As rolling resistance would be a function of the weight they bear, one
would think that the larger distributed weight area (contact patch) would
result in a lower rolling resistance, as there would be less overall
rubber sidewall distortion.
However, they may have a lower fill pressure as well, which might be a
reason for a claim of higher rolling resistance.
Increasing contact patch length, whether by increasing weight or
decreasing pressure or decreasing tire width, increases sidewall
distortion.
If tire width is increased but pressure and weight are unchanged, the
contact patch length will decrease. I think that would decrease rolling
resistance.
One factor that appears significant is ratio of radii from axle center
to tire surface, at center of contact patch and at forward/rearward
extremes of the contact patch. The closer this ratio is to 1, the less
rolling resistance will be. One reason I see: Less rubbing of tread
against the road surface upon meeting and leaving the road surface. Also
less deformation counts at least a little, even other than by actual
scraping/rubbing of tread on road surface. It appears to me that 1 minus
this radius ratio strongly influences the rolling resistance coefficent.
And tread duration goes largely as:
* Directly proportionately with contact patch width
* Directly proportionately with depth of rubber to wear away
* Inversely proportionately with pressure
* Inversely proportional to either the above {1 minus radius ratio}, or to
actual rolling resistance coefficient (ratio of rolling resistance to
weight supported by the tire including its own)
As pressure increases, tire life tends to increase as long as product of
rolling resistance coefficient (or the above radius ratio) and pressure
decreases more than contact patch width decreases.
In general, tires for vehicles having 4 or more wheels have contact
patch width varying only slightly inversely with pressure, unless the
contact patch width is narrowed by a large pressure uptick, and then maybe
only after the tire has been "broken in" at the lower pressure. That
problem tends to require either overinflation or proper pressure after
experiencing significant wear while underinflated.
One more thing: Assuming tire design that has contact patch width not
varying much with pressure (or ratio of pressure to weight loading), the
above radius ratio is approximating the cosine of the angle between
straight down and from axle center to either forward or rearward end of
the contact patch.
This (1-minus-radius_ratio) in such case tends to be close to inversely
proportional to square of tire pressure (above atmospheric pressure).
That appears to me to indicate that things tend to get better with such
tires as pressure gets higher, as long as the pressure is not exsceeding
the pressure capability of the tire.
One more thing: Tire pressure is supposed to be measured "cold" - when
the tires are not heated up by using them. If the "cold" pressure does
not exceed the maximum rating for the tire, then the "warmed up pressure"
is supposed to be unable to be "excessive" unless the weigt loading
exceeds the tire rating for that, vehicle speed is excessive, or the
ambient temperature takes a jump big enough to be a considerable factor.
If you have excessive weight loading on your tires, then they are unsafe
at any pressure - their deformation varies directly with ratio of weight
loading to pressure, and any fatigue effects of that get worse when the
deformation is greater or the when "same deformation" occurs at higher
pressure - excessive weight loading is bad no matter what you do.
However, in my experience of overloading bicycle tires (by being
adventurous as a bicycle messenger by carrying heavier packages or
clusters thereof), I have found "least-worst" (still "living dangerously")
results from having pressure measured-cold at or a little above the
pressure marking on the sidewall of the tire - I don't see tires having
tolerance of excessive weight load improved by lower pressure; lower
pressure with excessive weight increases bigtime flexing of the sidewall,
and fatigue in the sidewall towards the bead.
- Don Klipstein (don@misty.com)
