Thread: 25s vs. 23s
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Old 02-14-19, 09:22 PM
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79pmooney
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In line with HTupolev's post - the faster you ride, the smaller (percentage-wise) rolling resistance is to the total resistance the rider has to overcome. Rolling resistance increases linearly with speed, air resistance with the square of speed. To show this trend, look at this scenario (totally fictional). Say at 15 mph rolling resistance is 10% of the total and air resistance is 30%. And that the rest, like rolling resistance varies linearly. Say we have 100 units of total resistance so rolling = 10, aero = 30, the rest = 60.

Now look at 20 mph. Rolling = 20/15+x 10 = 13.3. Aero = (20/15)^2 X 30 = 53.3. The Rest = 20/15 x 60 = 80. Rolling = 9%, Aero = 36% of the total.
Now look at 25 mph. Rolling = 25/15+x 10 = 16.6. Aero = (25/15)^2 X 30 = 83.3. The Rest = 25/15 x 60 = 100. Rolling = 8%, Aero = 42% of the total.
Now look at 30 mph. Rolling = 30/15+x 10 = 20. Aero = (30/15)^2 X 30 = 120. The Rest = 30/15 x 60 = 120. Rolling = 7.7%, Aero = 46% of the total.

(I'm guessing my figures are high for rolling resistance and quality tires and the "Rest" by a lot but that air resistance is considerably higher. I'm not going through the math again, but I think you'll get the point.)

So you can see that as speed increases, the tires matter less and less. Yes they are important enough that you won't even get to high speeds with clunker tires unless you are a muscular bull, but you can see that the difference between a high quality 25c and the same 23c is going to be very little, but that at high speeds, you might want to look at the air resistance of the tires. (But also the possible cost of flats from poor road surface and the effort required to chase back on if you are racing - a very real cost - and the confidence of cornering a little harder and faster on wider, more supple (due to lower pressure) tires.

An example - the Sky team last year raced the Tour Down Under with the whole team on 25c tires, not the usual 23c's citing reconnaissance of the road surfaces.

Ben

Last edited by 79pmooney; 02-14-19 at 09:37 PM.
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