jens wrote:
Tom A. wrote:
For some reason, I couldn't get as consistent results from my PT wheel data as from the crank PM...I think it might have to follow with the relatively low power levels of the testing and the resolution of the PT readings. I still use the PT wheel for most testing, but not its power output.
I'm still waiting on my Garmin speed sensor. In the meantime, I ran both tires again at 120 PSI and 25.2 mph.
Here's what I got:
Vittoria Corsa: 57 watts (crank); 55 watts (hub)
GP TT: 66 watts (crank); 65 watts (hub)
This is on the TT bike instead of the road bike, so the weight distr. and wattage are a little different for both.
Based on the crank power, that gives me CRRs of .030 for the GP TT and .0259
Since I ran the GP TT second this time, I figured I'd run it a bit more. I did observe that the by the 5th minute, the power had dropped from 66 watts to 63 watts at the crank and 65 to 63 at the hub. As has been suggested above, it could be that Conti tires improve disproportionately with temperature. That would certainly explain why historically they have done relatively poorly on my tests. I use the first stable reading, rather than allowing the tires to warm up.
IME, it's not so much that one type of tire heats up differently than another, than it is that higher Crr tires heat up more (in an absolute sense) than lower Crr tires...which makes sense from a physics standpoint.
That's also why my temperature compensation is in terms of %change per degree. I also found that when riding on the road, the tires DO end up hotter than ambient (with the delta larger for higher Crr tires), with the absolute number controlled by the ambient air temp. That's why I "correct" my results to 20C ambient.
All tires take quite a bit of time to heat up and stabilize, which is why I've settled on the protocol of running the tires for 5 minutes at 95 rpm, followed by a quick power meter zeroing, and then 4 minutes at 90 rpm. The data reported is then the average of the final 2 minutes of the 90 rpm stretch. With that initial higher power "charge" the steady state temp at 90 rpm seems to occur earlier. It also seems to result in a fairly steady power requirement in the final 2 minutes, but I can usually still detect some power drop during the first 2 minutes of the 90 rpm run if I look at the power trace carefully (i.e. power axis magnified).
Btw, in your Crr numbers you're stating, are you missing a zero?
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