AeroPod, PowerPod and Newton

I don't mean "Would a different set of wheels or thicker tubes show up in the cda or crr?' (Since, I bought my Ibike on Ebay I question my Ibikes condition). What I AM asking is: Since I have high Crr(.58) and Cda(.01) values in some of my old profiles, with a bad mount SETUP (as mentioned in my other posts), and gusting winds (also mentioned), and the fact that I use thorn resistent tubes (when I am not trying to set personal records or race!). In what coefficient do the tubes, wheels or tires factor in to? I guess the tires and road surface factor in to the Crr. Another specific question, please! Is a Crr of .01 understandable if I use thorn resistent tubes? Or, if the TR tubes show up in the cda #'s, is a factor of .6 acceptable? Please, assume my mount is good, and I am going to do a set of coastdowns and 4 mile ride, but that if I use the thorn resistent tubes, which value would they factor in to. Thanks.

P.S. Last time I asked this I was told that thorn resistent tubes would slow me down. I know! That is why I made my question above so 'WORDY"! No offense to whoever you were!

The choice of wheels will have a mild impact on the CdA value.
The choice of tubes & tires will have a significant impact on Crr.

Personally, I don't have experience with different types of tubes because I always ride with Butyl, but this is a good resource of Crr measurements on rollers and shows data comparing Butyl to Latex tubes: http://www.biketechreview.com/tires/AFM_tire_crr.htm. This might give you an idea of what kind of magnitude to expect. For Butyl vs. Latex, you're looking at something in the ballpark of +0.0003 to +0.0006 on Crr.

(Note that real world Crr values will be slightly higher in general compared to what you see in the tables in the link above because tires run cooler on the road.)

But don't get lost in the details. All you need to do is get out and do a set of coast-downs and 4 mi ride the next time you have decent wind conditions. Get the good drag measurement and then you don't have to worry about it. Once you get some experience with a good profile, then it is time to start experimenting with the different things that impact drag.

It sounds like you had some not so good variables going on when you did your last profile. I would recommend going through the whole process again, making sure you have everything set up perfectly (mount tight, good tilt and wind offset calibration). Find a day with calm wind conditions and a good road with little to no traffic (there can not be any cars passing you in either direction when you do your coastdowns). When you do your coastdowns, try to find a good bowl shaped road so you can easily get up to speed on the downhill and let the uphill slow you down (max grade of about 3%).

Since the values you get for this will affect your accuracy for as long as you ride this profile, it's best to take the time to make sure that everything is set up good. If you want, you can send me the calibration file (when you make your profile, hit the button that says "save cal ride file") and I will take a look at it and tell you how it looks.

travispape wrote:(Note that real world Crr values will be slightly higher in general compared to what you see in the tables in the link above because tires run cooler on the road.)

I'd say on average you could add about .002 to those Crr numbers. I don't think it's the temperature (Al tries to correct for this) so much as scaling from a little roller to a flat surface is an inexact science... plus rollers are very flat and smooth and most roads aren't.

rruff wrote:

travispape wrote:(Note that real world Crr values will be slightly higher in general compared to what you see in the tables in the link above because tires run cooler on the road.)

I'd say on average you could add about .002 to those Crr numbers. I don't think it's the temperature (Al tries to correct for this) so much as scaling from a little roller to a flat surface is an inexact science... plus rollers are very flat and smooth and most roads aren't.

I've had pretty good experience with merely multiplying those numbers by 1.5 for the roads in my area...that's for the tires on the upper end of that list, of course.

rruff wrote:

travispape wrote:(Note that real world Crr values will be slightly higher in general compared to what you see in the tables in the link above because tires run cooler on the road.)

I'd say on average you could add about .002 to those Crr numbers. I don't think it's the temperature (Al tries to correct for this) so much as scaling from a little roller to a flat surface is an inexact science... plus rollers are very flat and smooth and most roads aren't.

Maybe Tom could comment on the temperature thing, but I think numbers in the tables are as measured on the rollers without temperature compensation. They had already done a lot of work before they observed the temperature phenomenon (and started measuring temperature) and decided against going back and remeasuring all the old data and rather to continue reporting at the elevated temperature.