Benjamin Lewis said:Quoted message said:http://w3.iac.net/~curta/bp/velocityN/velocity.html
Using the calculator above (the metric version offers
rolling resistance and frontal area), let's try a few values
and see what we get.
The default values predict a brisk 37.18 km/h.
Reduce the rolling resistance 5% from 0.0050 to 0.0045, and
the speed rises to 37.40 km/h, an increase of 0.22 km/h.
But if that 5% reduction in rolling resistance also
increased the frontal area from 0.4m^2 to 0.40544m^2, then
the speed drops to 37.24 km/h.
So the combination of a wider tire that rolls more easily is
predicted to increase a rider's speed only 0.06 km/h, or
sixty meters in an hour--a convenient 1 meter per minute.
I note that for these defaults, the two tires become equal at a little
below 44 km/h. (set the power to 473.5 watts)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Quoted message said:
(This is assuming no wind and no drafting)
Dear Benjamin,
I found this memo stuck to your post in my newsreader:
# To: Captain Picard
#From: Commander G. La Forge, Chief Engineer
#
#I'd like to make it so, sir, but I left my 473.5 watt
#dilithium crystals in my other pants. Maybe that big
#Klingon sprinter can help you.
Naturally, I harnessed the big Klingon, told him that Lance
doubted that anyone with a forehead like a turtle could put
out a thousand watts for an hour, and found that the two
tires are still just about equal:
57.26 km/h wide (0.40544 25mm frontal) 0.0045 rr (imagined)
57.37 km/h thin (0.40000 23mm frontal) 0.0050 rr (default)
Of course, the 10% reduction in rolling resistance is just a
vague guess at the best that could be expected in reducing
rolling resistance by making the same tire 2mm wider, just
as the 0.0054m^2 increase is just a vague guess at the wind
drag increase for the wider tire.
But if adding 526.5 watts only lets the thinner tire pull
ahead 0.11 km/h, then we know why Commander Spock is leaving
the calculations to Commander Data. At low and therefore
plausible bicycle power levels, there isn't much difference
between the two tires. At outlandishly high levels, the tiny
differences make even less difference.
Which wasn't what I thought. I expected the wind drag to
matter more and more relative to the rolling resistance (it
does, since the thin tire pulls ahead), but overall it
matters less and less in terms of how much it affects the
actual speed as power increases.
That is, at 1000 watts, neither the tiny extra wind drag of
the wider tire nor the little bit of extra rolling
resistance of the thin tire can have much effect on the
speedometer reading.
So thanks for pointing out where the two imaginary tires
match--it made me think again.
Carl Fogel
