In article <
[email protected]>,
Booker C. Bense
<bbense+rec.bicycles.tech.rec.bicycles.racing.May.03.06@t
elemark.slac.stanford.edu> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
>
> In article <[email protected]>,
> Michael Press <[email protected]> wrote:
> >In article
> ><[email protected]>,
> > Dave Larrington <[email protected]> wrote:
> >
> >>
> >> One explanation I have seen advanced is that rolling resistance depends
> >> on the /circumference/ of the contact patch. The closer to circular the
> >> patch, the shorter the circumference. All other things being equal
> >> (which natch, they never are) the tyre with the least rolling resistance
> >> would be a sphere.
> >
> >Rolling resistance depends on two things.
> >
> >1. Tire tread. The more tread, the more resistance. The
> >energy is dissipated in deforming the tread.
> >
> >2. Side wall flex. The stiffer and thicker the side wall,
> >the more resistance. Energy is dissipated bending the side
> >wall.
> >
>
> You've described the mechanisms, but not how to minimize
> them. Since the tire flexes at the circumference of the
> contact patch, minimizing that circumference to minimize
> RR seems to at least be consistant. Although after some
> thought I think that you'd want to minimize the edge
> length in the direction of the highest curvature.
> (ie. shorter and wider is faster than long and skinny ).
Up to a point. There are conflicting effects. Wider tire
requires lower air pressure. Lower air pressure means more
side wall flex. Very high pressure increases energy
dissipation by way of road bounce.
Contact patch geometry may be correlated with rolling
resistance (or not; I do not know). Contact patch geometry
does not cause rolling resistance; this I do know. Side
wall flex causes rolling resistance. Side wall flex is an
invariant. It occurs at the contact patch and it is the
change in the shape of the side wall that supports the
rim.
Here are tire design criteria for reducing rolling
resistance.
1. Slick tread. No channels, lumps, knurling, or cuts.
2. Thin side wall; high thread count; thin threads.
Beware of tire manufacturers that misrepresent the thread
count.
3. No belts for puncture resistance.
4. Thin tread.
Interestingly, designing a tire for decreased rolling
resistance also improves its cornering.
Ultimately, marginal reductions in rolling resistance will
not be worth the price paid in other areas according to
one's own economy. (puncture resistance, off road, price,
....)
--
Michael Press