Crr calculation

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prato-77
Posts: 5
Joined: Tue Mar 01, 2016 5:48 pm

Crr calculation

Post by prato-77 »

Hi all
I am thinking why we can't use aeropod also to determine Crr.
Crr is on the equation of motion of cyclist as well as CdA. So if we fix CdA, we could calculate Crr as only unknown factor of the equation.
This could be helpful when in example your tests go a little bit too long and temperature of ambient and asphalt change. Maybe in the middle we could correct this parameter and obtain the most precise value for CdA.
Of course could be interesting also know our real Crr of our tires. Thanks.
Velocomp
Velocomp CEO
Posts: 7354
Joined: Fri Jan 11, 2008 8:43 am

Re: Crr calculation

Post by Velocomp »

prato-77 wrote: Fri Aug 20, 2021 5:09 am Hi all
I am thinking why we can't use aeropod also to determine Crr.
Crr is on the equation of motion of cyclist as well as CdA. So if we fix CdA, we could calculate Crr as only unknown factor of the equation.
This could be helpful when in example your tests go a little bit too long and temperature of ambient and asphalt change. Maybe in the middle we could correct this parameter and obtain the most precise value for CdA.
Of course could be interesting also know our real Crr of our tires. Thanks.
You are 100% correct. If you hold the value of CdA constant, you can measure Crr.

Here is an example:

1) I am a bit lazy and normally ride with tires inflated to about 80 PSI. It's a soft ride... :-)

2) At 80 PSI, I use a value of 0.0054 for my Crr, and over time I've determined my measured, "normal" CdA is 0.408.

3) When I pump up my tires to 110PSI, as I ride I can feel the reduced road friction (I'm riding in the same position). Reduced road friction means reduced Crr.

4) On the bike, with the higher tire pressure my on-the-road CdA goes DOWN. Why? My AeroPod is using the "soft" value of 0.0054 for Crr, causing rolling resistance watts to be higher than actual DFPM watts causing total opposing watts measured by AP to be too high. How high? For this ride, my average opposing watts were 138.W (yes, I know this is not world beating--that's another story :D )
Screen Shot 2021-08-20 at 1.02.27 PM.png
Screen Shot 2021-08-20 at 1.02.27 PM.png (245.15 KiB) Viewed 535 times
and my average DFPM watts were 133.7W
Screen Shot 2021-08-20 at 1.03.43 PM.png
Screen Shot 2021-08-20 at 1.03.43 PM.png (378.18 KiB) Viewed 535 times
AP therefore adjusts reported on-the-road CdA DOWN to get to the target of 133.7W.

5) Isaac CdA analysis shows the same thing; for the condition of Crr = .0054, Isaac Tools/CdA shows a measured CdA of 0.395.
Screen Shot 2021-08-20 at 12.59.01 PM.png
Screen Shot 2021-08-20 at 12.59.01 PM.png (161.48 KiB) Viewed 535 times
6) I use the command "Analyze/Tweak CdA, Crr..."
Screen Shot 2021-08-20 at 12.59.28 PM.png
Screen Shot 2021-08-20 at 12.59.28 PM.png (66.85 KiB) Viewed 535 times
to tweak the value of Crr (downwards, because my tires are at higher pressure). My goal is to find a value of Crr that makes total opposing watts equal to DFPM watts of 133.7W

7) After a bit of experimentation, I determine that a tweaked value of Crr of 0.0047 makes opposing watts nearly equal to applied watts.
Screen Shot 2021-08-20 at 1.07.18 PM.png
Screen Shot 2021-08-20 at 1.07.18 PM.png (270.13 KiB) Viewed 535 times

So, for my tires and wheels, the benefit of the higher tire pressure is a reduction of Crr from 0.0054 to 0.0047.

There are two lessons from this exercise:

1) When doing CdA measurements, make sure your tires are inflated to the values you normally use

2) It is possible to do Crr measurements using your ride files and Isaac.
Attachments
Screen Shot 2021-08-20 at 12.59.01 PM.png
Screen Shot 2021-08-20 at 12.59.01 PM.png (161.48 KiB) Viewed 535 times
John Hamann
AN2019
Posts: 27
Joined: Sat Oct 05, 2019 6:31 pm

Re: Crr calculation

Post by AN2019 »

Velocomp wrote: Fri Aug 20, 2021 1:38 pm
prato-77 wrote: Fri Aug 20, 2021 5:09 am Hi all
I am thinking why we can't use aeropod also to determine Crr.
Crr is on the equation of motion of cyclist as well as CdA. So if we fix CdA, we could calculate Crr as only unknown factor of the equation.
This could be helpful when in example your tests go a little bit too long and temperature of ambient and asphalt change. Maybe in the middle we could correct this parameter and obtain the most precise value for CdA.
Of course could be interesting also know our real Crr of our tires. Thanks.
You are 100% correct. If you hold the value of CdA constant, you can measure Crr.

