Subject: 8b.31  Rolling Resistance of Tubular Tires
From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Tue, 2 May 2006 10:25:45 -0700

Rolling resistance or rolling drag caused by flexing tires has been
misunderstood, judging from some tires found on the market.  The cause
of rolling resistance has at times been attributed to scrubbing
between tread rubber and road, when in fact it arises almost entirely
from flexing of rubber, or better yet, the elastomers in a tire.  This
includes the binder between casing cords, tread rubber, and inner tube
rubber, all of which flex and absorb energy.

Most elastomers (mainly forms of rubber in this case) have hysteresis,
meaning they do not rebound with the same force with which they are
compressed.  As a tire rotates into contact with the road, its
deformation requires more force than is returned when it lifts off
again.  This puts the center of pressure ahead of the center of the
contact patch.  In contrast, sitting still the center of pressure lies
in the center of the contact patch.

This effect makes the wheel perform as though it were rolling up a
slight incline, even though the road is level.  This effect can be
measured in various ways, the most obvious of which is to measure drag
directly with a scale, a method that is difficult.  Another method is
to record the time a loaded freely rolling wheel on a test drum takes
to coast to a lower speed, typically from twenty to ten miles per
hour.  With a calibrated load and drum, rolling resistance can
accurately be determined from the measured time.

IRC (Inoue Rubber Co.) tested a series of tires at the time that their
smooth tread tires were being introduced in the USA by Avocet.  The
goal of these tests was to show that slick tires had lower rolling
resistance than similar tires with grooved tread patterns, and in
particular, tires that had a raised center rib that was intended to
reduce rolling resistance by making less contact with the road.

The results were plotted on graphs that showed differences and did so
convincingly because each tire was tested over a range of inflation
pressures.  This gave curves that show consistency of measurement and
tire response to inflation, instead of giving one value per tire.

The graph shows something that had previously been a mystery in the
days when tubular tires were still commonly used.  Tubular tires are
attached to rims with glue that is offered in two main kinds, reusable
tacky glue that allows tire changes on the road, and hard glue used
primarily on race tracks.  From the Italian labels, Tipo Strada and
Tipo Pista for road and track, the use was obvious...  but why?

The reason for track glue became apparent in the graph that shows two
high quality racing tubulars, known have excellent performance.  These
two tires had a higher rolling resistance than thicker clincher tires
whose similar curves could more or less all be generated from a single
tire by multiplying its rolling resistance and re-plotting the data.
However, they showed by their flat response to inflation pressure,
that they had low rolling resistance and that something else was
occurring.  Their offset was caused by something that does not respond
to inflation pressure, losses from squirming on rim glue.

That tubular tires move on rims, and often wear through their base
tapes where they bear on the glued rim, was known but that it made a
significant difference in rolling resistance was not.

Hard tubular glue prevents visco-elastic motion between tire and rim.
>From the existence of hard glue this effect was apparently understood
in the early days of racing, becoming a misunderstood oddity after
WWII.  Much of the technology that was present in those days was not
recorded or handed down to the next generation.  Interestingly the
question about the two types of rim glue was answered in these rolling
resistance tests long after common use of these tires had passed.