Subject: Rolling Resistance of Tires
From: Jobst Brandt
Date: August 8, 1996
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: the tread cannot be bulged laterally by road contact, because rubber, although elastic, is incompressible.
[The tread can, however, bulge into dips in the road surface, one reason rolling resistance is lower on a smooth surface. -- John Allen]
Small-cross-section tires experience more deformation than 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. Smaller tires usually have lower rolling resistance for these reasons, 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) counts were low in rolling resistance and improved little with increasing inflation pressure.
As is mentioned at "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.
[Small cross-section tires generally run at higher pressure (and must, to support the load), so their deformation is not necessarily greater than that of larger cross-section tires. However since Brandt wrote this article, experiments conducted by Jan Heine and collaborators at Bicycle Quarterly magazine have shown that extra-high tire pressure and narrow tires do not reduce drag. Much drag is due to vibration, which is reduced when tires are run at moderate pressure; also, the ride is more comfortable and, I'd add, there is less bulging of the tread into dips in the pavement -- John Allen]
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