Here is an example:

1) I am a bit lazy and normally ride with tires inflated to about 80 PSI. It's a soft ride... :-)

2) At 80 PSI, I use a value of 0.0054 for my Crr, and over time I've determined my measured, "normal" CdA is 0.408.

3) When I pump up my tires to 110PSI, as I ride I can feel the reduced road friction (I'm riding in the same position). Reduced road friction means reduced Crr.

4) On the bike, with the higher tire pressure my on-the-road CdA goes DOWN. Why? My AeroPod is using the "soft" value of 0.0054 for Crr, causing rolling resistance watts to be higher than actual DFPM watts causing total opposing watts measured by AP to be too high. How high? For this ride, my average opposing watts were 138.W (yes, I know this is not world beating--that's another story :D )

Screen Shot 2021-08-20 at 1.02.27 PM.png

and my average DFPM watts were 133.7W

Screen Shot 2021-08-20 at 1.03.43 PM.png

AP therefore adjusts reported on-the-road CdA DOWN to get to the target of 133.7W.

5) Isaac CdA analysis shows the same thing; for the condition of Crr = .0054, Isaac Tools/CdA shows a measured CdA of 0.395.

Screen Shot 2021-08-20 at 12.59.01 PM.png

6) I use the command "Analyze/Tweak CdA, Crr..."

Screen Shot 2021-08-20 at 12.59.28 PM.png

to tweak the value of Crr (downwards, because my tires are at higher pressure). My goal is to find a value of Crr that makes total opposing watts equal to DFPM watts of 133.7W

7) After a bit of experimentation, I determine that a tweaked value of Crr of 0.0047 makes opposing watts nearly equal to applied watts.

Screen Shot 2021-08-20 at 1.07.18 PM.png


So, for my tires and wheels, the benefit of the higher tire pressure is a reduction of Crr from 0.0054 to 0.0047.

There are two lessons from this exercise:

1) When doing CdA measurements, make sure your tires are inflated to the values you normally use

2) It is possible to do Crr measurements using your ride files and Isaac.
Excellent thread. I'm trying to get a sense of a good, practical number for my Crr. The pavement here locally is not great, probably slightly worse than the average here in the US. Bicycle Rolling Resistance puts my tires at around 0.0035 at 100psi, but this is on a steel drum. I also tend to run my tires at 90psi. Is there a good way to determine a reasonable, everyday Crr number?
Velocomp
Velocomp CEO
Posts: 7354
Joined: Fri Jan 11, 2008 8:43 am

Re: Crr calculation

Post by Velocomp »

AN2019 wrote: Sun Oct 10, 2021 11:50 am

Excellent thread. I'm trying to get a sense of a good, practical number for my Crr. The pavement here locally is not great, probably slightly worse than the average here in the US. Bicycle Rolling Resistance puts my tires at around 0.0035 at 100psi, but this is on a steel drum. I also tend to run my tires at 90psi. Is there a good way to determine a reasonable, everyday Crr number?
If you're running at 90PSI, I would use our default value of 0.0054. This has been established over the past 15 years to be a very consistent number.
John Hamann
AN2019
Posts: 27
Joined: Sat Oct 05, 2019 6:31 pm

Re: Crr calculation

Post by AN2019 »

Velocomp wrote: Sun Oct 10, 2021 12:20 pm
AN2019 wrote: Sun Oct 10, 2021 11:50 am

Excellent thread. I'm trying to get a sense of a good, practical number for my Crr. The pavement here locally is not great, probably slightly worse than the average here in the US. Bicycle Rolling Resistance puts my tires at around 0.0035 at 100psi, but this is on a steel drum. I also tend to run my tires at 90psi. Is there a good way to determine a reasonable, everyday Crr number?
If you're running at 90PSI, I would use our default value of 0.0054. This has been established over the past 15 years to be a very consistent number.
It’d be interesting to know how you all came to this number. Is there a general guideline for different tire sizes and PSI?

A totally different question: is there a function in Isaac that will output a table with power difference between Newton and DFPM over a specific period of time? There’s a graph, but difficult to extract numbers at specific points.
Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 8:43 am

Re: Crr calculation

Post by Velocomp »

I run my tires at about 90PSI, and I have been making comparative measurements for about 15 years now. So, you are getting the benefit of my experience.

You can export your ride files in Isaac into csv format. You will get both PowerPod and DFPM data in the exported file.
John Hamann
AN2019
Posts: 27
Joined: Sat Oct 05, 2019 6:31 pm

Re: Crr calculation

Post by AN2019 »

Velocomp wrote: Mon Oct 11, 2021 12:29 pm I run my tires at about 90PSI, and I have been making comparative measurements for about 15 years now. So, you are getting the benefit of my experience.

You can export your ride files in Isaac into csv format. You will get both PowerPod and DFPM data in the exported file.
That’s helpful.

Just out of curiosity, what kind of tires do you run?
Velocomp
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Re: Crr calculation

Post by Velocomp »

Continental GatorSkin
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