Why does low pressure cost mileage?

On Feb 2, 9:34*pm, RichD wrote:
Low air pressure in tires retards fuel milege, but why?

The obvious answer is, greater friction. *But even though
the tire is flatter, still it rolls, there's no relative movement
between the pavement and tire, *hence no sliding friction.

So where and how do the losses occur?

Friction losses within the tire's material is certainly one way. I
think if you lowered one of your car's tires to, say, 20 psi and drove
on the freeway for a mile, you could easily detect that the tire was
hotter, indicating that it's sucking up energy. Admittedly, I don't
plan to try that test myself.

Think about dribbling a basketball. If the ball's soft, it doesn't
rebound nearly as well. Obviously more energy is lost. It's probably
going into the rubber.

Also, I think there would be more relative movement between the
pavement and tire, due to a sort of scrubbing action. Tires that are
consistently run with pressure that's too low tend to wear faster,
mostly toward the edges.

What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. But a bike can take more energy to
move if you have too much pressure. The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike a
rigid, unsuspended vehicle.

- Frank Krygowski

On Feb 3, 4:58*pm, Frank Krygowski wrote:

What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure.

I did not know more tire pressure = more resistance.

The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

From here, the difference seems to be your ability to imagine you know
what you're talking about.
-----

- gpsman

On Feb 4, 10:19*am, gpsman wrote:
On Feb 3, 4:58*pm, Frank Krygowski wrote:



What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure.

I did not know more tire pressure = more resistance.

Generally, more pressure reduces rolling resistance.

It is possible that above some pressure, and depending on the road
surface roughness, that more vertical energy is transmitted to the
rider, who absorbs vibration energy and does not return it to forward
momentum.

The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

From here, the difference seems to be your ability to imagine you know
what you're talking about.

I don't know that there has been an awful lot of conclusive work done
in this field due to the variability of roads, riders and conditions,
that make measurements of this kind difficult. We are only talking
very small amounts of energy. Maybe 1% of the riders output? (3W in
300W to hazard a guess.)

Tyre rolling resistance tests are performed on a smooth steel drum, so
that the manufacturers can have a standardised test and end users can
compare, however few of us ride on smooth steel drums or on boards.

--
JS.

On Feb 4, 12:39*am, James wrote:
On Feb 4, 10:19*am, gpsman wrote:

On Feb 3, 4:58*pm, Frank Krygowski wrote:

What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure.

I did not know more tire pressure = more resistance.

Generally, more pressure reduces rolling resistance.

It is possible that above some pressure, and depending on the road
surface roughness, that more vertical energy is transmitted to the
rider, who absorbs vibration energy and does not return it to forward
momentum.

The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

From here, the difference seems to be your ability to imagine you know
what you're talking about.

I don't know that there has been an awful lot of conclusive work done
in this field due to the variability of roads, riders and conditions,
that make measurements of this kind difficult. *We are only talking
very small amounts of energy. *Maybe 1% of the riders output? *(3W in
300W to hazard a guess.)

Tyre rolling resistance tests are performed on a smooth steel drum, so
that the manufacturers can have a standardised test and end users can
compare, however few of us ride on smooth steel drums or on boards.

--
JS.

Is it that long, do we really need the story of "how I still havn't
learnt to stick a plaster on an inner tube" and "We woz so clever that
our poor tyre gluing was caused by our brakes.". Time for bed.

On Feb 3, 2:58*pm, Frank Krygowski wrote:

What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure. *The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

- Frank Krygowski

Frank, why don't you think about this a little bit and see if you
might want to write something more complete and accurate.
Unless, of course AFAYK there could be NO condition under which an
automobile might suffer greater decreased mileage from too high a
pressure.
Hint: Ever used a wheelbarrow?

DR

On Feb 3, 6:19*pm, gpsman wrote:
On Feb 3, 4:58*pm, Frank Krygowski wrote:



What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure.

I did not know more tire pressure = more resistance.

It depends on the relative roughness of the road surface, and on the
relative magnitude of suspension losses vs. hysteresis losses in the
rubber. On a board track, extremely high tire pressures are better;
but on a board track, suspension isn't needed.

For the opposite extreme, try riding a rigid mountain bike on gravel
about 3" diameter, as I've done on an abandoned railroad bed. Even 50
psi made it very difficult to ride. Lowering tire pressure to just
prevent pinch flats made it much easier.

Most roads are between those extremes. But if the road has any
roughness at all, there will be some tire pressure above which power
requirements will be greater due to greater suspension losses. If that
weren't true, we'd dispense with the air and go back to solid rubber
tires, as hard as possible.

The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

From here, the difference seems to be your ability to imagine you know
what you're talking about.

"Seems to be" speaks about the observer's opinion, and perhaps
capabilities. But take heart. You can still learn.

- Frank Krygowski

On Feb 4, 12:44*am, Frank Krygowski wrote:
On Feb 3, 6:19*pm, gpsman wrote:

I did not know more tire pressure = more resistance.

if the road has any
roughness at all, there will be some tire pressure above which power
requirements will be greater due to greater suspension losses. If that
weren't true, we'd dispense with the air and go back to solid rubber
tires, as hard as possible.

False premise; fails to consider weight. We needn't delve into the
relative "roughness" of road surfaces.

You can still learn.

I'm sure there's something you can teach me, but this ain't it.
-----

- gpsman

gpsman wrote:

Frank Krygowski wrote:

What seems to be different about bikes and cars is that, AFAIK, a
car's gas mileage doesn't decrease if you have tire pressure too high,
at least within reasonable limits. *But a bike can take more energy to
move if you have too much pressure.

I did not know more tire pressure = more resistance.

The difference seems to be
related to lack of mechanical suspension on a bike; the bike tire is
typically the only suspension, and too much pressure makes the bike *a
rigid, unsuspended vehicle.

From here, the difference seems to be your ability to imagine you know
what you're talking about.

No, it's you.

When you inflate tires hard enough to yield a harsh, jarring ride,
where do you think the energy required to jar you comes from? Shaking
a couple hundred pounds of sloppy flesh and bone can waste far more
energy than you can dissipate in tread rubber.

Chalo

hoop snakes.

less pressure equals greater contact surface

greater contact surface and less pressure deforms designed contact
surface
now surfaceS
surfaceS change shape=loss
surfaceS grip road channels as small road walls, opposing pothole
walls, deflect travel direction continuously
and on
low pressure morphs a designed solid surface into a road sponge
I cant believe I wrote this

On Feb 4, 4:51*am, Chalo wrote:

When you inflate tires hard enough to yield a harsh, jarring ride,
where do you think the energy required to jar you comes from? *Shaking
a couple hundred pounds of sloppy flesh and bone can waste far more
energy than you can dissipate in tread rubber.

Ipse dixit.
-----

- gpsman