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
)
- Screen Shot 2021-08-20 at 1.02.27 PM.png (245.15 KiB) Viewed 4850 times
and my average DFPM watts were 133.7W
- Screen Shot 2021-08-20 at 1.03.43 PM.png (378.18 KiB) Viewed 4850 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 (161.48 KiB) Viewed 4850 times
6) I use the command "Analyze/Tweak CdA, Crr..."
- Screen Shot 2021-08-20 at 12.59.28 PM.png (66.85 KiB) Viewed 4850 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 (270.13 KiB) Viewed 4850 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.