Originally Posted by
cyccommute
First, consider what the difference between 40mm, 37mm and 35mm tires are. 3, 2 and 5mm. 5mm isn't even a quarter of an inch. 3mm is just shy of 1/8" and 2mm is just shy of 1/16". Are those really that large a difference? Going from a 40mm to a 37mm is a trivial volume difference. Going from 40mm to 35mm is still trivial.
Consider the difference at the contact patch which is the only place it really matters. A 3mm change in height is like adding a nickel under your tire. Is it really that radical a change?
I think you're mistaken in saying "hey, 2, 3, 5mm is not that much". Consider the % increase in widths:
35-37: 5.7%
37-40: 8.1%
35-40: 14.3%
Consider the contact patch. Let's assume it's a square (and it depends on the rider weight, tire pressure, etc.) - now we're dealing with squaring:
35-37: 11.8%
37-40: 16.9%
35-40: 30.6%
That contact patch lets you run at a lower pressure but also probably results in a more tire deflection. If we were to calculate the 'trivial volume' difference in percentages, it would also be quite large.
Originally Posted by
Tourist in MSN
Weight has very little to do with it. A supple casing, especially if used with a thin tread requires less effort to roll down the road. But some tires have a stiff casing, or a thick tread, or a layer of soft rubber that consumes a lot of energy as you flex it, those tires are slower.
Bingo. IMO, if you're already moving, then you don't need to worry about acceleration all that much and air resistance is negligible given the surface area of your body plus your gear. But if you're working against a soft tire which is deflecting a lot, you're expending more energy to go the same speed. Yes, you can be fast on thick, flexible, or knobby tires, but the question is does it make you more tired in the end? A stiffer, thicker tire can ride as well or better than a soft, thinner - or underinflated - tire.
Tourists don't usually use power meters... so you wouldn't know it in the field. but if you wanted to do an experiment, take the thicker, flexier tire and do a 3-5 mile ride at, I don't know, 150 watts (or whatever round number you can hold that long). Then switch to thinner tire and under the same conditions repeat the ride with the same wattage. Compare the speeds. Or do it backwards, ride at the same speed and compare the wattage.