tire resistance vs width

In article <[email hidden]>,

Quoted message said:

They saw large differences across the spectrum of tires they compared,
which included a few race tires, some mid fats, and some 650Bs. The
fastest was a narrow tire, but one of their general findings was that
when comparing several widths of the same tire model, the widest one
had the lowest rolling resistance.

Did they use appropriate tire pressures for the wider tires or did they
over-inflate them? See the tire pressure discussion on Sheldon Brown's
web site.

On Tue, 26 Sep 2006 18:26:13 +0000 (UTC), [email hidden] (sally)

Quoted message said:

In article <[email hidden]>,

Quoted message said:

They saw large differences across the spectrum of tires they compared,
which included a few race tires, some mid fats, and some 650Bs. The
fastest was a narrow tire, but one of their general findings was that
when comparing several widths of the same tire model, the widest one
had the lowest rolling resistance.

Did they use appropriate tire pressures for the wider tires or did they
over-inflate them? See the tire pressure discussion on Sheldon Brown's
web site.

Dear Sally and Erik,

Here's the Bicycle Quarterly site:

http://www.vintagebicyclepress.com/biketests.html

Here's Sheldon's take on tire inflation:

http://sheldonbrown.com/tires.html#pressure

As for width of the same model tire, the area of the contact patch
remains the same at the same pressure, with a narrow tire producing a
longer, narrower contact patch and a wide tire producing a shorter,
wider contact patch:

XXXX (narrow tire)

versus

XX
XX (wide tire)

In general, the shorter, wider contact patch reduces rolling
resistance on smooth surfaces at the same pressure because a shorter
section of the sidewall is flexing.

The usual debate is over how much the tire inflation should be
adjusted for comfort.

Narrow, high-pressure tires reduce rolling resistance by reducing the
sidewall flexing--the harsher ride means that the sidewalls aren't
flexing much.

Wide, low-pressure tires increase comfort by alllowing lots of
sidewall flex without pinch flats--the sidewalls are working as
springs.

When worrying about such details, it's worth remembering that the
wider tire that reduces rolling resistance has a nasty habit of
increasing wind drag. Compared to a pair of 23mm 700c tires, a pair of
32mm wide tires looks like about 4 upright wind vanes (front and back
on each tire), each about 680mm high and 9mm wider. That works out to
4 x 9 x 680 = 24,480mm^2 extra profile, or about 38 square inches,
roughly a 6x6 square. At normal speeds, the increased wind drag from
the wider tire would overwhelm any decrease in rolling resistance.

Cheers,

Carl Fogel

Narrower tires can sustain higher pressures, giving
them less and a smaller contact patch. And still they come
out on the wrong end of the rolling-resistance
competition.

Which means that the higher pressure isn't the cure. Maybe
it's the cause. Maybe having a lower pressure and complying
with the shape of the road surface, rather than bouncing off
every rock, pebble, bump, stick, crack, feather, and dead
rabbit, is the reason wider tires see the road as smoother...

--Blair