Coastdown for crr

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hoosier1981
Posts: 27
Joined: Fri Sep 08, 2017 2:00 pm

Coastdown for crr

Post by hoosier1981 »

Would it be possible to use a coastdown to calculate crr for those of us with a wide variety of tires? I've seen something similar to this on Kurt Kinetic fluid trainers with their 'inRide' power meter. Without calibration it's 'close' within 20-40 watts but then hits within a few watts after using a coast down from high speed. -That particular meter reads based of speed of the trainer fly-wheel (similar to how the Powerpod works in trainer mode).
Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 8:43 am

Re: Coastdown for crr

Post by Velocomp »

They're not really measuring Crr; what they are doing is measuring the rolling resistance of their flywheel.
John Hamann
GooseRider
Posts: 58
Joined: Wed May 10, 2017 10:52 am

Re: Coastdown for crr

Post by GooseRider »

I did coast downs for CdA and Crr. I used the calculations described on: https://www.bikeforums.net/33-road-bike ... meter.html

Here are the equations:

alpha * Crr + beta * CdA = gamma

where

alpha = -mg*integral(v)
beta = -0.5*rho*integral(v^3)
gamma = 0.5 m(vf^2-vi^2) - mgh

By doing one coast down from high speed and another coast down from low speed on the exact same stretch of road with no wind you have two equations with two unknowns to solve. Unfortunately, to really know Crr, you need to know the elevation difference between beginning and ending 'h' to less than 1 foot accuracy. You can find lidar elevation maps at that accuracy. Unfortunately, I couldn't find for my area. Instead I did a coast down from high speed in the opposite direction on the same stretch of road and used 'h' as free parameter to get same Crr in both directions. The CdA estimation was much less sensitive to the input variables.

Ideally, I think you want to do many coast downs and then average out Crr and CdA.

I did this since I am running Schwalbe Pro One tubeless 700x28c with 70psi front and 85psi rear. My rear tire actually measures 32mm with a caliber due to my wide carbon rims. This tire has one of the lowest rolling resistances so I was curios. My best estimate based on measurements is Crr=0.0034 for this condition.

Morten
hoosier1981
Posts: 27
Joined: Fri Sep 08, 2017 2:00 pm

Re: Coastdown for crr

Post by hoosier1981 »

WOW, that's interesting. I think I'm comfortable enough with the default numbers recommended for my road bike.
But I might try this for my 5" snow tires with 7mm lugs, haha. And my 2.8" aggressive tires...
I see pretty crazy numbers for some of these fat tires...for instance:
https://www.bicyclerollingresistance.co ... helga-2016
Rolling Resistance Coefficient (Crr) 8 psi / 0.6 Bar 0.01529
Raise the pressure to 20, and the crr was back under .01 (I'd blow my tire off the rim).
I think my Surly Lou 4.8 might fall in that area.

That site does have my roadbike tire listed which falls right in the area that isaac set it to.
GooseRider
Posts: 58
Joined: Wed May 10, 2017 10:52 am

Re: Coastdown for crr

Post by GooseRider »

Would tend to agree about the default numbers being good enough, given the accuracy needed in the measurements to get better numbers and also considering that rolling resistance is not a big contributor -- only a few watt differences between plausible rolling resistances.

Morten
Velocomp
Velocomp CEO
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Joined: Fri Jan 11, 2008 8:43 am

Re: Coastdown for crr

Post by Velocomp »

GooseRider wrote:Would tend to agree about the default numbers being good enough, given the accuracy needed in the measurements to get better numbers and also considering that rolling resistance is not a big contributor -- only a few watt differences between plausible rolling resistances.

Morten
Your analysis is spot-on.
John Hamann
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