I've been seeing a lot about this sort of thing in the last couple of
years. The biggest argument in favor of thin tires is supposedly reduced
air drag, but...

With every tire at a given pressure and a specific weight, the contact
patch is the same in area. That is to say, if you are particularly slim
and ride a light bike and have 100 psi tires and the weight distribution
is exactly 50-50, each tire will have one half of one square inch in
contact with the ground. That's using a gross weight of 100 pounds.
I've seen vehicle scales that use exactly this technology.

The difference between a 23mm tire and a 28mm tire, and that of even
larger sizes, is the shape of that contact patch. On the skinny tire,
the patch is ovalized in the direction of travel, while the fatter tire
has an oval with the major axis perpendicular to travel. What this means
is that the tire must deflect more in the skinny tire as it rotates,
eating up energy in the process.

That's how I've been reading it, almost verbatim.

By the way, how do you get 23 cm tires to fit on a frame? That's almost
ten inches wide, isn't it? (big grin) I have seen tires that wide for a
specially constructed recumbent, for use in the desert sands and like
terrain.

fred
--- In usarecumbents@yahoogroups.com, "Terry Farrell" <mfarrel2@...>
wrote:
>
> I ride a 2006 Rans Stratus XP with dual 26" Velocity wheels and
> Schwalbe 559-28 (26" x 1.1") tires. I also have a set of the same
> Velocity wheels (same rims and hubs, but fewer spokes) in the 650
> size from my Bachetta that have Continental Ultra Gatorskins 571-23
> (650 x 23 cm) on them.
>
> I was curious if the 650 wheels with their thinner tires might have
> less rolling resistance than my 26-inchers. So I roll-tested both
> sets of wheels/tires. There is a hill near my home with a relatively
> steep initial slope and then levels off to a steady shallow slope. I
> started each run from a dead stop, accelerating by gravity alone. I
> noted the time elapsed at the bottom of the hill, 1.05 miles from
> the start. I did 3 runs with each wheel/tire combo. Top speed was
> 23.0 to 23.5 mph and the runs ended at around 12 mph.
>
> The only variable I could not control was wind. I had a very slight
> tail wind of about 3 mph - and it seemed fairly consistent - perhaps
> a little more tail wind with the 650 runs - there were a few flags
> with some movement, but when I tested with the 26" wheels, the flags
> were completely limp - so I may have had a tad more tailwind during
> the 650 tests.
>
> Below are the tabulated results:
>
> 571-23
> Distance Seconds Ave.Spd.
> 1.05 254 14.88
> 1.05 253 14.94
> 1.05 246 15.37
> 1.05 251 15.06 Average
>
> 559 - 28
> Distance Seconds Ave.Spd.
> 1.05 243 15.56
> 1.05 258 14.65
> 1.05 251 15.06
> 1.05 251 15.08 Average
>
> If these numbers are to be believed as meaningful, it would seem the
> two sets of wheels/tires have approximately the same rolling
> resistance. Both types of tires were pumped up to between 120 and
> 125 psi. Perhaps it is true what they say about fatter tires at the
> same pressure have lower rolling resistance than thinner ones.
>
> The wider tires have a noticeable smoother ride. I think I'll be
> keeping the 559s on rather than going to the 650s.
>
> Hope this is of interest to someone.
>
> Terry Farrell
> Tampa, Florida
> 80 degrees & sunny today!
>