for my effort on the dirt road.
That's why I was looking for a Practical Way of setting profile up.
in this regard too, someone suggested practical values in relation to Crr.
thanks all!

[rruff]

crr should be more like 0.0100-0.0130 range for a mtb. I used it on my mtb for 3 years. Bigger/wider/heavier tires, lower tire pressures etc give you a higher crr than 0.008.

Don't forget the bumps and the softness of the dirt being big influences.

[rruff]

That's why I was looking for a Practical Way of setting profile up.

Instead of fixing Crr and computing CdA, you might instead get a good handle on CdA with good tires on the road, then fix CdA (bump it up a little for fat knobby tires) and compute Crr, with your MTB tires. Then add some more to Crr depending on the type of dirt.

Power accuracy will never be good regardless. Suspension will play havoc with the tilt.