This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <firstname.lastname@example.org>
Date: Thu, 08 Aug 1996 17:17:57 PDT
The question often arises whether a small cross section tire has lower
rolling resistance than a larger one. The answer, as often, is yes
and no, because unseen factors come into play. Rolling resistance of
a tire arises almost entirely from flexural rubber losses in the tire
and tube. Rubber, especially with carbon black, as is commonly used in
tires, is a high loss material. On the other hand rubber without
carbon black although having lower losses, wears rapidly and has
miserable traction when wet.
Besides the tread, the tube of an inflated tire is so firmly pressed
against the casing that it, in effect, becomes an internal tread.
The tread and the tube together absorb the majority of the energy lost
in the rolling tire while the inter-cord binder (usually rubber) comes
in far behind. Tread scuffing on the road is even less significant.
Patterned treads measurably increase rolling resistance over slicks,
because the rubber bulges and deforms into tread voids when pressed
against the road. This effect, tread squirm, is mostly absent with
smooth tires because it cannot be bulge laterally by road contact
because rubber, although elastic, is incompressible.
Small cross section tires experience more deformation than a large
cross section tire and therefore, should have greater rolling
resistance, but they generally do not, because large and small cross
section tires are not identical in other respects. Large tires nearly
always have thicker tread and often use heavier tubes, besides having
thicker casings. For these reasons, smaller tire usually have lower
rolling resistance rather than from the 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 low 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 was mentioned in another item, tubular tires, 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.