Subject: 8b.14   Rolling resistance of Tires
From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Tue, 2 May 2006 10:25:45 -0700

The question often arises whether small cross section tires have lower
rolling resistance than a larger ones.  The answer, as often, is yes
and no, because unseen factors come into play.  Rolling resistance of
tires arises almost entirely from flexural rubber losses in the tire
and tube.  Rubber, especially with carbon black, as is commonly used
in tire treads, is a high loss material.  On the other hand rubber
without carbon black, although having lower flexural losses, wears
rapidly and has poor wet traction.

Similar to tread rubber, inner tubes are pressed so firmly against
tire casings by inflation pressure that they become integral parts
that flex with the tire.  Tread and tube together absorb the majority
of the energy lost in rolling tires while inter-cord binder (usually
rubber) of casings comes in behind.  This is perceptible manually by
flexing the casing in the tread and in the side wall.  Against these
losses Tread scuffing on the road is insignificant.

Patterned treads (tread grooves or knobs) measurably increase rolling
resistance over slicks, because with voids, tread rubber can bulge and
deform into these spaces when pressed against even a glassy smooth
road.  This effect, called tread squirm, is mostly absent with smooth
tires because rubber, being incompressible, cannot move laterally.

Small cross section tires flex more than larger ones at the same
pressure and therefore, should have greater rolling resistance, but
they generally do not, because large and small cross section tires are
not otherwise identical.  Large tires usually have thicker tread and
tubes, besides having thicker casings and allowing higher inflation
pressures.  Therefore, small cross section tires generally have lower
rolling resistance from thinner construction, rather than from a
smaller contact patch to which it is often attributed.

These comparative values were measured on various tires over a range
of inflation pressures that were used to determine the response to
inflation.  Cheap heavy tires gave the greatest improvement in rolling
resistance with increased pressure but were never as good as high
performance tires.  High performance tires with thin sidewalls and
high TPI (threads per inch) were low in rolling resistance and
improved little with increasing inflation pressure.

As is mentioned in FAQ: "Mounting Tubular Tires", tubulars, although
having lower tire losses, performed worse than equivalent clincher
tires because the tubular's rim glue absorbs a constant amount of
energy regardless of inflation pressure.  Only (hard) track glue
absolves tubulars of this deficit and should always be used in timed
record events.

See: http://bike.terrymorse.com/rolres.html