Originally Posted by
Racer Ex
Was your crr constant regardless of speed? Or was it based on a computation of the sum of all inputs? If the most common theory about why there's a tipping point at the top end of pressure holds true, than speed would be a large factor in crr.
As explained in the links above, the Crr was calculated using Robert Chung's "virtual elevation" method and holding the CdA constant. Many sources show that Crr, and thus the rolling resistance force (Rolling Resistance Force = Crr x mass x gravity), is constant with speed. The
power to overcome that resistance is, on the other hand, proportional to speed...and that's included in the calculation method.
Just out of curiousity, but what is your understanding of "the most common theory about why there's a tipping point"?
Originally Posted by
Racer Ex
As far as the "feeling" vs. actual performance, my data was based on PM readings from three different pressures. Like you, they were my tires on my bike on my road at my weight at that time.
Can you tell us more about your test methodology and the results?
Originally Posted by
Racer Ex
I'd be interested in reading Zipp's take on the pressures being solely to protect their rims from damage, I was running well above their recommended when I broke my two.
Although I don't fully "buy" everything "JoshatZipp" says in the link below, here's a key quote:
"...Finally, on a tubular, higher pressure actually protects the rim from impact damage on rough roads, but of course it also decreases comfort, increases rider fatigue and decreases grip..."
http://forum.slowtwitch.com/gforum.c...otect;#1322087
Originally Posted by
Racer Ex
Sure you've seen this, but others may not have.
Yup...and the Vittoria KS tires listed in that chart that were tested vs. pressure on a smooth drum, and also with an attempt to add some bump amplitude are actually THE EXACT SAME TIRES I used in the testing to produce that plot above. Al sent me the tires for the test
Originally Posted by
Racer Ex
For me the bottom line is that there are a tremendous amount of variables here, without testing each tire on a variety of surfaces under a variety of loads at a variety of pressures and speeds, we're making semi educated guesses. You're mileage may vary. In the two files from different tests I have, the Veloflex carbons either rock the world or get beat out badly by most other tires. Go figga.
Except, there are some general trends which can be gained from smooth roller testing that apply to non-smooth surfaces since the mechanisms which give low Crr on a smooth surface are the same mechanisms that give good bump absorption performance.
As far as different tests giving different results for certain tires goes...one always has to be careful that the tires are of the same construction (sometimes changes are made in the same model) and that the testing is consistent. But, in general, most of the testing reveals the same trends...Light, flexible casings with thin treads, latex tubes, and rigidly attached to a rim, make for a fast tire
Originally Posted by
Racer Ex
Having been through the wind tunnel I'm pretty convinced that field testing for crr is a fool's errand, though I'm sure we'll keep trying. Extremely subtle changes in head position, hand position, leg position can all effect cda by 20 watts or more.
Sounds like you need to work on your field testing protocol
I've found my repeatability for testing CdA to be within .002 m^2 of CdA for identical setups in separate test runs on the same day