2 questions

[jasonod]

1) what determines the Fric variable, the tires and anything else? The Fric for my road bike profile is 3.727 using GP 4000 Conti clinchers and for my TT bike it's 4.14 using new Conti Gator Skin tubulars. I would think I'd have a lower number with the tubulars b/c less rolling resistance with the higher PSI?

2) if I were to replace my 19mm front tubular with a different tire size, let's say 22mm, do I have to repeat the coast downs and 4-mile ride all over again or can I just put in the new wheel circumference and go from there?

Happy iBike/iAero user since 2006

[coachboyd]

Actually, clinchers have a better rolling resistance than tubulars, so the data you are seeing is just a confirmation of that. Tubulars definitely have their advantage with a rounder tire and ride. . .that's why I've been racing them since I was 15. But for pure rolling resistance, clinchers win.

The easy answer is just going to be to change the wheel diameter. The changes you are going to see due to a different size tire are going to be so small that it's not worth it to redo the whole calibration. When I switch from my training wheels to my race wheels I can't even detect the differences (so I just leave the same profile). I can't say that for my TT bike. . .there I see a big difference with the wheels and race setup.

[jasonod]

Well I'll be. I always thought tubulars had the better rolling resistance. Thanks for clarifying.

The tire change in question would be for my TT bike; I ride a 22mm front but carry a 19mm spare. Based on your response I will just change the wheel diameter *if* I ever need to put the 19mm on.

Thanks!

[coachboyd]

http://www.rouesartisanales.com/article-1503651.html

Here is a great article about rolling resistance for tires. Keep in mind that some of the numbers you see for crr are based on the tire being in a controlled environment. Asphalt conditions will affect crr as well, the good thing is most changes in asphalt will not account for a big enough change to even detect a change in watts. This means when changing from good to bad asphalt, you can't look at the power meter comparison and tell where that change happened.