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Rec.Bicycles Frequently Asked Questions Posting Part 3/5
Section - 8b.14 Rolling resistance of Tires

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Date: Fri, 13 Feb 2004 12:07:59 -0800

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 integral part of the
tire.  The tread and the tube together absorb the majority of the
energy lost in a 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 tread rubber bulges and deforms into voids in the tread
pattern when the tire bears on the road.  This effect, called tread
squirm, is mostly absent with smooth tread because tread rubber cannot
bulge laterally on road contact because rubber, although elastic, is
incompressible.

Small cross section tires experience more deformation than a large
cross section tires 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 but not 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 increasing 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 far less than poorer ones with increasing inflation pressure.

As is mentioned in another FAQ item "Mounting Tubular Tires", tubular
tires, although having lower tire losses, performed worse than
equivalent clincher tires because tubular 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.

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 3/5
Previous Document: 8b.13 Tires with smooth tread
Next Document: 8b.15 Wiping Tires

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