noisebeam

Just wanted to share some observations:

For a 26x1.95 Shimano Crossroads ex tire at 80psi:
Cateye Astrale 8 manual recommends setting circumference to 2050mm. By using the mark tire with paint and roll out with me on bike (235lbs inc. bike) I get 2057mm. A very minor (0.3%) difference.

For a 700x28C Continental gatorskin at 100psi:
Cateye Astrale 8 manual recommends 2136mm. Roll out method (220lbs inc. bike) I get 2093mm. That is a 2% difference, significantly larger, but still not huge (the difference between a 100mi and a 102mi ride)

I think I've read that some Continental tires are undersized to what is marked on tire. (That is a 28mm tire is more like 25mm in true size). Apparently some manufacturers do this so if folks compare tire weights for marked sizes the tire that is actually smaller will then be lighter. I didn't check, but I'll get the calipers out tonight.

But maybe there is something else going on. For what its worth my commute reads 9.58mi on multiple rides for both bikes after roll out calibration.

Any thoughts? Any other experiences?

Al

Pittrider

Read the article under Sheldon Browne for calculating settings. This really set me straight on what number should be used. I always used the Nashbar suggested numbers and found that I was 2% off as well. Every 20 miles I would gain .25 over my riding partner. Sheldon's formula for a 700 x 23 is 622 +23 + 23 =668 * 3.14 =2097

noisebeam

Interesting that you mention Sheldon's tire sizes. I looked at those as well here are the results:

Tire: ---------------- Cont. gator 700x28C --- Shimano C.Rd. 26x1.95
Cateye Manual: --------------- 2136 --------------------- 2050
Roll-out measured: ------------ 2093 --------------------- 2057
Sheldon - radius calculated: - 2111 --------------------- 2036
Sheldon - look up table: ----- 2136 --------------------- 2055

I trust the roll out measurement the most as it eliminates all other variables (tire pressure, unaccurate tire labeling, rider weight, etc.)

So is the continental gatorskin 700x28C really a 23mm tire?

Al

Don Cook

I have three bikes with computers. In each case the computer sizing chart was incorrect. I also roll out the tyre on the garage floor with me on the bike to get the correct reading.

P. B. Walker

I always do a roll out. A roll out is always going to be more accurate. My method is to do 3 separate roll outs and then average the 3.

noisebeam

Yeah, that is why I did the roll out and did average four tries (each was within 2mm of the other.) My neighbors must have thought I was nuts as I did it after dark with a flashlight. There I was closely inspecting the road on my hand and knees with a flashlight and tape measure.

The main reason for this posting was I was surprised at how big a difference from the recommended chart (2136) compared to the roll out (2093). It is like my 700x28 tires give the circumference of a 700x23 tire. Has anyone else noted such a difference?

Al

P. B. Walker

I quite looking at the recommended charts a long time ago, but back when I did, I noted the same thing... big differences, which is why I've always gone with the roll out. All tires are different. And when you add in rider weight, and different tire pressures, you almost HAVE to use a roll out if you want to be any where near accurate.

AndrewP

I think it is because your weight on the tire reduces the effective radius