In the tire rolling resistance data above, tires were mounted on the same rim with the tube offered for that
tire by the manufacturer. Tubulars (2) were glued onto the rim with
standard practice using tubular road glue.|
All tires were run against a smooth steel drum with the same load
at a range of inflation pressures. What is shown by fitting a power
curve to the points is that the data is consistent with little scatter,
and that all tires fit into a family of rolling resistance curves that do
not cross and are essentially the same equation. The exception was the
tubular tires that were offset by a constant from their proper curve
so that they crossed other tires.
This constant offset was causes by rim glue losses that depend on load
alone and are not affected by inflation pressure. You can prevent these glue losses
by gluing tires with hard glue such as "Tiop Pista" glue if
you find some and shellac works as well except that it must be built
up with several coats until it has some thickness. The tire base
tape must also be coated. The last coat on the rim is allowed to get
tacky hard and then the tire is mounted and pumped up hard. It's a
pain and was not done often, especially because no one knew why that
glue should be used.
Tires with low rolling resistance don't get much better the harder they are
inflated and have a flatter characteristic over the range of inflation
pressures while high rolling resistance tires have a steeper slope,
getting better the less they flex but never as good as a low RR tire
except toward infinite pressure where none would have any flex or RR.
Of course these tires all had smooth or nearly smooth tread patterns.
The casing is a thin walled fabric that supports the tensile stress of
containing the inflation pressure. This casing is circular in cross
section wherever it is not in contact with the rim for both tubulars
and clinchers. Where the tire is in contact with the rim, it has no
effect on tire compliance of supporting a wheel load. Therefore, the
open face of the clincher as well as the underbelly of the tubular have no
effect on tire deflection.
Rolling resistance is caused by rubber deformation losses in the
tread, the tube, and the casing. The tube is firmly attached to the
casing by inflation pressure so it is like a tread inside the tire.
The whole tire flexes in three dimensional space, X, Y and Z. You can
verify this typically by laying a standard business card between tire
and tube. It will shred to fine confetti when ridden. It is this
motion that causes hysteretic loss in the elastomer in the casing, the
tread, and the tube. If the tire has patterned tread, such as a knobby
tire has in the extreme, deformation of the tread into the voids will
cause additional loss. That is why knobby tires roll so poorly.
The folly of extremely high inflation pressures becomes obvious when
looking at the curves. Good tires do not effectively benefit from 10
or 20 PSI more pressure. The RR values are in the few grams of force.
You'll also notice that tire cross section does not equal more or less
rolling resistance but that material thickness is the loss. Smaller tires flex more
than larger ones of the same casing and have greater losses.