Re: 700/23 vs 700/25 tires ?

On Feb 11, 11:53 pm, "b...@xxxxxxxxxxxxxxxxx" <bjwei...@xxxxxxxxx>
wrote:
On Feb 11, 6:08 pm, Frank Krygowski <frkry...@xxxxxxxxx> wrote:



On Feb 11, 4:27 pm, Peter Cole <peter_c...@xxxxxxxxxxx> wrote:

What's the CdA difference between a pair of 28mm and 21mm tires? I get
around 0.01m^2. If a typical bike & rider CdA is between 0.3 & 0.5, it
looks like 2-3%, which puts it in the 4-6W range. If you sub the ~3W
from the better Crr, the $17 28mm tires are only ~3W lossier than the
$60 21mm tires. Am I missing something? (besides the 160g weight diff
for the pair).

One possible factor would be that a slightly wider tire might better
shield the spokes from the relative wind.  IOW, the tire's frontal
area would tell you more drag; but the net drag for the wheel could go
the other way.  Testing with a moving wheel - or a wind tunnel with a
moving floor surface - would be the only way to tell, I think.

And of course, the rear wheel sees a much different, more shielded
environment.  Another reason to consider two different sizes front &
back?

I think if there is a transition between tire and
rim that is not smooth, you're going to increase
the dirtiness of the airflow downstream of it,
and the spokes are in that.  This is likely to
be more of an issue than shielding - for a
relatively bluff body, which the tire+rim is, you
can't really shield something that is more
than a few body diameters downstream.
Just as one really needs to be pretty close
to a rider to effectively draft him or her.

Yes, that's possible. Again, I think wind tunnel tests with a moving
floor surface would be the only way to tell.

- Frank Krygowski
.

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