coast down values
coast down values
I just did 15 coast downs and a 4 mile ride. I came up with Aero .280, CDA .322, Fric. 8.125 and Crr .0062. This is my 3rd attempt due to weather to get a good profile. Do these look reasonable for riding in the hoods with 140 psi 700x23 tires?
Last edited by scittkit on Wed Feb 27, 2008 5:50 pm, edited 1 time in total.
Re: coast down values
I would say they look reasonable......................................if you are a bit chubby and kinda slow.
My numbers - Aero 0.477, CdA 0.391, Fric 4.068, Crr 0.0042
Unfortunantly I can't tell you what any of that really means.
My numbers - Aero 0.477, CdA 0.391, Fric 4.068, Crr 0.0042
Unfortunantly I can't tell you what any of that really means.
Re: coast down values
Chubby? You mean my Jan Ulrich type thighs? The friction was my concern. The profile for the Bianchi has the same friction value(.4), but the Kuoat profile ends up with twice the friction(.8).
Re: coast down values
The sample profile uses a friction of 7.760 so I would guess that is an average number....... So just think of your Kuota as being above average!
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davidpuglia
- Posts: 13
- Joined: Fri Jan 11, 2008 1:12 pm
Re: coast down values
This looks reasonable - I've seen frictional numbers in my profiles ranging from 5.5xx to 8.xx.
Re: coast down values
You want chubby and slow, I'll give you chubby and slow.
Aero .380
CdA .323
Fric 10.552
Crr 0.0056
This is the average from my second set of coastdowns after I lost a couple of pounds and got better tires. Not bad for a beach cruiser with a surf board strapped to the side.
JK
Aero .380
CdA .323
Fric 10.552
Crr 0.0056
This is the average from my second set of coastdowns after I lost a couple of pounds and got better tires. Not bad for a beach cruiser with a surf board strapped to the side.
JKFernando
Re: coast down values
When we first shipped the iBike we reported "iBike drag coefficients" only: aero and frictional drag. You can see these numbers in the Totals screens of your iBike.
Since introducing iBike 2 and the new calibration procedure, (multiple coast down and 4 mile ride), you can also see the drag numbers reported in the much more conventional expression of CdA (Cd = coefficient of drag, A = frontal surface area of the rider, CdA = Cd x A) and Crr (coefficient of rolling resistance).
The iBike aero drag number is close, but not identical, to CdA. The iBike frictional drag is much more complicated because it also factors in rider weight and bike-specific tilt calibration. The relationship between iBike frictional drag and Crr is not at all obvious...
Note that it is totally appropriate to compare CdA and Crr numbers between iBike owners, but it is NOT appropriate to compare the old iBike drag numbers between riders.
CdA varies conventionally between 0.3 and 0.45, depending on rider size, bike type, and riding position (lower is BETTER). Crr varies mostly due to road type: wood track is about 0.001, rough asphalt is about 0.008 (again, lower is better).
Scittkit has a CdA of 0.322 and Crr of 0.006: both very reasonable. Robert has a significantly higher CdA of 0.391. And remember that of the two drag numbers, CdA is the most important because aerodynamic resistance varies as the CUBE of the speed. In practical terms, this means that it requires 8 times as much power to overcome a 20 mph headwind as a 10mph headwind. However, the lower the CdA, the less this cubic truth hurts the legs...
The secret benefit of the iBike CdA and Crr numbers is this: if you learn to pay attention to your drag coefficients, and work to reduce them, you can go faster, EASIER.
We will be saying a LOT more about this in the coming months. Stay tuned!
Since introducing iBike 2 and the new calibration procedure, (multiple coast down and 4 mile ride), you can also see the drag numbers reported in the much more conventional expression of CdA (Cd = coefficient of drag, A = frontal surface area of the rider, CdA = Cd x A) and Crr (coefficient of rolling resistance).
The iBike aero drag number is close, but not identical, to CdA. The iBike frictional drag is much more complicated because it also factors in rider weight and bike-specific tilt calibration. The relationship between iBike frictional drag and Crr is not at all obvious...
Note that it is totally appropriate to compare CdA and Crr numbers between iBike owners, but it is NOT appropriate to compare the old iBike drag numbers between riders.
CdA varies conventionally between 0.3 and 0.45, depending on rider size, bike type, and riding position (lower is BETTER). Crr varies mostly due to road type: wood track is about 0.001, rough asphalt is about 0.008 (again, lower is better).
Scittkit has a CdA of 0.322 and Crr of 0.006: both very reasonable. Robert has a significantly higher CdA of 0.391. And remember that of the two drag numbers, CdA is the most important because aerodynamic resistance varies as the CUBE of the speed. In practical terms, this means that it requires 8 times as much power to overcome a 20 mph headwind as a 10mph headwind. However, the lower the CdA, the less this cubic truth hurts the legs...
The secret benefit of the iBike CdA and Crr numbers is this: if you learn to pay attention to your drag coefficients, and work to reduce them, you can go faster, EASIER.
We will be saying a LOT more about this in the coming months. Stay tuned!
John Hamann
Re: coast down values
Thank you John! That explains alot of what I am looking at. Plus I have always known Robert was a bigger drag.
Re: coast down values
Thanks for the great explination! I think the more people understand about the coast down and 4 mile ride info the better! Too often I have seen it referred to as more of a set-up chore instead of a valuable tool that can give you some great data!
You should be thankful I have a higher CdA since you are always benifiting from my draft..... Now that I know what to work on, your view won't change, but you will get to see me ride away faster!
You should be thankful I have a higher CdA since you are always benifiting from my draft..... Now that I know what to work on, your view won't change, but you will get to see me ride away faster!