[python] Re: another newcomer
- From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Fri, 26 Jan 2007 12:40:51 +1100
On Fri, 2007-01-26 at 00:57 +0100, Ronald Hongsermeier wrote:
> While agreeing with you in the main, I think there may be a gray area
> where a thin layer of stuff that's not so stiff may have the effect of
> counteracting a negative rough surface effect, though I have neither
> numbers nor specific theory for this idea /*i.e., the stiffness of the
> surface of the tire may transmit energy into vertical motion which takes
> away from forward speed.*/
There are two factors here. One is loss of energy through flexing of
the tyre, pumping the air inside it, rubbing of cords and so on. The
other is loss of energy through the CoG of the bike rising and falling
over road bumps.
A tyre body dissipates energy when flexed, whether by deforming around a
bump or rolling on a smooth surface (because the different parts of the
body flex and unflex in turn, as the tyre rolls). The stiffer the tyre
body, the more energy it absorbs when it flexes, and converts into heat.
This flexing is reduced by inflating the tyre hard (but the knobs on
knobbies still flex).
Because the volume of a tyre rolling on a smooth surface is essentially
constant, the air inside it is not compressed and decompressed as it
rolls. Even when the air is pressurized by a bump, the air in the tyre
absorbs less energy than the body of the tyre when the tyre flexes, and
returns more of it when the tyre returns to its previous shape (no
internal friction, and modest pressure change and heat losses).
The overall effect is that a tyre with a compliant body which is
rendered stiff by the pressure of air inside it wastes less energy than
either the same tyre run at low pressure, or a tyre with the same
deformation, but a stiffer construction. On a perfectly smooth road,
you save energy by buying tyres with soft sidewalls and no knobs (like
Comp Pools) and pumping them up as hard as they will stand. The
softness of the rubber in the tread seems to be less important for this
purpose than how thick it is and how stiff the sidewalls are.
No matter why it is stiff, however, when a tyre rides up over a bump in
the road (a stone, say) the bike dissipates energy, by converting
kinetic energy into potential energy, most of which is wasted as heat
when you come down on the other side of the bump. If the tyre deforms
around the bump, the CoG of the bike is lifted less, and less energy is
wasted by this bouncing.
On rough roads, the energy lost by bouncing at high tyre pressure is
often more than is lost by tyre deformation at lower pressure. So for
rough roads, it may be worth while reducing tyre pressure to a point at
which the tyre will usefully cushion the bumps, although that means the
tyre will absorb more energy between bumps. This is a compromise, which
depends on how rough the road is. For instance, I generally run
Schwalbe Marathon Slicks on smooth sealed roads at 100psi (7 bar), but
after a friend did some roll-down tests and pointed out this issue to
me, I find that on some rough sealed roads, the same tyres run better at
85psi (6 bar). On gravel, even lower pressures might be better.
--
George Durbridge
+61 3 9481 3500
+61 409 413 945
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